ASM Metals Handbook - Desk Edition (ASM_ 1998) WW part 14 doc

14 Hydrogen embrittlement crack growth rate as a function of applied stress intensity for two different hardnesses and environments for an AISI 4340 steel contoured double-cantilever be

Constant displacement (K-decreasing) tests do not have the problems of the K-increasing tests The plastic zone ahead of

the crack tip does not increase with increasing crack size, so that the stress condition always remains in the plane-strain mode Also, the constant displacement tests can be self-loaded, and thus external testing equipment is not needed

Because in these tests the stress-intensity factor, KISCC, can be easily determined by exposing a number of specimens

loaded to different initial KI values This can even be accomplished by crack arrest in one specimen

A major problem with this test method occurs when corrosion products form in the crack, blocking the crack mouth and interfering with the environment at this crack tip Moreover, the oxide can wedge open the crack and change the originally applied displacement and load

Measurement of Crack Growth. In order to quantify the crack growth behavior in precracked stress-corosion

specimens, the crack length needs to be monitored, so that the crack velocity (da/dt) can be calculated, and the relationship between the increasing K and the crack velocity can be determined There are basically three methods to

monitor the growth of stress-corrosion cracks: visual/optical measurements, measurement of the crack-opening displacement using clip gages, and the potential drop measurement, which monitors the increase in resistance across two

on either side of the propagating crack

Tests for Hydrogen Embrittlement

HYDROGEN EMBRITTLEMENT is a time-dependent fracture process caused by the absorption and diffusion of atomic hydrogen into a metal, which results in a loss in ductility and tensile strength Hydrogen embrittlement is distinguished from stress-corrosion cracking generally by the interactions of the specimens with applied currents Cases where the applied current makes the specimen more anodic and accelerates cracking are considered to be stress-corrosion cracking, with the anodic-dissolution process contributing to the progress of cracking On the other hand, cases where cracking is accentuated by current in the opposite direction, which accelerates the hydrogen-evolution reaction, are considered to be hydrogen embrittlement

Tests for hydrogen embrittlement are performed to determine the effect of hydrogen damage in combination with residual

or applied stresses In the past decade, conventional testing methods have been modified to incorporate fracture mechanics, and the various types of hydrogen damage have been classified further in terms of crack nucleation, crack growth rates, and threshold stress-intensity measurements

Testing Methods

As described in the section "Tests for Stress-Corrosion Cracking" in this article, the cantilever beam test and the

wedge-opening load test result in a parameter called KISCC, which is the threshold stress intensity for SCC Many different

designations, such as Kth, KIHE, and KSH, denote this parameter for steels that undergo a similar phenomenon in which the mechanism is internal hydrogen embrittlement

The threshold stress intensity for hydrogen stress cracking is designated by KIHE, and KISCC is used for SCC The mechanisms are different in that SCC occurs under anodic polarization conditions, whereas hydrogen embrittlement and hydrogen stress cracking occur under cathodic polarization conditions, which normally are generated to protect steels from corrosion Such is the case when a sacrificial anode is galvanically coupled to the steel hull of a ship to prevent the hull from corroding In such a couple, the steel is the cathode and hydrogen is produced at the cathode in an electrochemical reaction This results in a steel structure, apparently free of corrosion (with a clean, metallic luster), that fails by intergranular cracking due to internal diffusion of hydrogen generated at the surface This type of hydrogen embrittlement is found in types 410 and 17-4PH stainless steel and AISI type 4340 steel

The cantilever beam test is a constant-load test in which a V-notched specimen is inserted along a portion of the beam and enclosed by an environmental chamber (Fig 9) A crack at the root of the V-notch is initiated and extended by fatigue before testing Notch-root thickness is prescribed by ASTM, although the requirement often is excessive for high-toughness steels The specimen is subjected to a constant load over a preset time period As the crack grows, the stress intensity increases Time to time failure is plotted versus applied stress intensity The lower limit of the resultant curve is

a threshold for hydrogen embrittlement (Fig 10)

Fig 9 Fatigue-cracked cantilever beam test specimen and fixtures

Fig 10 Procedure to obtain KIHE with precracked cantilever beam test specimen

The KIHE results of a cantilever beam test depend on how much time elapses before the test is terminated Recommended testing periods to establish the true stress-intensity threshold vary, ranging from 200 h, which is typical for hydrogen embrittlement testing, to as long as 5000 h Another limitation of this testing method is that it can be expensive in terms

of materials and machining As many as 12 specimens, placed under different loads in separate test machines, are needed

per test to obtain valid KIHE values

The wedge-opening load test applies a constant wedge or crack opening displacement; as the crack extends, stress

intensity decreases until crack arrest occurs (Fig 11) The initial load is assumed to be slightly above KIHE The specimen

is maintained under these conditions for about 5000 h to establish the threshold The crack grows to a point after which

further growth is not measured (KIHE) However, it is difficult to determine precisely when the "no growth" criterion is met Crack tip opening displacement should also be monitored Corrosion reactions accompanied by expansion in volume may occur at the crack tip This changes the opening displacement and increases the load, thus altering desired testing conditions

Fig 11 Schematic showing basic principle of modified wedge-opening load test specimen

As subcritical crack extension occurs, stress intensity increases in the cantilever beam test and decreases in the opening load test (Fig 12) Generally, the threshold stress intensity measured with the wedge-opening load test is lower than with the cantilever beam test The advantage of the wedge-opening load test is that only a single specimen is required

wedge-to measure KIHE

Fig 12 Influence of time, crack extension, and load on stress-intensity behavior of modified wedge-opening

load, cantilever beam, and contoured double-cantilever beam test specimens

The contoured double-cantilever beam test is used to measure crack growth rate a constant stress-intensity factor This test simplifies the calculation of stress intensity by using a contoured specimen so that stress intensity is proportional to the applied load and is independent of the crack length Under a constant load, stress intensity also remains constant with crack extension For the test geometry shown in Fig 13, the stress-intensity factor equals 20 times

the load (K = 20P)

Fig 13 Dimensions and configuration for double-cantilever beam test specimen Specimen contoured to 3a2/h3

+ 1/h = C, where C is a constant All values given in inches (1.0 in = 25.4 mm)

Data on hydrogen embrittlement can be obtained with subthickness specimens, even in excess of the ASTM requirement

of < 0.4 B/(YS)2 (where B is thickness and YS is yield strength of the specimen), by using side grooves, which

provide additional constraint on the material being tested Side grooves enable the maintenance of a plane-strain condition

in a thin specimen by enhancing stress triaxiality This method has been used extensively to study the effect of heat treatment (hardness) and environment on hydrogen stress cracking of AISI type 4340 steels (Fig 14)

Fig 14 Hydrogen embrittlement crack growth rate as a function of applied stress intensity for two different

hardnesses and environments for an AISI 4340 steel contoured double-cantilever beam test specimen

The contoured double-cantilever beam test has also been used to study the stress-history effect that produces an incubation time before hydrogen stress cracking Figure 15 shows that incubation time is dependent on the type of steel A decrease in the stress-intensity factor from 44 to 22 MPa (40 to 20 ksi ) may change the incubation time from less than 1 h for AISI type 4340 steel to about 1 year for type D-6AC steel

Fig 15 Incubation time prior to hydrogen stress cracking for AISI type 4340 and type D-6AC steel contoured

double-cantilever beam test specimens as a function of decrease in stress intensity

Three-Point and Four-Point Bend Tests. The contoured double-cantilever beam test uses a constant load to maintain a constant stress-intensity factor with crack extension The same effect can be produced by using a three- or four-point bend test under displacement control These tests use heavily side-grooved Charpy V-notch specimens (Fig 16) Because crack opening displacement is constant as the crack extends, the load decreases, so that there is a slight initial increase in stress intensity to a maximum value that drops slightly as the ratio of crack depth to specimen width exceeds 0.5 Typically, stress intensity is constant, within a small range Figure 17 compares the change in stress-intensity factor with crack extension as a function of load control to that of displacement control for a three-point bend specimen

Fig 16 Standard side-grooved Charpy V-notch test specimen used for three- and four-point bend tests

Fig 17 Use of three-point bend displacement control as constant-K specimen

The rising step-load test provides a stress intensity that is different at each load but remains constant with crack extension as each load level is sustained Crack initiation is signaled by a drop in load (Fig 18) The rising step-load test was developed as an accelerated low-cost test to measure resistance of steels (particularly weldments) to hydrogen embrittlement The threshold obtained by this method will be somewhat high, as test duration at each load is short

Fig 18 Typical load-time record for four-point rising step-load test

To index susceptibility to hydrogen-assisted cracking, the test should last no longer than 24 h, and the hydrogen source should reflect the most aggressive environment In one experiment, a 3.5% sodium chloride solution was selected to simulate seawater, and a cathodic potential of -1.2 V (saturated calomel electrode) was used to generate hydrogen to reproduce the extreme conditions of sacrificial anodic protection generally found on a ship hull

A Charpy specimen was chosen, because such specimens are small and easy to machine and handle In this test, however, the specimen was modified Instead of using a fatigue precrack, the notch-root radius was machined to less than 7.6 m (3 mil) This was done to lower the cost and give less ambiguous environmental conditions at the crack tip Also, hydrogen cracks nucleate below the surface

The specimen was deeply side grooved, a common practice used in hydrogen stress cracking tests to prevent the crack

from branching Side grooves are also used in crack opening displacement or J-integral testing to cause load-displacement

curves to increase monotonically to fracture by inducing a highly triaxial stress field at the crack tip Because a Charpy specimen is small, deep side grooves produce a triaxial stress field at the notch to promote hydrogen stress cracking The extent of the side grooving is such that the remaining ligament is only 40% of the original thickness The modified Charpy specimen dimensions are shown in Fig 16

The specimen was loaded by means of beams and an instrumented bolt (Fig 19) Four-point bending under constant

displacement control and stress intensity produced crack growth Once cracking initiated at the notch (a/W = 0.2, where a

is crack length and W is width of the specimen), arrest did not occur until the crack was nearly through the specimen The

load was increased manually at 1 h intervals An environmental chamber encompassed the specimen and included a potentiostat to produce hydrogen while under stress

Fig 19 Loading frame used for rising step-load test

The rising step-load test was used to evaluate high-strength HY ship steels and weldments in an environment simulating seawater under conditions of cathodic protection commonly used to protect ship hulls Samples from the heat-affected zone and other locations in the weld metal were tested Interlayer gas tungsten arc heating was evaluated as a means of providing a refined, homogeneous, tempered microstructure with improved resistance to hydrogen stress cracking As a baseline, comparison was made between HY-130 and HY-180 steels

Figure 20 plots rising step-load test results for HY-130 and HY-180 base metals, in addition to combinations of modified

HY steel compositions and programmed-cooling-rate thermal cycles for the base metal and weld wire The vertical axis is

a plot of a parameter derived from the specimen strength ratio in ASTM E 399, "Test Method for Plane-Strain Fracture

Toughness of Metallic Materials" i.e., 6 Pmax/B(W - a)2YS, where Pmax is the maximum load that the specimen is able to

sustain, B is the specimen thickness, W is the specimen width, a is the crack length, and YS is the yield strength in tension For the data shown in Fig 13, Pmax was replaced by the crack initiation load The horizontal axis is a ratio of

KIHE/YS, measured in a separate test program with cantilever beam and wedge-opening load specimens

Fig 20 Analytical correlation of strength ratio with threshold stress-intensity data

The resistance to hydrogen embrittlement of the two base metals and six locations in HY-130 weldments was ranked using this testing method Test results showed that HY-180 is more susceptible to hydrogen stress cracking than HY-130 and that the resistance to hydrogen embrittlement of specimens taken from the heat-affected zone and fusion line is consistently higher than that of weld-metal specimens The resistance of the weld metal is affected by the grain structure; interlayer gas tungsten arc reheating homogenized the weld structure, but did not temper the weld metal Specimens from the gas tungsten arc reheated weldment consistently exhibited higher hardness and lower resistance to hydrogen embrittlement than similar specimens from the standard HY-130 weld metal

The disk-pressure testing method measures susceptibility to hydrogen embrittlement of metallic materials under a high-pressure gaseous environment The test is used for the selection and quality control of materials, protective coatings, surface finishes, and other processing variables

A thin disk of the metallic materials to be tested is placed as a membrane in a test cell and subjected to helium pressure until the bursts Because helium is inert, the fracture is caused by mechanical overload; no secondary physical or chemical action is involved An identical disk is placed in the same test cell and subjected to hydrogen pressure until it bursts Metallic materials that are susceptible to environmental hydrogen embrittlement fracture under a pressure lower than the helium-burst pressure; materials that are not susceptible fracture under the same pressure for both hydrogen and helium

The ratio (S ) between the helium-burst pressure (PHe) and the hydrogen-burst pressure (P ) indicates the susceptibility of the material to environmental hydrogen embrittlement:

If S is equal to or less than 1, the material is not susceptible to environmental hydrogen embrittlement When S is greater than 2, the material is considered to be highly susceptible At values between 1 and 2, the material is moderately susceptible, with failure expected after long exposure to hydrogen; therefore, the material must be protected against exposure

Slow strain-rate tensile test can be used to evaluate many product forms, including plate, rod, wire, sheet, and tubing, as well as welded parts Smooth, notched, or precracked specimens can be used The principal advantage of this standardized test is that the susceptibility to hydrogen stress cracking for a particular metal-environment combination can

be assessed rapidly

A variety of specimen shapes and sizes can be used; the most common is a smooth bar tensile coupon, as described in ASTM E 8, "Methods of Tension Testing of Metallic Materials." The specimen is exposed to the environment and is stressed under displacement control For stainless steel in chloride solution, the strain rate is 10-6s One or more of the following parameters are applied to the tensile test at the same initial strain rate; time-to-failure; ductility, as assessed by reduction in area or elongation to fracture, for example; maximum load achieved; and area bounded by a nominal stress-elongation curve or a true stress-true strain curve

Potentiostatic Slow Strain-rate Tensile Testing. The use of dissociated water under potentiostatic conditions that produce hydrogen on the surface of the tensile test specimen while under slow strain-rate displacement control has been studied Results suggest that hydrogen is the most significant parameter in stress cracking under conditions of hydrogen sulfide stress-corrosion cracking found in oil fields

Selected References

• R.H Jones, Ed., Stress-Corrosion Cracking: Materials Performance and Evaluation, ASM

International, 1992

• G.H Koch, Stress-Corrosion Cracking and Hydrogen Embrittlement, Fatigue and Fracture, Vol 19,

ASM Handbook, ASM International, 1996, p 483-506

• G.M Ugianski and J.H Payer, Ed., Stress-Corrosion Cracking The Slow Strain-Rate Technique,

STP 665, ASTM, 1979

Metallographic Practices Generally Applicable to All Metals

Edited by George F Vander Voort, Buehler Ltd

Metallographic Methods

THE METHODS AND EQUIPMENT described in this article cover the preparation of specimens for examination by light optical microscopy (LOM), scanning electron microscopy (SEM), electron microprobe analysis (EMPA) for microindentation hardness testing, and for quantification of microstructural parameters, either manually or by the use of image analyzers

In this article, it is assumed that the specimen or specimens being prepared are representative of the material to be examined Random sampling, as advocated by statisticians, can rarely be performed by metallographers Instead, systematically chosen test locations are employed based on convenience in sampling In failure studies, specimens are usually removed to study the origin of the failure, to examine highly stressed areas, and to examine secondary cracks

All sectioning processes produce damage; some methods (such as flame cutting) produce extreme amounts of damage Traditional laboratory sectioning procedures using abrasive cut-off saws introduce minor damage that varies with the material being cut and its thermal and mechanical history This damage must be removed if the true structure is to be examined However, because abrasive grinding and polishing steps also produce damage, where the depth of damage decreases with decreasing abrasive size, the preparation sequence must be carefully planned and performed Otherwise, preparation-induced artifacts will be interpreted as structural elements A properly prepared specimen has the following characteristics:

• Deformation induced by sectioning, grinding, and polishing is removed or shallow enough to be removed by the etchant

• Coarse grinding scratches are removed; fine polishing scratches are tolerated in routine metallographic studies

• Pullout, pitting, cracking of hard particles, and smear are avoided

• Relief (i.e., excessive surface height variations between structural features of different hardness) is minimized

• The surface is flat particularly at edges (if they are to be examined) and at coated surfaces to permit examination at high magnifications

• Specimens are cleaned adequately between preparation steps, after preparation, and after etching

Preparation of metallographic specimens generally requires five major operations: sectioning, mounting (optional), grinding, polishing, and etching (optional)

Sectioning

Many metallographic studies require more than one specimen For example, a study of deformation in wrought metals usually requires two sections one perpendicular and the other parallel to the direction of deformation A failed part may best be studied by selecting a specimen that intersects the origin of the failure, if the origin can be identified Depending

on the type of failure, it may be necessary to take several specimens from the area of failure and from adjacent areas

Sampling. Bulk samples for sectioning may be removed from larger pieces or parts using methods such as core drilling, band or hack sawing, flame cutting, etc However, when these techniques are used, precautions must be taken to avoid alteration of the microstructure in the area of interest Laboratory abrasive-wheel cutting is recommended to establish the

desired plan of polish In the case of relatively brittle materials, sectioning may be accomplished by fracturing the specimen at the desired location

Abrasive-Wheel Cutting. By far, the most widely used sectioning devices in metallographic laboratories are abrasive cut-off machines (Fig 1) All abrasive-wheel sectioning should be done wet An ample flow of water, with a water-soluble oil additive for corrosion protection, should be directed into the cut Wet cutting will produce a smooth surface finish and, most importantly, will guard against excessive surface damage caused by overheating Abrasive wheels should

be selected according to the recommendations of the manufacturer Specimens must be fixtured securely during cutting, and cutting pressure should be applied carefully to prevent wheel breakage

Fig 1 A tabletop automated abrasive cutoff saw

Mounting of Specimens

The primary purpose of mounting metallographic specimens is for convenience in handling specimens of difficult shapes

or sizes during the subsequent steps of metallographic preparation and examination A secondary purpose is to protect and preserve extreme edges or surface defects during metallographic preparation The method of mounting should in no way

be injurious to the microstructure of the specimen Mechanical deformation and heat are the most likely sources of injurious effects

Clamp Mounting. Clamps have been used for mounting metallographic cross sections in the form of thin sheets Several specimens can be clamped conveniently in sandwich form This method is quick and convenient for mounting sheet type specimens; and when done properly, edge retention is excellent There is no problem with seepage of fluids from crevices between specimens The outer clamp edges must be beveled to minimize damage to polishing cloths If clamps are improperly used so that gaps exist between specimens, fluids and abrasives can become entrapped and will seep out obscuring edges The problems can be minimized by proper tightening of clamps, by use of plastic spacers between specimens, or by coating specimen surfaces with epoxy before tightening

Compression Mounting. The most common mounting method uses pressure and heat to encapsulate the specimen with a thermosetting or thermoplastic mounting material Common thermosetting resins include phenolic, such as Bakelite (Union Carbide Corp., Danbury, CT) and diallyl phthalate, while methyl methacrylate is the most common thermoplastic mounting resin Both thermosetting and thermoplastic materials require heat and pressure during the molding cycle; but after curing, mounts made of thermoplastic resins must be cooled to ambient under pressure, while mounts made of thermosetting materials may be ejected from the mold at the maximum molding temperature However, cooling thermosetting resins under pressure to at least 55 °C (130 °F) before ejection will reduce shrinkage gap formation

A thermosetting resin mount should never be water cooled after hot ejection from the molding temperature Thermosetting epoxy resins provide the best edge retention of these resins and are less affected by hot etchants than phenolic resins Mounting presses vary from simple laboratory jacks with a heater and mold assembly to full automated devices (Fig 2)

Fig 2 Mounting presses vary from a simple laboratory manual device (a) to a fully automated press (b)

Cold mounting materials require neither pressure nor external heat and are recommended for mounting specimens that are sensitive to heat and/or pressure Acrylic resins are the most widely used castable resin due to their low cost and fast curing time; however, shrinkage is somewhat of a problem Epoxy resins, although more expensive than acrylics, are commonly used because epoxy will physically adhere to specimens and can be drawn into cracks and pores, particularly if

a vacuum impregnation chamber is employed Hence, epoxies are very suitable for mounting fragile or friable specimens and corroded or oxidized specimens Dyes or fluorescent agents are added to some epoxies for the study of porous specimens, such as thermal spray coated specimens Most epoxies are cured at room temperature, and curing times can be

as long as 6 to 12 h Some epoxies can be cured at slightly elevated temperatures in less time Hard filler particles have been added to epoxy mounts for edge retention, but this addition is really not a satisfactory solution

Taper sectioning (mounting) generally is regarded as a special mounting technique; it enables the metallographer to examine in greater detail the immediate subsurface structure or surface topography of a specimen Microhardness determinations and thickness measurements of thin surface coatings or diffusion zones may be performed on taper-sectioned specimens Taper sectioning (Fig 3) is accomplished by establishing a plane of polish at a small angle to the surface of the specimen

Fig 3 Schematic of taper sectioning (mounting), as applied to a coated specimen Taper magnification equals

the cosecant of taper angle,

Edge preservation is a long-standing metallographic problem and many "tricks" have been promoted (most pertaining

to mounting, but some to grinding and polishing) These methods include the use of backup material in the mount, the application of coatings to the surfaces before mounting, and the addition of a filler material to the mount Plating of a

compatible metal on the surface to be protected (electroless nickel has been widely used) is generally considered to be the most effective procedure

However, introduction of new technology has greatly reduced edge preservation problems First, use of mounting presses, which cool the specimen to near ambient temperature under pressure, produces much tighter mounts Gaps that form between specimen and resin are a major contributor to edge rounding Second, use of semi-automatic and automatic grinding/polishing equipment, rather than manual (hand) preparation, increases surface flatness and edge retention Third, the use of harder, woven or nonwoven, napless surfaces for polishing with diamond abrasives, rather than softer cloths, such as canvas, billiard, and felt, maintains flatness Final polishing using low-nap cloths for short times introduces very little rounding compared to use of higher nap, softer cloths

Grinding

Grinding should commence with the finest grit size that will establish an initially flat surface and remove the effects of sectioning within a few minutes An abrasive grit size of 180 or 240 grit is coarse enough to use on specimen surfaces sectioned by an abrasive cut-off wheel Hack-sawed, band-sawed, or other rough surfaces usually require abrasive grit sizes from 120 to 180 grit The abrasive used for each succeeding grinding operation should be one or two grit sizes smaller than that used in the preceding operation A satisfactory fine grinding sequence might involve grit sizes of 240,

320, 400, and 600 grit This sequence is known as the "traditional" approach

As in abrasive-wheel sectioning, all grinding should be done wet provided that water has no adverse effects on any constituents of the microstructure Wet grinding minimizes loading of the abrasive with metal removed from the specimen being prepared and minimizes specimen heating

Each grinding step, while producing damage itself, must remove the damage from the previous step The depth of damage decreases with the abrasive size but so does the metal removal rate For a given abrasive size, the depth of damage introduced is greater for soft materials than for hard materials

Besides SiC paper, a number of other options are available to circumvent their use One option, used chiefly with automatic and automatic systems, is to grind a number of specimens placed in a holder simultaneously using a conventional grinding stone generally made of coarse grit alumina This step, often called "planar grinding," has the second goal of making all of the specimen surfaces coplanar This process requires a special-purpose machine because the stone must rotate at a high speed, 1500 rpm, to cut effectively The stone must be dressed regularly with a diamond tool

semi-to maintain flatness

Other materials have also been used both for the planar grinding stage or, afterwards, to replace SiC paper For very hard materials such as ceramics and sintered carbides, two or more metal-bonded or resin-bonded diamond disks with grit sizes from about 70 to 9 m can be used An alternate type of disk has diamond particles suspended in a resin applied in small blobs, or spots, to a disk surface These disks are available with diamond sizes from 120 to 6 m Another type of disk available in several diamond sizes uses diamond attached to the edges of a perforated, screen-like metal disk Another approach uses a stainless steel woven mesh "cloth" on a platen charged with coarse diamond, usually in slurry form, for planar grinding Once planar surfaces have been obtained, there are several single-step procedures available for avoiding the finer SiC papers These processes include the use of platens, woven polyester thick cloths, or rigid grinding disks With each of these, a coarse diamond size, most commonly 9 m, is used

Grinding Media. The grinding abrasives commonly used in the preparation of metallographic specimens are silicon carbide (SiC), aluminum oxide (Al2O3), emery (Al2O3-Fe3O4), composite ceramics, and diamond All except diamond are generally bonded to paper or cloth-backing materials of various weights in the form of sheets, disks, and belts of various sizes Limited use is made of grinding wheels consisting of abrasives embedded in a bonding material The abrasives may

be used also in powder form by charging the grinding surfaces with loose abrasive particles or with abrasive in a premixed slurry or suspension

Grinding Equipment. Although it is rarely used in industry, students still use stationary grinding paper that is supplied

in strips or rolls (Fig 4) The specimen is slid against the paper from top to bottom Grinding in one direction is usually safer than grinding in both directions While this can be done dry for certain delicate materials, water is usually added to keep the specimen surface cool and to carry the swarf away

Fig 4 A simple stationary grinding apparatus

Belt grinders (Fig 5) are usually present in most laboratories They are mainly used to remove burrs from sectioning, to round edges that need not be preserved for examination, to flatten cut surfaces to be macroetched, or to remove sectioning damage Generally only very coarse abrasive papers grits from 60 to 240 grits are used Most grinding work is done on rotating wheels; that is, a motor-driven platen upon which the SiC paper is attached (Fig 6)

Fig 5 Single and dual belt grinders for rough grinding

Fig 6 Laboratory flush mounted semi-automatic grinder/polisher system

Lapping is an abrasive technique in which the abrasive particles roll freely on the surface of a carrier disk During the lapping process, the disk is charged with small amounts of a hard abrasive, such as diamond or silicon carbide Lapping disks can be made of many different materials; cast iron and plastic are used most commonly Lapping produces a flatter specimen surface than grinding, but it does not remove metal as in grinding Consequently, lapping is not commonly employed in metallography Some platens, referred to as laps, are charged with diamond slurries Initially, the diamond particles roll over the lap surface (just as with other grinding surfaces), but they soon become embedded and cut the surface, producing chips

Polishing

Polishing is the final step in producing a deformation-free surface that is flat, scratch-free, and mirror-like in appearance Such a surface is necessary for subsequent metallographic interpretation, both qualitative and quantitative The polishing technique used should not introduce extraneous structures, such as disturbed metal, pitting, dragging out of inclusion,

"comet tailing," and staining Polishing usually is conducted in several stages Rough polishing generally is traditionally done with 6 or 3 m diamond abrasive charged onto napless or low-nap cloths Hard materials, such as through-hardened steels, ceramics, and cemented carbides, may require an additional polishing step For such materials, initial rough polishing may be followed by polishing with 1 m diamond on a napless, low-nap, or medium-nap cloth A compatible lubricant should be used sparingly to prevent overheating or deformation of the surface Intermediate polishing should be performed thoroughly so that final polishing may be of minimal duration Manual, or "hand," polishing is usually conducted using a rotating "wheel" (Fig 7)

Fig 7 Manual "hand" polishing setup

Mechanical Polishing

The term mechanical polishing is frequently used to describe the various polishing procedures involving the use of fine

abrasives on cloth The cloth may be attached to a rotating wheel or a vibrating bowl (Fig 8) The specimens are held by hand, held mechanically, or merely confined within the polishing area

Fig 8 Vibratory polishing unit

Hand Polishing. Aside from the use of improved polishing cloths and abrasives, hand-polishing techniques still follow the basic practice established many years ago:

preference, and is rotated in a direction counter to the rotation of the polishing wheel In addition, the specimen is continuously moved back and forth between the center and the edge of the wheel, thereby ensuring even distribution of the abrasive and uniform wear of the polishing cloth Some metallographers use a small wrist rotation while moving the specimen from the center to the edge of one side of the wheel The main reason for rotating the specimen is to prevent formation of "comet tails." This polishing artifact (Fig 9) is a result of directional polishing of materials containing inclusions, fine precipitates, voids, or other similar features

general, firm hand pressure is applied to the specimen in the initial movement

• Washing and Drying The specimen is washed and swabbed in warm running water, rinsed with ethanol,

and dried in a stream of warm air Scrubbing with cotton soaked with an aqueous soap solution followed

by rinsing with water is also commonly employed Alcohol usually can be used for washing when the abrasive carrier is not soluble in water or if the specimen cannot tolerate water

Fig 9 "Comet tails" due to directional polishing and pull out of hard particles 200×

For routine metallographic work, a fine diamond abrasive (1 m) may be used as the last step Traditionally, aqueous fine alumina slurries have been used for final polishing with medium-nap cloths Alpha alumina (0.3 m) and gamma alumina (0.05 m) slurries (or suspensions) are popular for final polishing, either in sequence or singularly Colloidal silica (basic

pH about 9.5) and acidic alumina suspensions are newer final polishing abrasives being used for difficult to prepare materials Vibratory polishers (Fig 8) are often used for final polishing, particularly with more difficult to prepare materials, for image analysis studies, or for publication-quality work

Automatic Polishing. Mechanical polishing can be automated to a high degree using a wide variety of devices ranging from relatively simple systems to rather sophisticated minicomputer- or microprocessor-controlled devices (Fig 10) Units also vary in capacity from a single specimen (Fig 11) to a half dozen or more at a time (Fig 12) Most units can be used for all grinding and polishing steps These devices enable the operator to prepare a large number of specimens per day, often with a higher degree of quality than that of hand polishing and at reduced consumable costs Automatic polishing devices also are desirable for preparing radioactive specimens by remote control or for using corrosive-attack polishing procedures safely without hand contact

Fig 10 A fully automatic polishing system

Fig 11 An automatic single-specimen grinder/polisher

Fig 12 A semi-automatic grinder/polisher

Polishing Cloths. The requirements of a good polishing cloth include the ability to hold an abrasive, long life, absence

of any foreign material that may cause scratches, and absence of any processing chemical (such as dye or sizing) that may react with the specimen More than a hundred cloths of different fabrics (woven or nonwoven) with a wide variety of naps (or napless) are available for metallographic polishing Napless or low-nap cloths are recommended for rough polishing using diamond abrasive compounds Low-, medium-, and occasionally high-nap cloths are used for final polishing, but this step should be as brief as possible to minimize relief

Polishing Abrasives. Polishing usually involves the use of one or more of the following abrasives: diamond, aluminum oxide (Al2O3), magnesium oxide (MgO), and/or silicon dioxide (SiO2) For certain materials, cerium oxide, chromium oxide, or iron oxide may be used With the exception of diamond, these abrasives normally are used in a distilled-water suspension If the metal to be polished is not compatible with water, other suspensions, such as ethylene glycol, alcohol, kerosene, or glycerol, may be required The diamond abrasive should be extended only with the carrier recommended by the manufacturer

Electrolytic Polishing

Even with the most careful mechanical polishing, some disturbed metal, however small the amount, will remain after preparation of a metallographic specimen This remainder is no problem if the specimen is to be etched for structural investigation, because etching is usually sufficient to remove the slight layer of disturbed metal If the specimen is to be examined in the as-polished condition using polarized light or if no surface disturbance can be tolerated, either electrolytic polishing (also called "electropolishing") or chemical polishing is preferred Alternatively, vibratory polishing

with (basic) colloidal silica, acidic alumina suspensions, or attack polishing agents added to these abrasives (or to - or -alumina suspensions) will remove minor amounts of residual damage providing good polarized light response A simple laboratory setup (Fig 13) is sufficient for most electropolishing requirements, and the more sophisticated commercial units (Fig 14) are all based on the same principle Direct current from an external source is applied to the electrolytic cell under specific conditions, and anodic dissolution produces leveling and brightening of the specimen surface

Fig 13 Simple laboratory system for electropolishing and electroetching

Fig 14 A commercially available electrolytic polishing and etching system

Not all materials respond equally well to electrolytic polishing Wrought solid solution-type alloys, such as aluminum, nickel, nickel-iron, and titanium alloys, are particularly good candidates for electrolytic polishing Electropolishing is usually reserved for single-phase alloys, because second phases and inclusions may be preferentially attacked during polishing

Chemical Polishing

Chemical polishing involves simple immersion of a metal specimen into a suitable solution to obtain a metallographic polish The results of chemical polishing are similar to those of electropolishing They vary from an etched specimen surface that has been macrosmoothed but not brightened to a bright dipped surface that has been brightened but not macrosmoothed

Etching

Metallographic etching encompasses all processes used to reveal particular structural characteristics of a metal that are not evident in the as-polished condition Examination of a properly polished specimen before etching may reveal

structural aspects, such as porosity, cracks, and nonmetallic inclusions In certain nonferrous alloys, grain size can be revealed adequately in the as-polished condition using polarized light

Electrolytic Etching. The procedure for electrolytic etching is basically the same as for electropolishing, except that voltage and current densities are considerably lower The specimen is connected to be the anode, and some relatively insoluble but conductive material, such as stainless steel, graphite, or platinum, is used for the cathode Direct-current electrolysis is used for most electrolytic etching, and for small specimens (13 by 13 mm, or by in., surface to be etched), one or two standard 1 V flashlight batteries provide an adequate power source A setup like the one shown in Fig 13 is usually all that is required

Etching for Macrostructure. Macroscopic examination differs from microscopic examination in that it employs very low magnifications (up to approximately 50×) and is used for the investigation of defects and structure of a large area as opposed to a microscopic portion of that area This technique is used to reveal solidification structure, flow lines, segregation, and structural changes due to welding, general distribution of sulfide inclusions, porosity, ingot defects, and fabricating defects It is important that the investigator be aware that macroetching can exaggerate the size of inhomogeneities or defects that could lead to misinterpretation of the actual condition of the material

Etching for Microstructure. In this article, microscopic examination is limited to a maximum magnification of 2500× the approximate useful limit of light microscopy Microscopic examination of a properly prepared specimen will clearly reveal structural characteristics, such as grain size, segregation, and the shape, size, and distribution of the phases and inclusions, that are present The microstructure revealed also indicates prior mechanical and thermal treatment that the metal has received

Etching is done by immersion or by swabbing (or electrolytically) with a suitable chemical solution that basically produces selective corrosion Swabbing is preferred for those metals and alloys that form a tenacious oxide surface layer with atmospheric exposure, such as stainless steels, aluminum, nickel, niobium, and titanium and their alloys It is best to use surgical grade cotton that will not scratch the polished surface Etch time varies with etch strength and can only be determined by experience In general, for high magnification examination, the etch should be shallower, while for low magnification examination a deeper etch yields better image contrast Some etchants produce selective results in that only one phase will be attacked or colored A vast number of etchants have been developed; the more commonly used etchants will be listed in this article (see also ASTM E 407)

Macroscopic Examination

There are aspects of structure, termed macrostructure, which can be observed with the unaided eye Macrostructural examination is often a prelude to microstructural examination but also includes routine quality tests for material acceptance These tests may also be useful for failure studies and research programs The most common macrostructural tests include macroetching, sulfur printing, lead exudation, and fracture tests

Macroetching is used to detect variations in soundness, chemistry, hardness, or strain A disk is cut from the part, smooth ground, and subjected to etching with an appropriate solution In a wrought product, a disk cut perpendicular to the deformation axis (i.e., transverse) is most commonly chosen as it reveals conditions that vary around the product Less commonly, a section along the centerline parallel to the deformation axis will be chosen to better reveal segregation, which usually elongates with deformation and is more readily detected in this way A finely polished surface is not required On the other hand, a saw-cut surface, due to its roughness, will permit observation of only the coarsest features

A smoothly ground surface is best For small products, a mounted and polished metallographic section will reveal most macrostructural features when standard microstructural etching solutions are used

Macroetchants tend to be rather strong in concentration compared to microetchants, so that contrast is adequate for visual examination Generally, the depth of etch attack of a macroetchant is substantially greater than for microetchants Macroetchants may be used at room temperature or at temperatures up to about 80 °C (180 °F) depending upon the purpose of the study and the nature of the material As an example, to study the macrostructure of weldments in ferrous alloys, macroetching with an aqueous 10% nitric acid solution at room temperature for 30 to 60 s is preferred However,

to evaluate the macrostructural soundness of a steel billet, hot acid etching with an aqueous 50% HCl solution at 70 to 80

°C (160 to 180 °F) for 15 to 30 min is preferred Macroetching solutions for many metals can be found in ASTM E 340 and in other standard compilations

Evaluation of the macrostructure of steel forgings, billets, bars, and continuously cast products is defined in various standards, such as ASTM E 381, MIL-STD-430A, ASTM A 561, and ASTM A 604 Features are classified based on their location (center, random, or subsurface) and nature (bursts, inclusions, flake, pipe ring patterns, freckles, white spots, cracks, etc.) Macroetching can also be applied to cast metals to reveal the grain structure, dendrites, blowholes, pin holes, sand holes, shrinkage cavities, etc In forged components, particularly closed die forgings, the flow pattern is often evaluated by macroetching a disk cut across the component and etching to reveal the flow lines Welding processes are often evaluated by macroetching sections taken through test welds In failure analysis, macroetching often detects unusual

or unexpected features that contribute to or cause failures

Contact Print Methods

Although a number of special contact print methods have been developed, the most commonly employed is the sulfur print, used to reveal the distribution of sulfur in a steel product A smoothly ground disk, carefully cleaned, is covered by

a sheet of photographic paper that has been soaked for 1 to 5 min in an aqueous 2% sulfuric acid solution (1 to 5% solutions can be used) The excess solution is allowed to drip off the paper Then, the emulsion side of the paper is placed against the ground surface of the steel specimen and left in contact for 2 to 10 min, depending on the sulfur content of the steel The lower the sulfur content, the longer the time of contact Any bubbles under the paper must be moved to the edge using a roller, squeegee, or sponge being careful not to move the paper The print is then peeled off carefully, washed in running water, fixed, washed, and dried Details of the sulfur print method are given in ASTM E 1180

The distribution of sulfur is shown by the presence of darkly colored areas of silver sulfide on the print The print is a mirror image of the sulfur distribution Voids, holes, or cracks may be represented by dark spots or lines due to hydrogen sulfide gas becoming trapped in these openings under the paper Other printing methods have been developed, but their use is infrequent

Microscopic Examination

Metallurgical microscopes differ from biological microscopes primarily in the manner by which the specimen is illuminated Unlike biological microscopes, metallurgical microscopes must use reflected light Figure 15 is a simplified ray diagram of a metallurgical microscope, while Fig 16 shows a typical inverted metallurgical microscope ("metallograph") The prepared specimen is placed on the stage with the surface perpendicular to the optical axis of the microscope and is illuminated through the objective lens by light from the source The light is focused by the condenser lens into a beam that is made approximately parallel to the optical axis of the microscope by the half-silvered mirror The light is then reflected from the surface of the specimen through the objective, the half-silvered mirror, and the eyepiece to the observer's eye

Fig 15 Image formation in a metallurgical microscope employing brightfield illumination

Fig 16 Example of an inverted metallurgical reflecting microscope for photomicroscopy (referred to as a

metallograph) Courtesy of C Brandmaier, Nikon Inc

Light Sources

The amount of light lost during passage from the source through a reflecting type of microscope is appreciable because of the intricate path the light follows For this reason, it is generally preferable that the intensity of the source be high, especially for photomicroscopy Several light sources are used, including tungsten-filament lamps, tungsten-halogen lamps, quartz-halogen lamps, and xenon arc bulbs

Tungsten-filament lamps generally operate at low voltage and high current They are widely used for visual examination because of their low cost and ease of operation

Tungsten-halogen lamps are the most popular light source today due to their high light intensity They produce good color micrographs when tungsten-corrected films are employed

Xenon arc lamps produce extremely high intensity, and their uniform spectra and daylight color temperature makes them suitable for color photomicrography The first xenon lamps produced ozone, but modern units have overcome this problem Light output is constant and can only be reduced using neutral density filters

Microscopic Techniques

Most microscopic studies of metals are made using brightfield illumination In addition to this type of illumination, several special techniques (oblique illumination, darkfield illumination, opaque-stop microscopy, phase-contrast microscopy, and polarized-light microscopy) have particular applications for metallographic studies

Köhler Illumination. Most microscopes using reflected or transmitted light use Köhler illumination, because it provides the most intense, even illumination possible with standard light sources The reflected light microscope has two adjustable diaphragms, the aperture diaphragm and the field diaphragm, located between the lamp housing and the objective Both diaphragms are adjusted to improve illumination and the image To obtain Köhler illumination, the image

of the field diaphragm must be brought into focus on the specimen plane This situation normally occurs automatically when the microstructural image is brought into focus The filament image must also be focused on the aperture diaphragm plane This focus produces uniform illumination of the specimen imaged at the intermediate image plane and magnified

by the eyepiece

In brightfield illumination, the surface of the specimen is normal to the optical axis of the microscope, and white light is used Figure 15 shows the ray diagram for this type of illumination in a standard type of bench microscope Light that passes through the objective and strikes a region of the specimen surface perpendicular to the beam will be reflected back up the objective through the eyepieces to the eyes, where it will appear to be bright or white Light that strikes grain boundaries, phase boundaries, and other features not perpendicular to the optical axis will be scattered at an angle and will not be collected by the objective These regions will appear to be dark or black in the image Brightfield is the most common mode of illumination used by metallographers

Oblique illumination reveals the surface relief of a metallographic specimen This process involves offsetting the condenser lens system or, as is more usually done, moving the condenser aperture to a position slightly off the optical axis Although it should be possible to continually increase the contrast achieved by oblique illumination by moving the condenser farther and farther from the light axis, the numerical aperture of a lens is reduced when this happens because only a portion of the lens is used For this reason, there is a practical limit to the amount of contrast that can be achieved

Illumination also becomes uneven as the degree of obliqueness increases Because differential interference contrast systems have been available, oblique illumination is rarely offered as an option on new microscopes

Darkfield illumination (also known as dark ground illumination) often is used to distinguish features not in the plane

of the polished-and-etched surface of a metallographic specimen This type of illumination gives contrast completely reversed from that obtained with brightfield illumination: the features that are light in brightfield will be dark in darkfield, and those that are dark in brightfield will be light in darkfield This highlighting of angled surfaces (namely, those of pits, crack, or etched grain boundaries) allows more positive identification of their nature than can be derived from a black image under brightfield illumination Due to the high image contrast obtained and the brightness associated with features

at an angle to the optical axis, it is often possible to see details not observed with brightfield illumination

Polarized-Light Microscopy. is particularly useful in metallography, because many metals and metallic and nonmetallic phases are optically anisotropic Polarized light is obtained by placing a polarizer in front of the condenser lens of the microscope and placing an analyzer before the eyepiece (Fig 17) The polarizer produces plan-polarized light that strikes the surface and is reflected through the analyzer to the eyepieces If an anisotropic metal is examined with the analyzer set 90° to the polarizer, the grain structure will be visible However, viewing of an isotropic metal (cubic metals) under such conditions will produce a dark, extinguished condition Polarized light is particularly useful in metallography for revealing grain structure and twinning in anisotropic metals and alloys and for identifying anisotropic phases and inclusions

Fig 17 Basic components of a polarizing light microscope

Differential Interference Contrast Microscopy. When crossed polarized light is used along with a double quartz prism (Wollaston prism) placed between the objective and the vertical illuminator, two light beams are produced that exhibit coherent interference in the image plane This occurrence leads to two slightly displaced (laterally) images differing in phase ( /2) that produces height contrast The image produced reveals topographic detail somewhat similar

to that produced by oblique illumination but without the loss of resolution Images can be viewed with natural colors similar to those observed in brightfield, or artificial coloring can be introduced by adding a sensitive tint plate

High-Temperature Microscopy. Several microscopes have been developed with devices that allow simultaneous heating and examination of specimens These have permitted direct examination of thermal effects in metals, such as grain growth, precipitation reactions, phase changes, sintering, diffusion, and certain types of surface reactions For some limited studies of this type, it is possible simply to surround the specimen with a small furnace and microscopically examine a polished surface An alternative means of heating the specimen is by electrical resistance Unfortunately, these simple methods have very limited usefulness because most metals and alloys oxidize at elevated temperatures, and oxidation must be avoided if unobstructed observations are to be made

Heating in a vacuum presents a problem in that certain phases exposed on the surface of the specimen can evaporate and then condense on the viewing window thereby hindering observation This situation can be partly overcome by using a

double viewing window with the window nearer the specimen being removable and the remaining window being used for photography Evaporation usually can be eliminated by operating in an inert-gas atmosphere; however, to prevent surface oxidation or contamination, the inert gas generally must be of very high purity

A serious problem in high-temperature microscopy is potential damage to the microscope parts, particularly the objective lens, by the high temperatures This danger is partly corrected by water cooling the parts of the hot stage near the objective; however, high magnifications are still not possible because the short working distances of most objectives do not permit examination through the viewing windows To overcome this, long-working-distance objectives have been used The most widely used type employs a reflecting concave mirror in conjunction with a standard objective

Low-Temperature Microscopy. Certain reactions that occur in metals at low temperatures can be observed by microscopy Stages have been constructed for this purpose; most are either adaptations of high-temperature stages or similar to them Generally a refrigerant, such as liquid nitrogen, cools the stage that holds the specimen A thermocouple

on the specimen measures the temperature controlled by the supply of refrigerant Low-temperature microscopy has found only limited use in metallography

Straining Stages. Several other devices can be fitted to the stages of microscopes so that a specimen can be viewed while experiments are being performed on it Stages constructed so as to allow straining of a specimen while it is being viewed have been particularly useful for studies of deformation, twinning, slip, and strain-induced transformations Because of the development of scanning electron microscopes (SEM), these types of experiments are more commonly performed within the SEM

Interferometry is the most sensitive and accurate optical method of measuring the microtopography of surfaces Two interference methods are in common use in metallography: the two-beam and the multiple-beam methods

Figure 18 shows the principles of the two-beam method In a two-beam interferometer, monochromatic light from the source is split into two beams One beam travels through the microscope objective to the specimen and then is reflected back through the objective and into the eyepiece The other beam passes through an identically matched objective onto an optically flat reference plate and then back through the same objective and is directed by the beam splitter to the eyepiece The two beams meet in the eyepiece and either reinforce each other (where the optical-path difference between them is equal to, or a multiple of, half the wavelength of the monochromatic light) or interfere with each other (where the optical-path difference does not satisfy the above conditions) From this reinforcement or interference, contour lines are formed with each line connecting points of the same level The difference in height between fringes is one-half the wavelength of the light Thallium light, which has a wavelength of 540 nm, is commonly used It is usually possible to measure to an accuracy of one-tenth of a displacement, which means that differences in height of about 27 nm can be measured

Fig 18 Principle of two-beam interferometry

Figure 19 shows the principles of multiple-beam interferometry Instead of creating interference between two light beams, the multiple-beam method produces interference among many beams An optically flat reference plate that is partly transmitting and partly reflecting to light is placed onto the surface of the specimen The plate and the specimen are positioned slightly out of parallel but usually by not more than one or two wavelengths Several objectives with reference plates built into them are commercially available for use on standard metallurgical microscopes Monochromatic light is directed perpendicular to the specimen surface through the objective Some of this light is reflected from the surface of

the reference plate and into the eyepiece of the microscope, whereas most of the light passes through the plate and onto the specimen A series of reflections occurs between the reference plate and the specimen, such that with each reflection some of the light passes through the reference plate and into the eyepiece of the microscope This light either reinforces or interferes with the light reflected from the surface of the plate, and contour lines result If the components are correctly positioned, the multiple-beam method can produce such fine fringes that displacements as small as 1/100 of the fringe displacement can be measured, thereby allowing measurements of differences in level as small as about 3 nm

Fig 19 Principle of multiple-beam interferometry

Macrophotography

Metallographers frequently need to take low magnification photographs of components, etch disks, fractures, and so forth, using either black and white or color film A wide variety of approaches are possible If the pictures are made on site, for example at an accident investigation, ordinary 35 mm medium-format or large-format cameras may be used, aided by various light sources, if needed A macrotype lens for close-up work is very useful

In the laboratory, it is helpful to use a camera stand, such as the Kaiser stand (Fig 20) Generally, the most critical aspect

of this work is adjustment of the lighting so that it is uniform, the desired details are visible, and undesirable shadows are minimized A light box is a useful aid for such work

Fig 20 Versatile camera stand for macrophotography

Many stereoscopic microscopes can be equipped with a camera for taking pictures at magnifications under 50× Some microscopes have a trinocular head where the camera is inserted into the third port (the other two are for the eyepieces)

In all work, it is important to use a light meter to gauge the proper exposure This precaution greatly reduces film waste and improves results Generally, depth of field is an important criterion in macrophotography, because many objects to be

photographed are not flat Basically, depth of field improves as the f-number of the lens is increased and is reduced as the

magnification increases The focal length of the lens has a minor influence on the depth of field However, as long focal length lenses produce a greater working distance between subject and lens, they are preferred The depth of detail (the

ability to separate detail throughout the depth of focus), is usually optimal between about f/10 and f/16, although this

range varies with magnification

A wide range of films can be used as well as electronic media Historically, wet-processed films have yielded the finest results, and that is still true today If enlargements are required, particularly for sizes greater than 8 by 11 in (21.5 by 28 cm), a medium-format film or a large-format film is better Generally, panchromatic films are best for black-and-white work For color work, the film type must be compatible with the color temperature of the lighting Digital cameras are becoming popular, and rather high pixel densities are available at a reasonable price Alternatively, a charge-coupled device (CCD) camera can be mounted on a macro lens to provide images to a capture system Digital formats are very convenient especially for annotation of images and ease of image storage, but very high quality printers, approaching the resolution of photographs, are still rather expensive

Microphotography

All metallographs come equipped with one or more camera ports, as well as provisions for attaching CCD cameras (or other types, although the CCD is by far the most common type used) While biologists frequently use 35 mm cameras to record microstructures, they are less popular with metallographers A small percentage of metallographers still prefer to use wet processed sheet film, usually 4 by 5 in (10 by 12.5 cm) size Orthochromatic film is no longer available in this size, and panchromatic films must be employed These are less convenient to use because loading, unloading, and developing must be done in total darkness Otherwise, results are the same Contact printing is most commonly performed

The majority of metallographers switched to instant films (Polaroid), which were introduced in the 1960s At that time, few (if any) metallographs had exposure meters, and wastage was significant because instant films have no latitude (exposures must be exactly controlled to get good images, unlike wet processed films) Instant films are convenient because dark room work is avoided However, except for the P/N type, there is no negative so extra prints cannot be made

in the same way as by traditional photography Instead, multiple photographs must be made anticipating future needs

Electronic photography is becoming very popular and will eventually become the dominant mode as it features all the convenience of instant photography with none of the disadvantages The biggest problem now is the cost of a high quality, high resolution printer, but that should become less of a problem in the future

Principles of Technique Selection in Mechanical Polishing

THREE DISTINCT OPERATIONS are involved in determining the microstructure of metals with the use of a light optical microscope: preparation of a section surface; development of the structure on that surface, usually by chemical etching; and microscopic examination The effectiveness of the examination is dependent upon these prior steps Improper specimen preparation will impair examination and may result in artifact structures being confused as true structure Etching must also be performed properly, although poor etch results are usually obvious and can be easily corrected Selection of the best etchant for a given specimen is more difficult, generally requiring trial and error based on past experience

The primary objective of a preparation procedure must be to produce a surface that fully represents the microstructure as

it existed in the metal before sectioning All structural features characteristic of the metal must be detectable, and false structures must not be introduced This requirement is more demanding than the mere production of what appears to be a highly polished surface

The main purpose of this section is to illustrate how objective experiments and comparisons can be used to develop preparation procedures that not only give better results but also are simpler and less laborious to use The emphasis is on principles that can be used as guides in the development of practical preparation procedures, rather than on the details of those procedures

Abrasion Damage and Abrasion Artifacts

Figure 21(a) shows the general pattern of a surface layer that has been plastically deformed for abraded cartridge brass (Cu-30%Zn), an alloy in which the effects of prior plastic deformation, generally from sectioning or grinding, can readily

be revealed by a range of etchants This micrograph highlights several characteristic features The shallow, dark-etching, unresolved band contouring the surface scratches is known as the outer fragmented layer; it is a layer in which the strains have been very large Beneath this layer extends a layer in which the strains have been comparatively modest, and in which the strains tend to concentrate in rays extending beneath individual surface scratches The bands of etch markings, known to develop at the sites of slip bands, show the strains as well as the more diffuse rays, known to indicate the presence of kink bands These effects extend to many times the depth of the surface scratches

Fig 21 Annealed cartridge brass (Cu-30%Zn) (a) Taper section (horizontal magnification, 600×; vertical

magnification, 4920×) of surface damage from abrasion with 220-grit silicon carbide paper (b) Micrograph taken after abrasion on 220-grit silicon carbide paper and then polished until about 5 m of metal was removed The banded markings are abrasion artifacts (false structures) (c) Micrograph showing the true microstructure taken after abrasion on 220-grit silicon carbide paper and then polished until about 15 m of metal was removed All three specimens were etched in an aqueous ferric chloride solution

Figures 21(b) and 21(c) show the importance of the surface damage shown in Fig 21(a) A specimen of annealed cartridge brass was abraded on 220-grit silicon carbide paper and then polished to remove a surface layer about 5 m (200 in.) thick All traces of the abrasion scratches were removed; and, ostensibly, a satisfactory surface was produced, but the bands of deformation etch markings (Fig 21b) appeared when the surface was etched When layers of greater thickness were removed during polishing, these bands gradually reduced in number and intensity, and eventually were eliminated, as shown in Fig 21(c), which shows the true structure

The deep abrasion-damage effects discussed thus far cause difficulties in only a limited range of alloys, but effects due to

an outer fragmented layer are likely to be found in all metals Certain distinctive artifacts caused by disturbance in the outer fragmented layer are observed in pearlitic steels Taper sections of abraded surfaces of these steels have shown that the cementite plates of pearlite may simply be bent adjacent to some scratches and may be completely fragmented adjacent to others

Austenitic steels generally are quite susceptible to abrasion artifacts, and the common etchants reveal effects due to prior deformation with considerable sensitivity The structure of a typical abrasion-damaged layer is comparable to that for brass A shallow, unresolved layer contours the surface scratches, and deep rays of deformation etch markings extend beneath the surface scratches Bands of these deformation etch markings may appear in a final-polished surface as abrasion artifacts Good abrasion practice and efficient polishing processes are required for removal of the abrasion artifacts in acceptable polishing time

Embedding of Abrasives. The points of the contacting abrasive particles of an abrasive paper fracture readily during abrasion, and these fragments may become embedded in the surface of a very soft metal, such as lead (Fig 22) or annealed high-purity aluminum Embedded particles are difficult to discern in the surface by light microscopy, but a surface with a high concentration of embedded abrasive characteristically has a rough, torn appearance

Fig 22 Embedding of SiC (arrows) abrasive in a nearly pure lead specimen (prepared up to 1 m diamond

finish) 200×

Preparation Damage

The mechanical preparation procedures commonly used in metallographic practice remove metal by mechanical cutting processes analogous to those occurring during abrasion Grinding and polishing always produce a series of scratch grooves on the surface of the specimen; these scratches may be difficult to detect by light microscopy, particularly with bright-field illumination Moreover, a plastically deformed, damaged layer is also introduced; the layer produced by mechanical polishing is much shallower than that produced by grinding, but its structure is similar A layer analogous to the outer fragmented layer in ground surfaces can be recognized contouring the surface scratches, and there are occasional rays of deformed metal extending to greater depths, many times the depth of the polishing scratches The presence of this damaged layer has important practical consequences it affects the response of the surface to etching

Even on well-prepared surfaces, a very fine layer of damaged material will remain that can be removed by chemical polishing ("attack" polishing) In mechanical-chemical polishing, a small amount of dilute etchant is used together with the abrasive suspension The etchant attacks the surface chemically while the abrasive removes the product

mechanical-of this chemical attack

An excessive amount of the chemical component in a mechanical-chemical process may cause detrimental effects, such as severe etch pitting Proper balance between the mechanical and chemical components can preserve most of the benefits provided by mechanical polishing and yet produce a damage-free surface a most desirable combination in a final-polishing stage

An alternative to attack polishing is provided by modern non-neutral polishing suspensions, for example, colloidal silica with a pH of 9.5 to 10 and acidic alumina suspensions with a pH of 2.5 to 3 A dilute etchant can also be added to these abrasive suspensions to further enhance metal removal These suspensions are particularly affective when used with a vibratory polisher

Enlargement of Polishing Scratches by Etching. It is a frequent annoyance in metallographic practice to find that

a surface, which appeared to be free of scratches when examined as-polished with brightfield illumination, turns out to be severely scratched after etching The scratches were actually there all the time, but they were too fine to be detected when the specimen was in the unetched condition; they were enlarged, or shown in greater contrast, by etching

In some cases, the etchant is revealing the damage present beneath scratches removed during preparation Scratches are attacked preferentially during etching because of the disturbed metal, or damaged layer, associated with them Severity of attack varies directly with the ability of the etchant to reveal deformation Color "tint" etchants are perhaps the most sensitive etchants to preparation-induced damage not fully removed during preparation The appearance of scratches also depends on the etching time A certain minimum etching time is necessary to develop the scratches fully; thereafter, the scratches recede with increasing etching time, because etching progressively removes the damaged layer It may be difficult to distinguish scratches enlarged by the final polishing stages from scratch traces introduced during the previous polishing stage The problem can be resolved by making the earlier set of scratches unidirectional and parallel to a known direction in the specimen surface The scratch traces can then be recognized This technique only works with manual preparation because automatic devices produce randomly oriented scratch patterns

Flatness. Quality in polishing practice means a surface that is adequately free from confusing polishing scratches and sufficiently flat for all constituents and local regions to be examined properly Different types of flatness problems are encountered in metallography Nonflat situations may arise in a variety of ways, but they are generally associated with microstructures that exhibit local hardness variations or at free surfaces Height differences can be markedly influenced

by the preparation practice This condition is usually called "relief." For example, hard particles (oxides, carbides, nitrides, or intermetallics) in a softer matrix stand above ("proud") the matrix in positive relief, while soft particles (sulfide inclusions, soft metal insoluble phases, graphite, etc.) in a harder matrix lie below the matrix in negative relief Negative relief is common at surfaces, such as exterior specimen surfaces, but also at interior surfaces, such as cracks or pores Even if there is no hardness difference at these edges, they can easily become "rounded" (negative relief) due to a lack of support during preparation

These conditions can be controlled to a large extent by the preparation procedure used Flatness is improved if napless, or very low-nap, surfaces are employed, particularly in the polishing stage Figure 23(a) shows the structure of a braze between Monel and type 304 stainless steel prepared using a procedure to minimize relief, while Fig 23(b) shows the same specimen using a procedure that will create height differences where there are hardness differences

Fig 23 Example of control of relief (a) and creation of excessive relief (b) in a braze between 304 stainless

steel and Monel Glyceregia etch, 200×

Retention of Graphite in Gray Iron. Graphite in cast iron can be damaged severely during the abrasion stage of

preparation; however, it is possible by suitable choice of abrasion process to obtain a reasonably true representation of the structure There remains the problem of retaining the graphite during polishing The solution to the problem depends heavily on the length of the nap of the polishing cloth

Graphite flakes in a gray iron invariably look much larger when long-nap cloths (Fig 24a) are used for polishing This apparent enlargement is caused by erosion, which occurs at the interface between graphite and matrix, producing an enlarged cavity from which the flake itself eventually is removed With a cloth of reasonably short nap, many of the flakes are well retained, although some may appear to be slightly enlarged Examination of sections of such a surface indicates that flakes aligned perpendicular to the surface are well sectioned but that slight erosion occurs around flakes that happen to be acutely aligned to the section surface Correct representation of the graphite flakes is obtained after polishing with a napless cloth (Fig 24b) Only a limited number of abrasives, notably diamond abrasives, produce

satisfactory results on napless cloths The use of napless cloths for all of the polishing steps yields the best retention of graphite

Fig 24 Retention of graphite in cast iron (a) Coarse graphite flakes in a gray iron specimen that was polished

with a long-nap cloth Note enlarged cavities where the graphite phase has been torn out (b) Well-preserved graphite flakes in a gray iron specimen that was polished with a napless cloth Both specimens not polished, not etched, and shown at 100×

Removal of Scratches. Only rarely is it required that final-polished surfaces be completely free of scratches, particularly in production work A more reasonable and practical requirement is that no coarse scratches should be detectable under the particular conditions of examination Attainment of this objective will depend on the specimen material (more difficult with soft materials), the etching conditions (more difficult with etchants that are sensitive to deformed structures), and the optical conditions (more difficult with optical conditions that are sensitive to surface irregularities, such as darkfield and polarized light) A metallographer should have available a variety of final-polishing processes capable of producing increasingly higher qualities of finish from which to select the most suitable for a particular need

Vibratory polishing methods are attractive for final polishing because they operate automatically An advantage of

vibratory polishing is that it can be adapted to chemical-mechanical polishing The important variables in vibratory polishing are the abrasive, the nature of the liquid in which the abrasive is suspended, and the load applied to the specimen

Not all polishing cloths can be used on a vibratory polisher Low- to medium-napped cloths are generally used Colloidal silica works very well on a vibratory polisher, partly because it stays in solution, being a colloid and partly because of its basic pH, around 9.5 to 10, which adds a chemical action to mechanical polishing

Electrochemical Differences. A further example of chemical effects arising during mechanical polishing is found in specimens containing constituents that differ considerably in their electrochemical characteristics In galvanized steels, marked electrochemical effects arise between the zinc of the coating and the steel base metal Severe etching of the coating occurs when the specimen is polished with an aqueous suspension or cleaned with water This effect can be eliminated by using a suspending liquid that has a pH very close to 7.0, thus suppressing electrochemical effects This pH can be achieved with the use of a standard buffer solution Others prefer to avoid the use of water in at least the final preparation step

Edge Retention. With few exceptions, the abrasion rates of the plastics in which metallographic specimens are mounted are much greater than those of metals The plastic abrades to a lower level than the metal, and rounding of the

specimen edge occurs to adjust for differences in level The degree of edge rounding may be increased or decreased during polishing; long-nap polishing cloths increase edge rounding

However, the abrasion rates of different types of plastics differ significantly, and edge retention can be improved by choosing a mounting plastic that has an abrasion rate matching as closely as possible that of the specimen For example, improved edge retention is obtained with the change from a phenolic to a thermosetting epoxy Metals that have very low abrasion rates, such as chromium and tungsten, show poorer edge retention than steels Metals that have higher abrasion rates, such as copper and aluminum, show good edge retention even when mounted in phenolic plastics

Attempts have been made to reduce the difference in abrasion rate between the specimen and mount to improve edge retention Mounting specimens using a press that cools the specimen back to near room temperature under pressure virtually eliminates the shrinkage gaps between specimen and mount, which has produced a vast improvement in edge retention

Layers and Coatings. Determination of the structure of a surface, coating, or a layer of oxide (scale) on a specimen is sometimes the principal reason for metallographic examination A specimen with such layers presents a problem in edge retention An oxide layer is usually friable, and thus susceptible to chipping and cracking during preparation In these cases, impregnation with epoxy resin is recommended Because the detection of porosity or cracking in the layer is an important feature of the examination, it is essential to avoid the development of preparation artifacts that might be mistaken for such features The development of such artifacts during abrasion is possible, because grinding with standard abrasive papers may result in extensive chipping of the oxide layer Polishing with diamond abrasives on a hard napless cloth ensures that a high degree of surface flatness will be maintained and that no polishing damage will be introduced

Electrolytic Polishing

ELECTROLYTIC POLISHING, also called electropolishing, is useful with stainless steels, copper alloys, aluminum alloys, magnesium, zirconium, and other metals that are difficult to polish by conventional mechanical methods The electrolytic technique can completely remove all traces of worked metal remaining from the cutting, grinding, and mechanical polishing operations used in preparing specimens When electropolishing is used in metallography, it is usually preceded by at least preliminary mechanical polishing and is often followed by etching

Current-Voltage Relations. In electropolishing, current-voltage relationships vary in different electrolytes and for different metals Figure 25 shows the simple relation wherein polishing occurs over an extensive continuous range of currents and voltages At low voltages, a film forms on the surface and little or no current passes; thus, etching occurs but not polishing At higher voltages, polishing occurs The perchloric acid electrolytes used for aluminum conform to this relation

Fig 25 Schematic relationship between current density and the single electrode potential for electrolytes

having polishing action over a wide range of voltages and currents

A more complex relation, frequently encountered, is shown by the curve in Fig 26 This curve depicts cell voltage as a function of anode current density for electropolishing of copper in an aqueous solution of orthophosphoric acid using a potentiometric circuit Five distinct regions can be distinguished on the cell voltage-current density curve In the region A-B, current density increases with the potential, some metal dissolves, and the surface has a dull etched appearance The region B-C reflects an unstable condition, while region C-D indicates a stable plateau at which the polishing film, previously formed, reaches a point of equilibrium and polishing occurs; during the latter stage, current density remains constant Optimum polishing conditions occur along C-D near D In the region D-E, gas bubbles evolve slowly, breaking the polishing film and causing severe pitting Polishing with rapid evolution of gas is represented by the region E-F

Fig 26 Cell voltage as a function of anode current density for electropolishing of copper in orthophosphoric

acid (900 g/L of water) using a potentiometric circuit

Electrolytes of the sulfuric-phosphoric acid and chromic-acetic acid types used for stainless steels also typify the complex, multistage relationship shown in Fig 26 In establishing voltage-current relationships like those in Fig 25 and

26, electrolysis must be allowed to proceed under fixed conditions until enough metal has dissolved to produce a steady state condition at the anode

Apparatus and Procedure

The electrical equipment used for electropolishing can vary from the simplest arrangement of dry cells to elaborate arrays

of rectifiers and electronic control devices Various types of apparatus are available commercially The choice of equipment depends on the number and type of specimens to be treated and the versatility and control desired

Current Source. Direct current is usually employed The current source may consist of a battery, a direct-current generator, or a rectifier In general, a battery supply is used for low voltages only, because a bank of batteries would be needed to produce higher voltages These three types of current source deliver a constant supply of direct current

Electrical Circuits. Two typical circuits, one for low and one for high current densities, are shown in Fig 27 For solutions in which a small drop in potential occurs across the cell, a potentiometric circuit, for low current densities, is

more suitable (Fig 27a) Conversely, when the drop in potential across the cell is high, a series circuit, for high current densities, should be used (Fig 27b) Provision must be made for controlling both voltage and current

Fig 27 Two electrical circuits and equipment arrangements for electropolishing metallographic specimens

Alternating current is used for electropolishing and electroetching the metals of the platinum group (platinum, iridium, palladium, rhodium, osmium, and ruthenium), using a series circuit and schematic arrangement as shown in Fig 27(b) with an alternating current source

The electrolytic cell is simply a container for the electrolyte, in which are suspended the cathode and the anode The cell is usually made of glass, but it may be polyethylene or polypropylene for solutions containing fluoride ions Sometimes a stainless steel cell is used, which may serve also as the cathode Frequently, the cell is surrounded by water

or an ice bath or is cooled in some other manner

The specimen to be polished (anode) should be arranged for quick removal from the electrolyte The electrical connection

to the specimen should be simple and easily broken so that the specimen can be rinsed immediately after polishing The cathode should be made of a metal that is inert in the electrolyte being used; stainless steel is satisfactory for most applications

For many applications, stirring, pumping, or air agitation of the electrolyte is necessary During electropolishing under steady-state conditions, the anodic reaction products accumulate on the surface of the polished metal Often, natural diffusion and convection processes cannot remove these products from the anode surface into the bulk of the electrolyte rapidly enough, and excessive accumulation of reaction products interferes with the electropolishing process The use of agitation usually requires an increase in the current density in order to maintain a sufficiently thick polishing film

Arrangement of Anode and Cathode. Two ways to position the specimen (anode) and the cathode are shown in

Fig 27 In each arrangement, only the portion of the specimen to be polished is exposed to the electrolyte In Fig 27(a), the surface to be polished is horizontal and facing upward, toward the cathode This arrangement helps to maintain a stable layer near the surface being polished, and is ordinarily used when polishing occurs under a viscous layer In Fig

27(b), the surface to be polished is vertical and facing toward the cathode This arrangement is sometimes used when polishing occurs with gas evolution, because it allows easy escape of the gas bubbles

Mounting of Specimens

Only the portion of the specimen to be polished should be in contact with the electrolyte Small specimens may be mounted by conventional molded-plastic-mounting procedures, for ease in handling for mechanical preparation and subsequent electropolishing Electrical contact can be made through a small hole drilled through the back of the mount into the metal specimen, or by the use of an indirect connection (Fig 28)

Fig 28 Example of a method for creating electrical conductivity of a small specimen in a polymeric mount

When specimens are mounted in plastic, the possibility of violent reaction between the plastic and some electrolytes must

be considered For example, phenol-formaldehyde mounting materials, acrylic-resin mounting materials, and base insulating lacquers and materials should not be used in solutions containing perchloric acid, because of the danger of explosion Polyethylene, polystyrene, epoxy resins, and polyvinyl chloride can be used as mounting materials in perchloric acid solutions without danger

cellulose-Mounting of specimens in dissimilar metals is undesirable, because the metal in contact with the electrolyte is likely to interfere with polishing and also because fusible mounting alloys containing bismuth may be dangerously reactive in certain electrolytes that contain oxidizing agents Bismuth-containing alloys may form explosive compounds in perchloric acid solutions

In preparing an unmounted specimen for electropolishing, a suitable chemically inert, electrically insulating coating can

be applied to all surfaces of the specimen (and specimen holder) except the surface to be polished Plastic electrician's tape is also an effective stop off, being impervious to most electrolytes and readily removable from the specimen after electropolishing

Development of Procedures for Electropolishing

Metallographers may be asked to electropolish a metal or alloy that has not previously been encountered In developing a suitable procedure, the problem should be viewed in comparison with known procedures and with information gained through previous experience

Effect of Alloy Type. It is generally helpful to compare the position of the major component of the alloy with elements

of the same general group in the periodic table, and to study the phase diagram, if available, to predict the number of phases and their characteristics Single-phase alloys are generally easiest to electropolish, whereas multiphase alloys are likely to be difficult or impossible to electropolish Even minor alloying additions to a metal may profoundly affect the response of the metal to polishing in a given electrolyte

General Principles. The possibility of polishing a metal and the conditions for polishing a metal in a given electrolyte can sometimes be ascertained by plotting current density versus electrode potential The curve in Fig 25 is typical of those electrolytes that polish over a very wide range or that will not polish at all The curve in Fig 26 is characteristic of those electrolytes that form an ionic film; polishing will occur between points C and D on this curve; polishing is usually best near point D

After the polishing range is determined, the other constants, such as preparation, agitation, and time, can be determined experimentally The amount of preparation required depends on the nature of the specimen and on the results desired

Specimen preparation can be accomplished in many cases by grinding through a series of graded abrasives up to a

600 grit SiC paper finish In other cases it is necessary or desirable to prepolish the specimen with 6 m diamond before starting to electropolish For some metals, it is best to completely polish the specimen mechanically and then use a brief electropolish stage to remove any remaining preparation-induced damage

The surface to be polished should be clean to allow uniform attack by the electrolyte To avoid contamination with oil from fingers, the specimen should be handled with forceps or tongs after final preparation for electropolishing

Advantages and Disadvantages of Electropolishing

When properly applied, electropolishing can be a useful tool for the metallographer The principal advantages of electropolishing include:

• For some metals, electropolishing can produce a high-quality surface finish that is equivalent to the best that can be obtained by mechanical methods

• Once a procedure has been established, good results can be obtained with less operator skill than that required for mechanical polishing

• There can be a marked saving of time if many specimens of the same material are to be polished sequentially

• Electropolishing is especially suited for the softer metals, which may be difficult to polish by mechanical methods

• No scratches are produced in electrolytic polishing The absence of scratches is a definite advantage in viewing high-quality electropolished surfaces of optically-active materials with polarized light

• Artifacts resulting from mechanical deformation such as disturbed metal or mechanical twins, which are produced on the surface even by careful grinding and mechanical polishing do not occur in electropolishing

• Surfaces resulting from electropolishing are completely unworked by the polishing procedure This feature is important in low-load hardness testing or x-ray studies

• With some electrolytes, etching can be accomplished by simply reducing the voltage to approximately one-tenth the potential required for polishing, then continuing electrolysis for a few seconds

• Electropolishing is frequently useful in electron metallography (where high resolution is often important) because it can produce thin undistorted metal surfaces

Metallographic preparation by electropolishing is subject to several limitations; these should be recognized to prevent misapplication of the method and disappointment in the results The principal disadvantages include:

• The chemicals and combinations of chemicals used in electropolishing are poisonous; many are highly flammable or potentially explosive Only well-trained personnel thoroughly familiar with chemical laboratory procedures should be permitted to handle or mix the chemicals or to operate the polishing baths

• The conditions and electrolytes required to obtain a satisfactorily polished surface differ for different alloys; hence, considerable time may be required to develop a procedure for a new alloy, if it can be developed at all This limitation does not apply if appropriate procedures exist

• In multiphase alloys, the rates of polishing of different phases often are not the same Polishing results depend heavily on whether the second or third phases are strongly cathodic or anodic with respect to the matrix The matrix is dissolved preferentially if the other phases are relatively cathodic, thus causing the latter to stand in relief Preferential attack may also occur at the interface between two phases These effects are most pronounced when phases other than the matrix are virtually unattacked by the polishing bath The effects are reversed when the matrix phase is relatively cathodic

• A large number of electrolytes may be needed to polish the variety of metals encountered by a given

laboratory

• Plastic or metal mounting materials may react with the electrolyte

• Electropolished surfaces are not flat but exhibit an undulating surface, and in some cases may not be suited for examination at all magnifications Under some conditions, furrowing and pitting may be produced

• Edge attack effects limit applications involving small specimens, surface phenomena, coatings, interfaces, and cracks

• Attack around nonmetallic particles and adjacent metal, voids, and various inhomogeneities may not be the same as that of the matrix, thus exaggerating the size of the voids and inclusions

• Electropolished surfaces of certain materials may be passive and difficult to etch

Electrolytes

Formulations of electrolytes, sorted into eight groups, may be found in Metallography, Structures, and Phase Diagrams, Volume 8, Metals Handbook, 8th ed., p 30-31; Metallography and Microstructures, Volume 9, ASM Handbook, p 52-53; and ASTM E 1558 Other compilations of electrolyte compositions may be found in Metallography: Principles and Practice, by G.F Vander Voort, McGraw-Hill, 1984 Preferred (or sometimes required) characteristics of an electrolyte

are:

• It should be somewhat viscous

• It must be a good solvent for the anode metal (the specimen) during electrolysis conditions

• It should preferably not attack the anode metal when no current is flowing

• It should contain one or more ions of large radii, such as (PO4)-3, (ClO4)-1, or (SO4)-3, and sometimes large organic molecules

• It should be simple to mix, stable, and safe to handle (many effective electrolytes are deficient in these respects)

• It should function effectively at room temperature and not be sensitive to temperature changes

Procedures and Precautions for Preparation and Handling of Etchants

IN THIS SECTION, the term etchant is used in its broadest sense, to include reagents used in metallographic work for

microetching, macroetching, electropolishing, chemical polishing, and similar operations

The formulations of etchants given elsewhere in this Volume are adequate for the majority of applications, but they may occasionally require modification Adjustments in etchant composition (as well as in etching time and technique), based

on the experience and skill of the metallographer, and magnification to be used, may be needed in order to obtain satisfactory results

Expression of Composition

Etchants are generally either aqueous or alcoholic solutions containing one or more active chemicals (acids, bases, or salts) Liquids other than water or alcohol are used as solvents in some formulations Compositions of most etchants described in this section are expressed in terms of the amounts of the substances to be used in preparing small quantities

of these reagents For etchants that are solutions of solid substances in liquids, the amounts of the solid substances are usually expressed in grams (g), and the amounts of liquids (or the total volumes of solution) are expressed in milliliters (mL) The liquids may be individual commercially available substances, or they may be stock solutions containing two or more substances Compositions of some etchants that are prepared by mixing together two or more liquids are given in parts by volume or percentage by volume Compositions of some etchants consisting of solutions of solid substances in liquids are described in terms of percentage by weight

In long-established (although nonstandard) usage in metallography, such terms as 1%, 2%, and 4% have been used to describe the approximate strength of an etchant, such as picral, and are understood to mean 1, 2, and 4 g, respectively, of picric acid per 100 mL of alcohol

Purity of Chemicals

In the preparation of solutions for microetching and electropolishing, recommended practice is to use chemicals meeting the requirements of NF (National Formulary), USP (U.S Pharmacopoeia), laboratory or purified grades, or grades of still higher purity (reagent, ACS, or certified grades) The commercial or technical grades of certain special-purpose industrial chemicals (such as CrO3 and synthetic methanol) are extremely pure and are equivalent to reagent, ACS, or certified grades for use in microetching and electropolishing Where water is specified, distilled water is preferred because of wide variations in the purity of tap water For macroetching, technical grades of chemicals are satisfactory, unless specifications indicate otherwise, and potable tap water of good quality is generally acceptable

Identification of Chemicals

The practices generally followed in the technical literature on metallography are used to identify the chemicals utilized in the preparation of etchants

Aqueous Acids. In identification of aqueous acids, the name or formula alone, sometimes followed by "conc" or

"concentrated," refers to the common commercially available concentrated laboratory grade (Table 1) Where more than one concentration is commonly available, the percentage by weight of the active constituent is shown after the name or formula

Table 1 Characteristics of aqueous liquid chemicals used in many metallographic etchants

Except for sulfuric acid, all data apply to both laboratory and technical or commercial grades of chemicals

Name Active constituent Nominal composition(a), wt% Specific gravity Degrees Baumé(b)

Sulfuric acid H 2 SO 4 96(d) 1.84(e) 66(e)

Miscellaneous aqueous chemicals

Hydrogen peroxide H 2 O 2

(a) Nominal percentage of the active constituent: remainder is water Reagents made by different

manufacturers may differ slightly in nominal concentration and allowable range of concentration

(b) Specific gravity as indicated on the Baumé scale: sometimes used for technical grades and in

laboratory measurements

(c) Technical grade is also called muriatic acid

(d) Laboratory grade Technical grade has concentration of 93%

(e) Specific gravity and degrees Baumé are nearly constant for 93 to 100% sulfuric acid

(g) Sometimes called "10 volume."

(h) Sometimes called "100 volume."

(i) Sometimes called "170 volume."

Where an acid is designated as "tech," the technical grade that has the same concentration as the common laboratory grade is meant The concentration of technical grades is sometimes expressed by suppliers in terms of specific gravity or degrees Baumé (°Bé), as shown in Table 1 Most technical-grade chemicals are available in several different concentrations

Miscellaneous Aqueous Chemicals. A variety of aqueous chemicals, such as ammonium hydroxide and hydrogen peroxide (Table 1), which are used in various etchants, are identified similarly to aqueous acids Concentration must always be specified for hydrogen peroxide, which is available in several widely differing concentrations

Alcohols. The alcohols most frequently used in etchants are methanol and ethanol (Table 2) It is important to use alcohol that has the desired water content (anhydrous or 95% alcohol, whichever is specified) in etchants that contain only

a small percentage of water

Table 2 Characteristics of pure methanol and ethanol

constituent

Nominal composition(a), vol%

Methanol (methyl alcohol) CH 3 OH 99.5(b)

Methanol (methyl alcohol), 95% CH 3 OH 95(c)

Ethanol (ethyl alcohol), anhydrous C2 H 5 OH 99.5(d)(e)

Ethanol (ethyl alcohol), 95% C 2 H 5 OH 95(e)

(a) Nominal percentage of the active constituent: remainder is

water unless otherwise specified

(b) Synthetic methanol: the commercial grade is of high purity

and is satisfactory for use in all ordinary metallographic

etchants where methanol is specified (wood alcohol has not

been manufactured commercially in the United States since

1969) Methanol is available only as an anhydrous (also

called absolute) grade containing less than 0.1 or 0.2%

water as packaged, and usually not more than about 0.5%

water at time of use, depending on storage and handling

(c) Where methanol, 95%, is called for, the ordinary

anhydrous grade must be diluted by the user with 5% water

by volume

(d) The anhydrous (also called absolute) grade of ethanol is

ordinarily used only where no significant amount of water

can be tolerated It contains less than 0.1 or 0.2% water as

packaged, and usually not more than about 0.5% water at

time of use, depending on storage and handling

(e) Available only with special government permit

Practice with regard to the substitution of methanol for ethanol, or vice versa, and with regard to the use of some grades

of denatured ethanol in etchants, varies greatly among metallographic laboratories In most alcoholic etchants, ethanol can

be substituted for methanol This substitution is desirable, because methanol is a cumulative poison, while ethanol is not Safety considerations may rule out alcohol substitutions in accepted formulations for electropolishing without a thorough chemical study Also, ethanol should not be substituted for methanol in nital containing more than 3% by volume of concentrated nitric acid In addition, it should not be substituted in other methanol-based etchants that contain strong oxidants and only a small percentage of water (if they are being stored in tightly stoppered bottles) due to possible

pressure buildup and explosion Mix these ingredients fresh and do not store them Never store mixtures based on

be suitable for general laboratory purposes and have been denatured with small percentages of volatile solvents; they may

be substituted for pure ethanol having the same water content, except where pure ethanol is required for some special reason

Table 3 Nominal compositions of various grades of denatured alcohol (ethanol) used in some metallographic etchants

See text for discussion of suitability of the various grades for use in etchants

Parts by volume in specially denatured alcohol(a) Formula SD-1 (b) Formula SD-3A Formula SD-30 Component

Anhydrous 95% (c) Anhydrous 95% (c) Anhydrous 95% (c)

Ethanol, anhydrous 100 95 100 95 100 95

Methyl isobutyl ketone 1 1

Parts by volume in proprietary solvent(d) Parts by volume in "reagent" alcohol(d) Component

(a) Specially denatured alcohol is available only with special government permit

(b) The formula shown here has replaced the old SD-1 formula in which wood alcohol was specified: wood

alcohol has not been manufactured commercially in the United States since 1969

(c) The designation of type of denatured alcohol as "95%" means that the denatured product contains 5 parts of

water for every 95 parts of anhydrous (absolute) ethanol, plus denaturants as specified

(d) Available without government permit from suppliers of laboratory chemicals, for scientific and general

laboratory purposes

The specially denatured (SD) alcohols described in Table 3 are generally suitable for use in etchants However, SD alcohol is obtainable only with special government permits and usually can be purchased only in larger quantities than the proprietary solvent and "reagent" alcohol in Table 3, and only from major suppliers of solvents

Water of Hydration. With some exceptions, it has been common practice since the earliest days of metallography to identify solid salts and acids used in etchants only by names and abbreviated formulas, without showing the presence or absence of water of hydration Historically, in developing and preparing etchants, the most stable hydrate (which was the common commercial form) was ordinarily used, except for salts that do not form hydrates Current practice varies from laboratory to laboratory

Using the specified amount of either the anhydrous or a hydrated form of a solid salt or acid in preparing an etchant will

in most cases produce essentially the same etching behavior Any difference in results will usually be small in comparison with the effects of normal differences in technique and other variables in specimen preparation Exceptions are the preparation of etchants that must be anhydrous or must contain only a small and fairly critical percentage of water for proper etching activity; for such etchants, the need to use specific anhydrous or hydrated forms of each component should

be clearly stated

Some salts (such as ferric nitrate) do not exist in an anhydrous form Conversely, some nominally water-free compounds contain a substantial percentage of water One of these is picric acid, for which the 10 to 15% water content found in laboratory grades is necessary for satisfactory performance of etchants based on it (Table 4) Always keep picric acid moist to eliminate the possibility of explosion

Table 4 Descriptions of miscellaneous chemicals used in metallographic etchants

aluminum chloride, anhydrous Solid; AlCl 3 ; reacts violently with water, evolving HCl gas; use of hydrated form, AlCl 3 ·6H 2 O, is preferred

ammonium molybdate Crystals; also called ammonium paramolybdate or heptamolybdate; (NH 4 ) 6 Mo 7 O 24 ·4H 2 O; can be used

interchangeably with molybdic acid, 85%

benzalkonium chloride Crystals; essentially alkyldimethyl-benzyl-ammonium chloride May not be readily available in this form;

see zephiran chloride

1-butanol See n-butyl alcohol

2-butoxyethanol See butyl cellosolve

n-butyl alcohol Liquid; normal butyl alcohol; also called butyl alcohol and 1-butanol

butyl carbitol Liquid; diethylene glycol monobutyl ether

butyl cellosolve Liquid; ethylene glycol monobutyl ether; also called 2-butoxyethanol

carbitol Liquid; diethylene glycol monoethyl ether

cellosolve Liquid; ethylene glycol monoethyl ether

chromic acid Dark-red crystals or flakes; CrO 3 ; also called chromic anhydride, chromic acid anhydride, and chromium trioxide

(See chromic oxide, Cr2 O 3 )

chromic anhydride See chromic acid

chromic oxide Fine green powder; Cr 2 O 3; a polishing abrasive Do not confuse with chromic acid (CrO3 ), which is a strong acid and a component of many etchants

cupric ammonium chloride Crystals; a double salt, CuCl 2 ·2NH 4 Cl·2H 2 O If not available, substitute 0.6 g CuCl 2 ·2H 2 O plus 0.4 g

NH 4 Cl for each gram of the double salt

diethylene glycol Syrupy liquid; also called 2.2'-oxydiethanol and dihydroxydiethyl ether; (HOCH 2 CH 2 ) 2 O More viscous than ethylene glycol; otherwise similar in behavior

diethylene glycol monobutyl ether See butyl carbitol

diethylene glycol monoethyl ether See carbitol

diethyl ether See ether

ether Liquid; also called ethyl ether and diethyl ether; very low flash point, highly explosive; boiling point is 34.4 °C (94 °F) ethylene glycol Syrupy liquid; also called 1,2-ethanediol and dihydroxyethane; (CH 2 ) 2 /(OH) 2 Less viscous than diethylene glycol; otherwise similar in behavior

ethylene glycol monobutyl ether Liquid; also called 2-butoxyethanol or butyl cellosolve

ethylene glycol monoethyl ether See cellosolve

ethyl ether See ether

ferric nitrate Crystals; Fe(NO 3 ) 3 ·9H 2 O There is no anhydrous form of this salt

fluoboric acid, 48% Liquid; HBF 4 ; if not readily available in small quantities, substitute 10.3 mL HF (48%) plus 4.4 g H 3 BO 3 for each 10 mL of 48% fluoboric acid specified

glycerol Syrupy liquid; also called glycerin or glycerine; C 3 H 5 (OH) 3 ; contains up to 5% (by weight) water

molybdic acid, 85% Crystals or powder containing the equivalent of 85% MoO 3 This misnamed chemical consists mostly of ammonium molybdate (or paramolybdate), which is (NH 4 ) 6 Mo 7 O 24 ·4H 2 O The two chemicals can be used interchangeably See

ammonium molybdate

muriatic acid Liquid; technical grade HCl (see Table 1)

picric acid Crystals; 2,4,6-trinitrophenol; crystals of laboratory chemical contain 10 to 15% water; explosive; its crystalline metallic salts are even more explosive Do not use grades that do not have the 10 to 15% water content

pyrophosphoric acid Crystals or viscous liquid; H 4 P 2 O 7 , anhydrous; hydrolyzes to phosphoric acid (H 3 PO 4 ) slowly in cold water and rapidly in hot water

zephiran chloride Aqueous solution; a proprietary material produced in grades containing about 12% and 17% (by weight) benzalkonium chloride (alkyldimethyl-benzyl-ammonium chloride) as the active constituent, plus some ammonium acetate; also

called sephiran chloride Available from pharmacies or pharmaceutical distributors See benzalkonium chloride

Miscellaneous Chemicals. Correct identification may present problems because of similarity in names of different chemicals or because of misleading or nonstandard nomenclature and trade names (Table 4) Also included are certain chemicals for which some aspects of composition or behavior are important

Metallographic Practices Generally Applicable to All Metals

Safety Precautions

All chemicals are potentially dangerous and persons formulating and using etchants should be thoroughly familiar with the chemicals involved, and with the proper procedures for handling and mixing them The discussion that follows indicates many of the potential hazards that attend the use of chemicals and describes precautions and safe practice for averting these hazards

Ventilation. All mixing, handling, and use of etchants should be done in a well-ventilated area, preferably within an exhaust hood, to prevent exposure to, or inhalation of, toxic and corrosive fumes Use of an exhaust hood is mandatory whenever large quantities of chemicals are handled or large areas of metal are etched (as in macroetching), and when carrying out lengthy electropolishing operations or electropolishing large areas Special hoods are available for work with perchloric acid

Protection of Personnel. When chemicals and etchants are being poured, mixed, or handled, and when etchants are being used, suitable protective equipment and clothing (glasses, face shield, gloves, apron, and other items, as appropriate) should always be worn, to prevent contact of chemicals with the eyes, skin, or clothing

If chemicals contact the skin, they should be washed off promptly with water and soap Medical attention should be obtained as soon as possible for chemical burns, especially if there are cuts or abrasions in the skin If chemicals contact the eyes, the eyes should be flushed at once with large quantities of water, and medical attention should be obtained without delay A face-and-eye fountain should be available for use wherever chemicals or etchants are stored or handled Wherever quantities large enough to be hazardous are stored or handled, a safety shower is needed also This washing equipment should be readily available and should be tested at scheduled intervals to ensure dependable performance in an emergency

Hydrofluoric and fluosilicic acids can cause painful and serious ulcers on contacting the skin, unless washed off immediately Also especially harmful to the skin are concentrated HNO3, H2SO4, CrO3, H2O2 (30 or 50%), NaOH, KOH,

Br2, and anhydrous AlCl3 Inhalation of vapors or mist from these chemicals or etchants containing them can also cause irritation or serious damage to the respiratory system

Container Material and Design. In preparing, storing, and handling etchants, use containers and equipment made of materials suitable for the chemicals used Glass is resistant to nearly all chemicals Polyethylene, polypropylene, and similarly inert plastics are resistant to hydrofluoric, fluosilicic, and fluoboric acids, unlike glass, as well as to solutions containing salts of these acids These inert plastics are also recommended for prolonged storage of strongly alkaline solutions and strong solutions of phosphoric acid, both of which attack glass (especially, ordinary grades of glass)

Certain mixtures of chemicals can generate gaseous reaction products over a period of time or if inadvertently exposed to heat, and can build up dangerous pressures if stored in tightly sealed containers The use of vented or pressure-relief types

of stoppers instead of tightly sealed screw caps or conventional stoppers on bottles of etchants that are prepared in quantity and stored is a worthwhile safety precaution

Heat Evolution in Preparing Etchants. Exercise caution and follow accepted laboratory procedures when mixing chemicals In general, heat is evolved, sometimes in large amounts, when strong acids (particularly H2SO4), alkalis (NaOH and KOH), anhydrous AlCl3, or their concentrated solutions are added to water, alcohols, or solutions of other chemicals, and when combining acidic with alkaline substances or solutions

The acid, alkali, or anhydrous AlCl3 should be introduced slowly to the water, alcohol, or solution, while stirring continuously to avoid local overheating Incomplete mixing can permit layering, with danger of a delayed violent reaction Special attention and special cooling procedures may be needed when large quantities of etchants are prepared and large areas of metal are etched, as in some macroetching, and when high currents are used in electropolishing

Mixing of Oxidizing Agents with Reducing Agents. Exercise special care in mixing oxidizing agents (HNO3,

H2SO4, HClO4, CrO3, salts of these acids, persulfates, Br2, and H2O2) with reducing agents (alcohols and other organic solvents, acetic acid, acetic anhydride, and most organic compounds); failure to follow accepted safe procedures can result in violent or explosive reactions Acetic anhydride cannot be used safely in electropolishing solutions except in limited ranges of composition and water content, and its use is not recommended

Care with Cyanides. The use of etchants that contain cyanides presents special toxicity hazards, because poisoning can result from inhaling hard-to-detect small amounts of HCN gas evolved from acidic solutions, from ingesting small amounts of cyanides, and from absorbing cyanides through the skin or exposed body tissues Careful handling and the use

of an effective exhaust hood are especially important Used cyanide-containing solutions should be made slightly alkaline with ammonia and poured into a chemically resistant waste-disposal drain, and the drain flushed thoroughly with a copious amount of water