Volume 09 - Metallography and Microstructures Part 10 pot

The procedures described in the section "Specimen Preparation" can be applied to metallographic preparation of niobium columbium, tantalum, molybdenum, tungsten, and their alloys.. Table

Fig 10 AZ31 B-F extrusion Longitudinal view of hot-worked structure Large, equiaxed recrystallized grains;

particles of manganese-aluminum compound and fragmented Mg17Al12 Etchant 8, Table 1 250×

Fig 11 AZ61A-F extrusion Longitudinal view of hot-worked structure Small, equiaxed recrystallized grains;

stringers of fragmented Mg 17 Al 12 See also Fig 12 Etchant 6, Table 1 250×

Fig 12 Same as Fig 11, except this specimen has not been etched, making the stringers of fragmented

Mg 17 Al 12 more easily visible As-polished 250×

Fig 13 AZ80A-F extrusion Longitudinal view of hot-worked structure Small, equiaxed recrystallized grains;

small amount of Mg 17 Al 12 discontinuous precipitate at the grain boundaries See also Fig 14 Etchant 3, Table 1,

15 s 250×

Fig 14 AZ80A-T5 extrusion Longitudinal view showing much mottled Mg17 Al 12 discontinuous precipitate near the grain boundaries, resulting from the artificial aging treatment Compare with Fig 13 Etchant 2, Table 1, 5

s 250×

Fig 15 ZK21A-F extrusion Longitudinal view of hot-worked structure Small equiaxed recrystallized grains at

the boundaries of and also within large, unrecrystallized elongated grains Etchant 6, Table 1 100×

Fig 16 ZK60A-F extrusion Longitudinal view of banded hot-worked structure Small, recrystallized grains;

light islands are solid solution deficient in zinc and zirconium (due to alloy segregation) and so more resistant to hot working See also Fig 17 Etchant 6, Table 1, then Etchant 4, Table 1 250×

Fig 17 Same as Fig 16, except artificially aged to the T5 temper Despite higher magnification, structure

appears same as Fig 16 (precipitate formed during aging is unresolvable by microscopy) Etchant 7, Table 1, 7

s, then Etchant 6, Table 1, 1 s 500×

Fig 18 HM31A-T5 extrusion Longitudinal view of banded hot-worked structure Small, recrystallized grains;

dark Mg 4 Th grain-boundary precipitate; light islands are solid solution rich in thorium and so more resistant to hot working; gray particle is manganese Etchant 5, Table 1 500×

Fig 19 AZ80A-T5 forging Longitudinal view of hot-worked structure, showing large, recrystallized grains and

spheroidized Mg17Al12 discontinuous precipitate mainly in the grains near the boundaries Etchant 5, Table 1 200×

Fig 20 ZK60A-T5 forging Longitudinal view Structure same as Fig 17, but with slightly larger grains and

increased alloy segregation Etchant 7, Table 1, 7 s, then Etchant 6, Table 1, 2 s 500×

Fig 21 HM21A-T5 forging Longitudinal view Structure is similar to that of sheet shown in Fig 6, except the

forging has smaller grains (Grain growth in sheet caused by solution heat treatment.) Etchant 5, Table 1 250×

Fig 22 AM60A-F die casting Small, cored grains of magnesium solid solution in which the aluminum content

increases toward the boundaries; passive Mg 17 Al 12 compound at grain boundaries Relief polishing causes dark areas See also Fig 23 Etchant 3, Table 1 500×

Fig 23 AS41A-F die casting Same structure as that shown in Fig 22, but with the addition of Mg2 Si in Chinese script and globular forms Etchant 3, Table 1, 5 s 500×

Fig 24 K1A-F die casting Small crystals of zirconium randomly dispersed in grains of magnesium that are

larger than those in more highly alloyed die castings (compare with Fig 22, 23, and 25.) Etchant 2, Table 1, 10

s 250×

Fig 25 AZ91A-F die casting Massive Mg17Al12 compound at the boundaries of small, cored grains Segregation (coring) in the grains and absence of precipitated discontinuous Mg 17 Al 12 are results of the rapid cooling rate of die castings See also Fig 26 and 27 Etchant 2, Table 1, 5 s 500×

Fig 26 AZ92A-F permanent mold casting Mg17Al12 compound: massive (outlined) at grain boundaries; precipitated (dark) near grain boundaries Slower cooling rate than that of die castings has resulted in larger grains than in structure shown in Fig 25 Etchant 2, Table 1, 5 s 250×

Fig 27 AZ92A-F sand casting Same microstructure as that shown in Fig 26, except the slower cooling rate, in

comparison with that of permanent mold castings, has resulted in larger grains See Fig 32 for effects of aging Etchant 2, Table 1, 5 s 250×

Fig 28 AZ92A-F sand casting The appearance of the interdendritic eutectic, a mixture of magnesium solid

solution and Mg 17 Al 12 , was retained in this form by a rapid quench from above the eutectic temperature See also Fig 29 Etchant 2, Table 1, 5 s 1500×

Fig 29 AM100A-F, as-cast Massive Mg17Al12 compound containing globular magnesium solid solution and surrounded by lamellar Mg17Al12 precipitate Normal air cooling produces this type of segregated eutectic Compare with Fig 28 and 30 Etchant 2, Table 1, 5 s 500×

Fig 30 AZ92A-F, as-cast Massive Mg17Al12 compound surrounded by lamellar Mg17Al12 precipitate Normal air cooling of zinc-containing magnesium-aluminum alloys produces this type of completely divorced eutectic Compare with Fig 29 Etchant 2, Table 1 500×

Fig 31 Massive Mg32 (Al,Zn) 49 (white) in as-cast alloy AZ63A-F Specimen etched with 50% picral to protect

Mg2Si (hexagonal particle) from HF, then with 5% HF to blacken Mg17Al12 and distinguish it from Mg32(Al,Zn)49, then with 10% picral to darken the matrix 500×

Fig 32 Alloy AZ92A-T6 sand casting Lamellar Mg17Al12 precipitate (light and dark gray) was produced throughout the grains of magnesium solid solution by artificial aging Some isolated islands of Mg2Si (white) are also present Etchant 2, Table 1 100×

Fig 33 Alloy AZ63A-T6 sand casting Lamellar Mg32(Al,Zn)49 discontinuous precipitate (dark) near some grain boundaries; some particles of Mg2Si and manganese-aluminum compounds Note that with 6% Al there is less precipitate than with 9% Al (compare with Fig 32) Etchant 2, Table 1, 5s 250×

Fig 34 EZ33A-T5 sand casting Interdendritic network of massive Mg9 R compound The precipitate in the dendritic grains of magnesium solid solution is not visible Etchant 2, Table 1 100×

Fig 35 ZK51A-T5 sand casting Fine, degenerate eutectic magnesium-zinc compound at the grain boundaries

The grains of magnesium solid solution are essentially homogeneous Etchant 2, Table 1, 5 s 250×

Fig 36 ZH62A-T5 sand casting Characteristic lamellar, or filigree, form of eutectic magnesium-thorium-zinc

compound at the boundaries of grains of magnesium solid solution 2% nital 250×

Fig 37 QE22A-T6 sand casting Massive Mg9R compound is present at the boundaries of grains of magnesium solid solution, resulting from partial solution and coalescence of the magnesium-didymium eutectic Etchant 2, Table 1 100×

Fig 38 HK31A-T6 sand casting Intergranular particles of massive Mg4Th compound (gray, outlined) The precipitate in the grains of magnesium solid solution is not visible See Fig 39 for effect of zinc addition Etchant 2, Table 1, 15 s 500×

Fig 39 HZ32A-T5 sand casting Intergranular Mg-Th compounds: bunches of acicular compound (dark gray)

and small areas of massive Mg 4 Th (see Fig 38) The precipitate within matrix grains is not visible 2% nital 250×

Fig 40 Fusion microporosity in an AZ63A-T4 sand casting Gray lamellar precipitate, present around black

voids despite solution heat treatment, indicates alloy segregation in these areas See also Fig 41 Etchant 2, Table 1, 10 s 100×

Fig 41 Fusion microporosity in an AZ63A-T4 sand casting The gray crackled film, formed around the black

voids by the acetic-picral etchant indicates alloy segregation in these areas See also Fig 40 Etchant 6, Table

1, 15 s 100×

Fig 42 Hot tear in an AZ91A-F die casting Tear occurred in an area of compound segregation that was last to

solidify and least resistant to stress caused by mold restriction during solidification shrinkage Etchant 2, Table

1, 5 s 75×

Fig 43 Shrinkage microporosity in an AZ92A-T6 sand casting The uniform dispersion of voids (black) in

particular areas of the casting is typical of this type of porosity, which results from improper feeding of molten metal to those areas See also Fig 44 Etchant 2, Table 1 100×

Fig 44 Shrinkage microporosity in an AZ92A-T6 sand casting Voids (black), resulting from withdrawal of

molten eutectic from between dendrites during solidification, are surrounded by areas low in alloying elements and containing no gray precipitate Etchant 2, Table 1, 5 s 100×

Fig 45 QE22A-T6 sand casting Alloy segregation (coring), characterized by intragranular precipitation of

didymium and zirconium hydrides (formed during solution treatment by reaction with water vapor) and by less

Mg 9 R at grain boundaries than normal Etchant 3, Table 1 500×

Fig 46 Segregation of zinc-zirconium-iron compound in a ZK61A-F sand casting This compound and Zr2Zn3form under similar conditions; the two can be distinguished by etching with 10% HF, which attacks Zr 2 Zn 3 but not zinc-zirconium-iron Etchant 2, Table 1, 10 s 250×

Fig 47 Segregation of layered oxide skin in a ZK61A-F sand casting This type of skin forms on molten metal

surfaces that are incompletely protected for several minutes Compare with the thin oxide skin shown in Fig

48 Etchant 2, Table 1, 10 s 250×

Fig 48 Segregation of thin oxide skin in an AZ91A-F die casting This type of skin forms whenever molten

metal surfaces are exposed to the atmosphere for a few seconds Compare with the layered oxide skin in Fig

47 Etchant 2, Table 1, 10 s 250×

Fig 49 Incomplete fusion in a two-pass gas tungsten-arc butt weld in 4-mm (0.160-in.) thick AZ31B-H2A

sheet Weld was made with alloy ER AZ61A filler metal Note the unfused area at the root of the second pass (top) Etchant 6, Table 1 3.8×

Fig 50 Shrinkage crack in the crater of a gas tungsten arc weld in an AZ92A-T6 casting caused by interruption

of welding without first reducing current to lower the temperature of the weld pool 2% nital 75×

Fig 51 Undercutting in a gas tungsten arc fillet weld in 4-mm (0.160-in.) thick AZ31B-H24 sheet The weld

was made with ER AZ61A filler metal Note undercut area in the edge of the top sheet of the lap joint Etchant

6, Table 1 3.8×

Fig 52 Incomplete joint penetration of a gas tungsten arc weld in a butt joint between 4-mm (0.160-in.) thick

AZ31B-H24 sheets The weld was made with ER AZ61A filler metal Note the unfused joint at the root of the weld Etchant 6, Table 1 3.8×

Fig 53 Crack in the heat-affected zone of a gas tungsten arc weld in an AZ92A-T6 casting, caused by use of

excessive welding current, producing too high a temperature gradient between base metal and weld pool 2% nital 75×

Fig 54 Gross gas porosity in gas tungsten arc weld joining 5-mm (0.190-in.) thick AZ31B-H24 sheets; ER

AZ61A filler metal Causes include dirty base metal and filler metal, inadequate coverage by shielding gas, and moisture in gas Etchant 6, Table 1 3.8×

Fig 55 Subsurface tungsten inclusion (large, round particle at top) in a gas tungsten arc weld in an AZ92A-T6

casting Filler metal is alloy ER AZ92A 2% nital 75×

Fig 56 Border area between zones of profuse (top) and sparse (bottom) shrinkage microporosity, in a gas

tungsten arc weld deposit of ER AZ101A filler metal Etchant 5, Table 1 65×

Fig 57 Shrinkage microporosity in heat-affected zone of gas tungsten arc weld made in an AZ91C-T6 casting

with ER AZ92A filler metal (The weld deposit is at top right.) Etchant 5, Table 1 75×

Nickel and Nickel-Copper Alloys: Metallographic Techniques and Microstructures

By William L Mankins, Process Development Manager, Huntington Alloys International

Introduction

THE PREPARATION of metallographic specimens and the microstructures of alloys containing 96% or more nickel (Nickel 200, Nickel 270, and Duranickel 301) and nickel-copper alloys (Monel 400, Monel R-405, and Monel K-500) are considered in this article Micrographs of these alloys are shown in Fig 1 to 15 in the section Atlas of Microstructures for Nickel and Nickel-Copper Alloys in this article

The procedures and materials for sectioning, mounting, grinding, and polishing specimens are essentially the same for all nickel alloys regardless of specimen size or sophistication of laboratory facilities In preparing specimens for metallographic examination, it is important to prevent working of the surface

Preparation for Microscopic Examination

The specimen to be examined is cut to a convenient size with a silicon carbide water-cooled cutoff wheel, then mounted

in a hard plastic, such as Bakelite or a hard epoxy resin Next, the mounted specimen is ground flat on a belt grinder using 120-grit abrasive and water coolant In general, it is preferable that the exposed area of the specimen not exceed about 1.6

cm2 (0.25 in.2)

Grinding may be performed manually or on power-driven wheels using silicon carbide paper disks, starting with grit and following with 320-, 400-, and 600-grit The specimen is then washed thoroughly and cleaned ultrasonically to remove any abrasive particles remaining on the surface

220-Polishing. All scratches from grinding are removed by polishing on a nylon cloth charged with 6-μm diamond paste and lubricated with lapping oil An alternate method is to polish on a broadcloth-covered wheel using 5-μm levigated alumina (Al2O3) powder suspended in water

Final polishing may be performed in one or two stages with a polishing wheel or vibratory polisher If a polishing wheel

is used, Microcloth and γ-Al2O3 powder (<0.1-μm particle size) suspended in water are recommended An alternative requires semifinal and final polishing using a vibratory polisher Semifinal polishing employs a nylon polishing cloth and

a slurry of 0.3-μm Al2O3 and distilled water A 350-g weight is placed on the specimen throughout the polishing cycle At the conclusion of each polishing cycle, the specimen is cleaned ultrasonically Final polishing employs a short-nap microcloth and a slurry of 0.05-μm Al2O3 and distilled water Polishing continues until the surface is free of scratches

Electropolishing. Nickel and nickel-copper alloys can be electropolished satisfactorily, although best results are generally obtained with specimens that first have been polished mechanically through 600-grit Recommended electrolytes and current densities for electropolishing these alloys are given in Table 1 A platinum cathode is suggested and the electrolyte should be water cooled and continuously stirred

Table 1 Electrolytes and current densities for electropolishing of nickel and nickel-copper alloys

Current density Electrolyte

composition

Applicable alloys

Composition of etchant Conditions for use

Etchants for Nickel 200 and 270; Permanickel; Duranickel 301; and Monel 400, R-450, and K-500

1 part 10% aqueous solution of NaCN (sodium cyanide), 1 part 10% aqueous solution of

(NH 4 ) 2 S 2 O 8 (ammonium persulfate) Mix solutions when ready to use

Immerse or swab specimen for 5-90 s(a)

1 part HNO 3 (conc), 1 part acetic acid (glacial) Use fresh solution For revealing grain boundaries Immerse

or swab specimen for 5-20 s

7.5 mL HF, 2.5 mL HNO 3 , 200 mL methanol Immerse sample 2-4 min

5 g FeCl 3 , 50 mL HCl, 100 mL H 2 O Immerse or swab specimen up to a few

minutes

Alternate etchant for Monel K-500

Glyceregia: 10 mL HNO 3 (conc), 20 mL HCl (conc), 30-40 mL glycerol Etch by immersing or swabbing the

specimen for 30 s to 5 min

(a) This cyanide-containing etchant is very hazardous in its preparation and use Cyanide, even in small quantities, as dust, solution, or fumes may be fatal when taken into the body A fume hood should be utilized

Preparation for Macroscopic Examination

Surfaces to be etched for macroscopic examination may be prepared by surface grinding to a fine finish with 180-grit and 240-grit silicon carbide paper Finer grinding, although unnecessary, yields a finer surface before etching, which requires less severe macroetching to reveal the metal structure

Etching of nickel alloys for macroscopic examination is performed by immersing or swabbing the ground specimen for 5

to 20 s in an etchant composed of equal parts (by volume) of concentrated nitric acid (HNO3) and glacial acetic acid

Macroetching of nickel-copper alloys is done by immersing or swabbing the ground specimen in concentrated HNO3 Colorless acid should be used to avoid staining Depending on the purpose of examination, etching time should be 3 to 5 min Within this range, shorter etching times will reveal sulfur embrittlement and details of welds in Monel; longer times will reveal general structure, including surface and subsurface cracks, porosity, and forging flow lines Macroetching can

be hastened by warming the specimen in hot water prior to etching

Microstructures of Nickel and Nickel-Copper Alloys

Nickel-base alloys are widely used as high-temperature materials Micrographs of such alloys are presented in the articles

"Wrought Heat-Resistant Alloys" and "Heat-Resistant Casting Alloys" in this Volume Micrographs of nickel-base alloys employed as magnetically soft materials are in the article "Magnetic and Electrical Materials" in this Volume The micrographs in this article show structures of nickel alloys that are used primarily for their resistance to corrosion and for other specialized applications As shown in Table 3, these alloys range in nickel content from 66.5% to 99.98%

Table 3 Nominal compositions of nickel and nickel-copper alloys

Alloy Composition

The microstructure of Nickel 200 typically contains some nonmetallic inclusions (principally oxide) Prolonged exposure

to temperatures from 425 to 650 °C (800 to 1200 °F) results in the precipitation of graphite from the nickel solid solution

Although Nickel 270 (99.98% Ni) is less likely than Nickel 200 to contain nonmetallic inclusions, their structures are similar, assuming that mechanical working and thermal treatments are similar (compare Fig 1 and 2 to Fig 3 and 4 in the section Atlas of Microstructures for Nickel and Nickel-Copper Alloys in this article)

Permanickel 300 is an age-hardening alloy that in the solution-annealed condition shows randomly dispersed particles of titanium nitride (TiN) and graphite when observed through an optical microscope When subsequently age hardened, the alloy has a similar appearance (Fig 5), but it also contains a fine granular precipitate This phase is not resolvable by optical microscopy in material aged at a normal aging temperature (480 °C, or 900 °F), but is visible in overaged material The phase or phases responsible for the age hardening of this alloy have not been positively identified The mechanism appears to be complex; carbon, magnesium, and titanium are required for full hardness Precipitation of a compound such

as Ni3(Mg,Ti)Cx seems likely during age hardening

Duranickel 301, an age-hardening alloy, combines the corrosion resistance of unalloyed nickel with increased strength and hardness After solution annealing, this alloy is age hardened by holding in the temperature range of 425 to 705 °C (800 to 1300 °F), which precipitates the phase Ni3(Al,Ti) throughout the structure In the solution-annealed and properly aged condition (see Fig 6), the precipitated phase is not resolved by an optical microscope Some particles of graphite, however, are usually visible

Nickel-Copper Alloys. Monel 400 is a stable solid solution of nickel and copper Non-metallic inclusions often appear

in the microstructure (see Fig 7 in the section Atlas of Microstructures for Nickel and Nickel-Copper Alloys in this article)

Monel R-405 is a free-machining grade of Monel 400 The microstructures of these two alloys are similar for the same mechanical and thermal treatment, except for the sulfide particles in Monel R-405, which improve machinability (Fig 8)

Monel K-500 is produced by adding aluminum and titanium to the basic nickel-copper composition Solution annealing and aging produce a γ' precipitate throughout the matrix In material aged at the normal temperature of 595 °C (1100 °F), this precipitate is not resolvable by an optical microscope (Fig 12 and 13) However, in material that is overaged by holding at 705 °C (1300 °F), for example the precipitate is visible by optical microscopy (Fig 14 and 15) In addition to the precipitate, particles of TiN are usually present in the microstructure

Atlas of Microstructures for Nickel and Nickel-Copper Alloys

Fig 1 Nickel 200, cold drawn and annealed in a continuous process at 830 °C (1525 °F) Structure: nickel solid

solution See also Fig 2 NaCN, (NH 4 ) 2 S 2 O 8 100×

Fig 2 Same as Fig 1, but at higher magnification Variation in shade of grains is caused by variation in grain

orientation NaCN, (NH4)2S2O8 500×

Fig 3 Nickel 270, hot rolled and annealed in a continuous process at 830 °C (1525 °F) Structure: nickel solid

solution See also Fig 4 NaCN, (NH 4 ) 2 S 2 O 8 100×

Fig 4 Same alloy and same processing as in Fig 3, but shown at a higher magnification The variation in shade

of the grains (dark, gray, and white) is the result of variation in grain orientation NaCN, (NH 4 ) 2 S 2 O 8 500×

Fig 5 Permanickel 300, solution annealed 1 h at 1205 °C (2200 °F) and water quenched, aged 10 h at 480 °C

(900 °F) and water quenched Dispersed particles of TiN and graphite (black dots) in nickel solid solution NaCN, (NH4)2S2O8 100×

Fig 6 Duranickel 301, solution annealed for 30 min at 980 °C (1800 °F) and water quenched, aged for 20 h at

480 °C (900 °F) and water quenched Microstructure: nickel solid solution; graphite particles (black dots) NaCN, (NH 4 ) 2 S 2 O 8 50×

Fig 7 Monel 400, cold drawn and annealed in a continuous process at 830 °C (1525 °F) Nickel-copper solid

solution with a few unidentified nonmetallic inclusions (black) NaCN, (NH4)2S2O8 100×

Fig 8 Monel R-405, cold drawn, and annealed in a continuous process at 830 °C (1525 °F) Microstructure:

nickel-copper solid solution with sulfide stringers (black constituent) NaCN, (NH4)2S2O8 100×

Fig 9 Monel K-500 in the hot rolled condition Structure: nickel-copper solid solution Variation in shade of

grains is the result of variation in grain orientation Glyceregia 100×

Fig 10 Monel K-500, solution annealed for 1 h at 1205 °C (2200 °F) and quenched in water Nickel-copper

solid-solution matrix See also Fig 11, 12, 13, 14, and 15 NaCN, (NH4)2S2O8 100×

Fig 11 Same as Fig 10, but at higher magnification Portions of only three grains are visible The black dots

are nitride particles See also Fig 10, 12, 13, 14, and 15 NaCN, (NH4)2S2O8 1000×

Fig 12 Monel K-500, held 1 h at 1205 °C (2200 °F), transferred to a furnace at 595 °C (1100 °F) and aged 4

h, water quenched Solid-solution matrix; nitride particles See also Fig 10, 11, 13, 14, and 15 NaCN, (NH4)2S2O8 100×

Fig 13 Same as Fig 12, but at higher magnification Structure contains precipitated Ni3(Al,Ti), resolvable only

by electron microscopy unless aging temperature is higher than 595 °C (1100 °F) See also Fig 10, 11, 12, 14, and 15 NaCN, (NH 4 ) 2 S 2 O 8 1000×

Fig 14 Monel K-500, held 1 h at 1205 °C (2200 °F), transferred to a furnace at 705 °C (1300 °F) and aged 4

h, water quenched Precipitated Ni3(Al,Ti) appears as tiny particles dispersed in the matrix solid solution See also Fig 10, 11, 12, 13, and 15 NaCN, (NH4)2S2O8 100×

Fig 15 Same as Fig 14 except at a higher magnification The Ni3 (Al,Ti) precipitate is better resolved When this precipitate is resolvable by optical microscopy, overaging is indicated See also Fig 10, 11, 12, 13, and 14 NaCN, (NH4)2S2O8 1000×

Refractory Metals and Alloys: Metallographic Techniques and Microstructures

John B Lambert, Vice President and Corporate Technical Director, Fansteel; Mortimer Schussler, Senior Scientist, Fansteel

Introduction

REFRACTORY METALS and their alloys are prepared similarly for metallographic examination Slight increases in grinding, polishing, and etching times may be required for alloys, because they are generally harder than the unalloyed metals Particular product forms, such as wire, may also necessitate special preparation techniques

The procedures described in the section "Specimen Preparation" can be applied to metallographic preparation of niobium (columbium), tantalum, molybdenum, tungsten, and their alloys Alternate procedures that have been used for each of these materials are also summarized Specific etchants for these metals and their alloys are listed in Table 1

Table 1 Etchants for metallographic specimens of refractory metals (a)

Etchant Composition Comments

Etchants for niobium and tantalum and their alloys

Swab 3 to 10 s; use fume hood

Additional etchants for niobium and niobium alloys

Additional etchants for tantalum and tantalum alloys

ASTM 177 10 g NaOH, 100 mL H 2 O Swab or immerse 5 to 15 s

ASTM 164 50 mL HNO 3 , 30 g

NH 4 HF 2 , 20 mL H 2 O

Swab 3 to 10 s; use fume hood

Etchants for tungsten and molybdenum and their alloys

Murakami's

reagent

(modified A)

30 g K 3 Fe(CN) 6 , 10 g NaOH, 200 mL H 2 O

Swab 5 to 60 s; immersion will produce a stain etch; follow with water rinse, alcohol rinse, dry

Additional etchants for molybdenum and molybdenum alloys

ASTM 129 10 mL HF, 30 mL

HNO 3 , 60 mL lactic acid

Swab 10 to 20 s; vary HF content to increase/decrease etching activity

(b) Adjust amount of H2O2 to obtain a reaction rate that will reveal the microstructure after etching for approximately 20-40 s

Information on the properties and selection of refractory metals and their alloys can be found in the article "Refractory

Metals and Alloys" in Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Volume 2, ASM

Handbook Production of refractory metal powders and the processing and applications of refractory powder metallurgy

parts are discussed in the article "Production of Refractory Metal Powders" in Powder Metal Technologies and

Applications, Volume 7, ASM Handbook

Specimen Preparation

Sectioning. The following abrasive wheels have been found to be efficient regarding cutting quality, cutting speed, and wheel wear:

• Niobium and tantalum (all sizes): A 180 PR (rubber-bonded, 180-grit alumina, Al2 O3)

• Molybdenum (larger specimens): A 180 PR or A 60 MR (rubber-bonded, 60-grit Al2O3), which cuts faster at the expense of wheel wear

• Molybdenum (thin specimens): A 180 PR

• Tungsten (thicker than 3 mm, or 1

8 in.): A 90 KR (rubber-bonded, 90-grit Al2O3) or C 120 KR bonded, 120-grit silicon carbide)

(rubber-• Tungsten (thinner than 3 mm, or 1

8 in.): A 180 PR (cut slowly)

• Tungsten (general): A 70 TB (resinoid-bonded, 70-grit Al2 O3)

Tungsten products, such as wire, can be cut using wet or dry abrasive wheels, preferably after a heavy nickel plate has been applied to the wire Nickel plating aids edge retention and helps keep wire sections flat during polishing Because tungsten products can delaminate, they should not be cut with shears or wire cutters

Initial Grinding. Irregularly shaped specimens that are not cut can be ground flat on an 80-grit belt sander It is generally preferable to remove excess stock in this manner before mounting

Mounting. Green Bakelite, the preferred thermosetting resin, is mounted in a heated press under 690 kPa (100 psi) pressure at 150 to 180 °C (300 to 355 °F) for 2 min The pressure is then released and the press opened If the melted but uncured Bakelite does not rupture at the top and release moisture, the small bubbles that form should be pricked Pressure

is then reapplied at 24 to 34 MPa (3500 to 5000 psi) for 3 to 5 min at the same temperature curing cycle

Grinding. Wet grinding is performed using 240- and then 400-grit silicon carbide paper on rotating brass laps, with running water as the lubricant Grinding time depends on specimen hardness and size, but is usually twice the time required to remove scratches from the previous operation

Rough and intermediate polishing is carried out in two steps using rotating brass laps covered with nylon and 30- and 9-μm diamond paste The lubricant is kerosene The mounts must be cleaned thoroughly with alcohol and soap between polishings

Final polishing is performed using a rotating brass lap covered with a bonded synthetic rayon polishing cloth (Microcloth), 1-μm diamond paste, and kerosene The medium nap of the synthetic rayon polishing cloth produces an excellent scratch-free finish; however, during grinding and rough polishing, refractory metals develop a disturbed surface layer that must be removed to observe the true structure This is accomplished by a series of light etchings just enough to cloud the specimen, alternated with very light repolishings on the final lap that do not quite remove the etch (see Table 1 for etchant compositions and procedures) Four to eight etch/polish operations are usually sufficient The true structure is revealed when two successive etchings reveal the same structure The final etching should be much deeper than the previous etchings

Vibratory polishing is an excellent alternative to the above procedures Niobium and tantalum are polished as described above, including the 1-μm diamond lap The mount is cleaned and placed in a 300-g holder for use in a vibratory lapping-polishing machine with a long-nap synthetic velvet (Rayvel) and an aqueous suspension of 0.3-μm

Al2O3 In a freshly cleaned machine, disturbed metal can be removed in approximately 18 h As metal sludge accumulates over a few weeks, the time required may double After a final etch, the true structure is revealed

For molybdenum and tungsten, the standard polishing procedure is used, through the 30-μm diamond lap The mounts are then cleaned and placed in the vibratory lapping-polishing machine as for tantalum and niobium, except nylon cloth is used Remaining scratches are removed, and the specimen is polished without disturbing the metal Depending on the cleanliness of the polishing apparatus, 6 to 15 h are required The true structure appears after one etch

Electropolishing. Unmounted larger specimens of molybdenum and tungsten can be electropolished adequately for routine examination The cutting wheels previously listed yield a surface smooth enough for this operation An excellent polish, particularly with difficult-to-polish alloys such as molybdenum alloys, TZM, and TZC can be obtained using an electrolyte consisting of 12.5% sulfuric acid (H2SO4) in alcohol, with nickel as the cathode and 8 to 50 V dc Voltage is easily monitored by inspection; an intense blue layer forms on the surface of the specimen when the correct voltage is applied Because alcohol has a relatively low boiling point, the electrolyte must be prepared carefully and must be cooled

by a water bath during electrolytic polishing

A 66% solution of chromic acid (CrO3) in water is a more stable electrolyte and is used at a high current density of approximately 2.3 A/cm2 (15 A/in.2) Therefore, the specimen must be removed frequently from the bath and cooled in running water Nickel is again used as the cathode

Electropolishing of higher quality can be obtained by first mounting the specimens, followed by grinding, and rough polishing through 30-μm diamond The power requirements in this case are approximately 3 A at a lower voltage, and the etching time is 30 to 40 s for a typical specimen

Aqueous electrolytes containing 1 to 10% sodium hydroxide (NaOH) or potassium hydroxide (KOH) can be used successfully for electropolishing of tungsten and tungsten alloys Best results are obtained if half the volume of water in the solution is replaced by glycerol and the NaOH content is approximately 5% With a 4% NaOH aqueous solution, a satisfactory polish should be obtained in 20 s using a current density of 2.3 A/cm2 (15 A/in.2)

Excellent polishes can be obtained over a wide range of current densities However, if the current density is too high, pitting will occur; if too low, the specimen will etch Heat is generated at high current densities and, if excessive, may cause uneven polishing, etching, or both Overheating can be minimized by frequently cooling the specimen in a stream

of cold water Electropolishing also can remove inclusions Large sections often do not polish evenly

Electrolytic Etching. Molybdenum and tungsten can be electroetched with the same equipment and electrolytes used for electro-polishing Power required is approximately 0.4 A for 8 s for molybdenum and 0.4 A for 5 s for tungsten Large specimens will etch more evenly in a chemical etchant The specimens should be cleaned prior to examination

Etchants. Etchant ASTM 163 in Table 1 is preferred for niobium and tantalum; Murakami's reagent (modified A), for molybdenum and tungsten

Alternate Preparation Procedures for Niobium and Tantalum

Polishing. A typical method of rough polishing mobium and tantalum uses a wax wheel and 15-μm levigated Al2O3 Intermediate polishing is performed using a synthetic rayon cloth-covered wheel and 1-μm Al2O3; final polishing, a synthetic rayon cloth-covered wheel and 0.3-μm Al2O3

Polish-etching, also known as chemical-mechanical polishing or attack polishing, is suited to niobium, tantalum, and their alloys, but the procedure is different from that for polish-etching of tungsten and molybdenum After grinding, the specimen is rough polished on a conventional corrosion-resistant horizontal polishing wheel covered with a chemotextile material (Pellon cloth) using 0.3μm Al2O3 The specimen is then polish-etched on synthetic velvet cloth using a slurry of 0.05-μm Al2O3 and a solution of hydrofluoric acid (HF) (2 mL for niobium, 5 mL for tantalum), 5 mL nitric acid (HNO3), and 30 mL lactic acid Because this mixture is hazardous and extremely corrosive, and because polish-etching is time consuming, the specimen should be held in a mechanical holder rather than by hand Initial polishing is performed for 1 to

4 h using a 160-rpm wheel and a 450-g (1-lb) weight on the holder For final polish-etching, the weight on the holder is reduced to 225 g (0.5 lb) and held 15 min

Etchants used for metallographic specimens of niobium, tantalum, and their alloys are listed in Table 1

Alternate Preparation Procedures for Molybdenum

Because molybdenum is relatively soft, scratches and distorted metal that develop in mechanical polishing are difficult to eliminate; therefore, electropolishing, as previously described, and electromechanical polishing are often used for molybdenum

Electromechanical polishing combines electrolytic and mechanical polishing It is excellent for retention of second phases and inclusions, for providing the most realistic representation of porosity, and for obtaining specimen flatness Polishing time is only slightly longer than for electropolishing

Operating conditions for electromechanical polishing of molybdenum and molybdenum alloys are not critical The following conditions are typical The polishing wheel must be covered with a material that will resist the electrolyte used; synthetic velvet is suggested Polishing wheel speed should range from 255 to 1100 rpm The abrasive/ electrolyte is 0.05-

m Al2O3 dispersed in a small amount of a 30% K3Fe(CN)6 aqueous solution The polarity of the 3 to 12 V dc power supply is slowly alternated approximately 30 cpm between the polishing wheel and the specimen Polishing time is 2 to

5 min

Electromechanical polishing is generally preferred for final polishing of large specimens and often is used as an intermediate or final polish for wire specimens Final polishing may also be performed electrolytically The etchants used

in this technique and in polish-etching can corrode brass laps, as can the metals being polished

Polish-etching. Although electropolishing and electromechanical polishing provide the most consistent results, etching is also satisfactory for molybdenum specimens Polish-etching substitutes chemical attack for the electrolytic action obtained in electrome-chanical polishing

polish-Polish-etching is recommended when the equipment required for electromechanical polishing is not available The slurry used in polish-etching is prepared by adding 0.05-μm Al2O3 to a solution containing 3.5 g K3Fe(CN)6, 1 g NaOH, and 300

mL H2O or to a solution containing 1 g CrO3 and 75 mL H2O Results are improved by chemical polishing using a solution of 30 mL lactic acid, 10 mL HNO3, and 5 mL HF, then polish-etching The slution is used fresh (it should not be stored) and is applied by swabbing with heavy pressure

Etchants. A modified Murakami's reagent, which provides good grain-boundary contrast and minimizes etch pitting, is recommended for etching molybdenum A typical mixture contains 15 g K3Fe(CN)6, 2 g NaOH, and 100 mL of H2O Specimens are immersed 5 to 10 s Other etchants for molybdenum and its alloys are given in Table 1

Alternate Preparation Procedures for Tungsten

Mounting. Most specimens of tungsten or tungsten alloys are mounted In mounting wires, however, it is difficult to obtain sections that are parallel to the longitudinal axis, because wires are seldom perfectly straight One method involves pressing the wires into a lead block, which then serves as the mounting block For wires less than 0 15 mm (0.006 in.) in diameter, a harder solder block, such as tin-lead (Alloy Grade 50A, 50Sn-50Pb), should be used to avoid losing the wires

in the mount as a result of the smearing action of unalloyed lead

This mounting technique is also used for coils of lamp wire and electronic wire Under pressure, the loops of a coil fold over, placing the longitudinal axis of the wire parallel to the polishing plane Fragile specimens can be pressed with less damage and distortion into solder blocks that have been preheated to 175 °C (345 °F)

Another technique includes placing the wires in an aluminum mold 25 to 40 mm (1 to 1.5 in.) in diameter Enough clear epoxy resin to make a thin disk is poured over the wires The mold is then heated on a hotplate at 70 to 90 °C (160 to 190

°F) for 20 min When the epoxy resin hardens, the disk containing the wires is removed from the mold and is cut transversely to the axis of the wires The transverse section is then placed upright in the mold, and more epoxy resin is poured in this time to a substantial depth The mold is again heated as described above When the epoxy resin has cured, the wires are held perpendicular to the plane of polish, and the section of the original epoxy-resin disk is completely fused into the new mount Grinding, polishing, and etching can now be performed as required

Grinding. Rough and finish grinding of tungsten and tungsten alloys is performed with conventional procedures using wet Al2O3 laps or papers from 60 through 600 grit Light pressure is recommended throughout grinding, and fresh abrasive laps or papers should be used during final grinding However, fine wire requires a less coarse initial grinding, because the depth of cold work resulting from grinding may exceed the diameter of the wire Wires less than 25 μm in diameter are polished using a cloth-covered lap and 0.05-μm Al2O3 Further mechanical polishing is not required if electropolishing or electromechanical polishing methods are to be used

Electromechanical Polishing. Conditions for electromechanical polishing of tungsten are similar to those used for molybdenum Voltage is increased to 5 to 15 V dc, and the speed of the polishing wheel is decreased to 160 to 550 rpm

Polish-etching. Tungsten is polish-etched in the same manner as molybdenum

Etchants. Murakami's reagent, conventional or modified, is most often used for etching tungsten and tungsten alloys, although other etchants are sometimes used In addition to the etchants shown in Table 1, electrolytic etching in a 4% NaOH aqueous solution with 10 to 50 A/cm2 (65 to 325 A/in.2) ac or 5 to 10 A/cm2 (30 to 65 A/in.2) dc has been used to improve grain-boundary contrast

Microstructures of Refractory Metals and Alloys

Refractory metals illustrated in this article include niobium, tantalum, molybdenum, and tungsten Several alloy modifications of these metals are also represented Micrographs cover wrought structures of products developed from powder metallurgy compacts and from ingots made by melting and casting

Niobium alloys were developed to provide greater oxidation resistance and elevated-temperature strength than are

obtainable with the commercially pure metal Niobium oxidizes less rapidly than the other refractory metals Although no single element can be added to niobium to make an oxidation-resistant alloy, combinations will produce various oxidation behaviors Many of the alloying additions that improve oxidation resistance also benefit high-temperature strength

Combinations of molybdenum, tantalum, tungsten, and titanium serve as solid-solution strengtheners when added to niobium; the ternary and quaternary solid-solution alloys exhibit complex strength-temperature responses Second-phase strengthening is achieved by controlled additions of zirconium, hafnium, or both These additions develop metal-nonmetal systems in which strengthening results from the formation of zirconium oxide (ZrO2) or of zirconium or hafnium carbide

Tantalum. In industry, tantalum, which closely resembles niobium, is produced as a powder or sponge and is further purified in the solid or the liquid state Solid-state purification occurs during sintering; liquid-state purification, during vacuum-arc melting or electron-beam melting in high vacuum, Consolidated high-purity tantalum has also been produced

by the thermal decomposition of a halide, such as tantalum bromide (TaBr5), on a hot wire Unalloyed tantalum has limited usefulness in high-temperature applications, because it has relatively low hot strength and low resistance to oxidation even at moderate temperatures

The elevated-temperature strength of tantalum is significantly increased by alloying Additions of up to 10% tungsten or molybdenum are effective in solid solution Additions of zirconium or hafnium to ternary alloys also contribute to solid-solution strengthening Alloying, however, does not significantly increase oxidation resistance

Molybdenum. Wrought products of molybdenum are developed from powders that are compacted and sintered Sintered ingots can be fabricated directly Purification of molybdenum is a major problem, however, and consolidation

and purification are often achieved concurrently by vacuum-arc melting, electron-beam melting, zone refining, or levitation melting Multiple melting to attain a desired level of purification is common

The earliest alloys of molybdenum contained less than 2% alloying elements, providing higher recrystallization temperatures and higher mechanical properties at elevated temperature than could be obtained with unalloyed molybdenum For several years, the development of molybdenum alloys with higher alloy content was limited by the inability of existing facilities to work the alloys On an experimental basis, however, binary alloys of molybdenum and niobium, tantalum, titanium, tungsten, vanadium, hafnium, and zirconium were prepared as arc-cast ingots and studied primarily in the cast condition

Binary alloys of molybdenum and tungsten have higher melting points than unalloyed molybdenum As tungsten content increases, the melting point, hardness, and density of binary alloys increase almost linearly between those of pure molybdenum and pure tungsten; however, the workability and machinability of binary alloys decrease gradually

Tungsten. Wrought products of tungsten, such as wire and sheet, are developed initially from high-purity powder that is pressed to form a compact, then sintered The sintered compact may be fabricated directly, which is the more common practice, or it may be used as an electrode in a melting process, such as consumable-electrode vacuum-arc melting Micrographs of sintered and wrought products are presented in this article

Although prepared from commercially pure powders, most wrought tungsten products, especially those used in lamp and electronic-tube applications, contain one or more useful additives "Non-sag," doped tungsten is prepared with a small amount of alkaline alummosilicate; most of the silicate evaporates in sintering, leaving a residue of approximately 100 ppm

Doping increases the recrystallization temperatures of tungsten by approximately 415 °C (750 °F), changes the recrystallized grains from equiaxed to elongated, and improves resistance to creep and high-temperature deformation, properties that are essential for tungsten wire used in incandescent lamps Thoriated tungsten, an alloy normally containing a dispersion of from 0.5 to 2.0% thorium dioxide, has greater resistance to impact and vibration in lamp filaments and improved thermionic emission in tungsten cathode electronic tubes than unalloyed tungsten Perhaps the best known solid-solution alloys are those containing rhenium The tungsten-rhenium alloys presented in this section exhibit higher electrical resistivity than unalloyed tungsten and are widely used in filaments for photographic flashbulbs

Atlas of Microstructures for Refractory Metals and Alloys

Fig 1 High-purity niobium (<10 ppm C, 30 ppm O, 20 ppm N), 1.6-mm (0.062-in.) thick sheet Electron-beam

melted, cold forged, cold rolled, 50 to 90% reductions between anneals Final anneal in vacuum at 900 °C (1650 °F) for 1 h Longitudinal section showing fully recrystallized structure Etchant: ASTM 163 250×

Fig 2 FS-80 niobium alloy tube 3.2-mm (1

8-in.) OD, 0.25-mm (0.010-in.) wall (after 70% reduction), vacuum annealed 1 h at 1150 °C (2100 °F) Longitudinal section Solid-solution matrix consists of large recrystallized grains (ASTM No 5-1

2) Intragranular precipitate is probably ZrO 2 30 mL each 50% HF, H 2 SO 4 , and H 2 O with 3

to 5 drops 30% H2O2 250×

Fig 3 FS-85 niobium alloy (Nb-28Ta-11W-0.8Zr), 2.8-mm (0.110-in.) thick sheet Arc melted, hot extruded,

warm rolled at 705 °C (1300 °F), 50 to 75% reductions between anneals Final anneal in vacuum at 1315 °C (2400 °F) for 1 h Longitudinal section of fully recrystallized structure showing typical banding ASTM grain size

7 Etchant: ASTM 163 250×

Fig 4 C-103 niobium alloy (Nb-10Hf-1Ti-0.5Zr), 6.4-mm (0.25-in.) thick plate, cold worked and annealed The

microstructure shows stringers, elongated in the rolling direction, of a dispersed phase consisting of HfO2 and ZrO 2 compounds Etchant: ASTM 163 150×

Fig 5 C-103 niobium alloy, 0.1-mm (0.040-in.) thick sheet Arc melted, hot extruded, warm rolled, and

annealed Cold rolled to finished size Final annealed in vacuum at 1290 °C (2350 °F) for 1 h Longitudinal section showing fully recrystallized structure ASTM grain size 7 Etchant: ASTM 163 250×

Fig 6 Nb-30Ti-20W alloy sheet Electron-beam melted ingot, arc remelted in vacuum Forged, rolled, annealed,

and gas nitrided Scanning electron micrograph of titanium-rich nitride phase (dark) in a titanium-depleted niobium alloy matrix Outer surface of specimen is shown at bottom of micrograph 33% HCl and 17% HF in glycerol 500×

Nb-46.5Ti, 13-mm (0.5-in.) diam rod Vacuum-arc melted into 2700-kg (6000-lb) ingot, press forged, rotary forged to 150-mm (6-in.) diam, annealed, and water quenched Extruded to 38-mm (1.5-in.) diam, annealed and quenched, surface conditioned, and drawn to size Rough polished on silk with acidified (1 to 2% CrO3) 1-μm Al2O3 slurry Final polished on Microcloth using a slurry of 3 to 4 g

Al2O3 in 150 mL H2O A 5- to 10-mL solution of 22 mL HNO3 and 3 mL HF in 250 mL H2O is added to the slurry on the wheel Fig 7: longitudinal section of wrought as-drawn microstructure Fig 8: longitudinal section of equiaxed, recrystallized structure resulting from annealing after final draw Both swab etched using 10 mL lactic acid, 10 mL H2O2, 3 mL HNO3, and 3 mL HF 400×

Fig 9 Fig 10

Unalloyed tantalum sheet Electron-beam melted, forged, cold reduced 60%, annealed in vacuum at

1095 °C (2000 °F) for 1 h Fig 9: final annealed in vacuum at 1010 °C (1850 °F) for 1 h Longitudinal cross section showing fully recrystallized, equiaxed grains of mixed size Average ASTM grain size 6 250× Fig 10: final annealed in vacuum at 2000 °C (3630 °F) for 1 h Longitudinal cross section of fully recrystallized structure showing effect of final annealing temperature on grain growth ASTM grain size 00 110× Etchant: ASTM 163

Unalloyed tantalum powder metallurgy sheet Bar cold rolled 85%, annealed, and finish rolled to

0.25-mm (0.010-in.) thick sheet Longitudinal sections showing fully recrystallized structure Fig 11: final annealed in vacuum at 1260 °C (2300 °F) for 1 h ASTM grain size 9 Fig 12: same as Fig 11, with added 30-min final anneal at 2000 °C (3630 °F) ASTM grain size 2 to 6 Compare with Fig 9 and 10 Etchant: ASTM 163 250×

Ta-250 ppm Y (added as Y2O3) powder metallurgy sheet, Bar cold rolled 85%, annealed, and finish rolled to 0.25-mm (0.010-in.) thick sheet Final annealed in vacuum at 1260 °C (2300 °F) for 1 h Fig 13: longitudinal cross section showing banding from residual cold work resulting from yttria stabilization ASTM grain size 9 Fig 14: same as Fig 13, with added 30-min final anneal at 2000 °C (3630 °F) Longitudinal cross section ASTM grain size 8 Etchant: ASTM 163 250×

Fig 15 Ta-150 ppm Si (silicon doped) 0.6-mm (0.023-in.) diam powder metallurgy capacitor wire Sintered bar

cold rolled up to 90% reduction between anneals, cold drawn approximately 75% Final annealed in vacuum at

1315 °C (2400 °F) for 1 h Longitudinal section, fully recrystallized structure ASTM grain size 10 Etchant: ASTM 163 250×

Fig 16 Ta-100 ppm V (vanadium doped) 25-μm (0.001-in.) thick powder metallurgy capacitor foil Bar rolled

to 60% reduction, annealed in vacuum 1 h, rolled to 0.45-mm (0.018-in.) thickness, final annealed, and finish rolled to final gage Scanning electron micrograph of a longitudinal cross section showing as-rolled elongated grains Etchant: ASTM 163 3000×

Fig 17 Ta-100 ppm V (vanadium doped) 25-μm (0.001-in.) thick powder metallurgy capacitor foil Bar rolled

to 60% reduction, annealed in vacuum 1 h, rolled to 0.45-mm (0.018-in.) thickness, final annealed, and finish rolled to final gage Scanning electron micrograph showing the cold-worked through-surface structure after etching Etched in electrolyte consisting of 30 g ammonium bromide, 5 g calcium chloride, and 18 mL deionized

H2O in 1000 mL methyl alcohol Current density: 18 mA/cm 2 (118 mA/in 2 ) for 20 min 2000×

Fig 18 Fig 19

Tantaloy "63" (Ta-2.5W-0.15Nb) tube, 0.6-mm (0.024-in.) thick wall, 19-mm (0.75-in.) OD beam melted, warm forged, and cold rolled 50 to 90% between anneals from ingot to final product Sheet annealed in vacuum at 1260 °C (2300 °F) for 1 h, press-brake formed, gas tungsten arc welded, and given final sizing pass Fig 18: transverse section showing fully recrystallized microstructure of base metal with some cold work resulting from the sizing operation Fig 19: transverse section through weld showing fine-grained base metal, coarsened grain size in heat-affected zone, and coarse- grained nondendritic weld zone Etchant: ASTM 163 50×

Electron-Fig 20 "61" metal (Ta-7.5W) 0.8-mm (0.032-in.) diam powder metallurgy spring wire Sintered bar, square

warm rolled at 315 °C (600 °F), 50 to 75% reduction, and cold drawn up to 50% between anneals Longitudinal section showing elongated grain structure Etchant: ASTM 163 250×

Fig 21 Ta-10W alloy 1.0-mm (0.040-in.) thick sheet Electron-beam melted, warm forged, cold rolled, and

annealed Final annealed in vacuum at 1480 °C (2700 °F) Longitudinal section showing fully recrystallized structure and banding ASTM grain size 6 Etchant: ASTM 163 250×

Fig 22 Ta-40Nb alloy 3.2-mm (0.125-in.) thick sheet Ingot warm forged and cold rolled, with 50 to 90%

reduction between anneals Final annealed in vacuum at 1205 °C (2200 °F) Micrograph shows fully

recrystallized, equiaxed grains ASTM grain size 9 Etchant: ASTM 163 250×

Fig 23 Unalloyed tantalum, phosphorus-deoxidized copper, and steel plate (not shown) that were explosively

bonded, then rolled Note irregular bond pattern 10% ammonium persulfate 100×

Fig 24 Unalloyed tantalum/Nickel 201 explosively bonded bimetal; both materials 3.2 mm (0.125 in.) thick

Cold rolled to size and annealed Scanning electron micrograph shows fully cold-worked tantalum (top) and fully recrystallized nickel (bottom) Etchant: ASTM 163, then ASTM 24 300×

Fig 25 Unalloyed 0.4-mm (0.015-in.) thick tantalum clad by explosive bonding on both sides with 5052

aluminum alloy Roller leveled after bonding Scanning electron micrograph of a longitudinal section showing aluminum surrounding slightly worked tantalum Etchant: ASTM 163 125×

Fig 26 Commercially pure molybdenum, pressed from powder and sintered The structure consists of an

aggregate of molybdenum grains; the black spots are voids Compare with Fig 27 Murakami's reagent 200×

Fig 27 Same as Fig 26, except extruded after sintering Extruding has elongated the grains of molybdenum

and closed most of the voids Remaining voids (black dots) are smaller Murakami's reagent 200×

Fig 28 Commercially pure molybdenum, rolled to 1.0-mm (0.040-in.) thick sheet, annealed at 900 °C (1650

°F) for 1 h Longitudinal section Partly recrystallized See also Fig 29 Murakami's reagent (mod) 200×

Fig 29 Same sheet as Fig 28, annealed 15 min at 1350 °C (2460 °F) Completely recrystallized No voids are

visible Murakami's reagent (mod) 200×

Fig 30 Mo-0.5Ti alloy, cold rolled and annealed by heating to 1315 °C (2400 °F) The structure consists of

elongated grains Murakami's reagent 200×

Fig 31 Mo-0.5Ti alloy, cold rolled; recrystallized by annealing Surface layer (top) is unrecrystallized because

of nitrogen contamination Etchant: ASTM 129 500×

Fig 32 TZM alloy, "warm" rolled at 1260 °C (2300 °F) Longitudinal section Structure is mostly

unrecrystallized solid solution Murakami's reagent 200×

Fig 33 TZM alloy, hot rolled at 1595 °C (2900 °F), which resulted in a completely recrystallized structure of

elongated grains Murakami's reagent 200×

Fig 34 70Mo-30W alloy, cold worked and annealed by heating to 1425 °C (2600 °F) Recrystallized, equiaxed

grains of solid solution Murakami's reagent 200×