Volume 20 - Materials Selection and Design Part 7 potx

Table 8 Typical tensile properties of selected aluminum cast products used in the automotive industry Tensile strength Yield strength Alloy Temper Casting MPa ksi MPa ksi... Staley, Alc

T7 Solution treated, quenched, and

overaged/stabilized

Wrought products that are overaged to carry them beyond a point of maximum strength to provide control of some other characteristic, usually corrosion resistance Applies to cast products that are artificially aged after solution heat treatment to provide dimensional and strength stability

T8 Solution treated, quenched, cold

worked, and artificially aged

Products that are cold worked after solution treatment and the effect of cold work

is recognized in mechanical property limits

T9 Solution treated, quenched,

artificially aged, and then cold

worked

Products that are cold worked to improve strength

Table 7 Typical tensile properties of selected 2xxx, 6xxx, and 7xxx aluminum alloy products

Tensile strength Yield strength Alloy Temper Product(a)

MPa ksi MPa ksi

(a) Properties of sheet and plate are for the long-transverse direction, and those of extrusions and forgings are for the longitudinal

direction

Table 8 Typical tensile properties of selected aluminum cast products used in the automotive industry

Tensile strength Yield strength Alloy Temper Casting

MPa ksi MPa ksi

O Temper. Ductility of all aluminum products is highest in this temper Factors determining properties of annealed

products depend on the alloy system The annealed strength of unalloyed aluminum, 1xxx series, generally increases and

ductility decreases with increasing impurity level, and the amounts of magnesium and manganese largely determine the

bulk properties of 3xxx and 5xxx products Strength increases as magnesium (solid-solution strengthening) and manganese

contents (dispersion and solid-solution strengthening) increase Ultimate tensile strength increases more significantly with increasing magnesium content than does tensile yield strength because of the potent effect of magnesium on work hardening Parts of heat-treatable alloy products that are difficult to form are often formed in the O temper, then heat treated to a final temper

H Tempers. Cold working of annealed material to H1 tempers increases the dislocation density This increases strength, particularly yield strength, and decreases ductility In unalloyed aluminum and in alloys containing little magnesium, cold working produces cells that have walls containing a high density of dislocations enclosing a volume of relatively strain-free material In alloys containing sufficient amounts of magnesium, however, the dislocations form a tangled forest In highly worked aluminum-magnesium alloys, rearrangement of the dislocation structure occurs over long times at room

temperature Stabilizing treatments, H3 tempers, prevent loss of strength in certain 3xxx and 5xxx alloys during

subsequent long-time exposure During partial annealing treatments, H2 tempers, cell walls either form or become more perfect, and dislocations within the cell migrate to cell boundaries If the temperature exceeds a critical level, which depends on alloy content and strain, the cold-worked product will either partially or completely recrystallize Materials in H2 tempers provide a combination of strength and ductility generally superior to that of material in H1 tempers Cold-worked alloys containing above approximately 3.5% Mg and annealed alloys containing above approximately 4.5% Mg can also suffer a degradation in corrosion resistance caused by precipitation of a continuous film of Al3Mg2 on grain boundaries at temperatures between ambient and approximately 205 °C (400 °F) Special H116 and H321 controlled hot-rolling tempers have been developed that either avoid precipitation of Al3Mg2 on grain boundaries or agglomerate the precipitate to increase corrosion resistance For a particular strength level, a higher resistance to stress corrosion is obtained by increasing magnesium and manganese rather than by increasing work hardening

W and T Tempers. The highest strengths are obtained by precipitation hardening The material is held for a sufficient time above the solvus to dissolve essentially all of the major alloying elements, quenched at a rate to retain most of these elements in solid solution, then aged either at room temperature (natural aging) or at a modestly elevated temperature

(artificial aging) The highest-strength alloys contain the largest concentration of the major alloying elements For a

particular alloy system, strength typically increases with increasing alloy content Most 2xxx and 6xxx wrought alloys and 2xx.x and 3xx.x cast alloys are strengthened during natural aging by Guinier-Preston (G-P) zones, which are precursors to

Al2Cu, Al2CuMg, Mg2Si, or Al4CuMg5Si4 phases Strength of these materials increases for about 4 days, then stabilizes

(T4 temper) In contrast, in 7xxx alloys containing G-P zone precursors to phases such as MgZn2, strength continues to

increase indefinitely at room temperature (W temper) The ductility in the freshly quenched (W < h temper) condition is high enough for many forming operations Consequently, many parts are formed shortly after quenching from the solution-heat-treatment temperature To prevent the formation of large grains during the solution treatment of formed parts, a critical amount of strain must be avoided Although this critical strain is alloy dependent, strains near 10% are particularly troublesome for most alloys In addition, ductility in the T4 or T3 tempers is sufficiently high that some parts can be formed successfully in this condition Strength, particularly yield strength, increases substantially with artificial aging (T6 temper) This increase is accompanied by a loss in ductility Strength of materials hardened by Al2CuMg, Al2Cu, or Al2CuLi precipitates may be increased by cold work prior to artificial aging, T8, treatments The increase in strength of these materials is attributed to a refinement of Al2CuLi and of the metastable precursors to Al2CuMg and Al2Cu Additions of silicon and other alloying elements can also serve to refine the size of precipitates in certain 2xxx alloys Cold-finishing rod and bar products after artificial aging increases their strength (T9 temper)

The solution treatment, in most cases, is a separate operation In particular circumstances, however, the heat from a shaping process may be sufficient to provide solution treatment These products can be cooled after the shaping process

and subsequently aged to develop useful properties (T5 temper) Some 6xxx alloys attain the same specified properties

whether furnace solution heat treated or cooled from an elevated-temperature shaping process at a rate rapid enough to maintain sufficient silicon and magnesium in solution In such cases, the T6 temper designation may be used

Aluminum Alloy Microstructural Features Not Inferred from the Alloy-Temper Designation Systems

The alloy designation system defines the alloy content, and the temper-designation system identifies many of the thermal and mechanical processes that control the microstructure and, hence, the bulk properties of aluminum alloy products Nevertheless, many metallurgical features are not specified by these systems The features include nonmetallic inclusions, porosity, second-phase particles, grain and dislocation structure, and crystallographic texture

Inclusions are typically oxides of aluminum and magnesium including spinel, MgAl2O4 Oxides form on the surface of molten aluminum and become entrapped when turbulent flow forces them below the surface Filtration of the molten metal is used to control inclusions Inclusions can give rise to problems ranging from pinholes in foil to reduced fatigue life in structural wrought products and castings

Porosity reduces ductility and increases susceptibility to the initiation of fatigue cracks Porosity may arise from either shrinkage during solidification or from hydrogen Hydrogen control during solidification is extremely important because

of the ten-fold decrease in the solubility of hydrogen in aluminum as it solidifies Hydrogen-induced porosity can also occur in solid aluminum products when they are heated to high temperatures in humid environments Provided that the hydrogen content is low enough, most of the porosity can be closed by thermomechanical treatments Isostatic pressure can be used to close the pores in castings, and conventional forging and extrusion are effective in healing ingot porosity Porosity in thick-rolled products is particularly difficult to close; tensile stresses in the short-transverse direction may arise during the initial rolling of thick plate because the amount of deformation per pass is limited This stress causes pores to enlarge With additional rolling to thinner plate, the pores heal

Second-phase particles are divided into four classes based on their mode of formation and their ability to be dissolved: primary particles, constituents, dispersoids, and precipitates

Primary Particles. These particles form when some phase other than aluminum solid solution separates first from the melt Primary silicon particles form in castings when hypereutectic aluminum-silicon alloys solidify by eutectic decomposition Ductility decreases with increasing size of the silicon particles, so size control is important The coarse, faceted primary silicon particles are refined to a fine spherulitic structure using additives containing phosphorus In

certain casting alloys and 8xxx wrought alloys, primary iron-bearing constituents can form if the alloying content is such

that the alloy is hypereutectic In wrought alloys, macroscopically large, undesirable primary particles of Al7Cr, Al3Ti, or Al3Zr can form by a peritectic reaction if chemical composition is not closely controlled

Constituents. These particles may be either intermetallic compounds or essentially pure silicon that forms during solidification of hypoeutectic aluminum-silicon alloys They range in size from a few micrometers to tens of micrometers Constituents can be classified either as virtually insoluble or soluble Because the low maximum solid solubility of iron in aluminum is further reduced by other alloying elements to 0.01 wt% or less, constituents containing iron are insoluble Iron-free constituents containing silicon can be either soluble or insoluble depending on the chemical composition of the alloy Major alloying elements can combine either with each other or with aluminum to form soluble constituent particles Most of these soluble constituents dissolve either during ingot preheating prior to deformation processing or during the solution heat treatment of cast shapes or wrought products Constituent size decreases with increasing solidification rate

In hypoeutectic 3xx.0 and 4xx.0 castings, modification by elements such as strontium significantly refine the flake

structure of the silicon particles to a finer fibrous morphology

Constituent particles are generally not beneficial and are detrimental to the fatigue resistance and fracture toughness of high-strength alloy products These particles fracture at relatively low plastic strains and provide low-energy sites for the initiation and growth of cracks Several high-purity (low iron and silicon) versions of 2024 and 7075 have been commercialized, and the maximum allowable impurity levels of all modern high-strength alloys are significantly lower than those of older alloys Despite the harmful effects of constituents in high-strength alloys, the ability of alloy 3004-H19 to make commercially successful beverage containers relies on careful control of size, volume fraction, and distribution of Al12(Fe,Mn)Si constituent particles These constituent particles serve to "scour" the die during the drawing operation so that galling is minimized Attempts to produce can stock from roll-cast sheet have generally not been successful because the particle size distribution in roll-cast sheet is not as effective in minimizing galling

Dispersoids. These form by solid-state precipitation, either during ingot preheating or during the thermal heat treatment

of cast shapes, of slow-diffusing supersaturated elements that are soluble in molten aluminum but which have limited solubility in solid aluminum Manganese, chromium, or zirconium are typical dispersoid-forming elements Unlike the precipitates that confer precipitation hardening, dispersoids are virtually impossible to dissolve completely, once precipitated In addition to providing dispersion strengthening, the size distribution of dispersoids in wrought alloys are a key factor in controlling degree of recrystallization, recrystallized grain size, and crystallographic texture Dispersoids in non-heat-treatable alloys also stabilize the deformation substructure during elevated-temperature exposures, for example, during paint baking

In contrast to the commercially significant dispersion strengthening provided by dispersoids in 3xxx and 5xxx alloys, the level of dispersion strengthening afforded by dispersoids in wrought heat-treatable alloys is trivial In 2x24 alloys,

Al20Cu2Mn3 dispersoids nucleate dislocations at the particle-matrix interface during the quench These dislocations serve

as nucleation sites for subsequent precipitation The newer 7xxx alloys contain zirconium, which forms coherent Al3Zr dispersoids while most of the older 7xxx alloys contain Al12Mg2Cr dispersoids which exhibit incoherent interfaces The

incoherent interfaces serve to nucleate MgZn2 precipitates during the quench, so alloys containing these precipitates lose a great deal of their potential to develop high strength after slow quenching (quench sensitivity) Nucleation is difficult on coherent interfaces, so the newer alloys are less quench sensitive A number of casting alloys, and some wrought alloys, contain elements that can form either constituents or dispersoids depending on the solidification rate

Precipitates can form during any thermal operation below the solvus In properly solution-heat-treated products, all precipitates dissolve during the solution-heat-treatment operation Depending on quench rate and alloy, precipitates can form during the quench from the solution-heat-treatment temperature at grain and subgrain boundaries and at particle-matrix interfaces These coarse precipitates do not contribute to age hardening and can serve to reduce properties such as ductility, fracture toughness, and resistance to intergranular corrosion After the quench, G-P zones form at ambient temperature (natural aging) These are agglomerates of atoms of the major solute elements with a diffuse, coherent boundary between the G-P zone and the matrix During elevated-temperature precipitation heat treatments (artificial aging) G-P zones may either nucleate metastable precipitates or they may dissolve, and metastable precipitates nucleate separately Cold working subsequent to quenching introduces dislocations that may serve to nucleate metastable or equilibrium precipitates With prolonged artificial aging, equilibrium precipitates may form Coarse equilibrium precipitates form during annealing treatments of heat-treatable alloy products, O temper They also form during most thermomechanical treatments prior to solution heat treatment

Grain Structure. The grain size of aluminum alloy ingots and castings is typically controlled by the introduction of inoculants that form intermetallic compounds containing titanium and/or boron During deformation processing, the grain structure becomes modified Most aluminum alloy products undergo dynamic recovery during hot working as the dislocations form networks of subgrains New dislocation-free grains may form between and following rolling passes (static recrystallization) or during deformation processing (dynamic recrystallization) During deformation, the crystal lattice of the aluminum matrix rotates at its interfaces between constituent and coarse precipitate particles These high-

energy sites serve to nucleate recrystallization This process is termed particle-stimulated nucleation and is an important mechanism in the recrystallization process of aluminum The particle size that will serve as a nucleus decreases as deformation temperature decreases and strain and strain rate increase Dispersoid particles retard the movement of high-angle grain boundaries Consequently, hot-worked structures are resistant to recrystallization and often retain the dynamically recovered subgrain structure in the interiors of elongated cast grain boundaries In heat-treated products containing a sufficient quantity of dispersoids the unrecrystallized structure of hot-worked-plate forgings and extrusions can be retained after solution heat treatment

Degree of recrystallization of hot-worked products has an effect on fracture toughness Unrecrystallized products develop higher toughness than do products that are either partially or completely recrystallized This behavior is attributed to precipitation on the recrystallized high-angle grain boundaries during the quench These particles increase the tendency for low-energy intergranular fracture Products such as sheet, rods, and tubing that are cold rolled invariably recrystallize during solution heat treatment or annealing to O temper

Decreasing the grain size can increase strength of 5xxx alloy products in the O temper by 7 to 28 MPa (1 to 4 ksi), but

grain size is not a major factor in increasing strength of other aluminum alloy products Several measures of formability are influenced by grain size, however, so grain size is controlled for this reason One particular use of grain size control is

to produce stable, fine grains, which are essential in developing superplastic behavior in aluminum alloy sheet

Crystallographic Texture. Cast aluminum ingots and shapes generally have a random crystallographic texture; the orientation of the unit cells comprising each grain are not aligned With deformation, however, certain preferred crystallographic orientations develop Many of the grains rotate and assume certain orientations with respect to the direction of deformation For flat-rolled products and extrusions having a high aspect ratio of width to thickness, the deformation texture is similar to that in pure fcc metals These orientations are described by using the Miller indices of the planes {nnn} in the grains parallel to the plane of the worked product and directions [nnn] parallel to the working direction The predominant textures are {110}[112], {123}[634], and {112}[111] During recrystallization, a high concentration of grains in the {001}[100] or {011}[100] orientations may develop Alternatively, if particle-stimulated nucleation is present to a large extent, the recrystallized texture will be random Control of crystallographic texture is particularly important for non-heat-treatable sheet that will be drawn If texture is not random, ears form during the drawing process In extruded or drawn rod or bar, the texture is a dual-fiber texture in which almost all grains are aligned

so that the grain directions are either [001] or [111] In heat-treatable alloys, texture has the most potent effect on the properties of extrusions that have the dual-fiber texture Strengthening by this process is so potent that the longitudinal yield strengths of extruded products exhibiting this texture are about 70 MPa (10 ksi) higher than strength in the transverse direction If this dual-fiber texture is lost by recrystallization, strength in the longitudinal direction decreases to that in the transverse directions

References cited in this section

2 H Baker, Ed., Alloy Phase Diagrams, Vol 3, ASM Handbook, ASM International, 1992

3 J.R Davis, Ed., ASM Specialty Handbook: Aluminum and Aluminum Alloys, ASM International, 1993

4 D Altenpohl, Aluminum Viewed from Within, an Introduction to the Metallurgy of Aluminum Fabrication,

Aluminium-Verlag, Dusseldorf, 1982

5 Aluminum Standards and Data, The Aluminum Association, 1993

6 C Brooks, Heat Treatment, Structure and Properties of Nonferrous Alloys, American Society for Metals,

1982

7 J Hatch, Ed., Aluminum: Properties and Physical Metallurgy, American Society for Metals, 1984

8 W.E Haupin and J.T Staley, Aluminum and Aluminum Alloys, Encyclopedia of Chemical Technology,

1992

9 Heat Treating of Aluminum Alloys, Heat Treating, Vol 4, ASM Handbook, ASM International, 1991, p

841-879

10 W Petzow and G Effenberg, Ed., Ternary Alloys: A Comprehensive Compendium of Evaluated

Constitutional Data and Phase Diagrams, VCH Verlagsgesellschaft, Weinheim, Germany, 1990

11 H.W.L Phillips, Equilibrium Diagrams of Aluminium Alloy Systems, Aluminum Development Association,

1961

12 I.J Polmear, Light Alloys, Metallurgy of the Light Metals, 3rd ed., Arnold, 1995

13 R.E Sanders, Jr., S.F Baumann, and H Stumpf, Non-Heat-Treatable Aluminum Alloys, Aluminum Alloys,

Their Physical and Mechanical Properties, Engineering Materials Advisory Services Ltd, 1986, p

1441-1484

14 T.H Sanders, Jr., and J.T Staley, Review of Fatigue and Fracture Research on High-Strength Aluminum

Alloys, Fatigue and Microstructure, American Society for Metals, 1979, p 467-522

15 J.T Staley, Metallurgical Factors Affecting Strength of High Strength Alloy Products, Proceedings of

Fourth International Conference on Aluminum Alloys, Norwegian Institute of Technology, Department of

Metallurgy and SINTEF Metallurgy, 1994

16 E.A Starke, Jr., and J.T Staley, Application of Modern Aluminum Alloys to Aircraft, Progr Aerosp Sci.,

Vol 32 (No 2-3), 1996, p 131-172

17 K.R Van Horn, Ed., Aluminum, Vol I, Properties, Physical Metallurgy and Phase Diagrams, American

Society for Metals, 1967

Effects of Composition, Processing, and Structure on Properties of Nonferrous Alloys

Ronald N Caron, Olin Corporation; James T Staley, Alcoa Technical Center

Copper and Copper Alloys

After iron and aluminum, copper is the third most-prominent commercial metal because of its availability and attractive properties: excellent malleability (or formability), good strength, excellent electrical and thermal conductivity, and superior corrosion resistance (Ref 18, 19, 20, 21, 22, 23, 24, 25) Copper offers the designer moderate levels of density (8.94 g/cm3, or 0.323 lb/in.3), elastic modulus (115 GPa, or 17 × 106 psi), and melting temperature (1083 °C, or 1981 °F)

It forms many useful alloys to provide a wide variety of engineering property combinations and is not unduly sensitive to most impurity elements The electrical conductivity of commercially available pure copper, about 101% IACS (International Annealed Copper Standard), is second only to that of commercially pure silver (about 103% IACS) Standard commercial copper is available with higher purity and, therefore, higher conductivity than what was available when its electrical resistivity value at 20 °C (70 °F) was picked to define the 100% level on the IACS scale in 1913 The thermal conductivity for copper is also high, 391 W/m · K (226 Btu/ft · h · °F), being directly related to the electrical conductivity through the Wiedemann-Franz relationship

Copper and the majority of its alloys are highly workable hot or cold, making them readily commercially available in various wrought forms: forgings, bar, wire, tube, sheet, and foil In 1995, copper used in wire and cable represented about 50% of U.S production and in flat products of various thickness another 15%, rod and bar about 14%, tube about 14.5%, with foundries using about 5% for cast products, and metal powder manufacturers about 0.6% Besides the more familiar copper wire, copper and its alloys are used in electrical and electronic connectors and components, heat-exchanger tubing, plumbing fixtures, hardware, bearings, and coinage

As with other metal systems, copper is intentionally alloyed to improve its strength without unduly degrading ductility or workability However, it should be recognized that additions of alloying elements also degrade electrical and thermal conductivity by various amounts depending on the alloying element, its concentration and location in the microstructure (solid solution or dispersoid) The choice of alloy and condition is most often based on the trade-off between strength and conductivity Alloying also changes the color from reddish brown to yellow (with zinc, as in brasses) and to metallic white or "silver" (with nickel, as in U.S cupronickel coinage)

Copper and its alloys are readily cast into cake, billet, rod, or plate suitable for subsequent hot or cold processing into plate, sheet, rod, wire, or tube via all the standard rolling, drawing, extrusion, forging, machining, and joining methods Copper and copper alloy tubing can be made by the standard methods of piercing and tube drawing as well as by the continuous induction welding of strip Copper is hot worked over the temperature range 750 to 875 °C (1400 to 1600 °F), annealed between cold working steps over the temperature range 375 to 650 °C (700 to 1200 °F), and is thermally stress relieved usually between 200 and 350 °C (390 and 660 °F) Copper and its alloys owe their excellent fabricability to the face-centered cubic crystal structure and its twelve available dislocation slip systems Many of the applications of copper and its alloys take advantage of the work-hardening capability of the material, with the cold processing deformation of the

final forming steps providing the required strength/ductility for direct use or for subsequent forming of stamped components Copper is easily processible to more than 95% reduction in area The amount of cold deformation between softening anneals is usually restricted to 90% maximum to avoid excessive crystallographic texturing, especially in rolling of sheet and strip

Although copper obeys the Hall-Petch relationship and grain size can be readily controlled by processing parameters, work hardening is the only strengthening mechanism used with pure copper Whether applied by processing to shape and thickness, as a rolled strip or drawn wire, or by forming into the finish component, as an electrical connector, the amount

of work hardening applied is limited by the amount of ductility required by the application Worked copper can be recrystallized by annealing at temperatures as low as 250 °C (480 °F), depending on prior degree of cold work and time at temperature While this facilitates processing, it also means that softening resistance during long-time exposures at moderately elevated temperatures can be a concern, especially in electrical and electronic applications where I2R heating

is a factor For applications above room temperature, but at temperatures lower than those inducing recrystallization in commercial heat treatments, thermal softening can occur over extended periods and characteristics such as the half-softening temperature should be considered; that is, the temperature for which the worked metal softens to half its original hardness after a specific exposure time, usually 1 h

A more useful engineering property for many electrical contact applications is stress-relaxation resistance, the property that characterizes the decrease in contact load supported by a mechanical contact over time at a given temperature, typically measured at room temperature between exposures to elevated temperature (Ref 20) Figure 3 illustrates the characteristics of the tensile-stress-relaxation property of drawn (worked) copper wire; the degree of relaxation increases with temperature and time It also increases with the initial temper or degree of cold work in the material The mechanism

is the thermally activated and applied-stress directed motion of crystal lattice defects, such as point defects and dislocations Consequently, the application of a thermal heat treatment (stabilization anneal) to induce recovery mechanisms to tie up mobile components of dislocations will improve the stress-relaxation resistance Alloying elements also restrict dislocation motion and provide a more potent remedy for improving stress-relaxation resistance of cold-worked metal in service For example, the improvement in stress-relaxation resistance obtainable by alloying copper with 5% Sn (alloy C51000) in combination with a low-temperature stabilization heat treatment and as a function of sheet orientation is illustrated by comparing the alloy data at 93 °C (200 °F) in Fig 4 with those for copper in Fig 3

Fig 3 Tensile-stress-relaxation characteristics of copper alloy C11000 Data are for tinned 30 AWG (0.25 mm

diam) annealed ETP copper wire; initial elastic stress, 89 MPa (13 ksi)

Fig 4 Anisotropic stress-relaxation behavior in bending for highly cold-worked C51000 copper alloy strip Data

are for 5% Sn phosphor bronze cold rolled 93% (reduction in area) to 0.25 mm (0.01 in.) and heat treated 2 h

at 260 °C (500 °F) Graphs at left are for stress relaxation transverse to the rolling direction; graphs at right, for stress relaxation parallel to the rolling direction Initial stresses: as rolled, parallel orientation, 607 MPa (88 ksi); as rolled, transverse orientation, 634 MPa (92 ksi); heat treated, parallel orientation, 641 MPa (93 ksi); heat treated, transverse orientation, 738 MPa (107 ksi)

Wrought Copper Alloys

The purpose of adding alloying elements to copper is to optimize the strength, ductility (formability), and thermal stability, without inducing unacceptable loss in fabricability, electrical/thermal conductivity, or corrosion resistance A list

of selected wrought copper alloy compositions and their properties is given in Table 10 In this table, the alloys are arranged in their common alloy group: the coppers (99.3% min Cu), the high-coppers (94% min Cu), brasses (copper-zinc), bronzes (copper-tin, or copper-aluminum, or copper-silicon), copper-nickels, and the nickel silvers (Cu-Ni-Zn) Composition and property data are given by the Copper Development Association (CDA) and are incorporated in the ASTM numbering system, wherein alloys numbered by the designations (now UNS) C10100 to C79900 cover wrought alloys and C80100 to C99900 apply to cast alloys Copper alloys show excellent hot and cold ductility, although usually not to the same degree as the unalloyed parent metal Even alloys with large amounts of solution-hardening elements zinc, aluminum, tin, silicon that show rapid work hardening are readily commercially processed beyond 50% cold work before a softening anneal is required to permit additional processing The amount of cold working and the annealing parameters must be balanced to control grain size and crystallographic texturing These two parameters are controlled to provide annealed strip products at finish gage that have the formability needed in the severe forming and deep drawing commonly done in commercial production of copper, brass, and other copper alloy hardware and cylindrical tubular products

Table 10 Compositions and properties of selected wrought copper alloys

The pure copper alloys, also called the coppers (C10100 to C15900), are melted and cast in inert atmosphere from

the highest-purity copper in order to maintain high electrical conductivity (oxygen-free, or OF, copper, C10200) Copper

is more commonly cast with a controlled oxygen content (0.04% O as in electrolytic tough pitch, or ETP, copper, C11000) to refine out impurity elements from solution by oxidation Included in this group are the alloys that are deoxidized with small addition of various elements such as phosphorus (C12200, Cu-0.03P) and the alloys that use minor amounts of alloy additions to greatly improve softening resistance, such as the silver-bearing copper alloys (C10500, Cu-0.034 min Ag) and the zirconium-bearing alloys (C15000 and C15100, Cu-0.1Zr)

High-copper alloys (C16000 to C19900) are designed to maintain high conductivity while using dispersions and precipitates to increase strength and softening resistance: iron dispersions in Cu-(1.0-2.5)Fe alloys (C19200, C19400), chromium precipitates in Cu-1Cr (C18200), and the coherent precipitates in the Cu-(0.3-2.0)Be-Co,Ni age-hardening alloys (C17200, C17410, and C17500)

Brass alloys are a rather large family of copper-zinc alloys A significant number of these are binary copper-zinc alloys (C20500 to C28000), utilizing the extensive region of solid solution up to 35% Zn, offering excellent formability with good work-hardening strength at reasonable cost The alloys below 15% Zn have good corrosion and stress-corrosion resistance Alloys above 15% Zn need a stress-relieving heat treatment to avoid stress corrosion and, under certain conditions, can be susceptible to dezincification Alloys at the higher zinc levels of 35 to 40% Zn contain the bcc beta phase, especially at elevated temperatures, making them hot extrudeable and forgeable (alloy C28000 with Cu-40Zn, for example) The beta alloys are also capable of being hot worked while containing additions of 1 to 4% Pb, or more recently bismuth, elements added to provide the dispersion of coarse particles that promote excellent machinability characteristics available with various commercial Cu-Zn-Pb alloys (C31200 to C38500) The tin-brasses (C40400 to C49000) contain various tin additions from 0.3 to 3.0% to enhance corrosion resistance and strength in brass alloys Besides improving corrosion-resistance properties in copper-zinc tube alloys, such as C44300 (Cu-30Zn-1Sn), the tin addition also provides for good combinations of strength, formability, and electrical conductivity required by various electrical connectors, such as C42500 (Cu-10Zn-2Sn) A set of miscellaneous copper-zinc alloys (C66400 to C69900) provide improved strength and corrosion resistance through solution hardening with aluminum, silicon, and manganese,

as well as dispersion hardening with iron additions

Bronze alloys consist of several families named for the principal solid-solution alloying element The familiar bronzes (C50100 to C54400) comprise a set of good work-hardening, solid-solution alloys containing from nominally 0.8% Sn (C50100) to 10% Sn (C52400), usually with a small addition of phosphorus for deoxidation These alloys provide an excellent combination of strength, formability, softening resistance, electrical conductivity, and corrosion resistance The aluminum-bronze alloys contain 2 to 15% Al (C60800 to C64200), an element adding good solid-solution strengthening and work hardening, as well as corrosion resistance The aluminum-bronzes usually contain 1 to 5% Fe, providing elemental dispersions to promote dispersion strengthening and grain size control The silicon-bronze alloys (C64700 to C66100) generally offer good strength through solution- and work-hardening characteristics, enhanced in some cases with a tin addition, as well as excellent resistance to stress corrosion and general corrosion

tin-Cupronickels are copper-nickel alloys (C70100 to C72900) that utilize the complete solid solubility that copper has for nickel to provide a range of single-phase alloys (C70600 with Cu-10Ni-1.5Fe, and C71500 with Cu-30Ni-0.8Fe, for example) that offer excellent corrosion resistance and strength The family of copper-nickel alloys also includes various dispersion- and precipitation-hardening alloys due to the formation of hardening phases with third elements, such as Ni2Si

in C70250 (Cu-3Ni-0.7Si-0.15Mg) and the spinodal hardening obtainable in the Cu-Ni-Sn alloys (C72700 with 8Sn, for example)

Cu-10Ni-Copper-nickel-zinc alloys, also called nickel-silvers, are a family of solid-solution-strengthening and work-hardening

alloys with various nickel-zinc levels in the Cu-(4-26)Ni-(3-30)Zn ternary alloy system valued for their strength, formability, and corrosion and tarnish resistance, and, for some applications, metallic white color

Strengthening Mechanisms for Wrought Copper Alloys

Solution Hardening. Copper can be hardened by the various common methods without unduly impairing ductility or electrical conductivity The metallurgy of copper alloys is suited for using, singly or in combination, the various common strengthening mechanisms: solid solution and work hardening, as well as dispersed particle and precipitation hardening The commonly used solid-solution hardening elements are zinc, nickel, manganese, aluminum, tin, and silicon, listed in approximate order of increasing effectiveness Commercial alloys represent the entire range of available solid-solution compositions of each element: up to 35% Zn, and up to (and even beyond) 50% Ni, 50% Mn, 9% Al, 11% Sn, and 4% Si The relative amount of solution strengthening obtained from each element or particular combination of elements is determined by the ability of the solute to interfere with dislocation motion and is reflected in the work-hardening rate starting with the annealed condition, as illustrated by the increase in tensile strength with cold work shown in Fig 5 and also Table 10

Fig 5 Tensile strength of single-phase copper alloys as affected by percentage reduction in thickness by rolling

(temper) Curves of lesser slope indicate a low rate of work hardening and a higher capacity for redrawing ETP, electrolytic tough pitch

Work hardening is the principal hardening mechanism applied to most copper alloys, the degree of which depends on the type and amount of alloying element and whether the alloying element remains in solid solution or forms a dispersoid

or precipitate phase Even those alloys that are commercially age hardenable are often provided in the mill hardened tempers; that is, they have been processed with cold work preceding and/or following an age-hardening heat treatment For the leaner alloys (below about 12% Zn, or about 3% Al, for example), processing generates dislocations that develop into entanglements and into cells, with some narrow shear band formation beyond about 65% cold reduction in thickness After about 90% cold work, the distinct "copper" or "metal" deformation crystallographic texture begins to develop With the richer solid-solution alloys that lower the stacking-fault energy, planar slip is the dominant dislocation mechanism, with associated higher work hardening Beyond about 40% cold work in these richer alloys, stacking faults, shear banding, and deformation twinning become important deformation mechanisms that, beyond 90% cold work, lead to the

"brass" or "alloy" type of crystallographic deformation texture and accompanying anisotropy of properties The variation

in tensile properties with cold working of an annealed Cu-30 Zn alloy (C26000) is shown in Fig 6 The degree of work hardening seen with cold working several selected single-phase copper alloys is illustrated by the cold-rolling curves in

Fig 5 Many copper alloys are used in wrought forms in a worked temper, chosen for the desired combination of hardened strength and formability, either for direct use in service or for subsequent component fabrication

work-Fig 6 The effect of cold rolling on the strength, hardness, and ductility of annealed copper alloy C26000 when

it is cold rolled in varying amounts up to 62% reduction in thickness

Dispersion strengthening is used in copper alloys for hardening, controlling grain size, and providing softening resistance, as exemplified by iron particles in copper-iron alloys, C19200 or C19400, and in aluminum bronzes, C61300

or C63380 Cobalt silicide particles in alloy C63800 (Cu-2.8Al-1.8Si-0.4Co), for example, provide fine-grain control and dispersion hardening to give this alloy high strength with reasonably good formability Alloy C63800 offers an annealed tensile strength of 570 MPa (82 ksi) and rolled temper tensile strengths of 660 to 900 MPa (96 to 130 ksi) Alloys offering exceptionally good thermal stability have been developed using powder metallurgy (P/M) techniques to incorporate dispersions of fine Al2O3 particles (3 to 12 nm in size) in a basically copper matrix, which is finish processed to rod, wire,

or strip products This family of alloys, C15715 to C15760, can resist softening up to and above 800 °C (1472 °F)

Precipitation Hardening. Age-hardening mechanisms are used in those few but important copper systems that offer a decreasing solubility for hardening phases The beryllium-copper system offers a series of wrought and cast age-hardening alloys, UNS C17000 to C17530 and C82000 to C82800 The wrought alloys contain 0.2 to 2.0% Be and 0.3 to 2.7% Co (or up to 2.2% Ni) They are solution heat treated in the range 760 to 955 °C (1400 to 1750 °F) and age hardened

to produce the beryllium-rich coherent precipitates when aged in the range 260 to 565 °C (500 to 1050 °F), the specific temperature being chosen for the particular alloy and desired property combination (Fig 7) The precipitation sequence during aging consists of the formation of solute-rich G-P zones, followed in sequence by coherent platelets of the metastable intermediate phases ' and '' Overaging is marked by the appearance of the B2 ordered equilibrium -BeCu phase as particles within grains and along grain boundaries, large enough to be seen in the light microscope The cobalt and nickel additions form dispersoids of equilibrium (Cu, Co, or Ni)Be that restrict grain growth during solution annealing in the two-phase field at elevated temperatures (Fig 7b) A cold-working step following solution annealing is often used to increase the age-hardening response Alloy C17200 (Cu-1.8Be-0.4Co), for example, can be processed to reach high strength: that is, tensile strengths after solutionization (470 MPa, or 68 ksi), after cold rolling to the hard temper (755 MPa, or 110 ksi), and after aging (1415 MPa, or 205 ksi) While they are commercially available in the heat-treatble (solutionized) condition, the beryllium-copper alloys are commonly provided in the mill-hardened temper with the optimal strength/ductility/conductivity combination suitable for the application

Fig 7 Phase diagrams for beryllium-copper alloys (a) Binary composition for high-strength alloys such as

C17200 (b) Pseudobinary composition for C17510, a high-conductivity alloy containing Cu-1.8Ni-0.4Be

Other age-hardening copper alloys include the chromium-coppers, which contain 0.4 to 1.2% Cr (C18100, C18200, and C18400); these alloys produce arrays of pure chromium precipitates and dispersoid particles when aged The Cu-Ni-Si alloys, C64700 and C70250, age harden by precipitating the Ni2Si intermetallic phase (Fig 8) Compositions in the Cu-Ni-Sn system, C71900 and C72700, are hardenable by spinodal decomposition, a mechanism that provides high strength and good ductility through the formation of a periodic array of coherent, fcc solid-solution phases that require the electron microscope to be seen Each of these alloys, including the beryllium-coppers can be thermomechanically processed to provide unique combinations of strength, formability, electrical conductivity, softening resistance, and stress-relaxation resistance

Fig 8 Photomicrograph showing the dispersion of Ni2 Si precipitates in the quenched and aged condition of copper alloy C64700, Cu-2Ni-0.7Si Magnification: 500×

Copper Casting Alloys

The copper casting alloys, numbered UNS C80100 to C99900, are available as sand, continuous, centrifugal, permanent mold, and some die castings (Ref 22, 23) They are generally similar to the wrought counterparts, but they do offer their own unique composition/property characteristics For example, they do offer the opportunity to add lead to levels of 25% that could not be easily made by wrought techniques in order to provide compositions in which dispersions of lead particles are useful for preventing galling in bearing applications The copper casting alloys are used for their corrosion resistance and their high thermal and electrical conductivity The most common alloys are the general-purpose Cu-5Sn-5Pb-5Zn alloy (C83600), used for valves and plumbing hardware, and C84400, widely used for cast plumbing system

components C83600 contains lead particles dispersed about the single-phase matrix and offers good machinability, with moderate levels of corrosion resistance, tensile strength (240 MPa, or 35 ksi), ductility, and conductivity (15% IACS)

While the Cu-Sn-Pb-(and/or Zn) casting alloys have only moderate strength, the cast manganese and aluminum bronzes offer higher tensile strengths, 450 to 900 MPa (65 to 130 ksi) As with the wrought alloys, the cast aluminum-bronze alloys commonly contain an iron addition (0.8 to 5.0%) to provide iron-rich particles for grain refinement and added strength In addition, at aluminum levels in the range 9.5 to 10.5% (or 8.0 to 9.5% Al with nickel or manganese additions) the alloys are heat treatable for added strength Depending on the section thickness and cooling rate of the casting, as well

as the alloy composition and heat treatments, the microstructures can be rather complex The aluminum-bronzes can be annealed completely or partially in the field and quenched to form martensite with needles Aging these alloys will temper the martensite by precipitation fine needles One of the aluminum-bronze alloys, Cu-10.5Al-5Fe-5Ni, for example, is used for its combination of high strength and good corrosion resistance Through heat treatment, the intermetallic -phase, with its complex composition (Fe,Ni,Cu)Al and CsCl crystal structure, provides a strengthening component in any of its morphologies: as globular particles, fine precipitates, or as a component of cellular eutectoid colonies

Copper Alloy Powders

Unique structural components are commercially made of copper and its alloys by P/M methods Copper and prealloyed powders are made by reduction of oxides, or by atomization, wherein the solidification of liquid droplets from a pour stream is broken up by an impinging jet of a liquid or gas Self-lubricating sintered bronze bearings are deliberately not pressed and sintered to 100% density in order to maintain an interconnected porosity to serve as an oil reservoir The P/M technique is uniquely suited to permit the addition of up to about 1.5% graphite in these bearings Likewise, various multiphase P/M copper alloys containing a mixture of hard and soft phases in a copper matrix (Cu-7Sn-3Fe-6Pb-6 graphite-3SiO2, for example) are made for friction materials The combination of thermal stability, wear resistance, and sliding friction properties make these materials suitable for use in clutch plates, and so forth Various structural parts are made of bronze, brass, and nickel-silver alloys In addition, P/M techniques are used to prepare the initial stages of the oxide-dispersion-strengthened (ODS) copper alloys, which are fabricated into finished forms by standard wrought methods to provide good softening resistance with excellent thermal or electrical conductivity These ODS materials are prepared by internal oxidation of powder copper-aluminum alloy to form a dispersion of fine Al2O3 particles, about 3 to

12 nm in size

References cited in this section

18 Properties of Pure Metals, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol

2, ASM Handbook, ASM International, 1990, p 1099-1201

19 D.E Tyler and W.T Black, Introduction to Copper and Copper Alloys, Properties and Selection:

Nonferrous Alloys and Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p

216-240

20 D.E Tyler, Wrought Copper and Copper Alloy Products, Properties and Selection: Nonferrous Alloys and

Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 241-264

21 P Robinson, Properties of Wrought Coppers and Copper Alloys, Properties and Selection: Nonferrous

Alloys and Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 265-345

22 R.F Schmidt and D.G Schmidt, Selection and Application of Copper Alloy Castings, Properties and

Selection: Nonferrous Alloys and Special-Purpose Materials, Vol 2, ASM Handbook, ASM International,

1990, p 346-355

23 A Cohen, Properties of Cast Copper Alloys, Properties and Selection: Nonferrous Alloys and

Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 356-391

24 E Klar and D.F Berry, Copper P/M Products, Properties and Selection: Nonferrous Alloys and

Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 392-402

25 J.C Harkness, W.D Speigelberg, and W.R Cribb, Beryllium-Copper and Other Beryllium-Containing

Alloys, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol 2, ASM

Handbook, ASM International, 1990, p 403-427

Effects of Composition, Processing, and Structure on Properties of Nonferrous Alloys

Ronald N Caron, Olin Corporation; James T Staley, Alcoa Technical Center

Nickel and Nickel Alloys

Nickel and nickel alloys are used in the chemical processing, pollution control, power generation, electronic, and aerospace industries, taking advantage of their excellent corrosion, oxidation, and heat resistance (Ref 18, 26, 27, 28, 29,

30, 31, 32) Nickel is ductile and can be made by the conventional processing methods into cast, P/M, and various wrought products: bar/wire, plate/sheet, and tube Commercially pure nickel has moderately high values of melting temperature (1453 °C, or 2647 °F), density (8.902 g/cm3, or 0.322 lb/in.3), and elastic modulus (204 GPa, or 30 × 106 psi)

It is ferromagnetic, with a Curie temperature of 358 °C (676 °F) and good electrical (25% IACS) and thermal conductivity (82.9 W/m · K, or 48 Btu/ft · h · °F) Elemental nickel is used principally as an alloying element to increase the corrosion resistance of commercial iron and copper alloys; only about 13% of annual consumption is used in nickel-base alloys Approximately 60% is used in stainless steel production, with another 10% in alloy steels and 2.5% in copper alloys Nickel is also used in special-purpose alloys: controlled expansion, electrical resistance, magnetic, and shape memory alloys

Effects of Alloying Elements in Nickel Alloys

Nickel has an fcc crystal structure, to which it owes its excellent ductility and toughness Because nickel has extensive solid solubility for many alloying elements, the microstructure of nickel alloys consists of the fcc solid-solution austenite ( ) in which dispersoid and precipitate particles can form Nickel forms a complete solid solution with copper and has nearly complete solubility with iron It can dissolve about 35% Cr, about 20% each of molybdenum and tungsten, and about 5 to 10% each of aluminum, titanium, manganese, and vanadium Thus, the tough, ductile fcc matrix can dissolve extensive amounts of elements in various combinations to provide solution hardening as well as improved corrosion and oxidation resistance The degree of solution hardening has been related to the atomic size difference between nickel and the alloying element, and therefore the ability of the solute to interfere with dislocation motion Tungsten, molybdenum, niobium, tantalum, and aluminum, when aluminum is left in solution, are strong solution hardeners, with tungsten,

niobium, tantalum, and molybdenum also being effective at temperatures above 0.6 Tm (Tm = melting temperature), where

diffusion-controlled creep strength is important Iron, cobalt, titanium, chromium, and vanadium are weaker hardening elements Aluminum and titanium are usually added together to form the age-hardening precipitate, Ni3(Al,Ti)

solution-Gamma Prime ( ') Precipitation. Gamma-prime ( '), Ni3(Al,Ti), and the closely related '', Ni3Nb, are the major precipitation-hardening phases in nickel alloys These precipitates are based on the intermetallic compound, Ni3Al, which

has an fcc L12 ordered crystal structure with a lattice parameter differing from the nickel austenite ( ) matrix by 1% This misfit allows the homogeneous nucleation and growth of rather stable arrays of coherent precipitates (Fig 9) Strengthening is provided in part by the hindrance to dislocations moving across the - ' interface and, more importantly, as they cut across the ordered precipitate, where they must split into partials to maintain the ordered crystal structure Moreover, Ni3Al is one of the unique phases that show a significant increase in flow stress with temperature In particular, the yield strength increases over the range 300 °C (572 °F) to above 900 °C (1650 °F), showing a broad peak at about 600 °C (1110 °F)

Fig 9 Replica electron micrograph of the nickel alloy Udimet 700 (Ni-15Cr-17Co-5Mo-3.5Ti-4Al-0.06C) in the

solution-annealed and aged condition, showing precipitation of carbide at grain boundaries and arrays of ' within grains of the solid-solution matrix 4500×

In addition, some alloying elements can partition to ', affecting the interface mismatch and precipitate-coarsening kinetics as well as contributing a solution-hardening component to strength, with titanium being the most effective at room and elevated temperatures However, titanium, niobium, and tantalum can influence mechanical properties still further by encouraging the formation of other similar types of precipitates With higher titanium content, ' will transform to the hexagonal close-packed (hcp) -phase, Ni3Ti, which has an acicular or cellular morphology With increased amounts of niobium, ' transforms to the commercially important metastable body-centered tetragonal (bct) phase ' A decrease in hardening will result if the equilibrium orthorhombic phase, Ni3Nb, is allowed to form The actual phases precipitated and their effectiveness in hardening the microstructure are dependent on the alloy composition, the applied heat treatments, the resulting precipitate volume fraction, and the service conditions

Carbides. Although not a carbide former, nickel dissolves many elements that readily form the carbides seen in nickel alloys (MC, M6C, M7C3, M23C6) The MC carbides (where M = W, Ta, Ti, Mo, Nb) are usually large, blocky, and undesirable The M6C carbides (M = Mo, W) can precipitate as small platelets in the grains or as blocky particles in boundaries useful for grain control, but deleterious for ductility and stress rupture properties The M7C3 (M = Cr) can be useful when precipitated as discrete particles, but more so are grain boundary particles of M23C6 (M = Cr, Mo, W), where they can enhance creep rupture properties (Fig 10) If carbides are allowed to agglomerate or form grain-boundary films during heat treatment or in service at elevated temperatures, they can seriously impair ductility and cause embrittlement

As in stainless steels, precipitation of chromium carbides at boundaries can lead to intergranular corrosion due to the chromium-depleted zone alongside the grain boundary becoming anodic to the rest of the grains This grain-boundary sensitization is controlled in several ways: (1) by avoiding the chromium-carbide aging temperature range (425 to 760 °C,

or 800 to 1400 °F) during processing, (2) with stabilization heat treatments to tie up carbon with more stable carbide formers (niobium, tantalum, titanium), and (3) by reducing the carbon level in the base alloy

Fig 10 Photomicrograph of the nickel alloy Udimet 700 (Ni-15Cr-17Co-5Mo-3.5Ti-4Al-0.06C) in the

solution-annealed and aged condition, showing precipitation of M 23 C 6 carbide at grain boundaries and arrays of ' within

grains of the solid-solution matrix 1000×

Nickel Alloys

Nickel is alloyed to extend the good corrosion resistance and good heat resistance of elemental nickel Even with extensive amounts of alloying elements, the tough, ductile fcc austenitic matrix is preserved It is convenient to describe nickel alloys by grouping them into their two broad application areas: corrosion resistance, especially in aqueous environments, and heat resistance Naturally, this artificial separation should not be considered a rigid barrier as the corrosion-resistant alloys have good strength above room temperature and the heat-resistant alloys have good corrosion resistance The unique, special-property alloys, many of which are also used for their good corrosion and heat resistance

as well as high strength, are described separately

Corrosion-Resistant Nickel Alloys. A list of selected corrosion-resistant nickel alloys with nominal values of mechanical properties is given in Table 11 The commercially pure nickel grades, Nickel 200 to 205, are highly resistant

to many corrosive media, especially in reducing environments, but also in oxidizing environments where they can maintain the passive nickel oxide surface film They are used in the chemical processing and electronics industries They are hot worked at 650 to 1230 °C (1200 to 2250 °F), annealed at 700 to 925 °C (1300 to 1700 °F), and are hardened by cold working For processed sheet, for example, the tensile properties in the annealed condition (460 MPa, or 67 ksi, tensile strength; 148 MPa, or 22 ksi, yield strength; and 47% elongation) can be increased by cold rolling up to 760 MPa (110 ksi) tensile strength, 635 MPa (92 ksi) yield strength, and 8% elongation Because of its nominal 0.08% C content (0.15% max), Nickel alloy 200 (UNS No 2200) should not be used above 315 °C (600 °F), because embrittlement results from the precipitation of graphite in the temperature range 425 to 650 °C (800 to 1200 °F) The more widely used low-carbon alloy Nickel 201 (UNS No 2201), with 0.02% max C, can be used at temperatures above 290 °C (550 °F) Higher-purity nickel is commercially available for various electrical applications

Table 11 Compositions and properties of selected corrosion-resistant nickel-base alloys

Ultimate tensile

strength

Yield strength

(0.2% offset) Alloy Nominal composition, wt %

MPa ksi MPa ksi

Nickel-molybdenum and nickel-silicon alloys

The nickel-copper alloys are strong and tough, offering corrosion resistance in various environments, including brine and sulfuric and other acids, and showing immunity to chloride-ion stress corrosion They are used in chemical processing and pollution control equipment Capable of precipitating ', Ni3(Al,Ti), with its 2.7Al-0.6Ti alloy addition, alloy K-500 adds

an age-hardening component to the good solution strengthening and work-hardening characteristics already available with the nominal 30% Cu in alloy 400 The composition of these alloys can be adjusted to decrease the Curie temperature to below room temperature

The Ni-Cr-Fe(-Mo) alloys might simply be thought of as nickel-base analogs of the iron-base austenitic stainless steel alloys, with an interchange of the iron and nickel contents In these commercially important alloys the chromium content

in general ranges from 14 to 30% and iron from 3 to 20% With a well-maintained Cr2O3 surface film, these alloys offer excellent corrosion resistance in many severe environments, showing immunity to chloride-ion stress-corrosion cracking They also offer good oxidation and sulfidation resistance with good strength at elevated temperatures These nickel-rich Ni-Cr-Fe alloys have maximum operating temperatures in the neighborhood of 1200 °C (2200 °F) Alloy 600 (UNS N06600, with Ni-15Cr-8Fe) is a single-phase alloy that can be used at temperatures from cryogenic to 1093 °C (2000 °F) The modest yield strength of strip in the annealed condition (207 to 310 MPa, or 30 to 45 ksi) can be readily work hardened by cold rolling to reach yield strengths of 827 to 1100 MPa (120 to 160 ksi) and can retain most of this strength

up to about 540 °C (1000 °F)

The Ni-Cr-(Fe)-Mo alloys consist of a large family of alloys that are used in the chemical processing, pollution control, and waste treatment industries to utilize their excellent heat and corrosion resistance Alloys in this commercially important family, such as C-276 and alloy 625, are made even more versatile by their excellent welding characteristics and the corrosion resistance of welded structures The molybdenum additions to these alloys improve resistance to pitting and crevice corrosion Aluminum improves the protective surface oxide film, and the carbide formers titanium and niobium are used to stabilize the alloys against chromium-carbide sensitization Even with the low-level additions of aluminum and titanium to alloy 800, for example, small amounts of ' can form in service during exposure to elevated temperatures The high molybdenum and silicon additions in Hastelloy B and D promote good corrosion resistance in the presence of hydrochloric and sulfuric acids

Heat-Resistant Nickel Alloys. Chemical compositions of selected heat-resistant superalloys are given in Table 12 A glance at this list reveals that these nickel-containing materials include nickel-, iron-nickel-, or cobalt-base alloys They can be made by wrought and P/M methods, and also with castings produced with carefully controlled conditions to provide the desired polycrystal, or elongated (directionally solidified), or single-crystal grain structure for improved elevated-temperature mechanical properties The majority of the nickel-base superalloys utilize the combined strengthening of a solution-hardened austenite matrix with ' precipitation The niobium-rich, age-hardening precipitate, '', offers the ease of heat treatment and weldability that has made alloy 718 the most important nickel-base superalloy for aerospace and nuclear structural applications Alloy 718 is a high-strength, corrosion-resistant alloy that is used at temperatures from -250 to 700 °C (-423 to 1300 °F) Some of the alloys, Hastelloy X for example, obtain additional strengthening from carbide precipitation instead of ' Others, MA 754 for example, utilize P/M techniques involving mechanical alloying (Ref 27) to achieve a dispersion of about 1 vol% of very fine (25 nm) inert oxide particles, such as Y2O3, to promote higher elevated-temperature tensile and stress-rupture strength

Table 12 Chemical compositions of selected superalloys

PWA 1484 (SC) bal 5.0 10.0 2.0 6.0 5.6 9.0 Ta, 3.0 Re, 0.1 Hf

CMSX-4 bal 6.0 9.0 0.6 6.0 1.0 5.6 7 Ta, 3 Re, 0.1 Hf

Powder metallurgy alloys

MA754 bal 20.0 0.5 0.3 0.05 0.6 Y O

MA 6000 bal 15.0 2.0 2.0 4.0 2.5 4.5 0.05 0.1 2Ta

MERL 76 bal 12.2 18.2 3.2 1.3 4.3 5.0 0.025 0.02 0.3 Hf, 0.06 Zr

Rene' 95 bal 12.8 8.1 3.6 3.6 3.6 2.6 3.6 0.08 0.01 0.053 Zr

(a) P, polycrystalline casting; DS, directionally solidified casting; SC, single-crystal casting; bal, balance

The iron-base Fe-Ni-Cr heat-resistant alloys are extensions of the iron-base stainless steels with higher nickel and additions of other alloying elements Retaining the fcc iron-nickel austenite matrix, these alloys (alloys A-286 and 901, for example) are workable into various wrought forms and are capable of precipitation hardening with ' Alloys 903 and

909 are controlled thermal expansion Fe-Ni-Co-base alloys that are capable of age hardening with Ni3(Nb,Ti) precipitation and are designed to have high strength and low coefficient of thermal expansion for applications in gas turbine rings and seals up to 650 °C (1200 °F) (Ref 26) These alloys are hot worked at about 870 to 1120 °C (1600 to

2050 °F) and solution heat treated at 815 to 980 °C (1500 to 1800 °F) The standard aging treatment consists of 720 °C (1325 °F) for 8 h, furnace cool at 55 °C (100 °F)/h to 620 °C (1150 °F) for 8 h, followed by air cooling Alloy 909 in the as-hardened condition, for example, retains much of its room-temperature yield strength (1070 MPa, or 155 ksi) at 540 °C (1000 °F), namely, 895 MPa (130 ksi) (Ref 30)

Specialty Nickel Alloys. Unique combinations of properties are available with other nickel-base alloys for special applications While some of these properties are also available to some extent with alloys described above, the alloys described below were developed to promote their rather unique properties

There are many electrical resistance alloys used for resistance heating elements They can contain 35 to 95% Ni, but invariably contain greater than 15% Cr to form an adherent surface oxide to protect against oxidation and carburization at temperatures up to 1000 to 1200 °C (1850 to 2200 °F) in air Examples are Ni-20Cr (UNS N06003), Ni-15Cr-25Fe (UNS N06004), and Ni-20Cr-3Al-3Fe These alloys are single-phase austenite and have the needed properties for heating elements: desirably high, reproducible electrical resistance; low thermal expansion to minimize thermal fatigue and shock; good creep strength; strong and ductile for fabrication (Ref 27, 28)

The ferromagnetic characteristics of nickel allow formulation of nickel-base alloys for corrosion-resistant soft magnets for a variety of applications, typified by Ni-5Mo-16Fe Low thermal expansion characteristics are shown by Fe-(36-52)Ni-(0-17)Co alloys, making these materials useful for glass-to-metal sealing and containment equipment for liquefied natural gas, for example The controlled thermal expansion alloys, typified by alloy 903 (Ni-42Fe-15Co + Nb,Al,Ti), are also '-precipitation hardenable, offering high strength and low, relatively constant thermal expansion coefficient for applications up to about 650 °C (1200 °F) With nearly 50-50 at.%, nickel forms a shape memory intermetallic alloy with titanium, which offers 8% of reversible strain via a thermoelastic martensitic transformation, along with good ductility and corrosion resistance

The L12 intermetallic compound, Ni3Al, has been the focus of development work to create a strong, corrosion-resistant

material for elevated-temperature applications Wrought and cast beryllium-nickel alloys are commercially available (UNS N03360 with Ni-2Be-0.5Ti, for example) and respond to processing and age-hardening heat treatments as readily

as the beryllium-copper alloys, but offer higher strength with better resistance to thermal softening and stress relaxation (Ref 25, 29, 30, 31, 32)

References cited in this section

18 Properties of Pure Metals, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol

2, ASM Handbook, ASM International, 1990, p 1099-1201

25 J.C Harkness, W.D Speigelberg, and W.R Cribb, Beryllium-Copper and Other Beryllium-Containing

Alloys, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol 2, ASM

Handbook, ASM International, 1990, p 403-427

26 W.L Mankins and S Lamb, Nickel and Nickel Alloys, Properties and Selection: Nonferrous Alloys and

Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 428-445

27 J.J deBarbadillo and J.J Fischer, Dispersion-Strengthened Nickel-Base and Iron-Base Alloys, Properties

and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol 2, ASM Handbook, ASM

International, 1990, p 943-949

28 R.A Watson et al., Electrical Resistance Alloys, Properties and Selection: Nonferrous Alloys and

Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 822-839

29 D.W Dietrich, Magnetically Soft Materials, Properties and Selection: Nonferrous Alloys and

Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 761-781

30 E.L Frantz, Low-Expansion Alloys, Properties and Selection: Nonferrous Alloys and Special-Purpose

Materials, Vol 2, ASM Handbook, ASM International, 1990, p 889-896

31 D.E Hodgson, M.H Wu, and R.J Biermann, Shape Memory Alloys, Properties and Selection: Nonferrous

Alloys and Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 897-902

32 C.T Liu, J.O Stiegler, and F.H Froes, Ordered Intermetallics, Properties and Selection: Nonferrous Alloys

and Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 913-942

Effects of Composition, Processing, and Structure on Properties of Nonferrous Alloys

Ronald N Caron, Olin Corporation; James T Staley, Alcoa Technical Center

Cobalt and Cobalt Alloys

Falling between iron and nickel in the periodic table, cobalt has many similar properties as these other two more familiar transition metals (Ref 18, 33) Its melting temperature (1493 °C, or 2719 °F), density (8.85 g/cm3, or 0.322 lb/in.3), thermal expansion coefficient (13.8/K, or 7.66/°F), thermal conductivity (69.0 W/m · K), and elastic modulus (210 GPa,

or 30 × 106 psi) are all rather similar to the respective values of iron and nickel All three are ferromagnetic, but the Curie temperature of cobalt, 1123 °C (2050 °F), is significantly higher than that of iron (770 °C, or 1418 °F) or nickel (358 °C,

or 676 °F) The crystal structure and chemical and mechanical properties differ enough to give cobalt a viable commercial life of its own

Cobalt is not as readily geologically available as its two companion metals are, making cobalt about 100 times more costly than iron and about 8 times more costly than nickel Cobalt is heavily used as an alloying element in nickel-base heat-resistant alloys and as the ductile binder phase (3 to 25%) for tungsten carbide particles in cemented carbides Nonetheless, the excellent wear resistance, elevated-temperature hardness, and corrosion resistance of cobalt alloys have been commercially utilized in gas turbine engines, earthmoving equipment, and as bearing materials Cobalt alloys are most often used for their wear resistance either in solid forms or as welded or thermally sprayed overlays to hardface other structural materials They are also available as P/M products and in wrought sheet, bar, and tube forms

Elemental cobalt has an hcp crystal structure ( -Co) at room temperature, transforming to fcc ( -Co) at 417 °C (783 °F) Although the principal alloying elements affect the temperature of this transition (chromium, tungsten, and molybdenum stabilize the hcp phase, and iron and nickel stabilize the fcc structure), the fcc-to-hcp transformation is notably sluggish especially in alloyed cobalt The alloys usually go into service at room temperature in the metastable fcc form The -to- transformation usually occurs by the strain-induced martensitic (or shear) reaction, which also contributes to the high work-hardening rates generally seen with cobalt alloys Carbon, one of the principal alloying elements, has a profound influence on hardness, elevated-temperature strength, and creep resistance, as well as resistance to abrasive wear through formation of carbide phases

Effects of Alloying Elements in Cobalt Alloys

The traditional cobalt alloys are known as Stellite alloys and were originally formulated around 1900 for their wear and hardness properties They are composed of significant amounts of chromium, tungsten, molybdenum, iron, nickel, and carbon The ferromagnetism of cobalt is suppressed by the heavy additions of chromium and retained as expected by

additions of iron and nickel Chromium is restricted to less than 25% to avoid precipitation of the chromium-rich phase The microstructures consist of hard complex alloy carbides (M7C3, M6C, and M23C6) in a tough solid-solution alloy matrix The morphology and volume fraction of the carbide phase are a function of alloy composition and fabrication method Cobalt-base alloys can be grouped into their three major property application areas: wear resistance, elevated-temperature strength, and corrosion resistance A representative sampling of the many commercially available cobalt-base alloys is listed in Table 13 In most applications, the cobalt alloy must also exhibit good properties in all three general property areas

-Table 13 Nominal compositions of selected cobalt-base alloys

Nominal composition, % Alloy tradename

Cobalt-base wear-resistant alloys

Stellite 1 bal 31 12.5 1 (max) 2.4 3 (max) 3

Stellite 21 bal 28 5.5 0.25 2 (max) 2.5 2 (max) 1 (max)

Haynes alloy 6B bal 30 4 1 1.1 3 (max) 2.5 0.7 1.5

Tribaloy T-800 bal 17.5 29 0.08

(max)

3.5

Stellite F bal 25 12.3 1 (max) 1.75 3 (max) 22 2 (max) 1 (max)

Stellite 4 bal 30 14.0 1 (max) 0.57 3 (max) 3

Haynes alloy 25 (L605) bal 20 15 0.10 3 (max) 10 1 (max) 1.5

Haynes alloy 188 bal 22 14 0.10 3 (max) 22 0.35 1.25 0.05 La

MAR-M alloy 509 bal 22.5 7 0.60 1.5

(max)

(max)

0.1 (max)

Wear-Resistant Cobalt Alloys

Cobalt alloys are used mostly for their excellent wear resistance (Ref 33, 34) The Co-Cr-W-C Stellite wear-resistant alloys contain generous amounts of carbide-forming elements chromium, tungsten, and molybdenum, with carbon varying from 0.1 to 3.3% to encourage the formation of hard carbide particles The microstructures generally consist of chromium-rich M7C3 carbides and, in the higher tungsten alloys, the tungsten-rich M6C type of carbide, with enough chromium and tungsten alloying element left in solid solution to provide a tough, solution-hardened matrix Stellite 6, for example, was reported (Ref 34) to contain about 12% Cr7C3 in a solid-solution matrix of about 58Co-18Cr-4W The somewhat richer alloy Stellite 1 contains about 27% M7C3 and 1.5% W6C, in a matrix of 45Co-11Cr-10W-0.3C In general, alloys with more than about 1.3% C can be made only by casting because of limited ductility These higher-carbon alloys are applied as hardfacings onto other structural metals using weld overlays or spray coatings The solidification microstructures (Fig 11), resulting from multipass weld overlays using the gas tungsten arc welding (GTAW) process, for example, contain arrays of eutectic carbides (dark etching phases) between alloy-rich dendrites (white etching phases in Fig 11a to d)

Fig 11 Microstructures of various cobalt-base wear-resistant alloys (a) Stellite 1, two-layer GTAW deposit (b)

Stellite 6, two-layer GTAW deposit (c) Stellite 12, two-layer GTAW deposit (d) Stellite 21, two-layer GTAW deposit (e) Haynes alloy 6B, 13 mm (0.5 in.) plate (f) Tribaloy alloy (T-800) showing the Laves precipitates (the largest continuous precipitate, some of which are indicated with arrows) All 500×

Haynes alloy 6B is available in plate, sheet, and bar wrought forms, with a more homogeneous microstructure, containing globular carbides in a worked or an annealed matrix (Fig 11e) In wrought form, alloy 6B offers tensile properties of 619 MPa (90 ksi) yield strength, 998 MPa (145 ksi) tensile strength, and 11% elongation On the other hand, the commercial alloy Tribaloy T-800 contains large additions of molybdenum and silicon to produce a large volume fraction (about 50%)

of a hard, corrosion-resistant Laves phase, an intermetallic compound of the type MgZn2 (Fig 11f) Depending on the cobalt alloy, this phase can have a complex composition beyond a simple AB2 stoichiometry (CoSiMo, or Co2W) Although the massive amounts of hard second-phase particles limit room-temperature ductility in the Tribaloy alloys, they are readily formed as thermally sprayed or as P/M components to provide exceptionally good wear resistance

Abrasion Resistance. The characteristics of both the carbide and the matrix give cobalt alloys their good resistance to the various types of wear encountered in service The hard second-phase carbide particles in cobalt alloys provide the resistance to abrasive wear whether of the high-stress type, where the abrasive medium is crushed after entrapment between metallic surfaces, or of the low-stress type, where the moving surfaces come into contact with packed abrasive particles such as soil and sand The resistance to the cutting/plowing abrasive wear mechanisms shown by cobalt-base alloys generally depends on the hardness of the solid-solution matrix as well as the hardness, volume fraction, and distribution of the carbide or Laves phases

Sliding Wear Resistance. Sliding wear, where two surfaces are forced together and move relative to one another especially without lubricant, consists of a complex set of mechanisms that can occur singly or in combination The three mechanisms involve, first, generation of localized high temperatures (at low contact forces) to form adherent oxide glazes that can reduce wear rates or, by spalling of the oxide glaze, create oxide debris that combines abrasion with sliding wear, called fretting The second type is related to high stress metal-to-metal contact (at high contact forces) resulting in cold welding and fracture of small surface pieces, transferring them from one surface to another (called galling) The third mechanism is subsurface initiation and growth of fatigue cracks The solid-solution matrix of cobalt-base alloys gives the good resistance to sliding wear through control of the oxidation behavior, the inherent resistance to deformation, fatigue, and fracture at elevated temperatures, independent of the presence of hard second-phase particles

Erosion Resistance. Cobalt alloys are resistant to the several types of erosive wear The ductility and toughness of the solid-solution matrix are important in resisting the cutting/plowing action of solid-particle erosion, that is, impingement of small, solid particles carried by a gas or by a liquid in a slurry against the metal surface Resistance to liquid-droplet erosion and cavitation erosion, however, requires the ability to absorb shock (stress) waves without microstructural fracture While liquid-droplet and cavitation erosion have the same effect on the surface, they are caused by different mechanisms Both result in a succession of shock (or stress) waves induced by liquid drops hitting the surface In cavitation erosion the metal surface is in contact with a liquid undergoing pressure changes The collapse of bubbles, momentarily formed by localized low-pressure areas or by evaporated pockets of the liquid, causes surface damage by the impact of liquid jets from the bubble implosion The good resistance to cavitation erosion shown by cobalt alloys is attributable to its tough matrix having the ability to absorb the shock waves through work hardening and the energy-absorbing, strain-induced to martensitic phase transformation The important role that the strain-induced phase transformation plays in the good erosion wear resistance shown by cobalt alloys is indicated by the detection of an increase in -cobalt on the alloy surface and the identification of this phase in the wear debris in wear test experiments with Stellite alloy 6

Heat-Resistant Cobalt Alloys

The high-temperature resistance of cobalt alloys is due to the stability of both the arrays of carbide particles and the matrix The hardness and creep rupture strengths are stable to about 800 °C (1472 °F) The carbide particles pin grain boundaries to suppress grain growth (hardness) and grain-boundary sliding (creep resistance) Moreover, the room-temperature hardness is essentially unchanged after exposure to relatively high elevated temperatures Cobalt alloys are used in aircraft gas turbines because they show good elevated-temperature strength, and resistance to thermal fatigue and thermal shock as well as good oxidation resistance The alloys also have the good resistance to sulfidation needed in land-base turbines that use lower-quality fuels

Both Haynes alloy 25 (L605) and alloy 188 are available in wrought forms as sheet, plate, bar, or pipe In either alloy, the chromium and tungsten provide solution hardening and nickel stabilizes the -phase for improved ductility Both alloys work harden rapidly and offer a small aging response, with good ductility shown after the aging/stabilization heat treatment For example, the strength of alloy 25 in the annealed condition 476 MPa (69 ksi) yield strength, 917 MPa (133 ksi) tensile strength, 41% elongation will increase after a 30% cold-working step to 1000 MPa (145 ksi) yield strength, 1345 MPa (195 ksi) tensile strength, 16% elongation When quenched from its annealing temperature (1175 to

1230 °C, or 2150 to 2250 °F), the alloy shows a small increase in hardness accompanied by a small decrease in ductility when aged or exposed to 480 to 650 °C (900 to 1200 °F) Alloy 25 retains good strength at elevated temperatures At 982

°C (1800 °F), for example, the annealed temper offers 165 MPa (24 ksi) yield strength, 255 MPa (37 ksi) tensile strength, and 72% elongation In Haynes 188, the carbides are precipitated during the aging treatment in the range 650 to 1175 °C (1200 to 2150 °F), with M6C forming at the higher aging temperature and M23C6 at the lower end of the range A small lanthanum addition gives this alloy its excellent oxidation resistance by its making the oxide scale more adherent and more impermeable to diffusion and further growth

MAR-M Alloy 509 is capable of being cast into complex shapes as an investment casting alloy that offers high strength, high creep strength, as well as good resistance to oxidation, sulfidation, and thermal shock This alloy is used in the cast condition at temperatures up to about 955 °C (1750 °F) In this type of alloy, chromium and tungsten are the main solid solution strengthening elements, nickel stabilizes the fcc phase to improve ductility, and the tantalum, titanium, zirconium additions form MC type carbides Its room-temperature tensile properties are typically 586 MPa (85 ksi) yield strength,

780 MPa (113 ksi) tensile strength, and 4% elongation Good strength is retained even at temperatures as high as 982 °C (1800 °F), where typical tensile properties are 190 MPa (28 ksi) yield strength, 248 MPa (36 ksi) tensile strength, and 26% elongation

Corrosion-Resistant Cobalt Alloys

Several cobalt-chromium-nickel-molybdenum alloys have been developed for providing excellent corrosion resistance along with high strength and toughness In these alloys, the carbon is minimized to avoid carbide precipitation along grain boundaries, maximizing corrosion resistance The presence of chromium in cobalt alloys permits them to passivate by forming adherent films of Cr2O3 as do stainless steels These alloys are provided in various wrought forms, in the work-hardened or work-plus-age-hardened condition.The alloys work harden rapidly due to the strain-induced transformation that provides a dispersion of fine hcp platelets The tensile strength of the annealed condition (937 MPa, or 130 ksi) can

be increased to 1585 MPa (230 ksi), with about 50% cold reduction and raised still further (2000 MPa, or 290 ksi) after a subsequent aging treatment at about 530 to 593 °C (1000 to 1100 °F) for 4 h In this aged condition the alloy retains

favorable levels of ductility and toughness, showing around 10% elongation and 46% reduction in area Good strength and ductility are available at elevated temperatures as well

Because of their biocompatibility, nonmagnetic cobalt-chromium-molybdenum alloys, such as Co-28.5Cr-6Mo (ASTM F

75 and F 799), have been used for orthopedic implants When made by investment casting, the alloys are strengthened by carbide particles When made by forging or hot isostatic pressing, the alloys are hardened by a combination of nitrogen and carbide strengthening mechanisms In addition to the excellent corrosion resistance for these applications, these alloys also have the required high-cycle fatigue resistance as well as strength, ductility, and wear resistance

References cited in this section

18 Properties of Pure Metals, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol

2, ASM Handbook, ASM International, 1990, p 1099-1201

33 P Crook, Cobalt and Cobalt Alloys, Properties and Selection: Nonferrous Alloys and Special-Purpose

Materials, Vol 2, ASM Handbook, ASM International, 1990, 446-454

34 K.C Antony, Wear Resistant Cobalt-Base Alloys, J Met., Vol 35, 1983, p 52-60

Effects of Composition, Processing, and Structure on Properties of Nonferrous Alloys

Ronald N Caron, Olin Corporation; James T Staley, Alcoa Technical Center

Titanium and Titanium Alloys

The good strength, low density (4.5 g/cm3, or 0.16 lb/in.3), relatively high melting point (1668 °C, or 3034 °F), excellent corrosion resistance, and good heat-transfer properties of titanium and its alloys have made them attractive to structural designers for use primarily in aerospace and chemical industries where the combination of unique properties can justify the cost (Ref 18, 35, 36, 37, 38, 39) Titanium is also seeing increased use for sporting goods, such as golf clubs The alloys are available as castings, P/M products, and in basically all wrought plate, sheet, tube, forging, bar, and wire forms Titanium exists in two crystallographic forms: the room-temperature hcp -phase transforms to the bcc -phase at 883

°C (1620 °F) Manipulating the morphology of these two allotropic phases through alloy additions and thermomechanical processing is commonly done to provide a wide range of useful mechanical property combinations The coefficient of thermal expansion, 8.41 m/m · K, is lower than that of steel or aluminum, while its elastic modulus (110 GPa, or 16.2 ×

106 psi), falls between the values of steel and aluminum Titanium can be used for cryogenic applications because it has

no debilitating ductile-brittle transition temperature (DBTT), although decreases in toughness below room temperature can be seen in alloys The maximum useful temperature range for structural applications is 425 to 580 °C (800 to 1100

°F), depending on the alloy and condition

Titanium readily forms a stable adherent oxide layer, TiO2, that passivates the metal and provides resistance to attack by most mineral acids and chlorides Titanium is nontoxic and resists human body fluids, making it biocompatible for use in the biomaterials field In an anhydrous or nonoxygen environment, however, the oxide film will not re-form if damaged, making titanium susceptible to crevice corrosion Titanium is also susceptible to hydrogen embrittlement, through its ability to form hydride by absorbing hydrogen in pickling solutions at room temperature or from reducing atmospheres at elevated temperatures Because of its excellent corrosion resistance and good heat-transfer characteristics, unalloyed (commercially pure) titanium is used in heat exchangers, condensers, jet engine shrouds, submarine components, reactor vessels, and storage tanks

Titanium is made by double, or triple melting in vacuum electric arc furnaces, with the ingots from the first melt used as consumable electrodes for subsequent melts In addition to homogenization, the purpose of this melt practice is to reduce the number of hard, embrittling oxygen or nitrogen-rich inclusions and to remove volatile elements especially hydrogen The impurity elements carbon, nitrogen, silicon, iron, and oxygen raise strength and lower ductility In particular, the iron and oxygen impurity content control the strength of unalloyed titanium, while the carbon and nitrogen impurities must be kept to a minimum to avoid embrittlement Extra-low-interstitial (ELI) grades are commercially available that maximize ductility and toughness

Effects of Alloying Elements in Titanium Alloys

The relatively low strength of the commercially pure grades of titanium are readily improved with the addition of alloying elements in conjunction with the application of thermomechanical processing during fabrication into final products (Table 14) The alloying elements used in titanium are classified by their individual effects on the phase diagram: whether they stabilize the - or the -titanium phase (Table 15) Titanium and its alloys are processed at temperatures both above and below the transus, the temperature above which the alloy is 100% , in order to manipulate the amount and morphology of the and phases during processing as well as the relative amounts retained in the final product The base composition and the processing temperatures dictate the microstructural constitution of the alloys, which are classified according to the dominant phase in the alloy Alpha alloys are predominately , usually with minor amounts of present Alpha-beta alloys will obviously contain both phases, with more than the -alloys The content at room temperature may be as low as 5 to 10%, and greater Beta alloys have sufficient stabilizer content that the alloys can be solution treated above the transus, water quenched, and retain 100% The commercial alloys are normally metastable and are aged to precipitate to increase strength The microstructure will thus consist of in a matrix

Table 14 Compositions and properties of selected wrought titanium and alloys

Tensile strength

(min)

0.2% yield strength

H (max)

Fe (max)

O (max)

(a) Mechanical properties given for annealed condition; may be solution treated and aged to increase strength

(b) Mechanical properties given for solution treated and aged condition; alloy not normally applied in annealed condition Properties may be sensitive to section size and processing

(c) Primarily a tubing alloy; may be cold drawn to increase strength

Table 15 Ranges and effects of some alloying elements used in titanium

Alloying element Range,

Zirconium 2-5 and strengthener

Silicon 0.05-0.5 Improves creep resistance

Aluminum in solution hardens the phase, but at levels >6 wt% it can form the embrittling intermetallic phase Ti3Al Zirconium and tin solution harden both the and the phases, and both elements retard the rates of phase transformation, permitting greater control of microstructures during heat treatment Niobium is added to titanium to improve oxidation resistance at elevated temperatures Chromium additions enhance corrosion resistance

Titanium Alloy Microstructure/Property Relationships

Alpha Phase. The and near- titanium alloys are processed in the or / phase fields at elevated temperatures to take advantage of the greater processibility of the bcc phase than the phase Beta is strain-rate sensitive and is processed at low strain rates to reduce the resistance to deformation Beta phase responds well to solutionizing (hardenability) and age hardening treatments The alloys are processed in the or the / two-phase field, cooled to room temperature, and heat treated to form controlled amounts and morphology of -to- phase transformation products For example, when the near- alloy, Ti-8Al-1Mo-1V, is forged at the appropriate elevated temperature within the / -two-phase field, the resulting microstructure (Fig 12b), consists of relatively fine equiaxed grains retained from their equilibrium presence at the processing temperature (primary ) in a matrix comprising acicular grains that

transformed from the grains that existed at the elevated temperature It is this microstructure (Fig 12b), that provides the best combination of strength and ductility for this alloy When this alloy is hot forged at a temperature much lower in the / two-phase field, the microstructure at room temperature consists of nearly all grains (Fig 12a), resulting in lower strength than the alloy is capable of providing This microstructure could also be achieved by processing at the same temperature as in Fig 12(b), but with slow cooling from the processing (or solution-treatment) temperature This microstructure in Fig 12(a) has better toughness than that in Fig 12(b) In contrast, hot forging of this alloy at a temperature in the all- regime will result in the relatively coarse array of acicular transformation products produced from a coarse grain parent, Fig 12(c) This structure provides the maximum fracture toughness, with a notable reduction in ductility and fatigue strength

Fig 12 Microstructures of near- alloy Ti-8Al-1Mo-1V after forging with different starting temperatures (a)

Equiaxed grains (light) in a matrix of and (dark) (b) Equiaxed grains of primary (light) in a matrix of transformed (dark) containing fine acicular (c) Transformed containing coarse and fine acicular (light) Etchant: Kroll's reagent (192) All micrographs at 250×

Alpha-Beta Structures. The - titanium alloys offer similar microstructures as the alloys, but with a greater amount of retained , which affects the properties For example, when the commercially important - alloy, Ti-6Al-4V, is cooled from an elevated-temperature processing operation or heat treatment, the phase can transform to the coarse acicular structure seen in Fig 12(c) or to a fine acicular structure via a diffusion-controlled nucleation and growth mechanism, or to needlelike ' via a martensitic reaction as it cools below the martensite start temperature, Ms (Fig 13) The coarseness of the transformed structure is controlled by cooling rate The martensite reaction produces the ' phase from the by a diffusionless or shear movement of the product grain boundary, mechanistically similar to what occurs during the quenching of steels The martensitic product has an hcp crystal structure and is in a supersaturated state with respect to the alloy content because it inherits the composition of the parent A second type of martensitic transformation product, the orthorhombic '', can occur either upon cooling (athermal) or during cold working (strain induced) When aged, both metastable martensites precipitate equilibrium amounts of +

Fig 13 Microstructures of alloy Ti-6Al-4V after cooling from different areas of the phase field shown in (a) The

specimens represented in micrograph (e) provided the best combination of strength and ductility after aging Etchant: 10 HF, 5 HNO3, 85 H2O All micrographs at 250×

The fine acicular may be difficult to distinguish from the martensite as illustrated in Fig 13 In general, acicular (or lamellar) improves creep resistance, fracture toughness, and crack growth resistance, but at a sacrifice in ductility and fatigue (with a slight drop in strength) The coarser the transformed- structure, the greater the improvement in toughness and crack-growth resistance, and the greater the loss in ductility and fatigue strength

Typical microstructures representative of those most commonly used for / alloys are shown in Fig 14 Proceeding from Fig 14(a) to (d) will generally result in progressively decreased tensile and fatigue strengths, with increasing improvements in damage tolerance type properties The difference in microstructure between Fig 14(a) and (b) is due to the differences in processing history The temperature during sheet rolling decreases as rolling proceeds, and the final rolling temperature is significantly lower than the final forging temperature Thus, there is less retained at the final working temperature for the sheet, and a predominantly "globular"-type microstructure (the black features in Fig 14a are retained or transformed ) The final forging temperature is significantly higher, with more retained , accounting for the higher amount of lamellar transformed microstructure The slow cooling of the recrystallized annealed structure permits the primary to grow during cooling, consuming most of the Retained is observed at some - boundaries and triple points The solution-treated and aged condition is not commonly used for Ti-6Al-4V, but it is the standard heat treatment for aerospace fasteners

Fig 14 Microstructures corresponding to different combinations of properties in Ti-6Al-4V forgings (a) 6%

equiaxed primary plus fine platelet in Ti-6Al-4V - forged, then annealed 2 h at 705 °C (1300 °F) and air cooled (b) 23% elongated, partly broken up plus grain-boundary in Ti-6Al-4V, forged and water quenched, then annealed 2 h at 705 °C and air cooled (c) 25% blocky (spaghetti) plates plus very fine platelet in Ti- 6Al-4V forged from spaghetti- starting structure, then solution treated 1 h at 955 °C (1750 °F) and reannealed 2 h at 705 °C (d) 92% basketweave structure in Ti-6Al-4V forged and slow cooled, then annealed 2 h at 705 °C Structures in (a) and (b) produced excellent combinations of tensile properties, fatigue strengths and fracture toughness Structure in (c) produced very poor combinations of mechanical properties Structure in (d) produced good fracture toughness, but poor tensile properties and fatigue resistance

Beta Phase. The alloys are rich enough in -stabilizing elements (Tables 14 and 15) that the beta phase can be retained at room temperature with appropriate cooling rates The martensite reaction does not occur because the Ms is below room temperature In this metastable condition, the retained is hardenable by precipitation of given the proper aging heat treatment The family of alloys are classified as "lean" and "rich" alloys, a useful oversimplification in spite of the continuum of phase changes that occur with increasing alloy content (Fig 15) The lean alloys are prone to form the embrittling, metastable phase during cooling or aging, while the rich alloys are stable enough to avoid formation of phase during processing In practice, this is not a problem as these alloys are normally aged at a temperature above the solvus so and are the only phases present However, this would preclude using alloys in the solution-treated condition for elevated-temperature application below about 425 °C (800 °F) where precipitation will occur Using aged alloys below this temperature should not be a problem as the aging stabilizes the sufficiently

to prevent formation The formation of phase can also be effectively suppressed by cold working the parent phase before aging to encourage extensive heterogeneous nucleation of the desired equilibrium phase

Fig 15 Schematic phase diagram of a -stabilized titanium system, indicating the compositional range that

would be considered alloys and the subdivision of this range into the lean and rich alloys

Besides shaping the alloy, the thermomechanical working is designed to provide the microstructure for the optimal combination of properties required by the application The morphology (shape/size/volume fraction) of the phase determines structural properties For the near- and the - alloys, in general, the equiaxed morphology promotes higher ductility and formability, higher strength, better low-cycle fatigue, and higher threshold stress for hot-salt stress corrosion compared to the acicular morphology However, when processed with a predominately acicular morphology, the alloy has superior creep resistance, higher fracture toughness, superior stress-corrosion resistance, and lower crack-propagation rates A general comparison of the structural property combinations available with either the or the - processing mode applied to the - alloys are qualitatively described in Table 16

Table 16 The effect of processing mode on the properties of - titanium alloys

Property

processed / processed

Tensile strength Moderate Good

Creep strength Good Poor

Fatigue strength Moderate Good

Fracture toughness Good Poor

Crack growth rate Good Moderate

Grain size Large Small

Wrought Titanium Alloys

Unalloyed titanium is all phase at room temperature The acicular form is readily obtained as the principal transformation product, but the equiaxed morphology is obtainable only by applying a recrystallizing heat treatment to

a cold-worked condition The commercially pure grades have relatively low strength (Table 14), but with good control of impurities it has good toughness, offering impact toughness values equivalent to quenched-and-tempered steels There is good creep resistance up to about 315 °C (600 °F) Because of the good corrosion resistance and the ability to maintain a clean surface in service, the base titanium metal/oxide system offers good heat-transfer properties making commercially pure grades particularly useful for heat-exchanger applications Commercially pure grades are also available with minor alloy elements For example, a small solid-solution addition of 0.2 Pd improves corrosion resistance still further The alloy Ti-0.3Mo-0.8Ni (UNS R53400) offers higher strength with corrosion resistance between that of the unalloyed and the titanium-palladium grades

Titanium and its alloys are processed at elevated temperatures to take advantage of the better forgeability of the bcc phase It also results in less crystallographic texturing and more uniform properties than does processing the hcp phase

In the initial ingot breakdown operations, the metal is forged in the field just above the transus However, as the metal

is taken down through the secondary processing steps, such as rolling, it is processed high in the / field just below the transus to avoid excessive grain growth

Many titanium alloys are placed in service in heat-treated conditions After being shaped into final form or before machining, the alloys are commonly given softening, stress-relieving, or solutionizing and aging thermal treatments The alloys are commonly given stress-relief anneals without adversely affecting strength or ductility in order to relieve the stresses induced by prior thermomechanical processing and forming operations However, care is taken to choose temperatures that are high enough to accomplish the stress relieving and low enough to avoid overaging (for the solution treated and aged condition) or strain aging, or to avoid recrystallizing of a cold-worked condition Cooling rates from stress relieving or aging treatments must be sufficiently slow and uniform to avoid reintroducing thermal stresses Aging treatments are carried out at a temperature in the range 425 to 650 °C (800 to 1200 °F) as appropriate for the specific alloy and desired properties

Heating rates to the aging temperature of alloys must be rapid enough to avoid precipitation of the phase The and

- alloys are solution treated just below the transus for optimal ductility, toughness, and creep strength of the aged condition On the other hand, annealing or processing is done well below the transus to encourage a fine equiaxed in the final product for high strength, good ductility, and resistance to fatigue crack initiation

The annealing treatments need to be carried out in inert atmosphere or in vacuum to avoid excessive oxidation of the surface When an oxygen-enriched layer is formed it is called " case", and it must be chemically or mechanically removed before being put into service Titanium should not be heat treated in a hydrogen atmosphere; the hydrogen content is limited to a maximum of 125 to 150 ppm to avoid embrittlement, which manifests itself as reduced impact strength, low notched tensile strength, and delayed cracking

Titanium alloys in general can be made to exhibit superplasticity under the right conditions, but the - alloys show the highest degree of this phenomenon Superplasticity is exceptionally high ductility when processed or tensile tested with very fine - structures, with a primary grain size of about 10 m (400 in.) at very high temperature and with controlled, (slow) strain rates The - alloy, Ti-6Al-4V, for example, has been superplastically formed commercially into complex parts at 870 to 925 °C (1600 to 1700 °F), under an argon atmosphere, and at strain rates in the range 1.3 ×

10-4 to 1.0 × 10-3/s

Alpha Alloys. The -titanium alloys are slightly less corrosion resistant but offer higher strength than unalloyed titanium The alloy Ti-5Al-2.5Sn (UNS R54520), for example, shows good ductility at a tensile strength level of 790 MPa (115 ksi) (Table 14) However, outside of grain size control of strength, it cannot be strengthened by heat treatment Its primary use is for cryogenic applications

The near- titanium alloys utilize a small amount of -stabilizer elements to retain some for additional microstructural and property control These alloys, Ti-8Al-1Mo-1V (UNS R54810) and Ti-6Al-2Sn-4Zr-2Mo, for example, are processed high in the / field to restrict grain growth The microstructures can range from equiaxed to acicular (Fig 12a to c) The latter alloy is used primarily for elevated-temperature applications, up to 540 °C (1000 °F)

Alpha-Beta Alloys. The - titanium alloys contain one or more with one or more -stabilizing elements, offering increased strength and a wider range of properties, with the usual trade-off in strength versus ductility These alloys can

be strengthened by heat treatment or by thermomechanical processing to produce the wide variety of microstructures described above (Fig 13 and 14) These alloys can be aged, with better aging response shown when the is more rapidly cooled to room temperature The most commonly used alloy in this family, Ti-6Al-4V (UNS R56400), can be processed

to provide a wide range of tensile properties The alloy exhibits yield strengths in the range of 830 to 970 MPa (120 to

140 ksi), tensile strengths of 900 to 1070 MPa (130 to 155 ksi), and elongations of about 10 to 15% in the annealed condition In the solution treated and aged condition the properties range from 1000 to 1100 MPa (145 to 160 ksi) yield strength, 1070 to 1170 MPa (155 to 170 ksi) tensile strength, and elongations of 5 to 8% The aged properties are strongly dependent on section thickness

Beta Alloys. With the titanium alloys, it is difficult to obtain a great variety of useful property combinations by processing, but they do offer better fracture toughness and better room-temperature forming characteristics than the - alloys The alloys also provide high strength where yield strength instead of creep strength is important The beta alloys are more hardenable and better able to retain the desired as-quenched condition in heavier sections than the - alloys Aging treatments are done at 450 to 650 °C (850 to 1200 °F) producing dispersions of fine alpha particles in the retained beta matrix Yield strengths approaching 1380 MPa (200 ksi) are available with alloys in the solution treated and aged condition The alloy Ti-3Al-8V-6Cr-4Mo-4Zr (UNS R58640) exhibits cold-drawn and aged tensile strengths of 1310 to

1450 MPa (190 to 210 ksi) with a minimum of 10% elongation for wire

Titanium Casting Alloys

The titanium casting alloys comprise less than 2% of total mill products and have been based on traditional wrought alloy compositions, with Ti-6Al-4V being used for about 85% of the total Casting permits the manufacture of increasingly complex parts and have been used primarily in pumps and valves for marine and chemical plant applications as well as for air frames, gas turbine engine components, and surgical implant prostheses The cast microstructure invariably consists of -transformation products formed during cooling of the casting: platelets, grain boundary , and colonies The inevitable shrinkage porosity seen in the as-cast microstructure has been successfully eliminated in commercial practice by applying subsequent hot isostatic pressing (HIP) operations With the application of pressure under argon of 103 MPa (15 psi) at temperatures of 815 to 980 °C (1500 to 1800 °F), the shrinkage porosity collapses and the interfaces diffusion bond to provide a high-quality product Stress-relief annealing is done at lower temperatures (730

to 845 °C, or 1350 to 1550 °F) and in vacuum to remove hydrogen and protect surfaces from oxidation

Industrial and marine applications for the cast alloys are mostly for corrosion resistance The - alloy Ti-6Al-4V is used in the annealed condition with tensile properties of 855 MPa (124 ksi) yield strength, 930 MPa (135 ksi) tensile strength, and 12% elongation The - alloy Ti-6Al-2Sn-4Zr-2Mo, making up of about 6% of casting products, is employed at higher operating temperature where its good creep resistance up to 500 to 600 °C (930 to 1110 °F) is needed The alloy Ti-15V-3Al-3Cr-3Sn, developed as a cold-formable, age-hardenable sheet alloy, is also heat treatable as a casting alloy, where it has tensile strengths of 1200 MPa (175 ksi)

The inherently large grains resulting from casting (0.5 to 5 mm, or 0.02 to 0.2 in.) are beneficial for fracture toughness, creep resistance, and fatigue crack propagation resistance, but they are harmful to fatigue strength and tensile elongation Heat treatments are applied to minimize the amount of large boundary for improved fatigue strength

Titanium Alloy Powders

Titanium and its alloys, particularly the - alloy Ti-6Al-4V, have been fabricated by P/M techniques to take advantage

of the net-shape manufacture and of the complex configurations that are possible with these methods Blended elemental powder components are cold pressed and sintered to achieve densities on the order of 95 to 99% of theoretical and can be