ASM Metals Handbook - Desk Edition (ASM_ 1998) Episode 10 doc

Table 5 Physical properties of an Al/SiC/xxp MMC for electronic applications Composite, SiC loading Property 55 vol% 70 vol% Typical aluminum alloys Coefficient of thermal expansion, 10 -6 /°C (10 -6 /°F) 8.5 (4.7) 6.2 (3.4) 22-24 (12-13) Thermal conductivity, W/m · K (Btu/h · ft · °F) 160 (93) 170 (99) 150-180 (87-104) Density, g/cm 3 (lb/in. 3 ) 2.95 (0.106) 3.0 (0.0108) 2.7 (0.097) Elastic modulus, GPa (10 6 psi) 200 (29) 270 (39) 70 (10) Spray deposition involves atomizing a melt and, rather than allowing the droplets to solidify totally as for metal powder manufacture, collecting the semi-solid droplets on a substrate. The process is a hybrid rapid solidification process because the metal experiences a rapid transition through the liquidus to the solidus, followed by slow cooling from the solidus to room temperature. This results in a refined grain and precipitation structure with no significant increase in solute solubility. The production of MMC ingot by spray deposition can be accomplished by introducing particulate into the standard spray deposition metal spray leading to codeposition with the atomized metal onto the substrate. Careful control of the atomizing and particulate feeding conditions is required to ensure that a uniform distribution of particulate is produced within a typically 95 to 98% dense aluminum matrix. A number of aluminum alloys containing SiC particulate have been produced by spray deposition. These include aluminum-silicon casting alloys and the 2xxx, 6xxx, 7xxx, and 8xxx (aluminum-lithium) series wrought alloys. Significant increases in specific modulus have been realized with SiC-reinforced 8090 alloy (Table 6). Products that have been produced by spray deposition include solid and hollow extrusions, forgings, sheet, and remelted pressure die castings. Table 6 Properties of conventionally processed aluminum alloys (ingot metallurgy) and spray- deposited aluminum MMCs Elastic modulus Density Material GPa 10 6 psi g/cm 3 lb/in. 3 Specific modulus Improvement, % 2014 72 10 2.8 0.101 25.7 0 8090 80 12 2.55 0.092 31.4 22 2014/SiC/15p (a) 95 14 2.84 0.103 33.5 30 (a) Spray codeposited, extruded, and peak aged P/M Aluminum MMCs. Powder metallurgy processing of aluminum MMCs involves both SiC particulates and whiskers, although Al 2 O 3 particles and Si 3 N 4 whiskers have also been employed. Processing involves (1) blending of the gas-atomized matrix alloy and reinforcement in powder form; (2) compacting (cold pressing) the homogeneous blend to roughly 80% density; (3) degassing the preform (which has an open interconnected pore structure) to remove volatile contaminants (lubricants and mixing and blending additives), water vapor, and gases; and (4) consolidation by vacuum hot pressing or hot isostatic pressing. The hot-pressed cylindrical billets can be subsequently extruded, rolled, or forged. Whisker-reinforced aluminum MMCs may experience some whisker alignment during extrusion or rolling (Fig. 5). Control of whisker alignment enables production of aluminum MMC product forms with directional properties needed for some high-performance applications. Cross rolling of sheet establishes a more planar whisker alignment, producing a two-dimensional isotropy. Fig. 5 SiC whisker- reinforced (20 vol% SiC) aluminum alloy sheet with the whiskers aligned in the direction of rolling The mechanical properties of whisker-reinforced aluminum MMCs are superior to particle-reinforced composites at any common volume fraction (Fig. 6). Tables 7 and 8 show the effects of whisker alignment on the properties of aluminum MMCs. Table 9 lists typical mechanical properties for particle-reinforced aluminum alloys. Table 7 Typical properties of MMC billet and extruded plate having density of 2.86 g/cm 3 (0.103 lb/in. 3 ) to show the effects of SiC whisker alignment Ultimate tensile Yield strength (a) Coefficient of thermal expansion ( ) MMC material form Test specimen orientation MPa ksi MPa ksi 10 -6 /K 10 -6 /°F Longitudinal (axial) 496 71.9 351 50.9 16.1 8.95 305 mm (12 in.) diam cylindrical billet Transverse 503 72.9 358 51.9 16.4 9.12 Longitudinal 737 107 448 64.9 13.0 7.23 13 by 125 mm ( by 5 in.) extrusion Transverse (long) 462 67.0 379 54.9 19.6 10.9 (a) 0.2% offset Table 8 Typical properties of SiC whisker-reinforced aluminum alloy sheet Sheet thickness Ultimate tensile strength Yield strength (a) Young's, modulus, E Fracture toughness, K c mm in. Test specimen orientation MPa ksi MPa ksi Elongation (e), % GPa 10 6 psi MPa ksi 2.54 0.100 Longitudinal (along roll direction) 718 104 573 83.1 5.3 114 16.5 55 50 2.54 0.100 Transverse (90° to roll direction) 559 81.0 386 56.4 8.5 95 14 59 54 Material: 2124-T6 reinforced with 15 vol% SiC whiskers. (a) 0.2% offset Table 9 Typical mechanical properties of SiC particulate-reinforced aluminum alloy composites Modulus of elasticity Yield strength Ultimate tensile strength Alloy and vol % GPa 10 6 psi MPa ksi MPa ksi Ductility, % 6061 Wrought 68.9 10 275.8 40 310.3 45 12 15 96.5 14 400.0 58 455.1 66 7.5 20 103.4 15 413.7 60 496.4 72 5.5 25 113.8 16.5 427.5 62 517.1 75 4.5 30 120.7 17.5 434.3 63 551.6 80 3.0 35 134.5 19.5 455.1 66 551.6 80 2.7 40 144.8 21 448.2 65 586.1 85 2.0 2124 Wrought 71.0 10.3 420.6 61 455.1 66 9 20 103.4 15 400.0 58 551.6 80 7.0 25 113.8 16.5 413.7 60 565.4 82 5.6 30 120.7 17.5 441.3 64 593.0 86 4.5 40 151.7 22 517.1 75 689.5 100 1.1 7090 Wrought 72.4 10.5 586.1 85 634.3 92 8 20 103.4 15 655.0 95 724.0 105 2.5 25 115.1 16.7 675.7 98 792.9 115 2.0 30 127.6 18.5 703.3 102 772.2 112 1.2 35 131.0 19 710.2 103 724.0 105 0.90 40 144.8 21 689.5 100 710.2 103 0.90 7091 Wrought 72.4 10.5 537.8 78 586.1 85 10 15 96.5 14 579.2 84 689.5 100 5.0 20 103.4 15 620.6 90 724.0 105 4.5 25 113.8 16.5 620.6 90 724.0 105 3.0 30 127.6 18.5 675.7 98 765.3 111 2.0 40 139.3 20.2 620.6 90 655.0 95 1.2 Fig. 6 Yield strength comparison between whisker- and particulate-reinforced aluminum MMCs Continuous Fiber Aluminum MMCs As shown in Fig. 1, aluminum MMCs reinforced with continuous fibers provide the highest performance/strength. Because of their high cost, however, most applications have been limited to the aerospace industry. Aluminum/boron is a technologically mature continuous fiber MMC (Fig. 7). Applications for this composite include tubular truss members in the midfuselage structure of the Space Shuttle orbiter and cold plates in electronic microchip carrier multilayer boards. Fabrication processes for aluminum/boron composites are based on hot-press diffusion bonding of alternating layers of aluminum foil and boron fiber mats (foil-fiber-foil processing) or plasma spraying methods. Selected properties of aluminum/boron composites are given in Table 10. Table 10 Room-temperature properties of unidirectional continuous-fiber, aluminum-matrix composites Property B/6061 Al SCS-2/6061 Al (a) P100 Gr/6061 Al FP/Al-2Li (b) Fiber content, vol% 48 47 43.5 55 Longitudinal modulus, GPa (10 6 psi) 214 (31) 204 (29.6) 301 (43.6) 207 (30) Transverse modulus, GPa (10 6 psi) . . . 118 (17.1) 48 (7.0) 144 (20.9) Longitudinal strength, MPa (ksi) 1520 (220) 1462 (212) 543 (79) 552 (80) Transverse strength, MPa (ksi) . . . 86 (12.5) 13 (2) 172 (25) (a) SCS-2 is a silicon carbide fiber. (b) FP is an alpha alumina ( -Al 2 O 3 ) fiber Fig. 7 Cross section of a continuous-fiber-reinforced aluminum/boron composite. Shown here are 142 m diam boron filaments coated with B 4 C in a 6061 aluminum alloy matrix Continuous SiC fibers are often used as replacements for boron fibers because they have similar properties (e.g., a tensile modulus of 400 GPa, or 60 × 10 6 psi) and offer a cost advantage. One such SiC fiber is SCS, which can be manufactured with any of several surface chemistries to enhance bonding with a particular matrix, such as aluminum or titanium. The SCS-2 fiber, tailored for aluminum, has a 1 m (0.04 mil) thick carbon rich coating that increases in silicon content toward its outer surface. Hot molding is a low-pressure, hot-pressing process designed to fabricate Al/SiC parts at significantly lower cost than is possible with a diffusion-bonding/solid-state process. Because the SCS-2 fibers can withstand molten aluminum for long periods, the molding temperature can be raised into the liquid-plus-solid region of the alloy to ensure aluminum flow and consolidation at low pressure, thereby eliminating the need for high-pressure die molding equipment. The hot-molding process is analogous to the autoclave molding of graphite-epoxy, in which components are molded in an open-faced tool. The mold in this case is a self-heating, slip-cast ceramic tool that contains the profile of the finished part. A plasma-sprayed aluminum preform is laid into the mold, heated to near molten aluminum temperature, and pressure- consolidated in an autoclave by a metallic vacuum bag. Aluminum/SiC MMCs exhibit increased strength and stiffness as compared with unreinforced aluminum, with no weight penalty. Tensile properties of 6061/SCS-2 composites are given in Table 10. In contrast to the base metal, the composite retains its room-temperature tensile strength at temperatures up to 260 °C (500 °F). Aluminum/graphite MMC development was initially prompted by the commercial appearance of strong and stiff carbon fibers in the 1960s. Carbon fibers offer a range of properties, including an elastic modulus up to 966 GPa (140 psi × 10 6 ) and a negative CTE down to -1.62 × 10 -6 /°C (-0.9 × 10 -6 /°F). However, carbon and aluminum in combination are difficult materials to process into a composite. A deleterious reaction between carbon and aluminum, poor wetting of carbon by molten aluminum, and oxidation of the carbon are significant technical barriers to the production of these composites. Two processes are currently used for making commercial aluminum MMCs: liquid metal infiltration of the matrix on spread tows and hot press bonding of spread tows sandwiched between sheets of aluminum. With both precursor wires and metal-coated fibers, secondary processing such as diffusion bonding or pultrusion is needed to make structural elements. Squeeze casting also is feasible for the fabrication of this composite. Precision aerospace structures with strict tolerances on dimensional stability need stiff, lightweight materials that exhibit low thermal distortion. Aluminum/graphite MMCs have the potential to meet these requirements, Unidirectional P100 Gr/6061 aluminum pultruded tube exhibits an elastic modulus in the fiber direction significantly greater than that of steel, and it has a density approximately one-third that of steel. Properties are listed in Table 10. Aluminum/Al 2 O 3 MMCs can be fabricated by a number of methods, but liquid or semi-solid-state processing techniques are commonly used. Aluminum oxide fibers, which include Fiber FP (99.5% Al 2 O 3 ) and Saffil (96Al 2 O 3 - 4SiO 2 ) are inexpensive and provide the composite with improved properties as compared with those of unreinforced aluminum alloys. For example, the composite has an improved resistance to wear and thermal fatigue deformation and a reduced CTE. Continuous fiber Al/Al 2 O 3 MMCs are fabricated by arranging Al 2 O 3 tapes in a desired orientation to make a preform, inserting the preform into a mold, and infiltrating the preform with molten aluminum via a vacuum assist. Reinforcement-to-matrix bonding is achieved by small additions of lithium to the melt. Table 10 gives the room- temperature properties of a unidirectional Al-2Li/Al 2 O 3 . Titanium-Matrix Composites Titanium is selected as a matrix metal because of its good specific strength at both room and moderately elevated temperature and its excellent corrosion resistance. Because titanium retains its strength at higher temperatures than aluminum, it has increasingly been used as a replacement for aluminum in aircraft and missile structures as the operating speeds of these items have increased from subsonic to supersonic. Continuous Fiber Titanium MMCs. Silicon carbide fibers are the reinforcement of choice for titanium MMCs. The SCS-6 fiber is a carbon-cored monofilament that is 142 m in diameter. A tungsten-cored fiber that has a carbon coating has also been developed with a diameter of 127 m. A tungsten-cored monofilament with carbon and titanium diboride coatings that is 102 m in diameter is also available. Fiber contents of 30 to 40 vol% are common (Fig. 8). Conventional matrix alloys include Ti-6Al-4V for low-temperature applications and Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) when higher creep resistance is required or when the temperature is higher than the maximum use temperature for Ti-6Al-4V. The Ti-6242 alloy is used in turbine engine actuator pistons and reinforced fan frames. More recently, titanium aluminide ordered intermetallics such as Ti-22Al-23Nb and Ti-22Al-26Nb have been used as matrix materials. These materials are being developed for rotating blades and impellers. Processing techniques for titanium MMCs used for aerospace applications include fiber-foil-fiber processing and tape casting or wire winding used in conjunction with hot isostatic pressing. Plasma spraying has also been employed to deposit a titanium matrix onto the fibers. Similarly, electron beam physical vapor deposition of metal on fiber has also been demonstrated. Table 11 gives properties for a representative unidirectional SiC/Ti laminate. Table 11 Room-temperature properties of a unidirectional SiC c /Ti MMC Property SCS-6/Ti-6Al-4V Fiber content, vol% 37 Longitudinal modulus, GPa (10 6 psi) 221 (32) Transverse modulus, GPa (10 6 psi) 165 (24) Longitudinal strength, MPa (ksi) 1447 (210) Transverse strength, MPa (ksi) 413 (60) Fig. 8 Typical fiber array in a SiC-reinforced titanium MMC. Actual fiber diameters are 127 m. Courtesy of Charles R. Rowe, Atlantic Research Corporation Particle-Reinforced Titanium MMCs are processed by P/M methods. Although a variety of materials have been studied, the most common combination is Ti-6Al-4V reinforced with 10 to 20 wt% TiC. These composites offer increased hardness and wear resistance over conventional titanium alloys. Properties of unreinforced and reinforced Ti-6Al-4V are compared in Table 12. Table 12 Properties of TiC particle-reinforced titanium MMCs Property Ti-6Al-4V 10 wt% TiC/Ti-6Al- 4V 20 wt% TiC/Ti-6Al- 4V Density, g/cm 3 (lb/in. 3 ) 4.43 (0.160) 4.45 (0.16) 4.52 (0.162) Tensile strength, MPa (ksi), at: RT 896 (130) 999 (145) 1055 (153) 540 °C (1000 °F) 448 (65) 551 (80) 620 (90) Modulus, GPa (10 6 psi), at: RT 113 (16.5) 133 (19.3) 144 (21) 540 °C (1000 °F) 89 (13) 105 (15.3) 110 (16) Fatigue limit (10 6 cycles), MPa (ksi) 517 (75) 275 (40) . . . Fracture toughness, MPa (ksi ) 55 (50) 44 (40) 32 (29) Coefficient of linear thermal expansion (RT to 540 °C, or 1000 °F), ppm/°C 8.5 8.1 8.0 Hardness, HRC 34 40 44 RT, room temperature Other MMCs of Importance Magnesium-matrix composites are being developed to exploit essentially the same properties as those provided by aluminum MMCs: high stiffness, light weight, and low CTE. In practice, the choice between aluminum and magnesium as a matrix is usually made on the basis of weight versus corrosion resistance. Magnesium is approximately two-thirds as dense as aluminum, but it is more active in a corrosive environment. Magnesium has a lower thermal conductivity, which is sometimes a factor in its selection. Magnesium MMCs include continuous fiber Gr/Mg for space structures, short staple fiber Al 2 O 3 /Mg for automotive engine components, and discontinuous SiC or B 4 C/Mg for engine components and low- expansion electronic packaging materials. Matrix alloys include AZ31, AZ91, ZE41, QE22, and EZ33. Processing methods parallel those used for the aluminum MMC counterparts. Copper-matrix composites have been produced with continuous tungsten, silicon carbide, and graphite fiber reinforcements. Of the three composites, continuous graphite/copper MMCs have been studied the most. Interest in continuous graphite/copper MMCs gained impetus from the development of advanced graphite fibers. Copper has good thermal conductivity, but it is heavy and has poor elevated-temperature mechanical properties. Pitch-base graphite fibers have been developed that have room-temperature axial thermal conductivity properties better than those of copper. The addition of these fibers to copper reduces density, increases stiffness, raises the service temperature, and provides a mechanism for tailoring the coefficient of thermal expansion. One approach to the fabrication of graphite/copper MMCs uses a plating process to envelop each graphite fiber with a pure copper coating, yielding MMC fibers flexible enough to be woven into fabric. The copper-coated fibers must be hot pressed to produce a consolidated component. Table 13 compares the thermal properties of aluminum and copper MMCs with those of unreinforced aluminum and copper. Graphite/copper MMCs have the potential to be used for thermal management of electronic components, satellite radiator panels, and advanced airplane structures. Table 13 Thermal properties of unreinforced and reinforced aluminum and copper Density Axial thermal conductivity Axial coefficient of thermal expansion Material Reinforcement content, vol% g/cm 3 lb/ft 3 W/m · °C Btu/ft · h · °F 10 -6 /°C 10 -6 /°F Aluminum 0 2.71 169 221 128 23.6 13.1 Copper 0 8.94 558 391 226 17.6 9.7 SiC p /Al 40 2.91 182 128 74 12.6 7 P120 Gr/Al 60 2.41 150 419 242 -0.32 -0.17 P120 Gr/Cu 60 4.90 306 522 302 -0.07 -0.04 Superalloy-Matrix Composites. In spite of their poor oxidation resistance and high density, refractory metal (tungsten, molybdenum, and niobium) wires have received a great deal of attention as fiber reinforcement materials for use in high-temperature superalloy MMCs. Although the theoretical specific strength potential of refractory alloy fiber- reinforced composites is less than that of ceramic fiber-reinforced composites, the more ductile metal fiber systems are more tolerant of fiber-matrix reactions and thermal expansion mismatches. When refractory metal fibers are used to reinforce a ductile and oxidation-resistant matrix, they are protected from oxidation, and the specific strength of the composite is much higher than that of superalloys at elevated temperatures. Fabrication of superalloy MMCs is accomplished via solid-phase, liquid-phase, or deposition processing. The methods include investment casting, the use of matrix metals in thin sheet form, the use of matrix metals in powder sheet form made by rolling powders with an organic binder, powder metallurgy techniques, slip casting of metal alloy powders, and arc spraying. Figure 9 compares the elevated-temperature tensile strength of a nickel-base superalloy (Waspaloy) reinforced with various refractory wires. As this figure indicates, a composite consisting of 50 vol% W-24Re-HfC had the highest strength at 1093 °C (2000 °F). Fig. 9 Elevated temperature (1093 °C, or 2000 °F) tensile strength of Waspaloy reinforced with 50 vol% refractory metal wire. 218 CS represents potassium-doped tungsten. ST 300 is a W-1.0ThO 2 alloy. Comparative data are included for unreinforced MarM 246, a nickel-base superalloy. Intermetallic-Matrix Composites. One disadvantage of superalloy MMCs is their high density, which limits the potential minimum weight of parts made from these materials. High melting points and relatively low densities make intermetallic-matrix composites (IMCs) viable candidates for lighter turbine engine materials. Aluminides of nickel, titanium, and iron have received the most attention as matrices for IMCs. Property data on TiB 2 -reinforced titanium aluminides can be found in the article "Structural Intermetallics" in this Section. Selected References • Aluminum-Matrix Composites, ASM Specialty Handbook: Aluminum and Aluminum Alloys, J.R. Davis, Ed., ASM International, 1993, p 160-179 • D.M. Aylor, Corrosion of Metal-Matrix Composites, Corrosion, Vol 13, ASM Handbook, ASM International, 1987, p 859-863 • M.E. Buck and R.J. Suplinskas, Continuous Boron Fiber MMCs, Engineered Materials Handbook, Vol 1, Composites, ASM International, 1987, p 851-857 • J.L. Cook and W.R. Mohn, Whisker-Reinforced MMCs, Engineered Materials Handbook, Vol 1, Composites, ASM International, 1987, p 896-902 • J.V. Foltz and C.M. Blackman, Metal-Matrix Composites, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Vol 2, ASM Handbook, ASM International, 1990, p 903-912 • D.M. Goddard et al., Continuous Graphite Fiber MMCs, Engineered Materials Handbook, Vol 1, Composites, ASM International, 1987, p 867-873 • J.J. Lewankowski and P.M. Singh, Fracture and Fatigue of Discontinuously Reinforced Aluminum Composites, Fatigue and Fracture, Vol 19, ASM Handbook, ASM International, 1996, p 895-904 • J.A. McElman, Continuous Silicon Carbide Fiber MMCs, Engineered Materials Handbook, Vol 1, Composites, ASM International, 1987, p 858-866 • P.K. Rohatgi, Y. Liu, and S. Ray, Friction and Wear of Metal-Matrix Composites, Friction, Lubrication, and Wear Technology, Vol 18, ASM Handbook, ASM International, 1992, p 801-811 • J.C. Romine, Continuous Aluminum Oxide Fiber MMCs, Engineered Materials Handbook, Vol 1, Composites, ASM International, 1987, p 874-877 Structural Intermetallics Introduction ALLOYS based on ordered intermetallic compounds constitute a unique class of metallic material that form long-range ordered crystal structures (Fig. 1) below a critical temperature, generally referred to as the critical ordering temperature (T c ). These ordered intermetallics usually exist in relatively narrow compositional ranges around simple stoichiometric ratio (see the phase diagrams shown in this article). Fig. 1 Atomic arrangements of conventional alloys and ordered intermetallic compounds. (a) Disordered crystal structure of a conventional alloy. (b) Long-range ordered crystal structure of an ordered intermetalli c compound The search for new high-temperature structural materials has stimulated much interest in ordered intermetallics. Recent interest has been focused on nickel aluminides based on Ni 3 Al and NiAl, iron aluminides based on Fe 3 Al and FeAl, and titanium aluminides based on Ti 3 Al and TiAl. These aluminides possess many attributes that make them attractive for high-temperature structural applications. They contain enough aluminum to form, in oxidizing environments, thin films of alumina (Al 2 O 3 ) that are compact and protective. They have low densities, relatively high melting points, and good high- temperature strength properties (Tables 1 and 2). Table 1 Properties of nickel, iron, and titanium aluminides Critical ordering temperature (T c ) Melting point (T m ) Young's modulus Alloy Crystal structure (a) °C °F °C °F Material density, g/cm 3 GPa 10 6 psi Ni 3 Al L1 2 (ordered fcc) 1390 2535 1390 2535 7.50 179 25.9 NiAl B2 (ordered bcc) 1640 2985 1640 2985 5.86 294 42.7 Fe 3 Al D0 3 (ordered bcc) 540 1000 1540 2805 6.72 141 20.4 B2 (ordered bcc) 760 1400 1540 2805 . . . . . . . . . FeAl B2 (ordered bcc) 1250 2280 1250 2280 5.56 261 37.8 Ti 3 Al D0 19 (ordered hcp) 1100 2010 1600 2910 4.2 145 21.0 TiAl L1 0 (ordered tetragonal) 1460 2660 1460 2660 3.91 176 25.5 TiAl 3 D0 22 (ordered tetragonal) 1350 2460 1350 2460 3.4 . . . . . . (a) fcc, face-centered cubic; bcc, body- centered cubic; hcp, hexagonal close packed Table 2 Attributes and upper use temperature limits for nickel, iron, and titanium aluminides Maximum use temperature, °C (°F) Alloy Attributes Strength limit Corrosion limit Ni 3 Al Oxidation, carburization, and nitridation resistance; high-temperature strength 1000 (1830) 1150 (2100) NiAl High melting point; high thermal conductivity; oxidation, carburization, and nitridation resistance 1200 (2190) 1400 (2550) Fe 3 Al Oxidation and sulfidation resistance 600 (1110) 1100 (2010) FeAl Oxidation, sulfidation, molten salt, and carburization resistance 800 (1470) 1200 (2190) Ti 3 Al Low density; good specific strength 760 (1400) 650 (1200) Nickel, iron, and titanium aluminides, like other ordered intermetallics, exhibit brittle fracture and low ductility at ambient temperatures. It has also been found that quite a number of ordered intermetallics, such as iron aluminides, exhibit environmental embrittlement at ambient temperatures. The embrittlement involves the reaction of water vapor in air with reactive elements (aluminum, for example) in intermetallics to form atomic hydrogen, which drives into the metal and causes premature fracture. Thus, the poor fracture resistance and limited fabricability have restricted the use of aluminides as engineering materials in most cases. However, in recent years, alloying and processing have been employed to overcome the brittleness problem of ordered intermetallics. Success in this work has inspired parallel efforts aimed at improving strength properties. The results have led to the development of a number of attractive intermetallic alloys having useful ductility and strength. Figure 2 illustrates the crystal structures showing the ordered arrangements of atoms in several of these aluminides. For most of the aluminides listed in Table 1, the critical ordering temperature is equal to the melting temperature. Others disorder at somewhat lower temperatures, and Fe 3 Al passes through two ordered structures (D0 3 and B2) before becoming disordered. Many of the aluminides exist over a range of compositions, but the degree of order decreases as the deviation from stoichiometry increases, Additional elements can be incorporated without losing the ordered structure. For example, in Ni 3 Al, silicon atoms are located in aluminum sites, cobalt atoms on nickel sites, and iron atoms on either. In many instances, the so-called intermetallic compounds can be used as bases for alloy development to improve or optimize properties for specific applications. [...]... (1 110) 600 (1 110) 20 High hcp/bcc At room temperature Ti3Al TiAl 4. 1-4 .7 10 0-1 45 (14. 5-2 1) 70 0-9 90 (10 1-1 44) 80 0-1 140 (11 6-1 65) 760 (1400) 650 (1200) 2-1 0 1 0-2 0 D019 3. 7-3 .9 16 0-1 76 (23. 2-2 5.5) 40 0-6 50 (5 8-9 4) 45 0-8 00 (6 5-1 16) 100 0 (1830) 900 (1650) 1-4 1 0-6 0 L10 Nickel-base superalloys 8.3 206 (30) 109 0 (1995) 109 0 (1995) 3-5 1 0-2 0 fcc/L2 Fig 17 Comparison of the creep behavior of conventional titanium... HT/FL G1/Ti-47Al-1Cr-1V2.6Nb P/M extrusion + HT/NL Forging + HT/duplex Forging + HT/FL Sumitomo/Ti-45Al1.6Mn ABB alloy/Ti-47Al-2W0.5Si 47XD/Ti-47Al-2Mn2Nb-0.8TiB2 45XD/Ti-45Al-2Mn2Nb-0.8TiB2 Reactive sintering/NL Casting + HT/duplex Casting + HIP + HT/NL + TiB2 Casting + HIP + HT GE alloy 204b/Ti46.2Al-x Cr-y (Ta,Nb) Casting + HIP + HT/NL Ti-47Al-2Nb-2Cr-1Ta Casting + HIP + HT/duplex Ti-47Al-2Nb-1.75Cr... with those of two high-speed tool steels Table 5 Chemical compositions of developmental FeAl (Fe-35.8 at.% Al) alloys Alloy(a) FA-350 FA-362 FA-372 FA-383 FA-384 FA-385 FA-386 FA-387 FA-388 FA-385M1 FA-385M2 FA-385M3 FA-385M4 FA-385M5 FA-385M6 FA-385M7 FA-385M8 FA-385M9 FA-385M10(b) (a) (b) (c) Composition, at.% Cr Nb Ti Mo 0.2 0.2 2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 2 0.2... steady-state regime over that of conventional alloy Ti- 1100 (Ti-6Al-3Sn-4Zr-0.4Mo-0.45Si) and two orders of magnitude over that of Ti-6Al-2Sn-4Zr-2Mo-0.1Si However, 0.4% creep strain in Ti-2 5-1 0-3 -1 is reached within 2 h Additions of silicon and zirconium appear to improve creep resistance, but the most significant improvement is attained by increasing the aluminum content to 25 at.% and limiting -stabilizing... been achieved with coating alloys based in the Ti-Al-Cr system These coatings have been applied by sputtering (Ti-44Al-28Cr on Ti-47Al2Cr-2Ta), hot isostatic pressing (Ti-44Al-28Cr and Ti-50Al-20Cr coatings), and low-pressure plasma spraying Figure 20 shows the results of interrupted weight gain oxidation data for Ti-48Al-2Cr-2Nb coated with Ti-51Al-12Cr These tests showed that the coating successfully... elastic moduli of alloys range from 160 to 176 GPa (23 × 106 to 25.5 × 106 psi) and decrease slowly with temperature Table 10 Tensile properties and fracture toughness values of gamma titanium aluminides tested in air Alloy designation and composition, at.% °C 4 8-1 -( 0.3C)/Ti-48Al-1V0.3C-0.2O 4 8-1 (0.2C)/Ti-48Al-1V0.2C-0.14O 4 8-2 -2 /Ti-48Al-2Cr-2Nb Processing and microstructure Forging + HT/duplex Casting/duplex... as long-range order, solid solution, and texture effects, also contribute Table 8 Properties of Alloy -2 Ti3Al alloys with various microstructures Microstructure(a) MPa Ti-25Al Ti-24Al-11Nb Ti-24Al-14Nb Ti-25Al-10Nb-3V-1Mo Ti-24.5Al-17Nb (a) (b) E W FW W W FW C+P W+P FW + P W W+P E, equiaxed Yield strength ksi Ultimate tensile strength MPa ksi 538 787 761 831 825 823 745 759 942 952 705 78 114 110 120... Ti-25Al, is stable up to approximately 109 0 °C (1995 °F) Fig 18 The titanium-aluminum binary phase diagram The semicommercial and experimental -2 alloys developed are two phase ( -2 + /B2), with contents of 23 to 25 at.% Al and 11 to 18 at.% Nb Alloy compositions with current engineering significance are Ti-24Al-11Nb, Ti-25Al-10Nb-3V1Mo, Ti-25Al-17Nb-1Mo, and modified alloy compositions such as Ti-24.5Al-6Nb-6(Ta,Mo,Cr,V)... as Ti-22Al-27Nb (at.%) Table 9 lists the elevated-temperature tensile properties of such alloys Table 9 Tensile properties of a two-phase (O + Test temperature °C 22 °F 72 540 100 0 650 1200 760 1400 Aging treatment None None 540 °C (100 0 °F), 100 h 540 °C (100 0 °F), 100 h 650 °C (1200 °F), 100 h 650 °C (1200 °F), 100 h 760 °C (1400 °F), 100 h 760 °C (1400 °F), 100 h None None 540 °C (100 0 °F), 100 h... 166 107 6 156 108 3 157 100 7 146 104 9 152 104 9 152 107 0 155 938 136 945 137 938 136 952 138 787 114 766 114 649 94 Elongation, % 3.4 2.2 3.3 2.8 2.6 2.5 5.2 5.0 14.3 14.3 17.9 16.1 14.3 12.5 10. 7 10. 7 10. 7 14.3 21.4 Gamma Alloys The -TiAl phase has an L10 ordered face-centered tetragonal structure (Fig 2), which has a wide range (49 to 66 at.% Al) of temperature-dependent stability (Fig 18) The -TiAl . expansion, 10 -6 /°C (10 -6 /°F) 8.5 (4.7) 6.2 (3.4) 2 2-2 4 (1 2-1 3) Thermal conductivity, W/m · K (Btu/h · ft · °F) 160 (93) 170 (99) 15 0-1 80 (8 7-1 04) Density, g/cm 3 (lb/in. 3 ) 2.95 (0 .106 ). and reinforced Ti-6Al-4V are compared in Table 12. Table 12 Properties of TiC particle-reinforced titanium MMCs Property Ti-6Al-4V 10 wt% TiC/Ti-6Al- 4V 20 wt% TiC/Ti-6Al- 4V Density, g/cm 3 . applications with the following composition range (in atomic percent): Ni-(1 4-1 8)Al-( 6-9 )Cr-( 1-4 )Mo-(0.0 1-1 .5)Zr/Hf-(0.0 1-0 .20)B In these aluminide alloys, 6 to 9 at.% Cr is added to reduce