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ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials Copyright © 1990 ASM International® ASM Handbook Committee, p 903-912 All rights reserved. DOI: 10.1361/asmhba0001101 www.asminternational.org

Metal-Matrix Composites

John V. Foltz, Metallic Materials Branch, Naval Surface Warfare Center, White Oak Laboratory Charles M. Blackmon, Applied Materials Technology Branch, Naval Surface Warfare Center, Dahlgren Laboratory

METAL-MATRIX COMPOSITES elastic modulus and strength of unidirec- Primary processing is the operation by (MMCs) are a class of materials with poten- tionally reinforced composites, are suffi- which the composite is synthesized from its tial for a wide variety of structural and ciently high in some MMCs to permit use of raw materials. It involves introducing the thermal management applications. Metal- the unidirectional lay-up in engineering reinforcement into the matrix in the appro- matrix composites are capable of providing structures. priate amount and location, and achieving higher-temperature operating limits than This article will give an overview of the proper bonding of the constituents. Second- their base metal counterparts, and they can current status of MMCs, including informa- ary processing consists of all the additional be tailored to give improved strength, stiff- tion on physical and mechanical properties, steps needed to make the primary compos- ness, , abrasion resis- processing methods, distinctive features, ite into a finished hardware component. tance, creep resistance, or dimensional sta- and the various types of continuously and Many reinforcement and matrix materials bility. Unlike resin-matrix composites, they discontinuously reinforced MMCs. More are not inherently compatible, and such are nonflammable, do not outgas in a vacu- information on the processing and proper- materials cannot be processed into a com- um, and suffer minimal attack by organic ties of MMCs is available in the Section posite without tailoring the properties of an fluids such as fuels and solvents. "Metal, Carbon/, and Ma- interface between them. In some compos- The principle of incorporating a high- trix Composites" in Composites, Volume 1 ites the coupling between the reinforcing performance second phase into a conven- of the Engineered Materials Handbook agent and the metal is poor and must be tional engineering material to produce a published by ASM INTERNATIONAL. enhanced. For MMCs made from reactive combination with features not obtainable constituents, the challenge is to avoid ex- from the individual constituents is well Property Prediction cessive chemical activity at the interface, known. In a MMC, the continuous, or ma- which would degrade the properties of the trix, phase is a monolithic alloy, and the Property predictions of MMCs can be material. These problems are usually re- reinforcement consists of high-performance obtained from mathematical models, which solved either by applying a surface treat- carbon, metallic, or ceramic additions. Re- require as input a knowledge of the proper- ment or coating to the reinforcement or by inforced intermetallic compounds such as ties and geometry of the constituents. For modifying the composition of the matrix the aluminides of , , and iron reinforced by straight, parallel con- alloy. are also discussed in this article (for more tinuous fibers, three properties that are fre- Solidification processing (Ref 4, 5), solid- information on intermetallic compounds, quently of interest are the elastic modulus, state bonding, and matrix deposition tech- see the article "Ordered Intermetallics" in the coefficient of , and niques have been used to fabricate MMCs. this Volume). the thermal conductivity in the fiber direc- Solidification processing offers a near-net- Reinforcements, characterized as either tion. Reasonable values can be obtained shape manufacturing capability, which is continuous or discontinuous, may consti- from rule-of-mixture expressions for economically attractive. Developers have tute from 10 to 60 vol% of the composite. Young's modulus (Ref 1): explored various liquid metal techniques Continuous fiber or filament reinforce- Ec = Ef vf +Em Vm (Eq 1) that use multifilament yarns, chopped fi- ments include graphite (Gr), silicon - coefficient of thermal expansion (Ref 2): bers, or particulates as the reinforcement. bide (SIC), , aluminum oxide A castable ceramic/aluminum MMC is now (A1203), and refractory metals. Discontin- otfvfEf + ot m v mE m commercially available (Ref 6); cast com- a¢ = (Eq 2) uous reinforcements consist mainly of SiC Ef vf + Em v m ponents of this composite are shown in in whisker (w) form, particulate (p) types Fig. 2. Solid-state methods use lower fab- of SiC, A1203, or (TiB2), and thermal conductivity (Ref 3): rication temperatures with potentially bet- and short or chopped fibers of A1203 or kc = kfvf + kmvm (Eq3) ter control of the interface thermodynam- graphite. Figure 1 shows cross sections of where v is volume fraction, and E, et, and k ics and kinetics. The two principal typical continuous and discontinuous rein- are the modulus, coefficient of thermal ex- categories of solid-state fabrication are dif- forcement MMCs. pansion, and thermal conductivity in the fusion bonding of materials in thin sheet The salient characteristics of metals as fiber direction, respectively. The subscripts form (Ref 7) and tech- matrices are manifested in a variety of c, f, and m refer to composite, fiber, and niques (Ref 8). Matrix deposition pro- ways; in particular, a metal matrix imparts a matrix, respectively. cesses, in which the matrix is deposited on metallic nature to the composite in terms of the fiber, include electrochemical , thermal and electrical conductivity, manu- Processing Methods plasma spraying, and physical vapor de- facturing operations, and interaction with position (Ref 7). A new method, metal the environment. Matrix-dominated me- Processing methods for MMCs are divid- spray deposition, is currently being inves- chanical properties, such as the transverse ed into primary and secondary categories. tigated (Ref 9). After deposition process- 904 / Special-Purpose Materials

(a) 100 ~m 100 I~m 10 iLm

(d) ~ (el (0 I I 10 i~m Cross sections of typical fiber-reinforced MMCs. (a) Continuous-fiber-reinforced boron/aluminum composite. Shown here are 142 i~m diam boron filaments Fig. 1 coated with BaC in a 6061 aluminum alloy matrix. (b) Discontinuous graphite/aluminum composite. Cross section shows 10 i~m diam chopped graphite fibers (40 vol%) in a 2014 aluminum alloy matrix. (c) A 6061 aluminum alloy matrix reinforced with 40 vol% SiC particles. (d) Whisker-reinforced (20 vol% SiC) aluminum MMC. (e) and (f) MMCs manufactured using the PRIMEXTM pressureless metal infiltration process. (e) An AI203-reinforced (60 vol%) aluminum MMC. (f) A highly reinforced (81 vol%) MMC consisting of SiC particles in an aluminum alloy matrix. The black specks in the matrix are particles of an inorganic preform binder and do not indicate porosity. (a) and (b) Courtesy of DWA Composite Specialties, Inc. (c) and (d) Courtesy of Advanced Composite Materials Corporation. (e) and (f) Courtesy of Lanxide Corporation

ing, a secondary consolidation step such as Aluminum-Matrix Composites Continuous Fiber Aluminum MMC. diffusion bonding often is needed to pro- Boron/aluminum is a technologically ma- duce a component. Most of the commercial work on MMCs ture continuous fiber MMC (Fig. la). Ap- Which secondary processes are appropri- has focused on aluminum as the matrix metal. plications for this composite include tubular ate for a given MMC depends largely on The combination of light weight, environmen- truss members in the midfuselage structure whether the reinforcement is continuous or tal resistance, and useful mechanical proper- of the Space Shuttle orbiter and cold plates discontinuous. Discontinuously reinforced ties has made aluminum alloys very popular; in electronic microchip carrier multilayer MMCs are amenable to many common met- these properties also make aluminum well boards. Fabrication processes for B/A1 al forming operations, including , suited for use as a matrix metal. The melting composites are based on hot-press diffusion forging, and rolling. Because a high percent- point of aluminum is high enough to satisfy bonding or plasma spraying methods (Ref age of the materials used to reinforce dis- many application requirements, yet low 13). Selected properties of a B/A1 composite continuous MMCs are hard oxides or car- enough to render composite processing rea- are given in Table 1. bides, machining can be difficult, and sonably convenient. Also, aluminum can ac- Continuous SiC fibers (SiC¢) are now methods such as sawing, electrical commodate a variety of reinforcing agents, commercially available; these fibers are discharge machining (Ref 10), and abrasive including continuous boron, AI203, SiC, and candidate replacements for boron fibers be- waterjet cutting (Ref 11) are sometimes uti- graphite fibers, and various particles, short cause they have similar properties and offer lized (see Machining, Volume 16 of the 9th fibers, and whiskers (Ref 12). The microstruc- a potential cost advantage. One such SiC Edition of Metals Handbook for more infor- tures of various aluminum matrix MMCs are fiber is SCS, which can be manufactured mation about these machining methods). shown in Fig. 1. with any of several surface chemistries to Metal-Matrix Composites / 905

Table 1 Room-temperature properties of unidirectional continuous fiber aluminum-matrix composites Property B/6061 AI SCS-2/6061 AI PI00 Gr/606l AI FP/AI-2Li(a) Fiber content, vol% ...... 48 47 43.5 55 Longitudinal modulus, GPa (106 psi) ...... 214 (31) 204 (29.6) 301 (43.6) 207 (30) Transverse modulus, GPa (106 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) FP is the proprietary designation for an alpha alumina (ct-Al203) fiber developed by E.I. Du Pont de Nemours & Company, Inc. Source: Ref 14-16

room-temperature tensile strength at tem- pultrusion is needed to make structural ele- peratures up to 260 °C (500 °F) (Fig. 3). This ments. Squeeze casting also is feasible for material is the focus of development pro- the fabrication of this composite (Ref 23). grams for a variety of applications; an ex- Precision aerospace structures with strict ample of an advanced aerospace application tolerances on dimensional stability need for an SCS/A1 MMC is shown in Fig. 4. stiff, lightweight materials that exhibit low Graphite/aluminum (Gr/A1) MMC devel- thermal distortion. Graphite/aluminum opment was initially prompted by the com- MMCs have the potential to meet these mercial appearance of strong and stiff car- requirements. Unidirectional P100 Gr/6061 bon fibers in the 1960s. As shown in Fig. 5, AI pultruded tube (Ref 15) exhibits an elas- carbon fibers offer a range of properties, tic modulus in the fiber direction significant- including an elastic modulus up to 966 GPa ly greater than that of steel, and it has a Discontinuous /aluminum cast- Fig. 2 ings. Pictured are a sand cast automotive disk (140 psi x 106) and a negative coefficient of approximately one-third that of brake rotor and upper control arm, a permanent mold thermal expansion down to -1.62 × 10-6/ steel (Table 1). Reference 24 contains addi- cast piston, a high-pressure die cast sprocket, an °C (-0.9 × 10-6/°F). However, carbon and tional data for P100 Gr/AI. investment cast aircraft hydraulic manifold, and three aluminum in combination are difficult mate- In theory, Gr/AI angle-plied laminates investment cast engine cylinder inserts. Courtesy of Dural Aluminum Composites Corporation rials to process into a composite. A delete- can be designed to provide a coefficient of rious reaction between carbon and alumi- thermal expansion (CTE) of exactly zero by num, poor wetting of carbon by molten selecting the appropriate ply-stacking ar- enhance bonding with a particular matrix, aluminum, and oxidation of the carbon are rangement and fiber content. In practice, a such as aluminum or titanium (Ref 14). The significant technical barriers to the produc- near-zero CTE has been realized, but ex- SCS-2 fiber, tailored for aluminum, has a 1 tion of these composites (Ref 19). Three pansion behavior is complicated by hyster- I~m (0.04 mil) thick carbon rich coating that processes are currently used for making esis attributed to plastic deformation occur- increases in silicon content toward its outer commercial Gr/AI MMCs: liquid metal infil- surface. tration of fiber tows (Ref 20), vacuum vapor Silicon carbide/aluminum MMCs exhibit deposition of the matrix on spread tows increased strength and stiffness as com- (Ref 21, 22), and hot press bonding of pared with unreinforced aluminum, and spread tows sandwiched between sheets of with no weight penalty. Selected properties aluminum (Ref 19). With both precursor of SCS-2/AI are given in Table 1. In contrast wires and metal-coated fibers, secondary to the base metal, the composite retains its processing such as diffusion bonding or

Temperature, o F O 200 400 600 800 1060 1200 1400 1600 1800 1,2, 250

1380 200 iC/Ti MMC SiC/AI MMC- % ~ ~- 1035 150 £ Unreinforced titanium

690 100

345 eeeeeeo,e ~ I 50 Unreinforced aluminum lee*eoIee,

0 0 -20 95 205 315 425 540 650 760 870 980 Temperature, °C Advanced aircraft stabilator spar made from Effect of temperature on tensile strength for two continuous fiber MMCs and two unreinforced metals. Fig. 4 an SCS/AI MMC. Courtesy of Textron Special- Fig. 3 Source: Ref 17 ty Materials 906 / Special-Purpose Materials

1200 I Temperatu re, o F [ -7 -240 -150 -60 32 120 210 300 I ~ ~Graphite single-crystal data -- 160 7' 300 I I o x 200 -"'-'- 1000 L_ --~)t 0 Mesophase-base carbon fiber -- 140 I -~ Start ~/ [O'p • Polyacrylonitrile-base carbon fiber E 0 .- F -- 120 800 tc~ ~- -10(~ L9 X -150 -100 -50 0 50 100 150 Temperatu re, o C -ioo ~- g o r|n Thermal expansion in the fiber direction of a E 600 k\_ H~. 6 P100 Gr/6061 AI single-ply unidirectional com- o -- 80 E posite laminate. Source: Ref 25 m 0z

400 "~, • -- 60 O <

~ -- 40

200

~ -- 20

0 0 -2 -1.8 -1.0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0 Longitudinal fiber coefficient of thermal expansion, 10-6/°C

r.'~ Carbon fiber axial modulus versus axial coefficient of thermal expansion for mesophase (pitch-base) Hb" S and polyacrylonitride-base (pan-base) graphite fibers. Source: Ref 18

ring in the matrix during thermal excursions is a designation that encompasses materials (Fig. 6). Full-scale segments of a Gr/AI with SiC particles, whiskers, nodules, space truss (Fig. 7) have been fabricated flakes, platelets, or short fibers in an alumi- and successfully tested. The advent of num matrix (see Fig. l). Several companies pitch-based graphite fibers with three times are currently involved in the development the thermal conductivity of (Ref 26) of powder metallurgy SiCa/Al, using either suggests that a high-conductivity Iow-CTE particles or whiskers as the reinforcement version of Gr/Ai can be developed for elec- phase (Ref 8). A casting technology exists tronic heat sinks and space thermal radia- for this type of MMC, and melt-produced tors. ingots can be procured in whatever form is Aluminum oxide/aluminum (AIEOa/AI) needed--extrusion billets, ingots, or rolling MMCs can be fabricated by a number of blanks--for further processing (Ref 29). Ar- methods, but liquid or semisolid-state pro- senault and Wu (Ref 30) compared powder cessing techniques are commonly used. metallurgy and melt-produced discontinu- Certain of the oxide ceramic fibers used as ous SiC/AI composites to determine if a reinforcements are inexpensive and provide correlation exists between strength and pro- the composite with improved properties as cessing type. They found that if the size, Space truss made of +12 ° lay-up graphite/ compared with those of unreinforced alumi- volume fraction, distribution of the rein- Fig. 7 aluminum tubes. Axial coefficient of thermal num alloys. For example, the composite has forcement, and bonding with the matrix are expansion is -0.072 x 10 6/°C per bay. Courtesy of an improved resistance to wear and thermal the same, then the strengths of the powder DWA Composite Specialties, Inc. fatigue deformation and a reduced coeffi- metallurgy and melt-produced MMCs are cient of thermal expansion. Continuous fi- the same. yield strengths of SiCo/AIcomposites are up ber Al203/A! MMCs are fabricated by ar- Whiskers in discontinuously reinforced to 60% greater than those of the unrein- ranging AI203 tapes in a desired orientation MMCs can be oriented in processing to forced matrix alloy. Selected properties of to make a preform, inserting the preform provide directional properties. McDanels SiCa/AI MMCs are given in Table 2. Studies into a mold, and infiltrating the preform (Ref 31) evaluated the effects of reinforce- of the elevated-temperature mechanical with molten aluminum via a vacuum assist ment type, matrix alloy, reinforcement con- properties of SiCa/A1 with either 20% whis- (Ref 27). Reinforcement-to-matrix bonding tent, and orientation on the tensile behavior ker or 25% particulate reinforcement indi- is achieved by small additions of lithium to of SiCo/AI composites made by powder cate that SiCd/A1 can be used effectively for the melt. The room-temperature properties metallurgy techniques. He concluded that long-time exposures to temperatures of at of a unidirectional A12OJA1 (FP/A1-2Li) are these composites offer a 50 to 100% in- least 200 °C (400 °F) and for short exposures given in Table 1; additional mechanical crease in elastic modulus as compared with at 260 °C (500 °F) (Ref 31, 33). properties of continuous AIzO3/AI are given unreinforced aluminum (Fig. 8). He also Discontinuous silicon carbide/aluminum in Ref 28. found that these materials have a stiffness MMCs are being developed by the aero- Discontinuous Aluminum MMCs. Discon- approximately equivalent to that of titanium space industry for use as airplane skins, tinuous silicon carbide/aluminum (SiCd/A1) but with one-third less density. Tensile and intercostal ribs, and electrical equipment Metal-Matrix Composites / 907

Table 2 Propertiesof discontinuous silicon carbide/aluminum composites 200 '~ Property SiCp/AI-4Cu-l.SMg(a) SiCw/AI-4Cu-l.$Mg(b) Reinforcement content, vol% ...... 20 15 ~" 150 .,,O ~" Longitudinal modulus, GPa (106 psi) ...... 110 (16) 108 (15.7) Transverse modulus, GPa (106 psi) ...... 105 (15) 90 (13) Longitudinal tensile strength, MPa (ksi) ...... 648 (94) 683 (99) ,..,E 100 ~ E Transverse tensile strength, MPa (ksi) ...... 641 (93) 545 (79)

Longitudinal strain to failure, % ...... 5 4.3 8 ~ -i0 ~=O Transverse strain to failure, % ...... 5 7.4 >- 50 >- 0 10 20 30 40 50 (a) 12.7 mm (0.5 in.) plate. (b) 1.8-3.2 mm (0.070-0.125 in.) sheet. Source: Advanced Composite Materials Corporation Silicon carbide content, vol% Effect of reinforcement content on the i=.'_rz$. 8 Young's modulus of a particulate-reinforced racks (Fig. 9). These composites can be agitated, partially solid aluminum alloy slur- SIC/2124-T6 AI MMC. Source: Ref 32 tailored to exhibit dimensional stability, ry has been used to produce a castable that is, resistance to microcreep, which is discontinuous MMC (Ref 36). Squeeze cast- cast; and the connecting rod was made important for precision mirror optics and ing has attracted much attention because using novel forming processes specifically inertial measurement units (Fig. 10). In the the process minimizes material and energy adapted for composites. Other possible au- electronics industry, metals such as iron- use, produces net shape components, and tomotive applications for this class of mate- nickel alloys that are now used for packag- offers a selective reinforcement capability rials include brake rotors, brake calipers, ing materials and heat sinks are candidates (Ref 37, 38). and drive shafts. for replacement by SiCd/AI. The composite A recent development in MMC fabrica- A diesel engine piston selectively has lower density, better thermal conduc- tion technology is the proprietary Lanxide reinforced with an aluminosilicate ceramic tivity (->160 W/m • K), and can be made to PRIMEXTM process, which involves pres- compact is currently in production (Ref 42). have a low coefficient of thermal expansion sureless metal infiltration into a ceramic Selective reinforcement of the all-aluminum (Fig. 11). Hybrid electronic packages made preform. This process has been used to piston with a ceramic fiber preform pro- from SiCd/AI are shown in Fig. 12. produce an AI203/A1 composite by the infil- vides wear resistance equal to that of a Most Gr/A1 MMC development work has tration of a bed of alumina particles with a piston with an iron insert, and the thermal focused on using continuous fibers as rein- molten alloy that was exposed to an oxidiz- transport is only marginally lower than that forcement. However, various solidification ing atmosphere. The matrix material of the of unreinforced aluminum. With the elimi- processing techniques have been investigat- resultant composite is composed of a mix- nation of the iron insert, piston weight is ed for use in the production of cast particle ture of the oxidation reaction product and reduced, and high-temperature strength and Gr/A1 for applications needing an inexpen- unreacted aluminum alloy (Ref 39). The thermal stability are enhanced. sive antifriction material (Ref 35). Lanxide process offers net shape capability Discontinuous A1203/AI MMCs are made (Fig. 13), and the properties of composites -Matrix Composites using short fibers, particles, or compacted produced by this method can be tailored to staple fiber preforms as reinforcements. fit specific applications. Magnesium composites are being devel- The addition of chopped A1203 fibers to an Aluminum oxide/aluminum MMCs are oped to exploit essentially the same proper- candidate materials for moving parts of au- ties as those provided by aluminum MMCs: tomotive engines, such as pistons (Ref 40), high stiffness, light weight, and low CTE. In connecting rods (Ref 16), piston pins, and practice, the choice between aluminum and various components in the cylinder head magnesium as a matrix is usually made on and valve train (Ref 41). Examples of auto- the basis of weight versus corrosion resis- motive parts fabricated from MMCs are tance. Magnesium is approximately two- shown in Fig. 14. These components were thirds as dense as aluminum, but it is more fabricated from aluminum-base composites active in a corrosive environment. Magne- with reinforcements typically of silicon car- sium has a lower thermal conductivity, bide or alumina in volume loadings ranging which is sometimes a factor in its selection. from 5 to 25%. A variety of processing Three types of magnesium MMCs are cur- techniques can be used to fabricate such rently under development: continuous fiber parts. For example, production of the com- Gr/Mg for space structures (Ref 43), short bustion bowl area of the diesel piston in- staple fiber AIzO3/Mg for automotive engine volved squeeze casting a ceramic preform components (Ref 44), and discontinuous with metal; the cylinder liner was sand mold SiC or B4C/Mg for engine components (Ref

~ 25(~, 0 x 10 ¢;

'~ 0 0 O r --o.. 0 C ""°~ ....o :_~ ,,v,- '*C~. 10! ~ -20 10 100 103 104 5 Cumulative compression time, h 0 10 20 30 40 50 Silicon carbide content, vol% Fig. 10 Microcreep behavior of 2124-T6 aluminum Lightweight aircraft equipment racks made of reinforced with 30 vol% SiC particulate. Effect of reinforcement content on the Fig. 9 particulate SiC/AI. Courtesy of DWA Com- Performance of composite indicates long-term dimen- Fig. 11 room-temperature coefficient of thermal posite Specialties, Inc. and Lockheed ASD sional stability. Source: Ref 32 expansion for a SiCp/2124-T6 AI MMC. Source: Ref 34 908 / Special-Purpose Materials

Titanium-Matrix Composites Table 3 Room-temperature properties of a unidirectional SiCc/Ti MMC Titanium was selected for use as a matrix Property SCS-6Fri-6AI-4V metal because of its good specific strength Fiber content, vol% ...... 37 at both room and moderately elevated tem- Longitudinal modulus, GPa 006 psi) ..... 221 (32) peratures and its excellent corrosion resis- Transverse modulus, GPa (106 psi) ...... 165 (24) tance. In comparison with aluminum, tita- Longitudinal strength, MPa (ksi) ...... 1447 (210) nium retains its strength at higher Transverse strength, MPa (ksi) ...... 413 (60) temperatures; it has increasingly been used Source: Ref 14 as a replacement for aluminum in aircraft and missile structures as the operating Electronic packages made from SiCd/AI (60 speeds of these items have increased from Fig. 12 vol% SiC) MMCs. Courtesy of I.anxide Cor- subsonic to supersonic. Efforts to develop Titanium MMCs with discontinuous rein- poration titanium MMCs were hampered for years forcements are in the early stages of devel- by processing problems stemming from the opment (Ref 52). This type of composite has 45) and low-expansion electronic packaging high reactivity of titanium with many rein- a moderate stiffness and elevated-tempera- materials (Ref 46). Processing methods for forcing materials. Reference 49 is a review ture strength advantage over monolithic ti- all three types parallel those used for their of titanium MMC technology. Silicon car- tanium alloys. It also offers a near-net- aluminum MMC counterparts. bide is now the accepted reinforcement; the shape manufacturing capability with the use The production of the continuous-fiber SCS-6 fiber is an example of one commer- of powder metallurgy techniques; therefore, Gr/Mg composite involves the titanium-bo- cially available type. The SCS-6 fiber has a it may be more economical to fabricate than ron coating method of making composite 140 gm (5.6 mil) diameter, a 33 t~m (1.3 mil) continuous fiber titanium MMCs. wires, physical vapor deposition of the ma- carbon core, and a carbon-rich surface (Fig. trix on fibers, or diffusion bonding of fiber- 15). Copper-Matrix Composites thin sheet sandwiches to make panels. A Although a number of processing tech- casting technology exists for Gr/Mg that niques have been evaluated for titanium Copper appears to have potential as a involves the deposition of an air-stable sili- MMCs, only high-temperature/short-time matrix metal for composites that require con dioxide coating on the fibers from an roll bonding, , and thermal conductivity and high-temperature organometallic precursor solution (Ref 47). vacuum hot pressing have been used to any strength properties superior to those of alu- Magnesium wets the coating, permitting in- substantial degree. Plasma spraying also is minum MMCs. Copper MMCs with contin- corporation of the matrix by near-net-shape employed to deposit a titanium matrix onto uous and discontinuous reinforcements are casting procedures (Ref 48). Testing of a the fibers (Ref 50). Properties for a repre- being evaluated. unidirectionally reinforced Gr/Mg MMC in sentative unidirectional SiC/Ti laminate are Continuous fiber reinforced cop- the fiber direction recorded modulus values given in Table 3. The elevated-temperature per composites were first fabricated in the in agreement with Eq 1 and a tensile strength of the SiC/Ti composite is signifi- late 1950s as research models for studying strength of 572 MPa (83 ksi) (Ref 43). A cantly greater than that of unreinforced stress-strain behavior, stress-rupture and PI00 Gr/AZ91C Mg unidirectional laminate titanium (Fig. 3). Potential applications for creep phenomena, and impact strength and was shown to have a lower CTE and smaller continuous titanium MMCs lie primarily in conductivity in MMCs (Ref 53). The com- residual strain than those of a PI00 Gr/6061 the aerospace industry and include major posites were made by liquid-phase infiltra- A1 MMC after both composites had under- aircraft structural components (Ref 51) and tion. On the basis of their high strength at gone thermal cycling between -155 °C and fan and compressor blades for advanced temperatures up to 925 °C (1700 °F), W/Cu 120 °C (-250 and 250 °F) (Ref 25). turbine engines. MMCs are now being considered for use as

(a) (bl

50 p.m

Discontinuous silicon carbide/aluminum MMC (60 vol% SiC) produced by the PRIMEXTM process. (a) Near-net-shape components fabricated from the Fig. 13 composite. (b) Composite microstructure. Courtesy of Lanxide Corporation Metal-Matrix Composites / 909

Superalloy-Matrix Composites Superalloys are commonly used for tur- bine engine hardware and, therefore, super- alloy-matrix composites were among the first candidate materials considered for up- grading turbine performance by raising component operating temperatures. Super- alloy MMCs were developed to their pre- sent state over a period of years, starting from the early 1960s. The following summa- ry is drawn from the review in Ref 59. High-temperature strength in superalloy MMCs has been achieved only through the use of refractory metal reinforcements (tungsten, molybdenum, tantalum, and nio- bium fibers with compositions specially modified for this purpose). The strongest fiber developed, a tungsten alloy, exhibited a strength of more than 2070 MPa (300 ksi) at 1095 °C (2000 °F), or more than six times the strength of the superalloy now used in the Space Shuttle main engine. Much of the early work on superalloy MMCs consisted of fiber-matrix compatibil- Automotive components fabricated from MMCs. Clockwise from left: experimental piston for a ity studies, which ultimately led to the use Fig. 14 gasoline engine, experimental cylinder liner, production piston for a heavy-duty diesel truck engine, of matrix alloys that exhibit limited reaction and experimental connecting rod. Courtesy of Ford Motor Company with the fibers. Tungsten fibers, for exam- ple, are least reactive in iron-base matrices, liner materials for the combustion chamber for thermal management of electronic com- and they can endure short exposures at walls of advanced rocket engines (Ref 54). ponents (Ref 55), satellite radiator panels temperatures up to 1195 °C (2190 °F) with Continuous Gr/Cu MMCs. Interest in con- (Ref 56), and advanced airplane structures no detectable reaction. tinuous Gr/Cu MMCs gained impetus from (Ref 57). Fabrication of superalloy MMCs is accom- the development of advanced graphite fi- In situ Composites. Discontinuous MMCs plished via solid-phase, liquid-phase, or bers. Copper has good thermal conductivi- formed by the working of mixtures of indi- deposition processing. The methods include ty, but it is heavy and has poor elevated- vidual metal phases exhibit strengths as investment casting, the use of matrix metals temperature mechanical properties. Pitch- much as 50% higher than those predicted in in thin sheet form, the use of matrix metals base graphite fibers have been developed theory from the strength of the individual in powder sheet form made by rolling pow- that have room-temperature axial thermal constituents (Ref 8). These materials are ders with an organic binder, powder metal- conductivity properties better than those of called in situ composites because the elon- lurgy techniques, slip casting of metal alloy copper (Ref 26). The addition of these fibers gated ribbon morphology of the reinforcing powders, and arc spraying. Iron-, nickel-, to copper reduces density, increases stiff- phase is developed in place by heavy me- and -base MMCs have been made, ness, raises the service temperature, and chanical working, which can consist of ex- and a wide range of properties have been provides a mechanism for tailoring the co- trusion, drawing, or rolling. This approach achieved with these MMCs, including ele- efficient of thermal expansion. One ap- has been applied to the fabrication of dis- vated-temperature tensile strength, stress- proach to the fabrication of Gr/Cu MMCs continuous refractory metal/copper com- rupture strength, creep resistance, low- and uses a plating process to envelop each posites, with niobium/copper serving as the high-cycle fatigue strength, impact strength, graphite fiber with a pure copper coating, prototype. Niobium/copper maintains high oxidation resistance, and thermal conduc- yielding MMC fibers flexible enough to be strength at temperatures up to 400 °C (750 tivity (Ref 59). The feasibility of making a woven into fabric (Ref 55). The copper- °F), and it remains stronger than high-tem- component with a complex shape was coated fibers must be hot pressed to pro- perature copper alloys and dispersion-hard- shown using a first-stage convection-cooled duce a consolidated component. Table 4 ened copper up to 600 °C (1110 °F) (Ref 58). turbine blade as a model from which a compares the thermal properties of alumi- These composites are candidates for appli- W/FeCrAIY hollow composite blade was num and copper MMCs with those of unre- cations such as electrical contacts that re- designed and fabricated. Additional infor- inforced aluminum and copper. Graphite/ quire good strength plus conductivity at mation on superalloy MMCs reinforced copper MMCs have the potential to be used moderate temperatures. with refractory metals can be found in the article "Refractory Metals and Alloys" in this Volume. Table 4 Thermal properties of unreinforced and reinforced aluminum and copper Axial coefficient of Intermetallic-Matrix Composites Reinforcement ~ Density ~ Axial thermal conductivity thermal expansion Material content, vol% ~cm 3 lb/it3 W/m - *C Btu/ft • h • OF 10-6/°C 10-6pF One disadvantage of superalloy MMCs Aluminum ...... 0 2.71 169 221 128 23.6 13.1 is their high density, which limits the po- Copper ...... 0 8.94 558 391 226 17.6 9.7 tential minimum weight of parts made from SiCp/AI ...... 40 2.91 182 128 74 12.6 7 these materials. High melting points and P120GffAI ...... 60 2.41 150 419 242 -0.32 -0.17 P120GffCu ...... 60 4.90 306 522 302 -0.07 -0.04 relatively low make intermetal- Source: Ref 34 lie-matrix composites (IMCs) viable candi- dates for lighter turbine engine materials 910 / Special-Purpose Materials

(a) (b)

t t 1 mm 100 Fm Continuous-fiber-reinforced titanium-matrix MMCs. (a) Hot-pressed SiC fibers (SCS-6, 35 vol%) in a Ti-6AI-4V matrix. Fiber thickness, 140 ixm; density, 3.86 Fig. 1 5 g/cm 3. (b) Chemical vapor deposited SiC fiber (SCS-6) showing the central carbon monofilament substrate and the carbon-rich surface. Fiber properties: thickness, 140 ixm; tensile strength, 3450 MPa (500 ksi); modulus of elasticity, 400 GPa (58 x 106 psi); density, 3.0 g/cm 3. (c) Fracture surface of a hot-pressed SCS-6 SiC/titanium MMC plate. (d) Close-up view of fractured SCS-6 fibers. Courtesy of Textron Specialty Materials, a subsidiary of Textron, Inc.

(Ref 60). An intermetallic compound dif- Aluminides of nickel, titanium, and iron mercially available intermetallic materials fers from an alloy in that the former has a have received most of the early attention as are SiC and A1203 fibers, refractory metal fixed compositional range, a long-range potential matrices for IMCs. Work on alu- fibers, and particulates such as titanium order to the arrangement of atoms within minide IMCs is concerned with developing carbide (TIC) and titanium diboride (TiB2). the lattice, and a limited number of slip methods to fabricate reproducible speci- Research is being done to find methods for systems available for plastic deformation. mens with useful properties; work is also growing advanced single-crystal fibers and At present, the IMC technology is in its being done on characterizing the interface using refractory metal aluminides and sili- infancy, and many critical issues remain to chemical reactions of fiber/matrix combina- cides as matrices (Ref 61). Key factors in be addressed. tions. Candidate reinforcements for com- selecting a reinforcement/matrix combina- Metal-Matrix Composites / 911 tion are chemical compatibility at the pro- 7. T.W. Chou, A. Kelly, and A. Okura, T. Ohkita, Studies on Ion-Plating Pro- cessing temperature and an approximate Fibre-Reinforced Metal-Matrix Com- cess for Making Carbon Fiber Rein- match of thermal expansion coefficients be- posites, Composites, Vol 16 (No. 3), forced Aluminum and Properties of the tween the material pair to minimize residual July 1985, p 187 Composites, in Proceedings of the 24th fabrication stresses. 8. D.L. Erich, Metal-Matrix Composites: National SAMPE Symposium, Vol 24, Reference 62 is an overview of the devel- Problems, Applications, and Potential Society for the Advancement of Mate- opment of nickel aluminide IMCs, and it in the P/M Industry, Int. J. Powder rial and Process Engineering, 1979, p describes the various processing techniques Metall., Vol 23 (No. 1), 1987, p 45 1417 used to make this composite. These tech- 9. J. White, T.C. Willis, I.R. Hughes, and 22. D.J. Bak, Vapor Deposition Improves niques include hot pressing, diffusion bond- R.M. Jordan, Metal Matrix Composites Metal Matrix Composites, Des. News, ing, hot extrusion, reactive sintering, and Produced by Spray Deposition, in Dis- 16 June 1986 liquid infiltration. Reference 63 presents persion Strengthened Aluminum Al- 23. R.J. Sample, R.B. Bhagat, and M.F. evidence that silicon carbide cannot serve loys, Y.W. Kim and W.M. Griffith, Amateau, High Pressure Squeeze Cast- as a reinforcement for nickel aluminide Ed., The Minerals, Metals and Materi- ing of Unidirectional Graphite Fiber Re- IMCs without the use of a diffusion barrier als Society, 1988, p 693 inforced Aluminum Matrix Composites, coating. A gas pressure liquid infiltration 10. M. Ramula and M. Taya, EDM Ma- in Cast Reinforced Metal Composites, technique has been used to produce contin- chinability of SiCw/AI Composites, J. S.G. Fishman and A.K. Dhingra, Ed., uous fiber A1203/NiA1 (Ref 64). Reference Mater. Sci., Vol 24, 1989, p 1103 ASM INTERNATIONAL, 1988, p 179 65 describes a powder cloth method for the 11. P.K. Rohatgi, N.B. Dahotre, S.C. 24. L. Rubin, "Data Base Development for fabrication of a 40 vol% continuous fiber Gopinathan, D. Alberts, and K.F. Neu- P100 Graphite Aluminum Metal Matrix SiC/Ti3A1 + Nb IMC. Data on IMC proper- sen, Micromechanism of High Speed Composites," Aerospace Report TOR- ties are very limited. Abrasive Waterjet Cutting of Cast Met- 0089 (4661-02)-1, Aerospace Corpora- The XD composites are a proprietary al Matrix Composites, in Cast Rein- tion, Sept 1989 class of discontinuous reinforcement in situ forced Metal Compos#es, S.G. Fish- 25. S.S. Tompkins and G.A. Dries, Ther- composites. The XD technology uses a man and A.K. Dhingra, Ed., ASM mal Expansion Measurements of Metal casting process to produce a fine, closely INTERNATIONAL, 1988, p 391 Matrix Composites, in Testing Technol- spaced, and uniform distribution of second- 12. F.A. Girot, J.M. 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