METALLURGICAL AND MATERIALS TRANSACTIONS A
ABSTRACTS
Volume 28A, No. 9, September 1997

This Month Featuring: Alloy Phases; Transformations; Mechanical Behavior; Solidification; Material Processing; Composite Materials. View September 1997 Contents.

ALLOY PHASES

A Thermodynamic Description of the Al-Mg-Zn System
H. LIANG, S.-L. CHEN, and Y.A. CHANG
A thermodynamic description of the Al-Mg-Zn system was developed based on critically evaluated experimental data. All binary intermetallic phases are assumed to have negligible ternary solubility except for MgZn2. Three different thermodynamic models are applied to three different types of phases in this system, i.e., disordered solution phases, stoichiometric compounds, and semistoichiometric phases. The model parameters are optimized based on the thermodynamic descriptions of the constituent binaries and experimental phase equilibrium and thermodynamic data available in the literature. The good agreement obtained between several calculated isopleths and thermodynamic values of the liquid phase and experimental data shows that the current description of this system is reasonable. The calculated phase equilibria in the Al-rich corner are believed to be reliable for practical applications, while those away from the Al-rich region are subjected to large uncertainty. Additional experimental investigations are needed to firmly establish the phase equilibrium of this system over wide ranges of composition and temperature.

Transformations in 2 + Titanium Aluminide Alloys Containing Molybdenum: Part I. Solidification Behavior
A.K. SINGH and D. BANERJEE
The solidification structures of 12 alloys in the Ti-Al-Mo system with Al contents ranging from 44 to 50 at. pct and Mo contents ranging from 2 to 6 at. pct have been characterized metallographically and, for composition gradients, by electron probe microanalysis. All alloys solidify dendritically through sequences that fall into four distinct categories: alloys that solidify completely into the phase field, alloys that solidify first into an L + phase field and finally into an + phase field through an L + + region, alloys that solidify first into a L + phase field and finally form + + structures, and alloys that solidify into a L + phase field. Postsolidification transformations occur as consequence of transitions from the high-temperature , + , and + phase fields to low-temperature + + or + region. A variety of phase distributions result, such as Widmanstätten 2 + B2 structures from the + reaction followed by ordering, lamellar 2 + structures from the + reaction followed by ordering of the phase, eutectoid B2 + structures from the B2 + reaction, and platelike structures from the + reaction. The former are observed in Al lean alloys, while the latter are present in Mo rich alloys.

Transformations in 2 + Titanium Aluminide Alloys Containing Molybdenum: Part II. Heat Treatment
A.K. SINGH and D. BANERJEE
The response of as-cast structures of 12 alloys in the Ti-Al-Mo system containing 44 to 50 at. pct Al and 2 to 6 at. pct Mo to simple single step heat treatments in the temperature range 1373 to 1673 K is described. The microsegregation patterns present in the cast structure persist to a large extent after heat treatment, especially below 1673 K. However, tentative conclusions regarding phase equilibria in this temperature and composition range are drawn from the results. High-temperature equilibria are dominated by the , + , and + phase fields, while the + phase field dominates equilibrium below 1473 K. Three major types of transformation behavior are observed: a massive to transformation, which occurs within the phase on quenching from 1673 and 1573 K in alloys centered around the 48 pct Al composition; a eutectoid transformation from to B2 + mixtures, which occurs at 1473 K and below in alloys centered around the 48Al-4Mo and 46Al-6Mo compositions; direct precipitation in , which occurs primarily in the 44Al-6Mo composition at 1273 K and below; and finally growth of lamellae in + lamellar structures with B2 precipitation on lamellar interfaces, which occurs over a broad range of alloy compositions and temperatures.

TRANSFORMATIONS

A Variant Selection Model for Predicting the Transformation Texture of Deformed Austenite
M.P. BUTRÓN-GUILLÉN, C.S. DA COSTA VIANA, and J.J. JONAS
The occurrence of variant selection during the transformation of deformed austenite is examined, together with its effect on the product texture. A new prediction method is proposed based on the morphology of the austenite grains, on slip activity, and on the residual stresses remaining in the material after rolling. The aspect ratio of pancaked grains is demonstrated to play an important role in favoring selection of the transformed copper ({311}<011> and {211}<011>) components. The extent of shear on active slip planes during prior rolling is shown to promote the formation of the transformed brass ({332}<113> and {211}<113>) components. Finally, the residual stresses remaining in the material after rolling play an essential part by preventing growth of the {110}<110> and {100}<uvw> orientations selected by the grain shape and slip activity rules. With the aid of these three variant selection criteria combined, it is possible to reproduce all the features of the transformation textures observed experimentally. The criteria also explain why the intensities of the transformed copper components are sensitive to the pancaking strain, while those of the transformed brass are a function of the cooling rate employed after hot rolling.

Precipitation Behavior in Ultra-Low-Carbon Steels Containing Titanium and Niobium
M. HUA, C.I. GARCIA and A.J. DeARDO
This work revealed the basic mechanism for the stabilization of carbon in ultra-low-carbon (ULC) steels that contain moderate S (0.004 to 0.010 wt pct), adequate Ti (0.060 to 0.080), and low Mn (0.20). During cooling through the austenitic region to the ferritic, the initially formed sulfide particles (TiS) undergo an in situ transformation into carbosulfides (H-Ti4C2S2) by absorbing C and Ti. The transformation from TiS to H may be considered as a hybrid of shear and diffusion, i.e., faulted Ti8S9 (9R) + 10[Ti] + 9[C] 41/2Ti4C2S2 (H). At low temperature (930°C), the stabilization process continues through epitaxial growth of carbides on H phase, i.e., [M] + x[C] + H epitaxial MCx (on H). This mechanism differs from the traditional view of stabilization, where the carbon is removed from solution by the formation of free-standing or independently nucleated H and/or MCN precipitates. While these two forms of carbon stabilization are now well known, this article presents a method of predicting which mechanism of stabilization will be operative in a given steel based on its bulk composition. Implications bearing upon new ULC steel design, considering the role of S, will be discussed.

Communication: Element Partitioning during Coarsening of (-') Ni-Al-Mo Alloys
M. FÄHRMANN, E. FÄHRMANN, T.M. POLLOCK, and W.C. JOHNSON

MECHANICAL BEHAVIOR

Strain Hardening Regimes and Microstructural Evolution during Large Strain Compression of Low Stacking Fault Energy Fcc Alloys That Form Deformation Twins
SIROUS ASGARI, EHAB EL-DANAF, SURYA R. KALIDINDI, and ROGER D. DOHERTY
Constant true strain rate simple compression tests were conducted on annealed, polycrystalline samples of -brass and MP35N, and the evolution of the true stress ()-true strain () response was documented. From these data, the strain hardening rate was numerically computed, normalized with shear modulus (G), and plotted against both ( - 0)/G (0 being the initial yield strength of the alloy) and . Such normalized plots for -brass and MP35N were found to be almost identical to each other, and revealed four distinct stages of strain hardening: stage A, with a steadily decreasing strain hardening rate up to a true strain of about -0.08; stage B, with an almost constant strain hardening rate up to a true strain of about -0.2; stage C, with a steadily decreasing strain hardening rate up to a true strain of about -0.55; and a final stage D, again with an almost constant strain hardening rate. Optical microscopy and transmission electron microscopy (TEM) were performed on deformed samples. The results suggested that stage A corresponded to stage III strain hardening (dynamic recovery) of higher stacking fault energy (SFE) fcc metals such as copper. The onset of stage B correlated with the first observation of deformation twins in the microstructure. Further straining in stage B was found to produce clusters of parallel twins in an increasing number of grains. Stage C correlated with the development of severe inhomogeneity of deformation within most grains, and with the development of significant misorientation between the twin/matrix interface and the {111} plane in the matrix of the grain, i.e., the matrix/twin interface lost coherency with continued deformation. Stage D correlated with extensive formation of secondary twins that resulted in twin intersections in many grains. Early in stage D, some strain localization in the form of shear bands was observed. Although formation of these shear bands had no detectable effect on the macroscopic strain hardening rate, it did correlate with a marked change in texture evolution. Based on these experimental observations, we have developed and presented a physical description of the microstructural phenomena responsible for the various strain hardening stages observed in low SFE fcc alloys.

Fracture Toughness and Fatigue Crack Growth in Rapidly Quenched Nb-Cr-Ti In Situ Composites
K.S. CHAN, D.L. DAVIDSON, and D.L. ANTON
In situ composites based on the Nb-Cr-Ti ternary system were processed by rapid solidification in order to reduce the size of the reinforcing intermetallic phase. Two-phase microstructures with small Cr2Nb particles in a Nb(Cr, Ti) solid solution alloy matrix were produced for several compositions that previous work showed to produce high toughness composites in cast materials. The fracture and fatigue behaviors of these composites were characterized at ambient temperature. The results indicate that the fracture resistance increases with a decreasing volume of Cr2Nb particles. Fracture toughnesses of the rapidly solidified materials with their smaller particle sizes were lower than for conventionally processed composites with larger particles of the intermetallic compound. The fatigue crack growth rate curves exhibit steep slopes and a low critical stress intensity factor at fracture. The lack of fracture and fatigue resistance is attributed to the contiguity of the intermetallic particles and the absence of plastic flow in the Nb solid solution matrix. The matrix alloy appears to be embrittled by (1) the rapid solidification processing that prevented plastic relaxation of residual stresses, (2) a high oxygen content, and (3) the constraint caused by the hard Cr2Nb particles.

Thermal Cycling Behavior of As-Quenched and Aged Ti-6Al-4V Alloy
HONGBIN GENG, SHIYU HE and TINGQUAN LEI
Thermal cycling tests between 77 and 623 K were performed on Ti-6Al-4V alloy; the tensile properties were evaluated, and transmission electron microscopy (TEM) microstructural analysis was performed both before and after thermal cycling. Thermal cycling (1000 cycles) promptly increases the strength of the as-quenched alloy, induces a slight decrease in strength for the near-peak-aged alloy, and makes no change for that of the overaged alloy. The elongation of the alloy in all heat- treated conditions decreases after 1000 thermal cycles. The loss of fracture elongation of the as- quenched alloy is the largest, but the residual ductility is the highest. The loss of fracture elongation for the near-peak-aged alloy is lower, and the residual plasticity is higher than those for the overaged alloy.

Elevated Temperature Fracture Toughness of Al-Cu-Mg-Ag Sheet: Characterization and Modeling
MICHAEL J. HAYNES and RICHARD P. GANGLOFF
The plane-strain initiation fracture toughness (KJICi) and plane-stress crack growth resistance of two Al-Cu-Mg-Ag alloy sheets are characterized as a function of temperature by a J-integral method. For AA2519 + Mg + Ag, KJICi decreases from 32.5 MPa at 25°C to 28.5 MPa at 175°C, while KJICi for a lower Cu variant increases from 34.2 MPa at 25°C to 36.0 MPa at 150°C. Crack-tip damage in AA2519 + Mg + Ag evolves by nucleation and growth of voids from large undissolved Al2Cu particles, but fracture resistance is controlled by void sheeting coalescence associated with dispersoids. Quantitative fractography, three-dimensional (3-D) reconstruction of fracture surfaces, and metallographic crack profiles indicate that void sheeting is retarded as temperature increases from 25°C to 150°C, consistent with a rising fracture resistance. Primary microvoids nucleate from smaller constituent particles in the low Cu alloy, and fracture strain increases. A strain- controlled micromechanical model accurately predicts KJICi as a function of temperature, but includes a critical distance parameter (l*) that is not definable a priori. Nearly constant initiation toughness for AA2519 + Mg + Ag is due to rising fracture strain with temperature, which balances the effects of decreasing flow strength, work hardening, and elastic modulus on the crack-tip strain distribution. Ambient temperature toughnesses of the low Cu variant are comparable to those of AA2519 + Mg + Ag, despite increased fracture strain, because of reduced constituent spacing and l*.

A Thermodynamic Analysis of the Empirical Power Relationships for Creep Rate and Rupture Time
A.J. KRASOWSKY and L. TOTH
A correlation between the empirical parameters describing power law of creep and the creep-rupture properties of materials is shown based on the thermodynamics of the steady-state creep or creep damage. Analytical relationships are suggested to explain this correlation, assuming that the activation energy depends on the logarithm of the stress. This approach relates the cohesive energy of material to the steady-state creep and rupture time data. As a result, the origin of the Monkman-Grant rule becomes clear. A simple formula is presented to estimate a priori the slope of the logarithm of rupture time vs the logarithm of rupture stress at a given temperature, using the melting temperature of the material.

High-Temperature Tensile Ductility in WC-Co Cemented Carbides
I.C. LEE and T. SAKUMA
High-temperature tensile deformation in WC-Co was investigated at temperatures between 1150°C and 1250°C. The flow stress is sensitive to temperature, strain rate, volume fraction of binder, and the addition of other carbides. The stress-strain rate relationship is divided into three regions at each temperature as in superplastic metals. A large tensile elongation over 100 pct was first obtained in WC-6Co and WC-13Co (wt pct) at temperatures of 1200°C. Contrary to superplastic metals, the largest tensile elongation is not obtained in region II but on the border of regions I and II. The failure mode changes from necking in region I to sharp cracking in region II.

Compressive Creep Behavior of Spray-Formed Gamma Titanium Aluminide
B. LI, J. WOLFENSTINE, J.C. EARTHMAN, and E.J. LAVERNIA
The creep behavior of spray-formed -TiAl with a fine, equiaxed fully lamellar (FL) microstructure was studied in a temperature-stress regime of 780°C to 850°C and 180 to 320 MPa. An apparent stress exponent of 4.3 and an activation energy of 342 kJ/mol were observed in the high-temperature high-stress regime. Compared with the FL -TiAl which was obtained through conventional casting + heat treatment processes, the spray-formed -TiAl exhibited higher creep resistance. The higher creep resistance observed in the present study was discussed in light of the interstitial level, the chemical composition, the grain size, and the interlocking of lamellae at the grain boundary, which in turn may be a function of interlamellar spacing and the step height of the serrated grain boundaries. It was suggested that the small interlamellar spacing and possibly larger step height may contribute to the higher creep resistance observed in the present study.

Strengthening Effects in AC8A/Al2O3 Short-Fiber Composites as a Function of Temperature and Strain Rate
C.S. LIAUO and J.C. HUANG
The strengthening aspect of AC8A/Al2O3 short-fiber composites is examined under the framework of a modified shear lag model over the range of 298 to 723 K and 10-3 to 103 s-1. The strength sustained by the composite at high temperatures is much higher than for the alloy. As the strain rate rises, the portion of strength that the composite or alloy can sustain is drastically increased. Also, the composite shows a lower strain rate sensitivity, likely to be caused by the higher tendency of fiber damage and local stress concentration. As the temperature is higher, the strain rate sensitivity becomes considerably higher. The composite strength can be theoretically calculated using the Friend and Modified Tsai-Hill formulas. By closer examination, the experimental data agree better with the prediction of the Modified Tsai-Hill 2D (min) or 2D (max) model. Nevertheless, all of the predictions give quite reasonable strength values as well as the trend as a function of temperature and strain rate. Overall, test temperature governs the strengthening efficiency. High temperatures give the best efficiency. Influence from strain rate exists, but is less significant. It is observed that the strengthening effect is more pronounced when the matrix strength is lower, such as at higher temperatures and lower strain rates. Calculations from the critical fiber volume fraction Vcrit and load transfer coefficient both show an increasing trend with increasing temperature and decreasing strain rate, also suggesting that the strengthening effect by adding short fibers into the matrix is more apparent and efficient at high temperatures and low strain rates.

Wear Characteristics of TiNi Shape Memory Alloys
H.C. LIN, H.M. LIAO, J.L. HE, K.C. CHEN, and K.M. LIN
The wear characteristics of TiNi shape memory alloys against Cr-steel have been studied. Experimental results indicate that the Ti49Ni51 alloy can exhibit a better wear resistance than Ti50Ni50 alloy due to their higher hardness and pseudoelastic behaviors. Four main mechanisms, adhesion, abrasion, surface fatigue, and brinelling, are found to have important contributions to the wear characteristics of TiNi alloys. The weight loss increases with increasing wear load and sliding distance but decreases with increasing sliding speed. The contact area during sliding wear will be increased due to the variant accommodation and/or pseudoelasticity and, hence, will reduce the average compressive stress and wear damage. Variant accommodation and/or pseudoelasticity can also stabilize the crack tips and hinder crack propagation, hence improving the wear characteristics of TiNi alloys.

Microtexture and Grain Boundary Evolution during Microstructural Refinement Processes in SUPRAL 2004
TERRY R. McNELLEY and MICHAEL E. McMAHON
Electron backscatter pattern (EBSP) analysis of as-processed, processed and annealed, and superplastically deformed specimens of commercially processed SUPRAL 2004 material has been employed to reveal the boundary misorientation distribution and evolution. Earlier studies using X-ray diffraction (XRD) and transmission electron microscopy on this alloy have attributed the transition to microstructures capable of supporting extensive superplastic flow to continuous recrystallization occurring early in the deformation process. The micro- and mesotextural data of the present study show that the deformation texture evident in the as-processed material persists without the formation of recrystallization texture components and remains up to the apparent onset of the grain boundary sliding (GBS) regime. Comparison of the correlated and uncorrelated boundary misorientation data illustrates that the development of boundaries misoriented by ~5 to 15 deg is not random in nature. There is no evidence of recrystallization involving the formation and migration of high-angle boundaries during the refinement process. Microtextural and boundary data from this study provide evidence that the microstructural transition enabling superplastic mechanical behavior of SUPRAL 2004 may be described by a recovery-dominated, continuous process involving the development of moderately misoriented boundaries and leading to a refined microstructure with a boundary distribution of low interfacial energy character.

Communication: Formation of the Al-Rich Phase on Grain Boundary and the Creep Damage Mechanism in Directionally Solidified Ni-Base Superalloy
S. KOMAZAKI and T. SHOJI

SOLIDIFICATION

Solidification Paths and Carbide Morphologies in Melt-Processed MoSi2-SiC In Situ Composites
DANIEL J. TILLY, JAN P.A. LÖFVANDER, and CARLOS G. LEVI
The present investigation was undertaken to elucidate the microstructural evolution of MoSi2-SiC in situ composites produced by melt processing. An assessment of the existing liquidus projection was performed by a combination of thermodynamic modeling, analysis of solidification microstructures, and measurements of the thermal history during solidification. Results show that the quasibinary MoSi2-SiC eutectic occurs at ~2 at. pct C and 2283 K, rather than 8 at. pct C and 2173 K, as previously reported. The ensuing L + MoSi2 + SiC monovariant line runs almost parallel to the Si- MoSi2 binary and terminates at a ternary L Si + MoSi2 + SiC eutectic calculated at 1.5Mo- 0.84C (at. pct) and ~1670 K. The maximum amount of SiC that may be produced by solidification along the quasibinary isopleth is ~37 vol pct, of which ~35 vol pct grows as primary. Analysis of solidification microstructures shows SiC grows with the cubic polytype structure (B3), while MoSi2 grows with the tetragonal C11b structure. Primary SiC may grow as equiaxed particles, platelets, and hopper crystals. Coupled growth with MoSi2 leads to SiC in the shape of thin ribbons, sheets, and needles. The facets of the SiC crystals were identified to be of the {111} and {002} type, in agreement with the periodic bond chain analysis. The predominant platelike morphology was shown to develop due to a re-entrant twin mechanism similar to that observed in Si and Ge.

Interfaces in MoSi2-SiC In Situ Composites Synthesized by Melt Processing
DANIEL J. TILLY, JAN P.A. LÖFVANDER, MARC DEGRAEF, and CARLOS G. LEVI
Interfaces between the primary -SiC and the surrounding MoSi2 matrix in melt-synthesized in situ composites have been investigated, with emphasis on the chemistry and crystallographic relationships developed during solidification. Primary SiC growth occurs with {002} and {111} facets, both of which are found to template the subsequent nucleation and epitaxial growth of the MoSi2 matrix. Eight independent orientation relationships (ORs) were identified, involving the following combinations of planes:

{002}SiC || (001)MoSi2 (3 rotational variants), or {101)MoSi2

{111}SiC || (001)MoSi2, or {100)MoSi2 (2 rotational variants), or {110)MoSi2

The interfacial relationships were rationalized using coincident site lattice arguments as well as energetic simulations based on the Grey-Bohr algorithm. The latter analysis suggests that the multiplicity of relationships arises from local effects associated with the size and shape of the adsorbate layers preceding the formation of the MoSi2 nuclei. An amorphous carbon layer, 2- to 5-nm thick, was detected at all interfaces and some of the matrix grain boundaries. This interphase is believed to evolve by solid-state precipitation of C during postsolidification cooling and is, in principle, metastable. The C interphase enables easy debonding and thus may have important implications for the mechanical performance of materials involving SiC/MoSi2 constituents.

MATERIALS PROCESSING

High Strain Rate Torsional Behavior of an Ultrahigh Carbon Steel (1.8 Pct C-1.6 Pct Al) at Elevated Temperature
MANUEL CARSI, FELIX PEÑALBA, OSCAR A. RUANO, and OLEG D. SHERBY
The elevated temperature mechanical properties of a 1.8 pct C-1.6 pct Al ultrahigh carbon steel (UHCS-1.8C-1.6Al) is described in the temperature range from 750°C to 1150°C and in the strain rate range from 0.2 to 26 s-1. A torsion test apparatus was used which permitted rapid cooling (50°C per second) immediately after fracture to establish microstructure-processing-property relations. The strength-strain rate relation of the UHCS-1.8C-1.6A1 material correlates well with a lattice diffusion-controlled dislocation creep process. The present data, together with other high carbon steels data, predict that austenite containing a high amount of carbon in solution has a high stacking fault energy. The ductility of the UHCS-1.8C-1.6A1 is maximum at 1050°C. This indicates that successful deep die forging and other mechanical processing operations at high strain rates should be performed at this temperature. The microstructure of the deformed samples consisted of a matrix of pearlite with some undissolved spherical carbides when rapidly cooled from 900°C and 1050°C and of a thin network of proeutectoid carbides when cooled from 1150°C. High hardnesses in the range of Rockwell C 42 to 50 are obtained for such structures.

Optimization of Microstructure Development during Hot Working Using Control Theory
JAMES C. MALAS III, W. GARTH FRAZIER, S. VENUGOPAL, ENRIQUE A. MEDINA, STEVEN MEDEIROS, RAGHAVAN SRINIVASAN, R. DENNIS IRWIN, W. M. MULLINS, AND ANIL CHAUDHARY
A new approach for controlling microstructure development during hot working processes is proposed. This approach is based on optimal control theory and involves state-space type models for describing the material behavior and the mechanics of the process. The effect of process control parameters such as strain, strain rate, and temperature on important microstructural features can be systematically formulated and then solved as an optimal control problem. This method has been applied to the optimization of grain size and process parameters such as die geometry and ram velocity during the extrusion of plain carbon steel. Experimental results of this investigation show good agreement with those predicted in the design stage.

Communication: Nitridation of Ti-Al Alloys: A Thermodynamic Approach
R. SCHMID-FETZER and K. ZENG

COMPOSITE MATERIALS

In Situ Ceramic Particle-Reinforced Aluminum Matrix Composites Fabricated by Reaction Pressing in the TiO2(Ti)-Al-B (B2O3) Systems
Z.Y. MA and S.C. TJONG
Particulate TiB2 reinforced aluminum-based metal matrix composites (MMCs) were successfully fabricated by means of the reaction processing method. TiB2 particulates were formed in situ through the reaction of Ti and B in Ti-Al-B, TiO2 and B in TiO2-Al-B, and TiO2 and B2O3 in TiO2-Al-B2O3 systems. The results showed that in situ TiB2 particulates formed in the Ti-Al-B system had a size of 5 µm and they exhibited block and rodlike structures. Moreover, coarse Al3Ti blocks several tens of micrometers in size were also formed simultaneously. On the other hand, equiaxed Al2O3 and TiB2 particulates with a size of less than 2 µm were formed in situ in the TiO2-Al-B and TiO2-Al- B2O3 systems. The Al3Ti phase was completely eliminated in the TiO2-Al-B system with increasing B content. Tensile tests revealed that the Al2O3 · TiB;i2/Al composite fabricated from the TiO;i2-Al-B system exhibits excellent mechanical properties. The yield strength of the Al2O3 · TiB2/Al composite appeared to increase with increasing TiB2 content. The yield strength of the Al2O3 · TiB2/Al composite could be further increased by introducing CuO into the TiO2-Al-B system. Such an increment in mechanical strength arose from the strengthening effect caused by the Al2Cu precipitates. The incorporation of CuO had no effect on the in situ reaction process of the TiO2-Al-B system. Finally, the effect of SiC addition on the microstructure and mechanical properties of the composites fabricated from the TiO2-Al-B and TiO2-Al-B-CuO systems was also investigated.

Communication: Wear Behavior of As-Cast ZnAl27/SiC Particulate Metal-Matrix Composites under Lubricated Sliding Condition
S.C. TJONG and F. CHEN

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