Solidification paths and carbide morphologies in melt-processed MoSi 2 -SiC In Situ composites
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INTRODUCTION
PROGRESS in the understanding of toughening mechanisms in the past 2 decades, coupled with demands for higher operating temperatures and a shrinking menu of materials with the requisite oxidative stability, have motivated a re-examination of MoSi2. Its potential for advanced structural applications is limited primarily by brittleness at low homologous temperatures and a relatively high coefficient of thermal expansion (CTE), which impair its performance under thermal stresses. Toughening approaches based on crack bridging by ceramic fibers[1] or ductile phases[2] have been proposed, and the latter successfully demonstrated using refractory metal particles and wires.[3–8] However, the incorporation of second phases can produce matrix cracking during processing or thermal cycling owing to CTE mismatch.[9] Maloney and Hecht[8] proposed that addition of SiC particulate could be used to lower the CTE of MoSi2 and demonstrated elimination of matrix cracking in composites reinforced with Mo wires. SiC is also attractive as a second phase because it is thermochemically compatible with MoSi2 at temperatures up to the solidus.[10] Further, it is reported to improve the flexural strength[11] and creep resistance of MoSi2[12] without deleterious effects on the oxidation resistance of the matrix.[13,14] MoSi2 composites with SiC whiskers, platelets, and equiaxed particulate have been produced by dry powder blending,[11,15] and slurry methods have been successfully DANIEL J. TILLY, formerly Graduate Student Researcher, Materials Department, University of California, is with General Electric Aircraft ¨ FVANDER, formerly Engines, Evendale, OH 45215-6301. JAN P.A. LO Assistant Research Engineer, Materials Department, University of California, is with E. Khashoggi Industries, Santa Barbara, CA 931091419. CARLOS G. LEVI, Professor, is with the Materials Department, Engineering II, University of California, Santa Barbara, CA 93106-5050. Manuscript submitted January 25, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
demonstrated for discontinuous[16] and continuous fibers.[17] Unfortunately, powder processing often results in intergranular glassy phases, which arise from surface oxidation of the particulate and degrade the creep properties of the intermetallic matrix.[5,18,19] Moreover, incorporation of SiC in amounts exceeding the percolation threshold introduces densification constraints that require the use of sintering aids,[8] which in turn may increase the content of deleterious glassy phases. Powder methods have also been used to generate MoSi2-SiC composite microstructures in situ, either by solid state displacement reactions,[20,21,22] mechanical alloying,[23] or reaction of C with SiO2 in the prealloyed MoSi2 powder.[24] Major concerns in these methods are the control of chemical homogeneity, as well as the SiC size and morphology. The available phase diagram for the Mo-Si-C system in Figure 1, indicates that MoSi2-SiC alloys can be directly synthesized by melt processing.[25] If the microstructural characteri
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