Interfaces in MoSi 2 -SiC In Situ composites synthesized by melt processing
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INTRODUCTION
THE addition of SiC to MoSi2 has been explored as a means of enhancing strength and creep resistance,[1,2] as well as for modifying the thermal expansion coefficient of the disilicide.[3] The first goal is partly motivated by the need to counteract the deleterious effects of intergranular glassy phases commonly found in materials prepared from commercial (even nominally pure) MoSi2 powder.[4,5] In this case, the SiC may be added by blending[1] or produced in situ by solid-state reduction of the glassy oxide with C additions to the disilicide powder.[6] The second goal arises from the need to reduce thermal-mismatch-induced matrix cracking[7] in MoSi2 matrix composites toughened by ductile refractory metal wires or particles.[3,4,8–10] Understanding of the MoSi2/SiC interfaces is evidently important in the development of these materials concepts. However, little is known beyond the fact that SiC and MoSi2 are thermochemically compatible at all temperatures below ;2280 K.[11] It is also known that SiC can be grown directly from Mo-Si-C melts with the appropriate chemistry.[12,13] Depending on the alloy composition and the stage of solidification in which it evolves, the SiC may be present 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. MARC DeGRAEF, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890. 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
as primary and/or secondary particles, each exhibiting a variety of distinct morphologies. Regardless of the morphology, all the melt-grown SiC is of the b (B3) type[12] (cF8, F 43m, a 5 436 pm), while the surrounding MoSi2 matrix exhibits the C11b structure (tI6, I4/mmm, a 5 320 pm, c 5 785 pm).[14] The phase equilibria and solidification paths pertaining to the understanding of microstructure evolution in this process have been explored in a companion article.[15] In the course of the cited investigation, there were found clear orientation relationships (ORs) between the SiC and MoSi2 phases in the as-cast structure. This article discusses these ORs as well as interfacial characteristics relevant to the nucleation, growth, and mechanical behavior of these in situ composites.
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EXPERIMENTAL TECHNIQUES
Four different MoSi2-SiC alloys with the compositions listed in Table I were produced using high purity elemental components (99.97Mo, 99.995Si, and 99.9C) via nonconsumable arc melting within a purified argon atmosphere (,1027 ppm O2). Further details of the synthesis technique are given in Reference 15. Alloys 2 through 4 were
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