Creep Behavior of SiC-Reinforced XD TM MoSi 2 Composite
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CREEP BEHAVIOR OF SiC-REINFORCED XDTM MoSi 2 COMPOSITE M. SUZUKI', S. R. NUr'I'' AND R. M. AIKIN, Jr.2 t Brown University, Division of Engineering, Providence, RI 02912 2 Case Western Reserve University, Department of Materials Science and Engineering, Cleveland, OH 44108 ABSTRACT The compressive creep behavior of MoSi2 reinforced with 30v/o SiC fabricated by in situ XDTM process was investigated at 1050"C-1300"C in anaerobic and aerobic test ambients. Creep experiments performed with the composite in dry nitrogen and in air showed power-law type constitutive behavior and a stress exponent of -3.5. Creep deformation occurred by dislocation glide accompanied by cavitation, and the apparent activation energy for creep at 11l00"C-1300"C was bi-vaiued with a threshold temperature of -1 170"C. Microstrucural observations by TEM indicated that the rate-controlling process changed from dislocation glide to dislocation climb at higher temperature, corresponding to the change in activation energy. Creep damage occurred by cavitation at SiC-matrix interfaces and at grain boundaries within polycrystalline SiC particles. This process was apparently facilitated by the accumulation of glassy phase at these sites during creep. Creep experiments in air showed there was no appreciable atmospheric effect on the response of the composite, while an increased strain rate was observed in the base alloy due to an increase in glass phase resulting from thermal oxidation. INTRODUCTION MoSi2 is a candidate material for high-temperature structural applications in oxidizing ambients (>1200"C). The compound has a high melting point of 2030°C and exhibits excellent oxidation resistance, which arises from the formation of a protective SiO)2 film. Several attempts have been made to fabricate MoSi2-based composites with the goal of improving strength and creep resistance at high temperatures, and enhancing fracture toughness and ductility at room temperature. However, there have been relatively few studies of the relation between creep properties and microstructure of these composites [1, 2]. Recently, a novel process for in situ fabrication of composites was developed at Martin Marietta Laboratories. The process, designated XDTM, can be used to fabricate SiC-reinforced MoSi2 composites with substantially less glass phase than silicides produced by conventional powder-processing. This SiC-reinforced MoSi2 composite showed improved compressive strength (2x) compared to the MoSi2 base alloy [3]. Atmospheric effects on the high-temperature creep behavior are often dramatic for carbidereinforced composites. Creep rates in aerobic and anaerobic test ambients can differ by factors of 5-I0 because of thermal oxidation and the associated reaction products [4]. However, the creep behavior of MoSi2 may show only minor atmospheric effects because of the excellent oxidation resistance described above. In this report, we present results showing atmospheric effects on the compressive creep behavior of SiC-reinforced XDTM MoSi 2 and the base alloy. Microstructur
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