The Effect of Second Phase Properties on the Compression Creep Behavior of MoSi 2 Composites
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THE EFFECT OF SECOND PHASE PROPERTIES ON THE COMPRESSION CREEP BEHAVIOR OF MoSi 2 COMPOSITES
A.K. GHOSH, A. BASU and H. KUNG Department of Materials Science & Engineering, University of Michigan, Ann Arbor, MI 48109-2136 ABSTRACT In an effort to enhance the toughness and creep strength of MoSi2, the role of various metallic and ceramic reinforcements is being examined. In this work, the effects of an oxide, a carbide and a nitride reinforcement on the compression creep behavior of MoSi2 are explored. Variations in the deformability of reinforcements and their relative strength and flaw population appear to influence the creep strength of the composites. Refinements in grain size also improve crack tolerance of the composite during deformation at 1200*C. INTRODUCTION The silicides of refractory metals such as Mo, Nb and W have great potential as matrix materials for composites with service capabilities at temperatures greater than 1000'C. Of these materials, molybdenum disilicide exhibits excellent high temperature oxidation resistance, approaching that of SiC, because of the formation of a protective silica film.[1,2] While a high melting point of 2030'C makes it an attractive choice as a high-temperature material, MoSi2 is extremely brittle at low temperatures. At -1000°C MoSi2 begins to yield and deforms by dislocation motion, similar to the behavior of metals.[3] At an even higher temperature (-1200*C) the yield strength of MoSi2 drops further and its creep resistance is not extremely high.[4] Consequently, attempts are being made to improve the mechanical properties of MoSi2 by synthesizing composites with hard ceramic phases as well as ductile metallic phases.[5-8] It is believed that by variations in the strength, deformability, and the coefficient of the thermal expansion (CTE) of particulate reinforcements, it would be possible to alter composite strength and interfacial stress. Consequently, the toughness of the composite may also be influenced. Additionally, reinforcements, when distributed in a fine scale through the microstructure, can change the grain size of the matrix material. The objective of this study was to examine the effect of reinforcement properties and the microstructural scale in MoSi2 composites reinforced by oxide, nitride and carbide reinforcements, and their effects on creep behavior. This report describes progress to date, while a comprehensive view will be presented later. Material Selection SiC and Si 3 N 4 particulate reinforcements were selected because of their high strength (or hardness) and compatibility with silicides. Since MoSi2 is known to contain small amounts of Si0 2 impurities, a CaO reinforcement was selected partly because of its ability to clean the matrix of this impurity by compound formation with Si0 2 , and secondly, because of its easier deformability. Table I lists the melting points, room temperature hardness and CTE of the matrix and the various reinforcement phases. In addition to the strength issue, the difference in CTE was believed to induce tensile vs.
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