Creep and fatigue properties of high temperature silicides and their composites

A review of creep and fatigue behavior of high temperature silicides and their composites is presented along with new results pertaining to titanium silicides and functionally graded materials. The emphasis was placed on the molybdenum disilicides and their composites in comparison to other silicides and high temperature ceramics. It was shown that the grain size effects on creep are significant in these materials. The effects were present not only in the Newtonian creep but persists even in the power-law creep, with grain size exponents of the order of 4.2. Factors that influenced the grain size effects were examined. This anomalous behavior, which appeared to be exhibited by some stoichiometric intermetallics, could not be explained by the currently known theories of grain boundary creep. Effects of alloying and reinforcement on creep rates were also discussed. The creep rates in other high temperature silicides were compared. It was shown that molybdenum disilicide with silicon nitride has a potential for high temperature applications in terms of its creep and oxidation resistance, although its creep resistance appears to be much less than that of an advanced Si3N4, such as AS800. Published results on fatigue crack growth in MoSi2 and its composites were analyzed using the recently developed Unified Approach for Fatigue. (C) 1999 Elsevier Science S.A. All rights reserved.