Processing and mechanical behavior of MoSi(2)-based composites.

Abstract

Recent interest in MoSi$\sb2$-based materials has been generated due to their potential as structural materials in high temperature environments, particularly for turbine engine applications. This research on the development of MoSi$\sb2$-based materials was carried out in two main areas: Initial efforts concentrated on developing processing techniques for these new materials and their composites, and subsequently effort was placed on the characterization of mechanical properties and understanding their mechanical reponse and its relationship to microstructure. MoSi$\sb2$-based composites containing different particulate reinforcements have been made by variation in powder metallurgy techniques to produce a range of grain sizes and reinforcement distribution within the composite. The mechanical behavior of these composites have been examined both in compression and in tension in the temperature range from 25$\sp\circ$C to 1400$\sp\circ$C. Monolithic MoSi$\sb2$ shows limited ductility below 1000$\sp\circ$C and fails due to localization of intergranular and transgranular microcracks. The addition of SiCp reinforcement causes refinement of the microstructure and improves composite strength and toughness by distribution of microcracks, thereby delaying fracture. At elevated temperatures MoSi$\sb2$-matrix composites deform primarily by dislocation creep with some concurrent grain boundary sliding which is accommodated by a combination of grain boundary cavitation and diffusional creep. Additionally a grain size dependence of creep strength is observed in MoSi$\sb2$ for grain sizes less than 30 $\mu$m. A creep deformation model involving grain boundary sliding accommodated by dislocation creep at grain boundary triple points is found to explain the creep behavior of these materials. Matrix microcracking during low temperature deformation and void formation during high temperature creep observed in these materials has also been investigated in the present work.