Mechanical behavior of molybdenum silicide and molybdenum silicide-titanium carbide composites.

Abstract

MoSi$\sb2$ is an intermetallic compound with potential use as a structural material in high temperature aerospace applications. The material exhibits both an excellent resistance to oxidation at high temperatures and a substantial plastic deformation capability in both single crystalline and polycrystalline form when tested in compression at temperatures in the range of 1200$\sp\circ{\rm C}$ and above. However, MoSi$\sb2$ also exhibits with decreasing temperature below 1200$\sp\circ{\rm C}$ a brittle to ductile transition such that in polycrystalline form it exhibits a very limited plastic deformation capability at 850$\sp\circ{\rm C}$ when tested in compression. The mechanical behavior of polycrystalline MoSi$\sb2$ in compression from 25$\sp\circ{\rm C}$ to 1300$\sp\circ{\rm C}$ has been characterized in this investigation. The effects of elevated temperature prestrain, which increases the initial mobile dislocation density in the material, on the mechanical behavior in the range of 750$\sp\circ{\rm C}$ to 950$\sp\circ{\rm C}$ have been examined. MoSi$\sb2$ exhibits a yield point phenomenon from 900$\sp\circ{\rm C}$ to 1000$\sp\circ{\rm C}$ which disappears when the initial mobile dislocation density is increased, which indicates that MoSi$\sb2$ is a dislocation density limited material. The use of elevated temperature prestrain on MoSi$\sb2$ results in the attainment in this material of plastic deformation at temperatures as low as 750$\sp\circ{\rm C}$, as compared to the 900$\sp\circ{\rm C}$ possible without the use of prestrain. There exists prevalent microcracking in MoSi$\sb2$ over a wide range of temperatures. The characteristics of these microcracks in relation to strain and temperature have been studied quantitatively in this investigation. The characteristics of the dislocation substructure have also been studied quantitatively, with the relative occurrences obtained for the various types of Burgers vectors, of slip planes and of screw/edge character as a function of temperature. The mechanical behavior in compression of polycrystalline MoSi$\sb2$ strengthened with particulate TiC has been studied as a function of temperature and TiC volume fraction. The use of elevated temperature prestrain increases the attainable plastic deformation in MoSi$\sb2$-TiC composites in the range of 850$\sp\circ{\rm C}$ to 950$\sp\circ{\rm C}.$.