4 - Titanium aluminides

Abstract

For last few decades, research on molybdenum silicides-based multiphase alloys and composites has been triggered by an ever-increasing demand for high-performance materials in aerospace, automotive, and industrial components operating at temperatures in the range of 1100–1400°C. These materials are attractive because of their high melting points, ability to retain strength at elevated temperatures, and oxidation resistance observed for selected compositions. The major limitation for widespread use of these materials is their insufficient ductility and low room temperature fracture toughness. Presence of suitable amount of discontinuous ceramic reinforcement has been found to improve the fracture toughness marginally, whereas more significant toughening has been observed on reinforcing with continuous SiC fibers or due to the presence of Moss in Moss+Mo3Si+Mo5SiB2 composites (Mo-Si-B alloys). The high temperature deformation behavior of the Mo-silicides depends on composition, crystal structure, character of bonding and orientation, microstructural constitution (nature of phases present and volume fraction), as well as grain size. Considerable increase in both high temperature yield strength and creep resistance is observed on addition of ceramic reinforcement. Research on Moss-Mo3Si-Mo5SiB2 composites has shown that solid solution strengthening and presence of Mo3Si and Mo5SiB2 phases contribute to strengthening, whereas formation of borosilicate scale protects against oxidation at high temperatures. Therefore, a desirable combination of room temperature fracture toughness, high temperature strength, and oxidation resistance is possible to obtain in case of the Moss-Mo3Si-Mo5SiB2 composites.