Phase Stability and Microstructure Control in High Temperature (Mo,Nb)-Si-B Alloys

Abstract

Abstract : The challenges of a high temperature environment (T>14O0 C) impose severe material performance constraints in terms of melting point, oxidation resistance and structural functionality. A number of ceramic materials, intermetallic compounds and refractory metals with high melting temperature are available as material choices. However, in a single component, single phase form, these materials rarely satisfy all the above requirements because of the brittleness of ceramic materials and intermetallic compounds at low temperatures and the oxidation problems and poor creep resistance of refractory metals at high temperatures. In this respect the evolutionary development of high temperature alloys over the past 4-5 decades represents a remarkable achievement and provides important lessons to guide future materials design efforts. One clear message is the importance of multiphase microstructures and the capability to control phase fractions and morphologies within the overall structure 87Sto,87Ros,90Dys. The flexibility in microstructure control has been shown to be critical in tailoring alloy performance in order to satisfy a number of mechanical property requirements that sometimes present conflicting demands 92Dim,9lKim. Besides the essential structural requirements, elevated temperatures also often involve aggressive environments that require a material to display an inherent oxidation protection that can be enhanced further by coating 79Mai.