Recent advances in aerospace refractory metal alloys

AbstractThe development of refractory metal alloys for aerospace applications is reviewed. Refractory metals are prime candidates for many high temperature aerospace components because of their high melting points and inherent creep resistance. The use of refractory metals is often limited, however, by poor room temperature properties, inadequate oxidation resistance at elevated temperatures, or difficulties associated with joining or welding. Recent advances in the understanding of the role of oxygen on these room temperature brittle behaviour problems, especially in molybdenum alloys, are described. The oxidation behaviour of refractory alloys at elevated temperatures in complex environments has also been re-evaluated recently and a clear understanding of the appropriate environments requiring protective coatings has emerged. Current research is emphasising the development of creep resistant niobium and tantalum alloys that are inherently oxidation resistant. The long term creep resistance of refractory alloys has been the subject of considerable research for space nuclear applications and progress in this area is discussed. The effectiveness of carbides in molybdenum and tungsten alloys designed for high temperature creep resistance has been reanalysed. Brittle behaviour after joining or welding of refractory metals has limited their applications. The origins of brittle behaviour in welded alloys are described. Examples are also given of novel solid state joining developments in tungsten and molybdenum alloys below and above their recrystallisation temperatures. Finally, novel developments are underway in particulate refractory metals, specifically encompassing rapid solidification processing. Although in its infancy, the application of rapid solidification to refractory metals is an exciting and fruitful area for further research.