Strengthening and toughening in refractory metal alloys

The development and application of refractory metal alloys for high-temperature aerospace structures is reviewed. Refractory metals are prime candidates for many high-temperature aerospace components because of their high melting points, fabricability, 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, welding, or forming. Materials selection criteria for high-temperature structures are reviewed with an emphasis placed upon the distinctions between refractory metals and competing high-temperature materials - including carbon, ceramics, and intermetallics. Each class of materials has inherent limitations which govern their application. In some cases, materials limitations are being solved through the use of engineered materials hybrids such as ceramic-coated refractory metals or ceramics toughened with a ductile refractory metal phase. For many applications, use of ceramics and intermetallics is limited by their technological immaturity. For example, current understanding and application of molybdenum-based materials is based upon nearly 40 years of application experience in high-temperature structures. As an example of a materials development and qualification program, a brief synopsis of molybdenum alloy development is presented. Topics surveyed include recent advances in the understanding of the role of oxygen on the room temperature brittle behavior problem in molybdenum alloys.