Selection of Intermetallic Compounds for High-Temperature Mechanical Use

Abstract

In the widespread search for structural materials for high temperatures, it is useful to first apply basic physical considerations to eliminate the least promising candidates. Certain basic quantities are indicators of some of the several properties that are demanded of hightemperature materials. Those properties are high strength and specific strength, high stiffness and specific stiffness, and low creep and thermal expansion coefficient. The basic quantities (structure-insensitive in that they vary little with processing history and consequent microstructural changes) which are chosen to measure these properties are melting temperature T_m, specific gravity ρ, and elastic moduli C_ij. Fortunately the most readily available of these structure-insensitive properties, T_m and ρ, are the most useful. As T_m increases, the general trend is for stiffness and strength to increase and thermal expansion and creep to decrease. For space and aircraft applications, knowledge of ρ allows relative values of specific stiffness and specific strength to be estimated from the ratio T_m/ρ. Data for more than 280 intermetallic compounds have been collected and subdivided by crystal class so as to allow the most promising compounds and groups of compounds to be selected. The Ll₂ structures tend to lie in the least desirable regions of T_m and ρ and therefore are not the first m candidates for maximizing high-strength at elevated temperatures. At present we have no simple structure-independent indicators of ductility and oxidation resistance--two other properties that are needed in hightemperature structural materials.