Intermetallic Compounds for Strong High-Temperature Materials: Status and Potential

Abstract

Materials of high strength have long been sought, but various needs - particularly those for aerospace applications - have added new dimensions: First, for use in aircraft and spacecraft parts of the least weight to do a given job are desired, i.e. materials of high specific strength (strength per unit density) or high specific stiffness (elastic modulus per unit density). Second, for inherently hot parts such as propulsion systems and their housings, strength and weight at elevated temperature are critical. The efficiency and total thrust of a jet engine increase with the peak temperature of the working fluid, and current limits are set by thermal constraints on materials. This paper reports that there are three features of the high- temperature intermetallics that give promise for better materials. First, many unexplored materials are available - about 400 binary compounds that melt above 1400{degrees}C. Second, the strong unlike-neighbor bonding that is responsible for ordering gives rise to high elastic moduli, and high moduli also imply high strength. Finally, some intermetallics have increasing strength with temperature (d{sigma}/dT {gt} 0) over a significant range. This behavior was first seen in Ni{sub 3}Al, later in other compounds of the same Ll{sub 2} structure, in B2more » and DO3 (cubic) compounds, and in tetragonal Be{sub 12}Nb(D2{sub b}), and TiAl (Ll{sub 0}). This property of d{sigma}/dT {gt} 0 is of obvious use in giving high strength at elevated temperature; it is equally important in not giving very high strength at low temperatures and thereby diminishing the chances of brittle failure at ambient temperatures.« less