Hardening behavior of nickel beryllides

Abstract

Future aerospace systems depend strongly on the availability of high temperature structural materials which are both strong and lightweight. Unfortunately, the mechanical strengths of almost all structural materials decrease as the temperature increases; an exception to this general rule is found in certain groups of long-range-order (LRO) alloys (or intermetallic compounds). A disadvantage of LRO alloys is that they are either intrinsically brittle because of their complex crystallographic structures, or they are too dense for aerospace applications. The brittleness problem has been studied heavily in recent years and some breakthroughs have been made. For example, polycrystalline Ni/sub 3/Al exhibits almost no ductility, but Ni/sub 3/Al containing a small amount (0.2 to 0.5 wt.%) of B can have a room temperature ductility of up to 40%. The high density of Ni/sub 3/Al remains an intrinsic problem. Amongst all the LRO alloys, those in the beryllide group probably have the lowest densities. In addition, it has been demonstrated that some transition beryllides exhibit attractive specific strengths at elevated temperatures, and these are of considerable interest for structural applications. The beryllide NiBe is of particular interest because it has a relatively simple ordered B2 crystal structure and exhibits a wide range of stoichiometry.more » These properties are similar to those of NiAl and CoAl. In the case of NiAl and CoAl, the alloys exhibit a hardness minimum at the equiatomic composition. The hardness increases observed in off-stoichiometric compositions have been attributed to defect structures developed in the alloys. In this paper, new data are presented on the hardness of NiBe for hypo- and hyper-stoichiometric compositions and comparisons are made with CoAl and NiAl.« less