STRUCTURE OF NITICU SHAPE MEMORY ALLOYS

A combined electron microscopy and X-ray diffraction study has been made of NiTiCu shape memory alloys where Cu is substituted for Ni in binary β NiTi. It is shown that the high temperature phase with the CsCl structure is retained for copper contents of up to 30 wt pct, and that this phase transforms on cooling to a monoclinic martensite with similar morphology and lattice parameters to those previously reported for the binary NiTi alloy. Initial substitution of Cu for Ni produced slight changes in the lattice parameters of both the high and low temperature phases, then these remain remarkably constant on further addition of Cu. The chief effect of the Cu substitutions is to reduce the distortion needed to form martensite from the parent austenite. An attempt to explain the effect of the Cu is made in terms of a few simple alloying theories, but it is concluded that an effective treatment must consider the interplay of several material parameters. © 1979 American Society for Metals and The Metallurgical Society of AIME.