Defects and diffusion in prototypes of shape memory alloys: Part I β₂' Au-Cd alloys

Abstract

We have reviewed the role of defects and diffusion in the Β2 Au-Cd alloys as prototypes of shape memory effect during the last fifty years. These alloys continue to be important vehicles of research for the shape memory effect involving martensitic transformations and related phenomena. Because these alloys remain ordered up to their melting points, irrespective of the large deviations from the equiatomic stoichiometry, they contain a variety of defects such as the vacancies, antisite atoms and their complexes. These defects manifest themselves in variable diffusion processes, particularly between the two sublattices. Initially, we have examined the self diffusion parameters with emphasis on the ratios of the diffusion coefficients between the two sublattices and compared them with the enthalpies of motion and formation of vacancies obtained in the quenching experiments. Diffusion in the equi-atomic stoiciometric alloy occurs primarily by the six-atom-vacancy interchanges between the two interpenetrating sublattices. The antisite bridge mechanism for diffusion, consisting of an Au antisite atom and a next nearest neighbor vacancy complex, contributes very significantly to diffusion at low temperatures in the offstoichiometric Au rich alloys. The diffusion and point defects kinetics are then related to the martensitic transformations, their aging and the rubber like behavior involving atomic rearrangements such as the symmetry conformation of the short range order. Diffusion is now recognized as a controlling force in these effects.