Abstract
An elongation strain as large as 5.5% was obtained in Cu–Zn–Al shape memory alloys upon heating under stress at moderate temperature. The amount and behavior of shape change strongly depended on the heating temperature, tensile direction, and amount of applied stress. The morphological examination revealed that the shape change was attributed to the formation of particular variants of bainite crystals. The shape change is thought to be produced by the martensitic process in the bainitic transformation. On the other hand, the shape change was found to occur after an incubation period and to fit with a curve by the Austin-Ricketts equation which represents the kinetics of a diffusion-controlled transformation. The activation energy obtained from the Arrhenius plot of the shape change, 8.7×104 J/mol, was almost the same as that of the diffusion of Zn atoms in the matrix crystal. Hence, the shape change is obviously controlled by a diffusion process. Nevertheless, this fact may not be contrary to the interpretation that the shape change is produced by the shear process, because the diffusion is thought to be necessary to prepare suitable surrounding conditions for the nucleation and growth of bainite crystals to be transformed by the shear process.
