Study of γ precipitates induced by the stabilized stress-induced martensite (SSIM) training method in Cu–Zn–Al alloys

It is known that the presence of small γ precipitates in Cu–Zn–Al alloys favours the appearance of the two-way shape memory effects (TWSME), especially via the training process of the stabilization of stress-induced martensite (SSIM). In order to investigate the effect of the γ precipitates trained by the SSIM method, the morphology and strain field of the γ phase in Cu–Zn–Al alloys have been studied by means of transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). It was found that the small γ precipitates (about 10–20 nm) are ellipsoidal, with a large strain field oriented in one direction in the β austenite grains, regardless of the grain orientation, and that the γ precipitates in the martensite relaxed their strain fields and changed their shape from ellipsoidal to spherical. HREM images revealed perfect coherency between the γ precipitates and the β matrix, and the scale of the strain field was estimated to be in the range of 80–150 nm. It was suggested that the oriented ellipsoidal γ precipitates are stress-induced during SSIM treatment in the austenite and the available elastic stress field causes the thermoelastic martensitic transformation. The effect of the fine γ precipitates on TWSME was also discussed.