Effect of small γ-precipitates on the two-way shape memory effect in Cu–Zn–Al alloys

The γ-precipitates in Cu–Zn–Al alloys, trained by the stabilization of the stress induced martensite (SSIM) method, have been studied. After the SSIM treatment, it was found that small γ-precipitates in the β-austenite are ellipsoidal, with a large strain field oriented in the same direction; while in the martensite the γ-precipitates changed their shape from ellipsoid to spheroid, and relaxed their strain fields. In order to check whether the strain field of the γ-precipitates is capable of producing a thermoelastic martensitic transformation, an in-situ observation, by heating a sample holder in TEM, was performed. It was found that during heating over a temperature A_s, the γ-precipitates with a spherical shape in the martensite recovered their strain field and elliptical shape. During cooling, the strain field of the γ-precipitates disappeared again. It was proposed that the strain field of the γ-precipitates, trained by the SSIM method, plays an important part in the thermoelastic martensitic transformation, and presents two-way shape memory effects.