Stabilization and the two way shape memory effect (TWME) in Cu–Zn–Al polycrystals

The influence of diffusion on the two way shape memory effect (TWME) in Cu–Zn–Al polycrystals has been studied. The excess vacancy concentration which controls the diffusion velocity has been monitored by different heat treatments: (a) a slow air cooling and aging in the austenite prior to the transformation, leading to a low vacancy concentration; (b) a quench from 800°C; and (c) a quench from 300°C with an increase in the excess vacancies. The TWME has also been studied at low temperatures where diffusion is absent. All these results show that in Cu–Zn–Al polycrystals, diffusion has little or no influence on the TWME, and that instead a spectrum of martensite plate configurations is created by the stress, which then serve as nuclei for the subsequent transformation without an applied stress. The TWME obtained in the polycrystals is in all cases lower than that in single crystals after the stabilization of the stress induced martensite variant.