On the origin of the two way shape memory effect in Cu–Zn–Al single crystals

The two way shape memory effect (TWME) can be obtained in single crystals of Cu–Zn–Al, Cu–Al–Ni and Cu–Al–Be alloys simply by a diffusion controlled stabilization of the martensite phase. After the retransformation to the austenite on heating some nuclei are retained which are responsible for the formation of preferential martensite variants on cooling without a stress. In this paper it is attempted to contribute to the clarification of the nature of these nuclei in Cu–Zn–Al single crystals. In the single crystals with martensite start temperatures between 0 and 20°C the martensite was stress induced and aged at 60°C. Samples which were retransformed on heating were studied by transmission electron microscopy. Thin martensite plates were observed to remain even after the bulk had retransformed owing to a diffusion controlled lowering of their free energy and probably to pinning effects. They acted as nuclei which on cooling first expanded laterally and then thickened, their high density leading to a perfect TWME.