Stabilisation of the 2H martensite in Cu-Zn-AI single crystals with e/a = 1.53

The 2H martensitic phase can be induced by tensile stressing the L2$_1$; ordered $\beta$ phase, provided the temperature is kept between M$_{\rm s}$ and M$_{\rm s}$+25°C, where M$_{\rm s}$ is the temperature for the spontaneous martensitic transformation $\beta \to 2$H. The resulting microstructure depends strongly on the orientation of the tensile axis : If its direction is close to the line joining the [1 0 0]$_\beta$ and the [1 1 1]$_\beta$ directions in the stereographic projection, a twinned martensite is obtained. For other tensile axis directions, a single variant martensite is induced on loading. When diffusional processes are permitted to occur in the 2H phase, by raising the temperature above room temperature, the retransformation temperature A$_{\rm s}$ is increased by more than 300°C in the latter case. For the twinned martensite the measured increase in As is similar to that reported for the 18R martensite. A possibly reason for these different behaviours will be presented