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

The 2H martensitic phase can be induced by tensile stressing the L2(1) ordered P phase, provided the temperature is kept between M-S and M-S+25degreesC, where M-S is the temperature for the spontaneous martensitic transformation beta --> 2H. 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(S) is increased by more than 300degreesC in the latter case. For the twinned martensite the measured increase in A(S) is similar to that reported for the 18R martensite. A possibly reason for these different behaviours will be presented.