The shape strain of the (225)_F martensite transformation

A new technique of shape strain measurement is described in which the displacement vector is determined from the displacement of individual reference points by using dimensionally stable plastic replicas of the untransformed surface. The method is used to examine the effect of austenite orientation on the shape strain of bulk (225)_F plates formed in an Fe-Cr-C alloy. The new measurements indicate a significant scatter of the displacement vector, confirming earlier reports of such scatter in measurements on surface plates, but show no systematic variation with austenite orientation. It is concluded that other factors besides austenite orientation contribute to the constraints on a martensite plate and hence influence the shape strain. The new measurements indicate that the expansion normal to the habit plane is significantly greater than the volume change implied by the lattice parameters. Earlier measurements reveal the same effect when reinterpreted without assuming a dilatation in the habit plane. This discrepancy seems to imply that the transformation mechanism involves the generation of dislocations which, on moving away from the interface, displace additional atoms into the transforming volume. The observed volume discrepancy agrees well with that expected from plastic accommodation of the contraction along [11̄0]_F.