MEASUREMENT OF SHAPE STRAIN FOR (225) AND (259) MARTENSITIC TRANSFORMATIONS

Measurements of the shape strain are reported for an Fe-Ni-C alloy having a (259)F habit plane and for Fe-Mn-C and Fe-C alloys, both of which have (225)F habit planes. The results for the Fe-Ni-C alloy are in excellent agreement with the predictions of the phenomenological theory of martensitic transformations. The results for the (225)F transformation differ from previous results in that the dilatation parameter δ is not significantly different from unity and the shape strains cannot be distinguished from invariant plane strains. The displacement direction, d, of the invariant plane strain is found to vary significantly in different plates. When the measurements are interpreted assuming δ = 1, the d results are found to lie approximately in a plane containing [11̄0]F. The new results for d lie on the opposite side of the (111)F plane to previous results and are compatible with the observed sense of rotation of the (111)F plane on transformation to martensite. The significance of this result in relation to the phenomenological theory of martensitic transformations is discussed. It is demonstrated that although the new d results differ from earlier ones they confirm the conclusion reached by Morton and Wayman that the phenomenological theory cannot account simultaneously for the measured shape strain and orientation relationship in the (225)F transformation. © 1969.