The concept of the habit plane and the phenomenological theories of the martensite transformation

Three different interpretations of a martensite habit plane are in use: 1. (a) the plane of the plate of a plate-shaped crystal, 2. (b) a semi-coherent plane glissile interface, 3. (c) the plane boundary of a plate shaped product. These are not necessarily the same; for surface martensite they are different. Several recent “generalized theories” of martensite transformation did not succeed in explaining the experimental observations on martensite of the {252}-type in a satisfying way. Results of observations on surface martensite in Fe-30 wt.% Ni are, among others: 1. (1) an orientation relationship, very close to the orientation predicted by the I.P.S. theory for the same alloy. The habit plane, however, is not the predicted (3, 15, 10) plane, but the (121) plane. 2. (2) A direction of the shape deformation predicted by a Frank type of matching. These results, together with the recent observation of bend faults in martensite of the (252) type, indicate that the transformation takes place in at least two stages. Therefore, a “generalized theory of martensite transformation”, which tries to predict the whole transformation (conceived as one single process) assuming a set of lattice invariant shears will, perhaps, never be very successful, because the transformation can proceed in different stages involving rather complex shears.