The role of plastic accommodation in the (225) martensite transformation

Abstract

Recent studies of the (225) F martensitic transformation in iron alloys indicate that the dilatation parameter, δ, is unity and the direction of the complementary shear is within a few degrees of [11̄0] F for the usual standard variant. When the habit plane is of the form [ hhl] F and δ = 1 it is impossible, according to the current phenomenological theory, for the complementary shear direction to be [11̄0] F because this line, which must contract by about 1.75%, would be invariant. To overcome this difficulty it is suggested that the contraction is accommodated by a lattice invariant plastic strain in the austenite ahead of the growing plate. Thus the shape strain is considered to be the resultant of a lattice strain and a lattice invariant accommodation strain. Phenomenologically this is equivalent to a relaxation of the usual condition that the complementary strain is a simple shear. A quantitative development is carried out by assuming a plausible multiple slip mechanism for the accommodation strain and it is shown that the predictions of the habit plane, orientation relationship and directions and magnitudes of the shape strain are in better agreement with observation than those of earlier theories. The experimentally observed variation in the direction of the shape strain is predicted and it is suggested that the orientation of the specimen surface determines which of the predicted directions operates in any particular case.