A new observation on the mechanism of martensitig transformations

Abstract

In an Fe-Ni-P alloy the influence of incoherent particles in the austenite on subsequent transformation to martensite is investigated. It is observed that small cone-shaped regions of retained austenite are left like “shadows” behind the particles after the transformation interface has passed. The geometrical and crystallographic features of these “austenite shadows” are determined and analysed by a comparison with the elements of the phenomenological theory of the transformation; these elements have been obtained by numerical calculations from the knowledge of the lattice constants, the orientation relationship and the habit plane. The main result of this analysis is that the direction of the “austenite shadows” is antiparallel to the direction of the macroscopic shear. From this new geometrical conditions are deduced which have to be satisfied by the mechanism responsible for the propagation of the austenite-martensite interface during the transformation. Furthermore it has been observed that the directions of the “austenite shadows” have opposite signs on either side of the plane defined by the midrib. From this it follows that the midrib plane is the plane of initiation of the transformation.