A generalized theory of martensite crystallography and its application to transformations in steels

Abstract

In view of recent experimental observations of martensitic structures exhibiting two independent transformation shears, a new theory of martensite crystallography has been developed which enables the features of transformations involving mechanisms of this kind to be predicted. Like earlier treatments, the generalized theory is based on an invariant plane strain criterion, the total shape deformation being resolved into a lattice deformation and two independent lattice invariant shears. The new theory is first described in terms of geometrical models followed by a formal algebraic version, and then used to investigate the effects on predictions of the crystallographic features of the martensite reaction in steels of various double shear mechanisms. Some general features of the results, such as the effect of reversing the order of the two shears and the influence of algebraic restrictions are first examined. Mechanisms giving rise to predictions of practical interest are then discussed. In particular, two mechanisms giving habit planes near {225}γ and other crystallographic features consistent with experimental results are analysed in detail. These involve small lattice invariant shears on the (011̄)[11̄1̄]α and (1̄12)[11̄1)α systems preceding the observed (112)[1̄1̄1]α twinning shear. Finally, the relation of the new theory to other theories, particularly those attempting to describe the {225}γ transformation in steels, is discussed, special mention being made of an equivalent theory developed concurrently by Acton and Bevis.