The crystallography and growth of partially-twinned martensite plates in Fe-Ni alloys

Abstract

Observations of the internal structure, morphology, and surface relief associated with partiallytwinned plates in Fe-Ni alloys containing near 30 wt. % Ni indicate that the growth of such plates involves two stages which are indistinguishable on the basis of the shape change of the transformed regions; however these stages differ in the transformation inhomogeneity and in the growth kinetics. Stage one corresponds to an internally twinned region of a plate and stage two corresponds to an untwinned region. It has been verified that the internal twinning resulting from first-stage growth is the inhomogeneous shear of the phenomenological crystallographic theory; i.e. the observed twinning shear gives rise to an invariant (habit) plane consistent with the observed midrib plane trace. The inhomogeneous shear associated with second-stage growth appears to be slip whose elements are the same as the twinning elements of stage one. The transformation inhomogeneity probably dictates the final martensite morphology since if only stage one is involved in the growth of a (completely-twinned) plate the austenite-martensite interface is planar in the classical sense; however partially-twinned plates do not exhibit planar interfaces and if no twinning occurs at all, plates are not formed. It is suggested that the local temperature rise produced by the (exothermic) transformation effects the transition from stage one to stage two.