On the growth of lenticular martensite

Abstract

It is proposed that the growth of lenticular martensite typically occurring in the FeNi and FeC systems takes place by the propagation of waves throughout the material. Two different types of waves are postulated: longitudinal transformation waves and transverse transformation waves, propagating at velocities of the order of elastic waves. The longitudinal transformation wave initiates the transformation process, once an embryo has reached a critical size whereupon it becomes a nucleus; it propagates radially along directions contained in the habit plane, forming the mid-rib. The martensitic disc generated by the longitudinal transformation wave acts as a second-order nucleus for the transverse transformation. The term ‘second-order nucleus’ is used to distinguish it from the ‘first-order nucleus’ that gives rise to the start of the transformation. The transverse transformation waves propagate perpendicularly to the habit plane, starting at the mid-rib. Accordingly, different defect generation mechanisms operate along the longitudinal and transverse propagation directions, due to the differences in stress state and substructure at the fronts and propagation velocities. The model allows the determination of the shape of a growing martensite plate, which closely resembles lenticular martensite. If xz is the habit plane, the direction of transverse propagation is oy and the major shear direction is ox, the martensite lens can be described, at time t, by the equation (x 2 + z 2) 3 2 x 2v ed+ z 2v es − 1 k ln(1 − k¦y¦ v es ) = t where v ed and v es are the velocities of longitudinal and shear elastic waves and k is a parameter. The arrest of growth takes place by uncoupling between the transformation front and the plastic waves that precede it. The implication of the pressure rise associated with the wave upon the nucleation is discussed.