An intersecting-shear model for strain-induced martensitic transformation

Strain-induced martensitic transformation in an austenitic 18Cr-10Mn-0.4N steel was investigated using neutron diffraction and transmission electron microscopy (TEM). Based on experimental evidence from neutron diffraction and TEM indicating that a sequential gamma -> epsilon -> alpha' transformation occurred and the epsilon intersection played a definite role in the alpha' formation, an intersecting-shear model for the strain-induced alpha' nucleation at the epsilon intersection is proposed. Apart from previous models for direct gamma -> alpha' transformation, two-step transformation composed of gamma -> epsilon followed by epsilon -> alpha' is regarded as a main transformation path. In this model, two invariant-plane strains are required to complete the epsilon -> alpha' transformation: the first shear is of the {0 0 0 1} < 1 0 (1) over bar 0 > type and amounts to one-half the twinning shear of gamma; the second shear whose magnitude is one-third the twinning shear of gamma is consecutively introduced parallel to the < 2 (1) over bar (1) over bar 0 > direction on the {0 1 (1) over bar 1} plane. An indirect verification of the model was provided by careful analysis of the precise rotational relationship involved in the epsilon -> alpha' transformation. It was found that a partial dislocation ([0 (1) over bar 1 0]) in moving the epsilon variant interacted with a partial dislocation ([1 0 (1) over bar 0]) in the stationary epsilon variant, and this interaction resulted in the formation of a stair-rod dislocation ([2 (1) over bar (1) over bar 0]) which connects two epsilon variants. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.