An intersecting-shear model for strain-induced martensitic transformation

Abstract 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 γ → e → α′ transformation occurred and the e intersection played a definite role in the α′ formation, an intersecting-shear model for the strain-induced α′ nucleation at the e intersection is proposed. Apart from previous models for direct γ → α′ transformation, two-step transformation composed of γ → e followed by e → α′ is regarded as a main transformation path. In this model, two invariant-plane strains are required to complete the e → α′ transformation: the first shear is of the {0 0 0 1} 〈 1 0 1 ¯ 0 〉 type and amounts to one-half the twinning shear of γ; the second shear whose magnitude is one-third the twinning shear of γ is consecutively introduced parallel to the 〈 2 1 ¯ 1 ¯ 0 〉 direction on the { 0 1 1 ¯ 1 } plane. An indirect verification of the model was provided by careful analysis of the precise rotational relationship involved in the e → α′ transformation. It was found that a partial dislocation ( [ 0 1 ¯ 1 0 ] ) in moving the e variant interacted with a partial dislocation ( [ 1 0 1 ¯ 0 ] ) in the stationary e variant, and this interaction resulted in the formation of a stair-rod dislocation ( [ 2 1 ¯ 1 ¯ 0 ] ) which connects two e variants.