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 γ → ε → α′ transformation occurred and the ε intersection played a definite role in the α′ formation, an intersecting-shear model for the strain-induced α′ nucleation at the ε intersection is proposed. Apart from previous models for direct γ → α′ transformation, two-step transformation composed of γ → ε followed by ε → α′ is regarded as a main transformation path. In this model, two invariant-plane strains are required to complete the ε → α′ transformation: the first shear is of the {0 0 0 1}〈101¯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 〈21¯1¯0〉 direction on the {011¯1} plane. An indirect verification of the model was provided by careful analysis of the precise rotational relationship involved in the ε → α′ transformation. It was found that a partial dislocation ([01¯10]) in moving the ε variant interacted with a partial dislocation ([101¯0]) in the stationary ε variant, and this interaction resulted in the formation of a stair-rod dislocation ([21¯1¯0]) which connects two ε variants.