The mechanical-induced martensite transformation in Fe–Ni–C alloys

The initiation of mechanically induced, mainly stress-assisted, martensite transformation is briefly reviewed and two methodologies are proposed to model it. The first methodology brings the influence of the austenite grain size, dγdγ, on the initiation of martensite transformation into the Patel and Cohen model. In order to do this, we took into account that the start of martensite propagation is associated with a zone of influence adjacent to the austenite grain boundaries. The modified model that bears a typical dγ-1/2 dependence showed good agreement with available data on a Fe–Ni–C alloy. The second methodology is based on an energy balance that is an entirely different and new approach to the problem of mechanically induced martensite initiation. This method balances the elastic energy density stored in the vicinity of a strain center within a certain grain with the free energy shortfall necessary to propagate the first martensite plate in that grain. This new model is not only able to describe the stress-assisted martensite, like the modified Patel and Cohen model, but may also be applied when the austenite undergoes plastic deformation, i.e. when austenite undergoes strain-induced martensite transformation.