Constitutive and transformation kinetics modeling of ε-, α′-Martensite and mechanical twinning in steels containing austenite

In the present study, a physically based constitutive and transformation kinetics model of stress assisted (SA) and strain induced (SI) ε-, α′-Martensite and twinning in steels containing austenite, is presented. The deformation behavior of γ-Austenite, α′-Martensite, α- and δ-Ferrite is modeled in terms of the dislocation density. The macroscopic elastoplastic response of the material in uniaxial tension is calculated with respect to the partitioned stress and strain in each phase using a homogenization method. Inelastic strain accommodation below the yield strength of austenite is considered due to the SA transformation. The kinetics of SA and SI ε-, α′-Martensite and twinning in austenite, are calculated in terms of the ε-, α′- and twin embryo nucleation rate and the subsequent growth of shear bands, promoted by the applied stress and strain, respectively. Heterogeneous nucleation of α′-Martensite only on ε and twin shear band intersections is considered, presenting a stress state dependence. Austenite stability against TRIP and TWIP is modeled as a function of the austenite stacking fault energy and the separation distance of Shockley partial dislocations. Fitting the model to experimental stress-strain data allows for the prediction of α′-, ε-Martensite and mechanical twin fractions, without a prior notion of the transformation kinetics. A MATLAB implementation and an Extended Methodology section are provided as Supplementary Material. The model could be used to aid in the design of novel alloys with exceptional properties, like medium Mn steels.