Multi-phase transformations at large strains – Thermodynamic framework and simulation

We develop a macroscopic material model for visco-plastic material behavior accompanied by phase transformations, which are important phenomena in steel production processes. The model is formulated within a thermodynamic framework at large strains, and it will be specialized and applied to a hybrid-forming process in steel production. Moreover, thermodynamic consistency of the model will be proved, taking multi-phase transformations into account. In a prototypical situation, TRIP is caused by bainitic and martensitic transformation. The phase transformations considered are: Transformation of the ferritic-pearlitic initial state into austenite, forming of bainite and of martensite. The modelling of bainite evolution takes the incubation via nucleation into account and allows for individual evolutions of upper and lower bainite. Furthermore, the numerical implementation of the constitutive equations into a finite-element program is described. Finally, in a numerical example we investigate the material behavior of a steel shaft subjected to thermo-mechanical loading in a hybrid-forming process.