A model for nodular graphite cast iron coupling aniso-thermal elasto-visco-plasticity and phase transformation

The spherical graphite ferritic cast iron undergoes the phase transformation during its cooling from solidus temperature to room temperature. This leads to complex behaviour with interactions between the mechanical response, thermal response and the microstructural change. In particular, the microstructural evolution effect on the mechanical behaviour causes a change in the properties of the studied material. This also induces other new mechanisms such as the phase transformation plasticity and the thermo-metallurgical expansion due to the microstructural change. The material is formed by one single phase in the no phase transformation domain. Its behaviour is described by an aniso-thermal elasto-viscoplastic model based on the Chaboche's model and developed within the framework of the thermodynamics with internal variables. The description of the behaviour of the two-phase material during microstructural change is based on a simplified approach to the mixture theory. This approach consists of taking the total deformation and the temperature as common variable to the two existing phases in the material (Voigt Model). Furthermore, in order to model the phase transformation mechanism, an additional internal variable representing the new phase volumetric fraction is introduced. The thermodynamical couplings used allow the interactions between the different phenomena that govern the material behaviour to be taken into account. Therefore, both the forms of thermodynamical potential and dissipation potential are accurately defined. Thus, the overall behaviour of the spherical graphite ferritic cast-iron is described by an aniso-thermal elasto-viscoplastic model coupling phase transformation. Different isothermal and aniso-thermal tests are carried out to identify the model parameters. The simulations of the verification tests, using the proposed model, provide a good agreement with the experimental data showing that such a model is successful in predicting the behaviour of the studied material during its cooling post-solidification phase. (author)