A unified internal state variable material model for inelastic deformation and microstructure evolution in SS304

At a fundamental level, the thermomechanical response of metals during inelastic deformation is governed by dislocation mechanics and associated microstructure evolution. However, the most commonly used material models in severe plastic deformation processes such as metal machining and forming are phenomenological (e.g. Johnson–Cook) or semi-phenomenological (e.g. Zerilli–Armstrong) and do not account for the dependence of material flow stress on dislocation and microstructure evolution processes in a unified manner. The latter negatively affects the accuracy and range of applicability of process simulation models. This paper presents a unified material model that explicitly accounts for dislocation and grain size evolution processes associated with inelastic deformation. The corresponding microstructure evolution laws are formulated and integrated into the constitutive model, which is derived from thermal activation theory and is an additive decomposition of athermal and thermal components representing the effects of long- and short-range obstacles, respectively. The unified model is seen to compare well with experimental data for the deformation of SS304 alloy. Different aspects of inelastic deformation including thermal softening and sensitivity, rate hardening and sensitivity, and strain hardening rate are reproduced quite accurately by the unified model. The unified model is further used to isolate the contribution of various microstructural features and it is found that the stress component representing the interaction of individual dislocations with dislocation forests accounts for the major contribution to the overall flow stress in SS304. The thermal stress does not vary with strain while the athermal stress varies with strain in addition to tracing the microstructure evolution. Moreover, the variation of both thermal and athermal stresses are analyzed as a function of the Zener–Hollomon parameter. It is seen that the thermal stress vanishes entirely below a critical value of the Zener–Hollomon parameter.