Unified modeling of flow behavior and microstructure evolution in hot forming of a Ni-based superalloy

In this paper, the flow behavior and microstructure evolution of a Ni-based superalloy were investigated by hot compression tests with true strain between 0.223 and 0.916, strain rate between 0.001 and 1 s−1 and deformation temperature between 1223 and 1373 K. Based on the experimental results, a set of internal-state-variable based unified constitutive equations were proposed to model the flow stress and microstructure evolution of the studied superalloy. The evolution models of dislocation density, average grain size, and dynamic recrystallization fraction were developed and embedded into the constitutive law, which was derived from thermal activation theory and composed of athermal and thermal stresses. The proposed model was calibrated using experimental flow stress and dynamic recrystallization fraction. The predicted flow stress and dynamic recrystallization fraction under different deformation conditions agreed well with the experimental results. Additionally, flow stress under step-strain rate condition was also precisely predicted by the model. The contributions of long and short range barriers to the overall flow stress, variation of corresponding stress components, and microstructure evolution with strain were further analyzed using the unified model. The thermal stress only varied with deformation temperature and strain rate, while the grain boundary strengthening component and stress contribution of dislocation interactions varied with the evolution of grain size and dislocation density, respectively.