A new model for dynamic recrystallization under hot working conditions based on critical dislocation gradients

Abstract Dynamic recrystallization (DRX) occurs during hot working of metals and alloys with a sufficiently low stacking-fault energy. It is characterized by the nucleation and growth of new grains, which takes place concurrently to plastic deformation. DRX processes are used in industrial hot forming processes to control the microstructure and properties of the workpiece. Various models have been developed that consider the coupled evolution of microstructure and flow stress during hot deformation processes. It is widely accepted that the onset of DRX is characterized by an inflection point of the strain hardening rate as a function of stress. This so-called second-derivative criterion was derived by Poliak and Jonas in the 1990s using results from the thermodynamics of irreversible processes. Recent results show that models which use classical Avrami kinetics to describe the evolution of the recrystallized volume fraction violate the criterion if the Avrami exponent assumes a value of 3 or less. This inconsistency has its root in the assumptions made for the nucleation and growth of DRX nuclei and was shown to affect the accuracy of models used to design and control metal forming processes. This paper presents a new, consistent model for the nucleation of DRX. Typical nucleation models assume a constant dislocation density throughout the parent grains to derive a critical dislocation density for nucleation. However, it is clear from crystal plasticity that the dislocation density is higher at the grain boundaries than inside the grains. In order to involve dislocation gradients, coupled evolution equations for mobile and immobile dislocation densities are derived and solved under hot working conditions in this work. A new nucleation criterion is developed based upon the strain-dependent dislocation density gradient in parent grains. Once the critical gradient is reached, nucleation starts. The results of the newly developed criterion are compared with conventional models and experimental data. The comparison shows that the new criterion predicts more realistic critical dislocation densities in contrast to conventional models which over-estimate the critical dislocation density. As a consequence, more accurate predictions in the design and control of metal forming processes may be achieved with the new model.