A continuum theory for dynamic recrystallization with microstructure-related length scales

A novel visco-plastic constitutive theory for large deformations is proposed to describe the microstructural evolution caused by dynamic recrystallization and grain growth processes which moderate-to-low stacking fault energy materials undergo during high temperature processing. The theory relies on scalar internal state variables explicitly linked to intrinsic microstructural length scales, i.e. mean dislocation spacing and average grain size, to describe the evolving microstructure. The formulation incorporates a material instability criterion to define the onset of recrystallization which depends on a critical mean dislocation spacing. The kinetics of grain size refinement during recrystallization and of the subsequent grain growth are controlled by the energy stored by the total dislocation density and by the grain boundary energies of newly recrystallized grains, respectively. The theory accurately describes the main macroscopic and microstructural features associated with recrystallization, including mean grain size and the flow stress oscillations typically observed at low strain rates in a range of carbon steels. Interpretation of experimental evidence of multiple peak recrystallization suggests that it is mainly controlled by a dynamic imbalance in favour of recrystallization-induced grain growth. The constitutive theory is numerically implemented into the finite element method and used to simulate the hot forging of a tapered cylinder to illustrate the ability of the model to predict recrystallization and inhomogeneous deformation phenomena under realistic forging conditions. (C) 1998 Elsevier Science Ltd. Ah rights reserved.