Extension of the internal state variables model for the multi-stage hot deformation verified by EBSD characterization

The key to model development lies in unifying the recrystallization behavior throughout multi-stage hot deformation processes. However, this integration has been hindered by three primary challenges: (1) the lack of a consistent criterion for the onset of dynamic recrystallization (DRX) during repeated deformation; (2) the insufficient consideration of the influence of DRX states on static recrystallization (SRX) mechanisms; and (3) the unclear treatment of recrystallization state transitions during successive deformation stages. To address these issues, this study introduces essential modifications to the internal state variable (ISV) model and extends its applicability to multi-stage hot deformation. Specifically, the critical dislocation density is employed as the criterion for DRX initiation, with a simplified method proposed for its determination. Furthermore, a SRX incubation fraction is incorporated to account for the influence of DRX states on static softening mechanisms. Additionally, a method is proposed to evaluate changes in the recrystallization fraction during repeated deformation by comparing the residual and critical dislocation densities, capturing the effects of recrystallized grain hardening and DRX recurrence. The predictive ability and underlying assumptions of the modified model are validated through double-pass hot deformation experiments. Furthermore, the applicability of the model under dynamically changing deformation conditions is verified. Finally, electron backscatter diffraction (EBSD) analysis of microstructural evolution under different inter-pass holding times is employed to elucidate the deformation mechanisms involved. The proposed model offers a theoretical framework for accurately predicting and controlling material states in multi-pass hot deformation processes.