Twin-boundary density evolution and its kinetics model of Ni80A alloy during two-stage deformation process

In the grain boundary engineering, a mixed grain boundary network (GBN) with abundant twin boundaries contributes to improving grain boundary-related properties of alloys. However, it is a difficult issue to achieve such mixed GBN in nickel-based alloys in thermal deformation process. In this work, a novel two-stage deformation method was employed. It involves prior cold deformation treatment (PCDT) to introduce stored energy, and promote dynamic recrystallization (DRX) during subsequent thermal deformation. A unified kinetics model, including twin density kinetics model, DRX kinetics model, grain growth model, dislocation-density model, and stored energy model, was derived to predict the evolution of mixed GBN during two-stage deformation process. Two-stage deformation experiments were conducted on Ni80A alloy under different prior cold and thermal deformation conditions. Microstructure analyses demonstrated that twin-boundary density is jointly governed by stored-energy release, DRX evolution, and grain-size evolution, resulting in a nonlinear dependence on the thermal-deformation parameters. At low temperatures and high strain rates, the desired mixed GBN with fine grain size and high twin density was obtained. PCDT enhances DRX process and Σ3n twin boundaries formation in thermal deformation. This results in desired mixed GBN under extended thermal parameter domains compared to alloys without PCDT. Based on the true stress-strain data and microstructural results of Ni80A alloy, the mixed GBN kinetics model was solved with the average absolute relative error of 7.2%. A finite element model coupling the mixed GBN model during two-stage deformation was constructed to uncover the evolution of microstructures in space-time domain. This work provides theoretical basis for controlling mixed GBN of alloys during thermal deformation.