Internal variable model for hot deformation and subsequent heat treatment of Ti6242S alloy considering discontinuous yielding and dynamic/static phase transformations

Hot deformation in the two-phase region combined with subsequent heat treatment is an effective approach for forming Ti6242S titanium alloy components and achieving excellent performance through prediction and control of deformation and microstructure; however, during the process, the materials undergo complex yielding, hardening, softening behaviors, microstructure evolution, and phase transition, and there are interactions. In this study, based on the investigation of the mechanisms of discontinuous yielding, dynamic and static recrystallization, as well as the analysis of phase transformation kinetics, an internal state variable model incorporating dislocation density, volume fraction and size of equiaxed α phase for the entire deformation and heat treatment process of Ti6242S alloy was established. By taking the calculated variables (e.g., dislocation density, microstructural parameters) after deformation as the initial state for heat treatment, the full-process modeling and simulation was realized. For the hot deformation stage, a constitutive model considering discontinuous yielding was built, and strain and stress partitioning between α and β phases was achieved using the mixture rule. A modified model was proposed to uniformly describe the Ostwald ripening behavior of the equiaxed α phase in both deformed and undeformed states. The predicted results of flow stress and microstructural evolution are in good agreement with the experimental ones, which indicates that during deformation, strain is mainly borne by the β phase, discontinuous yielding behavior is predominantly associated with the β phase, and the dislocation density of the α phase is more sensitive to temperature.