Unified modeling of flow softening and globularization for hot working of two-phase titanium alloy with a lamellar colony microstructure

In this paper, a set of physically based constitutive model coupling microstructure evolution was developed for unified prediction of flow stress and globularization evolution during hot working of two-phase titanium alloys with initial lamellar microstructure. The dislocation density variation, dynamic globularization and effect of Hall-Petch strengthening were considered in the microstructure model. The dynamic globularization was modeled by two parts: critical strain for initiation of globularization and globularization rate, both of which are function of processing conditions (temperature and strain rate); In the modeling of Hall-Petch strengthening, the dependence of Hall-Petch coefficient on processing conditions were considered and the loss of Hall-Petch strengthening with deformation process was molded. The microstructure model was implemented to a physically based constitutive model, composed of a thermally activated stress and an athermal stress, to realize the unified prediction of flow stress and globularization evolution. The material parameters were determined by calibration using the experimental flow stress and globularization fraction in isothermal compression tests through the genetic algorithm based optimization method. Based on the set of constitutive models, flow stress and globularization evolution of Ti-6Al-4V and TA15 alloys at different temperatures and strain rates were predicted. Good agreements between the experimental and computed results were obtained.