Microstructure Evolution and Deformation Behavior of Ti-55531 Alloy under Non-equilibrium Conditions during Near-Isothermal Forging

A near-isothermal thermomechanical processing scheme was put forward to obtain bi-modal microstructure for near β-Ti alloys. To this end, this paper focuses on the microstructure evolution and deformation behavior under non-equilibrium forging conditions. Two types of experiments were performed for the Ti-55531 alloy with an original equiaxed structure: (1) static cooling and (2) concurrent deformation/cooling. In static cooling, the formation mode of α_s phase is via α_p/β phase interface instability or sympathetic nucleation, which is determined by the deviation degree of Burgers orientation relationship (BOR) of α_p and β phases. In the near-isothermal deformation, formed low-angle boundaries and high-angle boundaries increase the available nucleation locations and cause the loss of BOR, which contribute to the morphology change for α_s phase and accelerated phase transformation kinetics of β to α_s. The bi-modal microstructure, including α_p phase and fine-equiaxed or rodlike α_s phase, respectively, is produced by a cooperation between dynamic precipitation of α_s phase and DRX and DRV of β phase. Moreover, the texture intensity of α phase is weakened, and flow stress is reduced by 30% compared to that in the isothermal deformation, due to a delay in α phase precipitation, and grain-boundary sliding at α_s–β interfaces. This finding provides a novel method to achieve bi-modal microstructure with weak material anisotropy and load-saving forming.