Elucidating the influence of temperature and strain rate on superplasticity of near-α Ti6321 titanium alloy

Abstract

The superplastic forming (SPF) technique presents new potential for the near-$\alpha$ Ti6321 titanium alloy in the fabrication of critical ultra-thick components for the marine industry. This study investigates the effects of temperature and strain rate on the flow behavior, microstructural evolution, and deformation mechanisms of the Ti6321 alloy during superplastic deformation. Optimal SPF conditions are identified at 850℃/0.0005–0.001 s^-1 and 900℃/0.0005–0.005 s^-1, under which elongations exceeding 370% and a strain rate sensitivity above 0.3 are achieved, indicating excellent superplasticity. The enhanced$\alpha \to \beta$ dynamic phase transformation with increasing temperature moderately contributes to superplastic performance. A maximum elongation of 824.4% is attained at 900℃/0.0005 s^-1 with an $\alpha /\beta$ phase ratio of 75/25, driven by synergistic interactions among grain boundary sliding, dynamic globularization, dynamic recrystallization, grain rotation, and active dislocation activity. However, at 950℃, a significantly higher $\beta$-phase content (~50%) induces dynamic grain coarsening and convergent evolution of microtexture in the $\alpha$ phase, which impedes grain rotation and leads to strain incompatibility between adjacent grains. Consequently, the macroscopic deformation is compromised due to the inability of existing mechanisms to effectively accommodate localized stress concentrations. At comparable $\beta$-phase fractions, a higher initial strain rate is found to promote grain refinement and orientation divergence, but also intensify dislocation multiplication. Pyramidal slips, particularly the first-order <$c+a$> slip, serve as the dominant slip mode in the primary $\alpha$ phase. These findings provide a mechanistic basis for optimizing SPF conditions in the Ti6321 alloy and offer novel insights into its marine applications.