Effect of Co addition on the microstructure evolution and superplastic behavior of Ti-4Al-3Mo-1V-0.1B alloy

Abstract. Temperature reduction of superplastic forming of titanium alloys is currently a significant issue. The present study focused on the effect of modifying the Ti-4Al-3Mo-1V-0.1B alloy with 0.5-2 wt% Co additions on the superplastic behavior and microstructure evolution. The results demonstrated that Co alloying promoted the formation of recrystallized and globular microstructure before the beginning of superplastic deformation due to the acceleration of diffusivity by Co in comparison with the Co-free alloy. The diffusivity acceleration also led to dynamic grain growth during superplastic deformation but promoted a stable superplastic flow of alloys with 0.5-2% Co at temperatures of 700-750 °C. This enhanced the strain rate sensitivity coefficient m from 0.35-0.4 to 0.5-0.65 and the elongation to failure from 200-350% to 500-1000% compared to the Co-free alloy. The 2 wt% Co alloying provided excellent low-temperature superplasticity in the temperature range of 625-775 °C with a coefficient m of 0.5-0.65 and elongation to failure of 800-1000% at a constant strain rate of 1 × 10-3 s-1.