Low-Temperature Superplastic Deformation Behavior of Bimodal Microstructure of Friction Stir Processed Ti–6Al–4V Alloy

For a long time, the conventional superplastic forming temperature for Ti alloys is generally too high (~ 900–920 °C), which leads to too long production cycles, heavy surface oxidation, and property reduction. In this study, an ultrafine bimodal microstructure, consisting of ultrafine equiaxed microstructure (0.66 μm) and 43.3% lamellar microstructure, was achieved in the Ti–6Al–4V alloy by friction stir processing (FSP). The low-temperature superplastic behavior and deformation mechanism of the FSP Ti–6Al–4V alloy were investigated at temperatures of 550–675 °C and strain rates ranging from 1 × 10⁻⁴ to 3 × 10⁻³ s⁻¹. The FSP alloy exhibited superplastic elongations of > 200% at the temperature range from 550 to 650 °C, and an optimal superplastic elongation of 611% was achieved at 625 °C and 1 × 10⁻⁴ s⁻¹. This is the first time to report the low-temperature superplasticity of the bimodal microstructure in Ti alloys. Grain boundary sliding was identified as the dominant deformation mechanism, which was effectively accommodated by the comprehensive effect of dislocation-induced β phase precipitation and dynamic spheroidization of the lamellar structure. This study provides a novel insight into the low-temperature superplastic deformation behavior of the bimodal microstructure.