Superplastic behavior of a metastable β-type Ti alloy governed by grain size: Microstructure evolution and underlying deformation mechanism

The superplastic forming (SPF) technology has gained widespread adoption in the manufacturing of intricately shaped components made from Ti alloys with high dimensional accuracy due to its near net forming capabilities. It is crucial to systematically investigate the superplastic behavior in metastable β-type Ti alloys. However, the microstructure evolution and underlying superplastic deformation mechanism governed by grain size in these alloys remain unclear. In this study, three types of fine-grained (<10 μm) Ti-15V-3Cr-3Sn-3Al (Ti-15-3) alloys were obtained using friction stir processing (FSP), and tensile tests were conducted at temperature of 650 °C with strain rates ranging from 1 × 10⁻⁴ s⁻¹ to 3 × 10⁻³ s⁻¹. The deformation behavior of the coarse-grained (>3 μm) Ti-15-3 alloy can be described as quasi-superplastic rather than strictly superplastic. In this microstructure, plasticity is synergistically contributed by grain boundary sliding (GBS), continuous dynamic recrystallization (CDRX), and dynamic phase transformation (DPT). For the fine-grained (<3 μm) Ti-15-3 alloy, deformation enters into the superplastic region with GBS becoming the predominant deformation mechanism accompanied by DPT. Additionally, strain/stress promotes α phase precipitation from the β matrix, which inhibits grain growth and serves as a supplementary stress accommodation mechanism facilitating continuous operation of GBS and ultimately achieving exceptional superplasticity.