Cyclic phase transformation-driven β-grain growth in TA1 pure titanium: Synergistic regulation via thermal grooves and sub-boundary energy

In titanium alloy, the β-phase grain size at high temperatures and its effect on subsequent α-phase precipitation was a key determinant of material properties. However, precisely controlling the high-temperature β-phase size remained a significant challenge. In this study, the growth kinetics of β-phase in TA1 pure titanium under single-phase and cyclic phase transformation were comparatively analyzed to elucidate the dual regulation mechanism of β-phase grain size by thermal groove and phase transformation. Experimental results indicated that when the temperature exceeded 850°C, α-phase growth was constrained by the grain-boundary thermal groove effect, while above the β-phase transformation temperature (882°C), β-phase growth was hindered by through-grain thermal grooves. Notably, the β-phase size did not increase continuously with temperature and time during single phase transition, whereas it exhibited sustained growth during cyclic phase transitions. Microscopic mechanism analysis revealed that atomic diffusion limitations induced by the β → α s phase (secondary α phase) transition led to the formation of α-sub-boundary networks. These α-subgrain boundaries facilitated the secondary growth of the β-phase by providing an additional energy drive of approximately 50%, effectively overcoming the migration barriers imposed by grain-boundary thermal grooves. Through the synergistic effect of thermal grooves and cyclic phase transformation, the limiting size of the β-phase (0.71-22.48 mm) could be accurately prepared by controlling the number of phase transformations (1-4 times). This study provides valuable insights for optimizing the microstructural design of titanium alloys.