Heterogeneous precipitate microstructure design in β-Ti alloys by regulating the cooling rate

Cooling after solutionizing or aging plays a crucial role in heat treatment and can greatly influence the final precipitate microstructure and mechanical properties of titanium (Ti) alloys. In this study, we employ phase field simulations to strategically engineer heterogeneous precipitate microstructures in near β-Ti alloys by manipulating the cooling rate, which leads to different phase transition mechanisms. By varying the cooling rate within the temperature range of 900 ^∘C to 500^∘C, we identify four distinct microstructures: uniformly distributed fine congruent α_c plates and fine α precipitates, heterogeneous and hierarchical α precipitates, and uniformly distributed coarse α precipitates. Further analysis reveals that these diverse precipitate microstructures originate from three distinct phase transition mechanisms, i.e., congruent transition, pseudospinodal decomposition, and classical nucleation and growth, each activated at different temperatures during continuous cooling. Utilizing the insights from the simulations, we successfully produce microstructures in Ti-5Al-5Mo-5V-3Cr-1Zr (Ti55531) with heterogeneous α precipitates by applying intermediate cooling rates (∼0.01^∘C/s), which activate classical nucleation and growth as well as pseudospinodal decomposition. These samples exhibit exceptional comprehensive mechanical properties, including an ultimate strength of approximately 1260 MPa and a total elongation of around 14 %. This investigation provides valuable guidance for designing novel heterogeneous precipitate microstructures by simply controlling the cooling rate, leading to significant enhancements in the mechanical properties of β-Ti alloys.