Abstract: The fabrication of ultrafine-grained microstructures (grain size below 1 μm) in titanium alloys is beneficial for improving their mechanical properties at room temperature and medium temperatures (400-550 °C). However, a long-standing challenge involves the low-cost manufacturing of bulk ultrafine-grained titanium alloys. In this work, we developed a facile strategy through martensite decomposition at thermal-mechanical coupling conditions, to fabricate an equiaxed microstructure in a Ti6Al4Mo4Zr1W0.2Si model alloy with an average α grain size of 315 ± 62 nm. The formation of the ultrafine-grained microstructure was because the lattice strain stored in the martensitic initial microstructure enhanced the nucleation rate of dynamic recrystallization, meanwhile, the pinning role of martensite decomposition products β and (Ti, Zr)₅Si₃ phases suppressed grain coarsening at high temperatures. Compared to conventional (α+β) alloys, the tensile strength of this alloy improved by 20%-30% at both room temperature and 550 °C, without decreasing its ductility. In situ SEM observations revealed that the ultrafine-grained microstructure would not only suppress dislocation motions but also contribute to the homogenous deformation in the matrix of the material, therefore, it resulted in higher mechanical performance. The research results may be of great significance to the development of next-generation aviation titanium alloys.

Key words: Ultrafine-grained, Titanium alloy, In situ observation, Deformation mechanism, Thermostability