Influence of ω and secondary α phase transformation on high-cycle fatigue behavior of Ti–2Al-9.2Mo–2Fe alloy

β-titanium alloys are ideal structural materials for automotive and aerospace applications due to their tunable mechanical properties. This study investigates the effects of precipitate control via aging treatment (450 °C, 500 °C, and 550 °C) on the microstructural evolution and high-cycle fatigue mechanism of the Ti–2Al-9.2Mo–2Fe alloy. The results revealed that the precipitation behavior significantly varied with temperature: isothermal ω phase dominated at 450 °C, while coarse secondary α phase formed at 550 °C. Notably, at 500 °C, a balanced coexistence of nanoscale ω and secondary α precipitates were achieved. This unique microstructure resulted in the best balance of mechanical properties, yielding a tensile strength of 1313 MPa and an elongation of 8 %. Furthermore, the high-cycle fatigue limit at 500 °C reached 500 MPa (at 10 7 cycles), which is superior to the specimens aged at 450 °C (450 MPa) and 550 °C (350 MPa). The fatigue fracture analysis indicated that the uniform distribution of precipitates at 500 °C maintained a uniform deformation distribution and stable crystallographic structure, effectively delaying crack initiation and stabilizing crack propagation, whereas localized stress concentrations at 450 °C and multi-crack initiations at 550 °C led to premature failure. These findings suggest that controlling the ω/α phase is critical for optimizing the fatigue performance of low-cost β-titanium alloys.