Manipulation of the α-phase precipitation behavior through dual-aging treatment and its correlation with mechanical properties in a metastable β titanium alloy

The precipitation behavior of α-phase and its crucial impact on the mechanical properties have been investigated in a metastable β-type Ti-4Al-6V-5Mo-3Cr-1Zr (wt%, Ti-46531) alloy. Through dedicated design of dual-step aging conditions, the nucleation mechanism, number density and morphology of the secondary α-phase precipitates can be modulated. After the first-step aging treatment at 300 °C for 5−100 h, isothermal ω-phase (ω_iso) particles precipitate and are uniformly dispersed in the β-phase matrix, with their number density gradually increasing with aging time. The subsequent second-step aging treatment at 600 °C for 2 h leads to the rapid dissolution of ω_iso and the precipitation of α-phase through the classical nucleation process with a steady number density. Lowering the second-step aging temperature to 520 °C promotes the heterogeneous nucleation of the α-phase with the assistance of uniformly dispersed ω_iso particles. This leads to a significant increase in the number density of α-phase precipitates. Consequently, the maximum yield strength increases from 1296 MPa for samples aged at 600 °C to 1623 MPa for samples aged at 520 °C. The precipitation of α-phase serves a dual role in enhancing the yield strength, both by providing precipitation strengthening and by increasing dislocation density. The contribution to yield strength enhancement from the former effect is considerably more substantial than the latter. This work underscores the crucial role of α-phase number density in determining the strength of metastable β titanium alloys, offering new insights into the tailoring of microstructures and mechanical properties through dual-aging treatments.