Influence of phase transformation kinetics on the microstructure and mechanical properties of near β titanium alloy

The influences of up-quenching aging (UQA) and step-quenching aging (SQA) on the microstructure development and mechanical properties of near β-Ti alloy Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe have been comparatively studied. The results showed that a large number of fine α precipitates were uniformly formed in UQA sample via ω-assisted nucleation, resulting in a higher aging hardening effect. Whereas under SQA treatment, the α precipitates had lower number density and became coarser in addition to inhomogeneous size distribution, in which plenty of α precipitates with large aspect ratio (>50) could be observed. With increasing the aging temperature from 480 °C to 650 °C, the number density of α precipitates with large aspect ratio was gradually reduced, and the hardness interval between UQA and SQA samples was decreased from 90 HV to 20 HV. When aged at high temperature (600 °C), a large number of ultrafine α precipitates were obtained in the UQA sample within 30 min, the number density, thickness and length of α precipitates were ~20 ppt/μm², 40 nm and 180 nm, respectively, resulting in very high peak hardness (473 HV), but a rapid over-aging occurred due to the ripeness of α precipitates. The SQA treatment showed no evident over-aging tendency, and could yield a good combination of ultimate tensile strength (1284 MPa) and elongation (8%) with the existence of multiscale α precipitates.