The stress-induced α″ and its effect on the aging microstructure and mechanical properties in an α + β titanium alloy

The stress-induced α″ martensite transformation always sacrifices the yield strength in titanium alloys with specific β stability. Here, a detailed analysis for the microstructure transformation of the stress-induced α″ martensite during isothermal aging is carried out using the traditional experimental characterizations based on the transmission electron microscope (TEM) in an α + β Ti-alloy, namely Ti-3Al-5Mo-4.5 V (wt%) alloy. The results show that the stress-induced α″ plates transform to the isothermal-α″ due to the local solute redistribution (Al, Mo, V) during the one-step isothermal aging at 400 °C. Subsequently, the ladder-like αs precipitates due to the decomposition of stress-induced α″ or isothermal-α″ after aging at 550 °C, eliminating the influence of stress-induced α″ martensite on the yield strength. Besides, the ultrafine αs is obtained by the refinement of isothermal ω during two-step aging at 550 °C. And the ultimate tensile strength increases slightly due to the precipitate of ladder-like and ultrafine αs after two-step aging at 550 °C. The present study provides a novel way to obtain the α lamellae and improve the mechanical properties of titanium alloys.