Phase Transformation and Kinetics in Metastable β Titanium Alloy During Isothermal Treatment

A metastable β titanium (β‐Ti) alloy, Ti–3Al–8V–4Mo–4Cr–4Zr–2Nb–1Fe, is subjected to solute and isothermal aging from 300 to 600 °C for 2–50 h. The microstructure evolution and precipitation mechanism of α‐phase during the isothermal treatment of the metastable β‐Ti alloy are studied. The different mechanisms of β → α phase transformation are revealed under several isothermal conditions. When the temperature is below 45 °C, the α‐phase nucleates at the β grain boundary and becomes plate‐like in shape. When the temperature is above 450 °C, the primary mechanism is homogeneous nucleation. When the temperature is increased to 550 °C, the shape of α‐phase changes from needle‐like to short rod‐like. The alloys exhibit phase separation during isothermal aging. Most of α phases observed in this investigation are 1α phases, which maintain the Burgers orientation relationship with β‐phase. The volume fraction and homogeneous distribution of fine α‐phase particles result in increase in the hardness of the alloy. The maximum value is related to aging at 450 °C, with peak hardness of 373 HV. The phase transformation kinetics of β → α are analyzed by the Johnson–Mehl–Avrami analysis during aging treatment at 450–600 °C.