Extremely Rapid Age Hardening in Ti-5Al-2Fe-3Mo Solution Treated at High α + β Temperature Region

To investigate the age hardening behaviors in an α + β titanium alloy, Ti-5Al-2Fe-3Mo, microstructures after solution treatment at a temperature in the high α + β region, and subsequent aging at temperatures below or around 500 °C were analyzed by transmission electron microscopy (TEM) and atom probe tomography (APT). Age hardening depended on both aging temperature and time, and at a very early stage of aging, fluctuations of Ti and alloying elements occurred in the β phase with an interval of several nanometers. It is considered that spinodal decomposition occurred. Subsequently, an extremely fine acicular hexagonal-close-packed (hcp) phase having width similar to that interval was formed, which contributed to age hardening together with the internal stress fields created by spinodal decomposition. A model was proposed to explain the extremely rapid hardening phenomenon: O suppresses martensite transformation during cooling from the solution treatment temperature, and the extremely fine hcp phase is formed in a short time by shear transformation in the O-lean area created by spinodal decomposition. In the O-lean area, the martensite start (M_s) temperature increases as the O concentration decreases. The effect of β-phase stability on the microstructural evolution in the spinodal modulated structure was also discussed: Shear transformation and β zone formation may occur depending on the β-phase stability and O concentration distribution.