Effect of double stage solution treatment on the volume fraction of massive phase (αm) as a new method to obtain a fine lamellar α/β in Ti–6Al–4V alloy

This study investigated the effect of double stage solution treatment (DSST) on the behavior of the massive phase (αm) transformation in a Ti–6Al–4V alloy. αm Phase transformation has been reported to occur by introducing a cooling rate between 20 °C/s and 410 °C/s before reaching the martensite start temperature (Ms). A featureless αm morphology is observed with optical microscopy (OM) because the α and β phases cannot be clearly distinguished. Higher magnification images with scanning electron microscopy (SEM) show that αm has a sub-lamellar morphology with two distinguishable regions: the clear region and the white-dot region. The volume fraction of αm was evaluated after DSST with a first annealing above the β-transus temperature, followed by a second annealing above Ms with varying holding times. The solution treatment at 850 °C for 1800 s yielded the highest fraction of αm (22.4%), which was confirmed with OM and further supported by the phase maps in the electron backscatter diffraction results.