Microstructural evolution and formation mechanism of FCC titanium during heat treatment processing

Highlights: ► The alloy with ST underwent the transformations: β → α + β + α″ + fcc phase. ► The lattice parameter "a" of the fcc phase was about 0.4385 nm. ► After aging at 450 °C, the highest strength was attained about 1740 MPa. ► The transus temperature of fcc-Ti was between 650 °C and 700 °C. -- Abstract: The phase transformation of Ti–20Zr–6.5Al–4V alloy was studied during different heat treatment processes. Due to the addition of zirconium possessing the high-stacking-fault energy, a small amount of Ti underwent allotropic transformation from hcp-Ti to face-centered cubic (fcc) Ti during heat treatment. The alloy, which was subjected to solid solution treatment at 950 °C for 60 min and subsequent water quenched, underwent the following phase transformations: β → α + β (residual) + α″ + fcc phase. The fcc phase was also found in the subsequent aging process, and when aging temperature reached 700 °C, the fcc-Ti completely decomposed into hexagonal close-packed α phase. The transus temperature of fcc-Ti was between 650 °C and 700 °C. It was found that the mechanical properties of the alloy are remarkably dependent on fcc phase content