Meta-stable region of Al5Ti3 single-phase in time-temperature-transformation (TTT) diagram of Ti–62.5 at.% Al single crystal

A time-temperature-transformation (TTT) diagram in an annealing temperature range between 500 and 900 °C within 100 h was examined focusing on the constituent long-period superstructures in Ti–62.5 at.% Al single crystals after the plate-like r-Al2Ti precipitates were removed by quenching at 1200 °C. The h-Al2Ti and Al5Ti3 superstructures with three and four-fold periodicities, respectively, with respect to a unit of the L10 structure appeared in accordance with annealing temperature and time. In the transition from h-Al2Ti to Al5Ti3, a modulated structure with 12-fold periodicity appeared and it was explained by a hypothetical structure overlapping h-Al2Ti and Al5Ti3. Although r-Al2Ti was a stable phase in Ti–62.5 at.% Al single crystals annealed at temperatures below 1200 °C, the Al5Ti3 single-phase single crystals without the L10 phase and other superstructural phases, which are never available under an equilibrium condition, were obtained in a wide time range for annealing at 700 and 750 °C.