Transmission electron microscopy on the phase equilibria among β, α and α2 phases in Ti–Al binary system

The phase relationships among β-Ti, α-Ti and α2-Ti3Al phases in Ti–Al binary system at temperatures around 1473 K have been examined by transmission electron microscopy, particularly paying attention to the size of anti-phase domains (APDs), using Ti–(25–35) at.%Al alloys quenched from various high-temperature phase regions. Regardless of the phases present at high temperatures, all alloys become α2 single phase in the quenched state. The region of α2 phase prior to quench exhibits no APDs in the quenched state. The prior β phase region shows finer APDs with an average size of 40 nm, whereas the prior α phase region exhibits large APDs of 150 nm in size. Based on the difference in APD size as a criterion, together with hardness and transformed morphology, the phase regions at given temperatures can be determined. The results suggest that the α2 phase is congruently in equilibrium with α phase at 1465 K (congruent type), rather than in equilibrium with β phase straight with two peritectoid reactions (peritectoid type). From thermodynamic consideration, oxygen content in alloys is not a factor to change the phase equilibria from congruent type to peritectoid type among these phases.