Transmission electron microscopy on the phase equilibria among β, α and α 2 phases in Ti–Al binary system

Abstract

The phase relationships among β-Ti, α-Ti and α 2-Ti 3Al 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 α 2 single phase in the quenched state. The region of α 2 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 α 2 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.