Phase equilibria in the Ti–Al binary system Original Research Article

New experimental results on the phase equilibria between the β-Ti (A2), α-Ti (A3), α2-Ti3Al (D019) and the γ-TiAl (L10) phases in the Ti–Al system using specimens with low levels of oxygen are presented. The results obtained on the α/γ and the α2/γ equilibria are in good agreement with the previous experimental and calculated phase boundaries, while the ones obtained on the α/β equilibrium deviate significantly from the previously proposed phase diagram. It is confirmed that the β phase field extends to higher aluminum contents and that the width of the α+β two-phase region is very narrow, less than 1 at.% Al. The presence of the A2/B2 order–disorder transition in the β phase is also confirmed by a combination of differential scanning calorimetric (DSC) analysis and extrapolation of ordering phase boundaries from the Ti–Al–X (X=Cr, Fe) ternary systems. A thermodynamic analysis has been carried out taking into account the ordering configurations in the β-Ti (A2)/β2-TiAl (B2), f.c.c.-Al (A1)/γ-TiAl (L10) and α-Ti (A3)/α2-Ti3Al (D019) equilibria. It is proposed that the anomalous α/β equilibrium is due to the A2/B2 ordering reaction.