Phase equilibria in the Ti–Al binary system

Abstract New experimental results on the phase equilibria between the β -Ti (A2), α -Ti (A3), α 2 -Ti 3 Al (D0 19 ) and the γ -TiAl (L1 0 ) phases in the Ti–Al system using specimens with low levels of oxygen are presented. The results obtained on the α / γ and the α 2 / 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)/ β 2 -TiAl (B2), f.c.c.-Al (A1)/ γ -TiAl (L1 0 ) and α -Ti (A3)/ α 2 -Ti 3 Al (D0 19 ) equilibria. It is proposed that the anomalous α / β equilibrium is due to the A2/B2 ordering reaction.