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 3Al (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 3Al (D0 19) equilibria. It is proposed that the anomalous α/ β equilibrium is due to the A2/B2 ordering reaction.