Phase equilibria in the Ti-Al binary system

New experimental results on the phase equilibria between the beta-Ti (A2), alpha-Ti (A3), alpha(2)-Ti3Al (D0(19)) and the gamma-TiAl (L1(0)) phases in the Ti-Al system using specimens with low levels of oxygen are presented. The results obtained on the alpha/gamma and the alpha(2)/gamma equilibria are in good agreement with the previous experimental and calculated phase boundaries, while the ones obtained on the alpha/beta equilibrium deviate significantly from the previously proposed phase diagram. It is confirmed that the beta phase held extends to higher aluminum contents and that the width of the alpha+beta two-phase region is very narrow, less than 1 at.% Al. The presence of the A2/B2 order-disorder transition in the beta 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 beta-Ti (A2)/beta(2)-TiAl (B2), f.c.c.-Al (Al)/gamma-TiAl (L1(0)) and alpha-Ti (A3)/alpha(2)-Ti3Al (D0(19)) equilibria. It is proposed that the anomalous alpha/beta equilibrium is due to the A2/B3 ordering reaction. (C) 2000 Acta Metallurgica Inc. Published bl, Elsevier Science Ltd. All Fights rt ser, ed.