On the B2→O phase transformation in Ti–Al–Nb alloys

The mechanisms of formation of the orthorhombic phase O (Cmcm) from B2 (Pm3̄m) in some alloys around the AlNbTi2 composition have been studied. The O-phase has been found to exist in two states: metastable and stable during thermal treatments performed both above and below the β/α and β/O transus followed by controlled cooling. The cooling rate limit, which separates the formation of the two types of O-phase, has been determined. Experimental data obtained for long annealing treatments performed from 1350 to 700 °C have been compared with thermodynamic calculation at equilibrium. A simple model giving account for the B2→O transformation has been proposed.