The ordering behaviour of the O phase in Ti2AlNb-based alloys

Based on the investigation of Muraleedharan et al. [Intermetallics 3 (1995) 187] concerning the order–disorder transition of O phase in Ti2AlNb-based alloy, the site occupations of the alloying elements are clarified and the ordering behaviour of the O phase in Ti-yAl–zNb (y⩾25 at.%) alloys is investigated with a Bragg–Williams model. As a result, the relationship among the long-range order parameter (s), temperature (T), and the composition of the O phase (y, z) is established. In the temperature and composition range where the O phase exists, there is a continuously ordering character in the ordering behaviour of the O phase. Also the first-order transition character is very ‘weak’.