THE ALPHA(2)-TO-O TRANSFORMATION IN TI-AL-NB ALLOYS

The alpha(2)-to-O transformation, with its characteristic microstructural features, occurs across the entire range of compositions in alloys based on the Ti3Al-Ti2AlNb system. In this paper, we describe the isothermal decomposition of the alpha(2)- to the O(O1)-phase in a Ti-28.5 at.% Al-13 at.% Nb alloy, in the first detailed study of this transformation. Both parent and product phases are ordered with an identical site occupancy. The transformation is accomplished by a lattice distortion and a small composition change. The microstructural features and patterns that evolve during the diffusional growth and coarsening of the O-phase are shown to be dominated by the strain energy associated with the transformation. The orientation relationships, habit plane and the self-accommodating nature of the product variants are therefore explained by simple analyses in terms of the phenomenological theory of martensitic transformations.