The formation mechanism of the O phase in a Ti3Al–Nb alloy

It is reported that there are several different transformation mechanisms of the O phase in different heat treatment conditions in the Ti3Al based alloys. However, very little work has been carried out on the α2→O phase transformation in the Ti3Al–Nb alloys of Nb amounts exceeding 12 at%. In this paper, the formation mechanism of the O phase in the Ti–24Al–14Nb–3V–0.5Mo (at%) alloy has been carried out by means of TEM and HRTEM. The results show that the O phase is directly derived from the primary equiaxed α2 grains with a fine streak contrast, and exists in multivariant forms owing to its different orientations after the alloy is solution treated at 1000°C for 1 h followed by water quenching (WQ) and aged at 650°C for 24 h. The O plates in the primary equiaxed α2 grains exist not only in the form of a single variant, but also in the form of fine α2+O mixtures. The analysis indicates that the formation of the O phase is the result of a phase decomposition, that is the introduction of niobium as the preferred β stabilizer makes the supersaturation of niobium in the primary α2 grains, and the α2 phase containing Niobium separates into Niobium lean and Niobium rich regions through the Niobium diffusion: α2→α2(Nb-lean)+O(Nb-rich). Niobium rich regions transform to the ordered orthorhombic phase (O phase) with a lattice distortion and only a very small composition change. It appears, therefore, that the transformation involves nucleation, growth and coarsening of the O phase by a diffusion mechanism.