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

It is reported that there are several different transformation mechanisms o f the O phase in different heat treatment conditions in the Ti3Al based all oys. However, very little work has been carried out on the alpha(2)-->O pha se 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-24A1-14Nb-3V- 0.5Mo (at%) alloy has been carried out by means of TEM and HRTEM. The resul ts show that the O phase is directly derived from the primary equiaxed alph a(2) grains with a fine streak contrast, and exists in multivariant forms o wing to its different orientations after the alloy is solution treated at 1 000 degrees C for 1 h followed by water quenching (WQ) and aged at 650 degr ees C for 24 h. The O plates in the primary equiaxed alpha(2) grains exist not only in the form of a single variant, but also in the form of fine alph a(2) + O mixtures. The analysis indicates that the formation of the O phase is the result of a phase decomposition, that is the introduction of niobiu m as the preferred beta stabilizer makes the supersaturation of niobium in the primary alpha(2) grains, and the alpha(2) phase containing Niobium sepa rates into Niobium lean and Niobium rich regions through the Niobium diffus ion: alpha(2)-->alpha(2)(Nb-lean) + O(Nb-rich). Niobium rich regions transf orm to the ordered orthorhombic phase (O phase) with a lattice distortion a nd only a very small composition change. It appears, therefore, that the tr ansformation involves nucleation, growth and coarsening of the O phase by a diffusion mechanism. (C) 2000 Elsevier Science Ltd. All rights reserved.