Acicular α₂ precipitation induced by capillarity at α/β phase boundaries in Ti-14Al-2Zr-3Sn-3Mo-0.5Si titanium alloy

The precipitation of α₂ phase based on Ti₃Al from Ti-14Al-2Zr-3Sn-3Mo-0.5Si titanium alloy homogenized at different temperatures in the (α + β) two-phase field has been characterized. Minimal lattice mismatch between the α and α₂ phase leads to a massive ordering transformation upon cooling and results in morphologies distinct from the ellipsoidal shape of the α₂ particles. Enrichment of Al in front of growing β grains due to the Gibbs-Thomson effect, together with preferential diffusion of Al along 〈1 1 2̄ 0〉 produces a layer of acicular α₂ particles apparently perpendicular to the α/β phase boundaries. Removal of the capillarity-induced composition inhomogeneity by subsequent aging at 750 °C results in disappearance of the acicular shape of the α₂ particles.