Continuous-cooling α-to-γ transformation behaviour of Ti–45.5 at.% Al–0.05 at.% B alloy

Abstract

The continuous-cooling transformation behaviour of Ti–45.5 at.% Al–0.05 at.% B alloy was quantitatively measured using a real-time resistivity–temperature–time measurement apparatus operating under a high vacuum. The addition of a small amount of B does not significantly alter the α–γ-phase equilibria but significantly raises the α–γ lamellar start temperature of Ti–45.5 at.% Al alloy at most cooling rates. Furthermore, it markedly increases the critical cooling rate for the ordering reaction. The effect of B addition, which greatly stabilizes the lamellar structure up to a fast cooling rate, is to accelerate the lamellar formation kinetics; the lamellar spacing was nevertheless distinctively larger in a B-doped alloy. This is because lamellae in B-doped alloy nucleate heterogeneously on titanium borides at the grain boundary; the borides are effective nucleation sites particularly since local Ti depletion can occur near the interface of the growing titanium borides during cooling. In the absence of B addition, the lamellar structure starts to form only at temperatures below T ₀, suggesting that a large undercooling is required for the nucleation of lamellae even at the grain boundaries. On the other hand, the B addition greatly retards the kinetics of the α-to-α₂ ordering reaction by markedly increasing its critical cooling rate without a large change in the ordering temperature. This is believed to be due to its tendency to segregate strongly to the antiphase boundaries.

Acknowledgement

The authors are grateful to the Korea Science and Engineering Foundation for their financial support of this research.