Two-step nucleation of the γ-phase in a Ti-45Al-18Nb alloy

Phase transformations in a high Nb Ti–Al–Nb alloy (Ti-45Al-18Nb at%) upon cooling were studied. This alloy solidifies as single β-phase, which upon cooling under equilibrium conditions, is expected to follow the β → β + γ → β + γ + σ → γ + σ transformation path. The DTA analysis confirmed two transformation events as manifested by two peaks upon cooling. However, compositional and structural characterization of samples with different heat treatment schedules, including slow cooling (9 °C/min), revealed that the σ-phase did not form upon cooling. The two DTA peaks observed in the cooling cycle of the alloy are proven to be associated with the two-stage formation of the γ-phase. Detailed microstructural evaluations showed that the γ-phase nucleated in two discrete stages and exhibited two different morphologies. The absence of the σ-phase is believed to be due to the lack of enough driving force at high temperatures. The aging of a fast cooled sample, yielding an equilibrated microstructure, substantiated the presence of the σ-phase.