Abstract
Phase transformations in a high Nb TieAleNb alloy (Ti-45Al-18Nb at%) upon cooling were studied. This
alloy solidifies as single b-phase, which upon cooling under equilibrium conditions, is expected to follow
the b / b þ g / b þ g þ s / g þ s 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 s-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 g-phase. Detailed microstructural
evaluations showed that the g-phase nucleated in two discrete stages and exhibited two
different morphologies. The absence of the s-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 s-phase.