Phase separation and formation of omega phase in the beta matrix of a Ti-V-Cu alloy

The formation of the omega phase in the presence of simultaneous development of compositional modulations (or phase separation) within the body-centered cubic beta matrix phase of a Ti-10V-6Cu (wt.%) alloy during continuous cooling has been investigated using a combination of transmission electron microscopy and atom probe tomography. While a water quench from the high-temperature beta phase field allows apparently athermal formation of omega domains without any significant partitioning of solute or modulation in matrix composition, subsequent annealing at 500 degrees C for just 60 s leads to unusually rapid growth of the omega domains concurrent with, but apparently independent of, a slower development of finer-scale modulations in solute composition occurring apparently uniformly across both omega and beta phases. In contrast, on slower air cooling from the solution treatment temperature, there are pronounced compositional fluctuations within the beta phase, presumably as a product of spinodal decomposition, that are detectable prior to the formation of omega phase. The to phase subsequently forms preferentially in solute-depleted regions of the matrix beta, with a composition reflecting the local matrix composition and a solute content significantly lower than the average matrix composition. As a result, it has a cuboidal morphology, distinguishably different from the elliposoidal form that is observed in samples water-quenched and annealed at 500 degrees C. (C) 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.