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

Abstract

The formation of the ω phase in the presence of simultaneous development of compositional modulations (or phase separation) within the body-centered cubic β 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 β phase field allows apparently athermal formation of ω domains without any significant partitioning of solute or modulation in matrix composition, subsequent annealing at 500 °C for just 60 s leads to unusually rapid growth of the ω domains concurrent with, but apparently independent of, a slower development of finer-scale modulations in solute composition occurring apparently uniformly across both ω and β phases. In contrast, on slower air cooling from the solution treatment temperature, there are pronounced compositional fluctuations within the β phase, presumably as a product of spinodal decomposition, that are detectable prior to the formation of ω phase. The ω phase subsequently forms preferentially in solute-depleted regions of the matrix β, 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 °C.