Formation of intermediate phases and their influences on the microstructure of high strength near-β titanium alloy

The influences of heating rate to aging temperature on intermediate phase transformation and final microstructures of near βTi alloy Ti–5Al–3Mo–3V–2Cr–2Zr–1Nb–1Fe (wt.%) have been studied in this work. The results showed that three nano-scaled intermediate phases, i. e. O′, ω_iso and O″ phases, were subsequently formed before α precipitation when heating by slow (10 ^°C/min) and fast (~100 ^°C/min) rates, depending on heating rate. The ω_iso and O″ phases become unstable and will dissolve before arriving the aging temperature, while plenty of O′ domains can always be observed accompanying with the precipitation of needle-like α phase. Whereas upon rapid heating (~400 ^°C/min), only O′ phase can be observed during continuous heating to target aging temperature. We suggest that the uniform and dense distribution of fine α precipitates formed even at fast heating rate ascribes for the precipitation of a large number of O′ precursors assist α nucleation. The slow and fast heating rates result in similar yield strength of the aged samples, however, the slow heating rate can obtain better ductility due to the formation of discontinuous grain boundary α colonies instead of continuous α layers.