Phase transformation during continuous heating in a β-quenched Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe alloy

Abstract

As a new near β titanium alloy designed by NIN (the Northwest Institute for Non-ferrous Metal Research), the phase transformations of the β-quenched Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe alloy during continuous heating have been investigated by means of in situ dilatometer test coupled with X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The β-quenched Ti-5321 alloy with an initial structure consisting of metastable β phase and dense ωath, which was subjected to continuous heating treatment and underwent the following transformations: β + ωath→ β + ωiso→ β + α+ ωiso→ β + α. The ωisophase precipitated at 300 ℃ with a ~3 nm ellipsoidal morphology. When continuously heated to 390 ℃, a coexistence of both ωiso and α phase was observed in the β matrix, the platelet-like α phase closely nucleating at the ω/β interface with ~1 nm in width and ~8 nm in length. The α phase subsequently developed into lamellar structure when heated to higher temperatures, with a width of ~20 nm at 570 ℃ and ~85 nm at 690 ℃ respectively. Meanwhile, these α laths were uniformly distributed and composed of two distinct orientations within the microstructure. Finally, it can be found that the α morphology is directly associated with the formation and decomposition of the metastable ωisophase which may lead to the homogeneous and fine distribution of α precipitates. Based on the study of phase transformations occurring during the continuous heating process presented in this paper, an efficient guidance to engineer the microstructures and mechanical properties of this new near β titanium alloy was offered.