Conjugated precipitation of twin-related α and Ti₂Cu phases in a Ti-25V-3Cu alloy

The formation of conjugated precipitates in a ternary Ti–25V–3Cu (wt.%) (Ti–24.4V–2.3Cu (at.%)) alloy subjected to isothermal aging at 500 C was investigated in this study. X-ray diffraction confirms that the equilibrium precipitates in the alloy are hexagonal α phase and bodycentered-tetragonal (bct) Ti₂Cu intermetallic compound. These precipitates are composed of two geometrically symmetrical components, which are verified by transmission electron microscopy (TEM) to be twin-related variants of α and Ti₂Cu phases. Atomic-level characterizations incorporating high-resolution TEM and atom probe tomography were used to investigate the early-stage nucleation process of the precipitates. The results reveal that precipitation occurs heterogeneously on plate-like (or disc-like) metastable Cu-enriched GP zones that preferentially inhabit {110}_β planes. Crystallographic orientation analyses suggest that the Ti₂Cu phase is related to the matrix via a definite orientation relationship of (013)_Ti2Cu||(011)_β and [100]_Ti2Cu||[100]_β, while the growth of the α phase is governed by the Potter’s OR such that (1101)_α||(011)_β and [1120]_α||[111]_β. (013)_Ti2Cu and (1101)_α are the twin planes of the respective phases. In spite of highly distinct crystal structures, the conjugated α and Ti₂Cu phases attain a nearly perfect lattice correspondence along certain crystallographic planes, which results in a low interfacial energy configuration and favours the co-development of these phases.