Observation of a new B2 structured phase in Ti-15Mo (wt%)

The formation of non-equilibrium and transient phases in metastable beta titanium alloys during low temperature thermal treatments is currently of great interest, as they provide a potential method of controlling the size and distribution of the equilibrium alpha phase and, hence, the resulting mechanical properties. Here, for the first time, we report on the formation of a new, B2 structured phase in the Ti-Mo system. The phase was observed in electron transparent material during in situ, and following ex situ, heat treatment at 300 °C. The B2 phase was enriched in Mo compared to the surrounding matrix material and formed in regular arrays of approximately square cross-section particles interspersed by thin beta channels. Electron diffraction indicated that the lattice parameter of this new phase was smaller than that of the parent phase, leading to significant strain in the beta channels. Critically, the B2 phase was only observed in material that had been electro-polished prior to heat treatment, and, therefore, it is hypothesised that this phase forms as a result of the preparation method and thin foil effects.