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Abstract
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.
Highlights
Metastable beta (β) titanium alloys, which retain the high temperature bcc phase when quenched from the β phase field, are used in many high strength applications, including the landing gear of large civil aircraft [1,2,3]
The results presented here are the first direct evidence of a B2 structure existing in the Ti-Mo system, similar microstructures comprising different phases have been reported before in Ti-Mo alloys with lower Mo concentrations
The phase and microstructural evolution of Ti-15Mo wt% during heat treatment at 300 °C has been characterised through transmission electron microscopy using both in situ and ex situ techniques
Summary
Metastable beta (β) titanium alloys, which retain the high temperature bcc phase when quenched from the β phase field, are used in many high strength applications, including the landing gear of large civil aircraft [1,2,3]. Following rapid cooling from high temperature, the microstructures of these alloys are almost always found to contain nanoscale particles of the athermal omega (ω) phase in addition to the metastable β phase [8, 9]. This phase forms through the consecutive collapse of pairs of {111}β as a result of a soft phonon mode in the β phase and the particles are compositionally indistinct from the surrounding matrix [10, 11]. The exact mechanism by which this occurs remains under debate [13,14,15,16] and some of the most recent work has suggested that α phase nucleation may, be independent of the athermal ω phase [17]
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