On the stability and formation of the α″ and ω phases in Ti-Nb alloys upon cooling

Metastable β-Ti alloys based on the binary Ti-Nb system can exhibit a stress induced transformation to the α″ phase allowing superelasticity and shape memory behaviours but they are also susceptible to the formation of the hexagonal ω phase. The presence of ω is widely reported to prevent the α″ transformation, and in certain forms embrittles the material. Most studies characterise the ω phase ex situ and only observation of the ω phase is used to predict its stability. This approach ignores different potential formation mechanisms and results in an ambiguous picture of its true stability. Here, through the use of in situ synchrotron X-ray diffraction we establish the true nature of the α″ and athermal ω phase transformations with respect to both temperature and composition in a series of cold rolled Ti-Nb alloys which initially contained both α″ and ω. Across the range of studied alloys, the α″ start temperature was always found to be above the ω start temperature but both phases were observed to grow simultaneously contradicting previous reports. In addition, the ω start temperature in all alloys was found to be ≤ 10˚C indicating that the ω present in the initial microstructures was metastable. These observations contradict a vast swathe of the conclusions drawn from ex situ data regarding the stability of the ω phase and categorically show that observation of this phase alone cannot be used to establish its stability and the relevant thermodynamic temperature, ωs.