On the stability and formation of the " and phases in Ti-Nb alloys upon cooling

Metastable beta-Ti alloys based on the binary Ti-Nb system can exhibit a stress induced transformation to the alpha '' phase allowing superelasticity and shape memory behaviours but they are also susceptible to the formation of the hexagonal omega phase. The presence of omega is widely reported to prevent the alpha '' transformation, and in certain forms embrittles the material. Most studies characterise the omega phase ex situ and only observation of the omega 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 alpha '' and athermal omega phase transformations with respect to both temperature and composition in a series of cold rolled Ti-Nb alloys which initially contained both alpha '' and omega. Across the range of studied alloys, the alpha '' start temperature was always found to be above the omega start temperature but both phases were observed to grow simultaneously contradicting previous reports. In addition, the omega start temperature in all alloys was found to be <= 10C indicating that the omega 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 omega phase and categorically show that observation of this phase alone cannot be used to establish its stability and the relevant thermodynamic temperature, omega s.