The total stress approach to martensitic transformations in Ti-Nb-based alloys.

Metastable β Ti alloys have potential for vibration damping and actuation applications within the aerospace industry due to thermal and mechanical hysteresis. However, variations in transformation parameters, which are also seen to change following thermal or mechanical cycling, significantly limit industrial acceptance. There is a widespread belief that these variations are a consequence of ⍵ phase formation. However, here we provide evidence to show that this is not necessarily the case. Instead, we show how residual stresses and defect structures are crucial to the transformation of these alloys and present an understanding of the mechanism that governs their behaviour. Importantly, we highlight the consequences for the design of new transforming alloys and component geometries, and how current design theories may need to be employed in conjunction with other methods to effectively prevent longer-term changes in behaviour. To this end, we demonstrate how functional properties could be periodically recovered by introducing short intercycle heat treatments and suggest possible next steps for advancing our understanding of these materials.