The interdependence of the thermal and mechanical cycling behaviour in Ti2448 (Ti-24Nb-4Zr-8Sn, wt%)

Metastable beta Ti alloys based on the Ti-Nb system have received increased attention in recent years due to an attractive range of properties. This includes their ability to undergo martensitic phase transformations on application of a stress, or on cooling below a critical temperature. However, the cyclic behaviour of these materials is unstable, which is a significant barrier to widespread industrial use. Recently, both the thermal and mechanical aspects of the transformation have been shown to be highly sensitive to the presence of internal stresses, which include a contribution from the dislocations produced as a necessary product of the transformation to provide geometric continuity at the phase boundary. It has been hypothesised that these dislocations may be responsible for the unstable cyclic behaviour. Redistribution of these dislocations and their associated stresses at elevated temperatures provides a novel mechanism by which unstable cyclic behaviour can be mitigated. Hence, in this work, in situ synchrotron characterisation is used to assess the effect of thermal cycling subsequent to mechanical loading on the superelastic transformation in a commercial Ti-Nb alloy. For the first time, it was shown that unstable cyclic behaviour can be avoided through the use of inter-cycle thermal treatments, which represents a significant breakthrough in our understanding of Ti-Nb superelastic materials.