Understanding the transformation behaviour of superelastic Ti-24Nb-4Zr-8Sn (wt%)

Some metastable b titanium alloys are able to undergo a fully reversible transformation to the martensitic a² phase giving rise to hysteretic loading behaviour. These alloys have potential application across a range of industries for uses including bone replacements in the biomedical industry, and as a part of vibration damping systems to reduce emissions and noise in the automotive and aerospace sectors. However, the commercial viability of these alloys is currently limited by large variations in reported transformation parameters, differences in the mechanical response between different studies and changes in properties which occur during mechanical cycling.

Differences in transformation parameters are often rationalised in the literature by preferential formation of the w phase. However, only the isothermal form of w (wiso) is thought to negatively impact the transformation from b to a², with the athermal form having relatively little effect. Despite this, there is limited evidence to suggest that wiso can form in sufficient volume fractions within the necessary timeframes to be responsible for this behaviour in all alloy compositions. Furthermore, the consequence of these discrepancies in transformation parameters on the evolution of a materials mechanical response is often not considered. As such, current theories rationalising the discrepancies in superelastic properties are insufficient to capture either variations in their initial behaviour or the evolution of their long-term stability.

Here, data from in situ and ex situ characterisation techniques will be presented and used to support an alternative mechanism governing the behaviour of these alloys, based on a combination of internal and external stresses in the material. These data show how local strain heterogeneities develop as a result of αʺ formation and influence both the phases present in the microstructure and the initial transformation parameters. This talk will further discuss how these features persist to influence the transformation behaviour during both thermal and mechanical cycles, and how regions of higher local strain can dominate the form of the macroscopic tensile response, paving the way towards a more complete understanding of the behaviour of these alloys.

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