Abstract
The superelastic behaviour of Ti-Nb alloys gives rise to properties that are attractive for specific applications in the biomedical and aerospace sectors. However, to date, industrial utilisation of these alloys has been limited due the inability to tailor the transformation conditions or achieve stable cyclic behaviour. Alloying is the primary method for modifying transformation conditions but significant variations exist between the results of different studies within the literature. Here, to try and provide increased clarity, the transformation behaviour of Ti-24Nb-(0-8)Ta and Zr (at.%) have been investigated using in situ synchrotron diffraction to directly assess the transformation behaviour of the alloys in response to both temperature and tensile loads. In contrast to previous reports, no evidence of the αʺ phase was found in any of the alloys when cooled from 350 to -196˚C. In addition, the β to αʺ transformation was observed to be reversible when loaded at both 30˚C and ‑196˚C. These observations suggest that β stability may be far greater than previously thought and requires a review of the current mechanistic understanding.
Highlights
Metastable β Ti alloys, based on the Ti-Nb system, have been the subject of considerable research over the past fifteen years due to their susceptibility to undergo the reversible β to αtransformation
These properties, in combination with low β phase elastic moduli, make these alloys attractive for a number of potential applications across the biomedical and aerospace engineering sectors. The uptake of these materials, in the aerospace sector is being limited by a number of as yet unresolved technical limitations. Examples of these limitations include, an inability to tailor the transformation behaviour to a specific application operating conditions, the difficulty of controlling the formation and evolution of the ω phase and the progressive change in behaviour that occurs with repeated transformation cycling [3, 4]
Additions of bio-compatible elements, such as Ta and Zr, have been the most widely studied and it is generally agreed that these elements stabilise the β phase and suppress the martensitic transformation
Summary
Metastable β Ti alloys, based on the Ti-Nb system, have been the subject of considerable research over the past fifteen years due to their susceptibility to undergo the reversible β to αtransformation. Depending upon the composition of the alloy and the conditions under which it is loaded, this martensitic transformation can give rise to superelastic or shape memory behaviour [1, 2]. These properties, in combination with low β phase elastic moduli, make these alloys attractive for a number of potential applications across the biomedical and aerospace engineering sectors. Modification of the transformation conditions and associated behaviour is generally achieved through the incorporation of additional elements into an alloy.
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