A Mechanistic Understanding of the β to α′′ Transformation in Superelastic Ti-24Nb-4Zr-8Sn (wt%)
Metastable β titanium alloys have potential for application in vibration damping systems, due to their wide mechanical hysteresis and large recoverable strains, arising from a re- versible martensitic transformation between a parent bcc β phase and an orthorhombic α′′ martensite. Of these alloys, those based on the Ti–Nb binary system such as Ti2448 (Ti-24Nb-4Zr-8Sn, wt%) are of particular interest, as with appropriate alloying and ther- momechanical processing their properties can be tuned for application over a wide range of temperatures. However, their use is currently limited by a significant variability in both thermal and mechanical transformation parameters. These are critical to defining the superelastic response and vibration damping potential of the material, which can additionally be shown to vary with both thermal and mechanical cycling. This study aims to rationalise how processing and loading parameters affects different aspects of the martensitic transformation, via a combination of in situ and ex situ techniques. Whilst the superelastic transformation was shown to largely be independent of grain size, it was found to be highly sensitive to other parameters. In particular the sensitivity of the microstructure to thermal history was identified, and it was shown that this sensitivity could not be rationalised by currently held theories. In light of this, the mechanical be- haviour of different microstructural conditions was identified to ascertain whether this sensitivity to thermal history was the origin of many discrepancies in the mechanical be- haviour. However, it was shown that the mechanical response was only dependent on the initial microstructure for low applied stresses, with microstructural changes occuring during testing of the material in response to the applied load. Consequently, an alterna- tive mechanism was proposed, consistent with a total stress approach, which was further shown to be consistent with variations in microstructure and mechanical properties for samples of varying thicknesses. In further characterising the mechanical response at low applied stresses, it was found that the behaviour of these materials actually changes via a two-step mechanism, driven by the accumulation of transformation related defects and their associated stress fields, again consistent with a total stress model.
This understanding highlighted that both the thermal and mechanical behaviours of the alloy may be mechanistically linked, where both effects could be rationalised by a total stress approach, driven by a changing distribution of dislocation strain fields. This link was confirmed through two experiments, that probed both mechanical and thermal aspects of the transformation. This understanding is able to rationalise many discrepan- cies within the literature and paves the way for more successful alloy design, which will ultimately make this class of alloy more commercially viable.