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Instruments / Technical Area	Publication Details	Published
I12-JEEP: Joint Engineering, Environmental and Processing	Functional fatigue during superelastic load cycling of Ti2448 (Ti-24Nb-4Zr-8Sn, wt%) N. I. Church , C. E. P. Talbot , L. D. Connor , S. Michalik , N. G. Jones Materialia 214 DOI: 10.1016/j.mtla.2023.101719 Diamond Proposal Number(s): [30411] Show Abstract ▼ Abstract: Superelastic alloys based on Ti-Nb have potential in the aerospace sector for vibration damping applications, due to their wide mechanical hysteresis and tuneable properties. However, their uptake is currently limited by functional fatigue, whereby a degradation in properties is seen on cyclic loading. To understand the mechanisms that underpin this change in behaviour, a combination of ex situ and in situ testing was used to characterise the evolution of the mechanical response in a commercial Ti-Nb based alloy. It was found that the behaviour of these materials changes via a two-step mechanism, driven by the accumulation of transformation related defects and their associated stress fields. This understanding rationalises many discrepancies within the literature and highlights how the overall shape of the load response of these alloys is dominated by changes occurring only in specific regions of the material.	Feb 2023
I11-High Resolution Powder Diffraction I12-JEEP: Joint Engineering, Environmental and Processing	The effect of sample size on the microstructure and mechanical properties of Ti2448 (Ti-24Nb-4Zr-8Sn wt%) N. I. Church , L. D. Connor , N. G. Jones Scripta Materialia 222 DOI: 10.1016/j.scriptamat.2022.115035 Diamond Proposal Number(s): [30411, 22960] Open Access Show Abstract ▼ Abstract: Ti-Nb based alloys are of commercial importance due to their damping capabilities via a superelastic transformation. However, the starting microstructure of these alloys is often not consistent between studies, even when produced via a nominally comparable methodology, which has the potential to limit the reversibility of the transformation. As such, samples were produced to investigate whether the thickness of the material influenced the microstructures and behaviours seen. It was shown that a single-phase layer exists close to the surface of a two-phase microstructure, and that in very thin material, this can span across the whole sample, promoting more recoverable transformation behaviour. This is important as it not only rationalises discrepancies in the literature, but may also influence component geometries necessary for industrial application.	Sep 2022
	On the role of internal stresses on the superelastic behaviour of Ti-24Nb (at.%) E. M. Hildyard , L. D. Connor , N.l. Church , T. E. Whitfield , N. Martin , D. Rugg , H. J. Stone , N. G. Jones Acta Materialia 273–275 DOI: 10.1016/j.actamat.2022.118161 Show Abstract ▼ Abstract: Ti-Nb-based alloys undergo a martensitic transformation that gives rise to superelastic and shape memory behaviours over a wide range of temperatures. The design of such alloys for industrial applications is commonly based around the reported martensite start temperature for a given composition. However, there are significant variations in these values within the literature that remain unexplained using current thermally driven transformation theories. Recently, a marked difference in the transformation behaviour of Ti-24Nb (at.%) in the cold rolled and solution treated conditions has been reported and a stress based mechanism has been postulated to rationalise these results. Here, the veracity of this stress based theory has been investigated through in situ studies of the transformation behaviour of Ti-24Nb (at.%) in the cold rolled condition using synchrotron X-ray diffraction. Upon heating to 350˚C, all αʺ initially present reverted to the β phase by ∼ 300˚C. When cooled from 350˚C, thermally induced martensite was observed below ∼ 270˚C and the intensity of the αʺ peaks increased as the temperature decreased. When loaded at temperatures between -150 and 30˚C, further transformation was observed to occur immediately upon the application of a stress. However, critically, αʺ under load was always found to be reversible upon unloading, despite being up to 420˚C below the observed martensite start temperature. This behaviour cannot be accounted for through conventional thermally driven transformation theories for these alloys. Comparison of the stress-strain curves for cold rolled and solution heat treated condition material indicated that their behaviours were similar but offset by a value equivalent to the difference in the critical transformation stress. This offset is readily explained by the total stress-based mechanism proposed and provides further evidence as to its fidelity.	Jul 2022

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