Search

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.

Metastable -Ti alloys based on the Ti-Nb system are of commercial interest as they undergo a reversible martensitic transformation that offers damping capabilities. However, their use is currently limited by a low yield stress compared to other alternatives such as NiTi. Here, we show how this limitation can be overcome by the in situ formation of the Ti3Sn phase concurrent with recrystalisation. The Zener pinning resulting from the Ti3Sn precipitates provides significant Hall-Petch strengthening leading to a yield stress in excess of 1 GPa, more than double that of similar alloys, whilst still allowing conventional processing routes. This has the potential to provide Ti-Nb yield stresses that can rival those of NiTi, addressing one of the biggest challenges for this class of alloys.

Superelastic alloys, such as those based on Ti2448 (Ti-24Nb-4Zr-8Sn wt%) have potential to be used in vibration damping systems in the aerospace industry. However, the accumulation of strain on superelastic load cycling, which can additionally be shown to depend on the cooling rate from the  phase, is not well characterised or understood and, as such, limits industrial acceptance. This study aims to rationalise the mechanical behaviour of Ti2448 for three sample conditions and suggests a mechanism to explain the evolution of this behaviour with mechanical cycling. By understanding the mechanisms that govern the mechanical response of these materials, alloys can be more specifically designed and processed to achieve the properties desired for industrial applications.

Superelastic Ti alloys have potential across a wide range of industrial sectors, but each application requires specifically tailored transformation behaviour. Composition is well known to influence transformation properties but for a given alloy, the effect of microstructural features, such as grain size, is far less clear. Within the literature, no consistent relationship between grain size and key parameters, such as the transformation stress, SIM, exist. This lack of clarity is a major barrier to engineering uptake of these materials. Therefore, in the present work, the transformation behaviour of Ti-24Nb-4Sn-8Zr (wt%) from two different starting stocks has been studied, with grain sizes varied by changing solution heat treatment time. Irrespective of the material stock, SIM was shown not to vary as a function of grain size. As such, this observation is believed to be a general effect, and as a result, microstructures can be tailored to improve other parameters such as yield stress, without altering the transformation behaviour.

The formation of non-equilibrium and transient phases in metastable beta titanium alloys during low temperature thermal treatments is currently of great interest, as they provide a potential method of controlling the size and distribution of the equilibrium alpha phase and, hence, the resulting mechanical properties. Here, for the first time, we report on the formation of a new, B2 structured phase in the Ti-Mo system. The phase was observed in electron transparent material during in situ, and following ex situ, heat treatment at 300 °C. The B2 phase was enriched in Mo compared to the surrounding matrix material and formed in regular arrays of approximately square cross-section particles interspersed by thin beta channels. Electron diffraction indicated that the lattice parameter of this new phase was smaller than that of the parent phase, leading to significant strain in the beta channels. Critically, the B2 phase was only observed in material that had been electro-polished prior to heat treatment, and, therefore, it is hypothesised that this phase forms as a result of the preparation method and thin foil effects.

The elastic moduli and acoustic loss behaviour of Ti-6Al-4V (wt.%) in the temperature range 5–298 K have been studied using Resonant Ultrasound Spectroscopy. A peak in the acoustic dissipation was observed at 160 K within the frequency range 250–1000 kHz. Analysis of the data acquired in this study, coupled with complementary data from the literature, showed that this was consistent with a Snoek-like relaxation process with an associated activation energy of 23 3 kJ mol$^{−1}$. However, the loss peak was broader than would be expected for a Snoek-like relaxation, and the underlying process was shown to have a spread of relaxation times. It is suggested that this effect arises as a result of variations in the strain experienced by the β phase due to different local microstructural constraint by the bounding secondary α phase.

An isothermal section of the Fe-Mo-Ti ternary system at 1000 °C has been constructed using data acquired from a series of seven alloys. The limit of solubility of Fe in the continuous A2 phase field between Ti and Mo has been determined, as have the extents to which Mo may be accommodated in the B2 TiFe phase, and Ti in the D8$_5$ Fe$_7$Mo$_6$ phase. The B2, D8$_5$ and C14 Fe$_2$ (Ti, Mo) intermetallics were found to have limited tolerance for non-stoichiometric compositions. The positions of the A2 + B2 + C14 and A2 + C14 + D8$_5$ three-phase fields were determined, along with the extents of the A2 + B2, A2 + D8$_5$, A2 + C14, C14 + B2 and C14 + D8$_5$ two-phase fields. No ternary phases were observed in any of the alloys studied.