Dr Edward Obbard (UNSW Dept. of Electrical Engineering and Telecommunications)Prof. Yulin Hao (Shenyang National Laboratory for Materials Science)
Research to optimize the biocompatibility of titanium alloys for orthopaedic applications focusses on minimizing the Young’s modulus of quenched, β-phase (Im-3m) titanium by adjusting the concentration of β-stabilizing elements. Ti-15Nb-2.5Zr-4Sn has the lowest Young’s modulus yet measured in any forged titanium alloy of less than 50 GPa. This composition also possesses a controllable thermal expansion coefficient that is influenced by prior plastic deformation and it is superelastic over a very wide temperature range. There is pronounced non-linear elasticity and asymmetric response to strain, associated with the stress induced, ferroelastic transition between the bcc β-phase and α" (Cmcm) martensite phase. The potential to tailor these remarkable properties to specific applications, as well as further progress in superelastic alloy development require a clear microscopic understanding of the underlying physical effects. Therefore, to investigate these “pre-martensitic” phenomena and quantitatively explain the non-linear elastic responses we carried out in-situ measurement of β-phase Ti-15Nb-2.5Zr-4Sn single crystals on ESRF beamline ID15B, equipped with a large position-sensitive detector and rotating load rig, that allows single-crystal x-ray scattering measurements with crystals as thick as several millimetres with simultaneous application of compressive or tensile stress. The results give a detailed picture of the large bcc crystal as it approaches and progresses through the structural transformation. Domains of ferroelastic α" and also ω instabilities form in response to the temperature and the direction and magnitude of the applied stress. We present these results and discuss some of the challenges of developing a quantitative interpretation of the data, an important one being how to properly treat coupling between elastic deformation and correlated structural distortions.
Dr Edward Obbard (UNSW Dept. of Electrical Engineering and Telecommunications)