Anomalous temperature dependence of the superelastic behavior of Ti-Nb-Mo alloys

The effect of test temperature on the superelasticity of Ti-27Nb and various Ti-Nb-Mo alloys is investigated. A deviation in the stress at which martensitic transformation starts (σβ-α″) from the behavior expected from the Clausius-Clapeyron relationship is confirmed in all alloys. The degree of deviation is found to be in inverse proportion to the electron-to-atom ratio. However, no deviation is observed in the stress at which the reverse transformation finishes (σα″-β). All alloys exhibit anomalous electrical resistivity during cooling. X-ray diffraction (XRD) and transmission electron microscopy investigations show that the volume fraction of the athermal ω (ωath) phase increases with a decrease in temperature. An in situ XRD experiment obtained during a loading-unloading cycle shows that the β and ωath phases transform into the α″ phase during loading. The annihilation of the ωath phase within the α″ phase allows σα″-β to obey the Clausius-Clapeyron relationship. As a result, a large hysteresis loop is produced.