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

Abstract 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 (σ β–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 (σ α″–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 (ω ath ) phase increases with a decrease in temperature. An in situ XRD experiment obtained during a loading–unloading cycle shows that the β and ω ath phases transform into the α″ phase during loading. The annihilation of the ω ath phase within the α″ phase allows σ α″–β to obey the Clausius–Clapeyron relationship. As a result, a large hysteresis loop is produced.