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 (sigma(beta-alpha '')) 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 (sigma(alpha ''-beta)). 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 omega (omega(ath)) phase increases with a decrease in temperature. An in situ XRD experiment obtained during a loading-unloading cycle shows that the beta and omega(ath) phases transform into the alpha '' phase during loading. The annihilation of the omega(ath) phase within the alpha '' phase allows sigma(alpha ''-beta) to obey the Clausius-Clapeyron relationship. As a result, a large hysteresis loop is produced. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.