Abstract
For pt.I see ibid., vol.15, p.507 (1985). An analysis of the variation of the elastic moduli and attenuation in the Nb-H(D) system with composition and temperature is carried out. The composition-dependent changes in elastic moduli are shown to consist of a temperature-independent part and a temperature-dependent part. The temperature-independent changes are analysed on the basis of contributions due to changes in volume and in the electronic structure of the alloy. The isotopic dependence of these contributions is shown to be very small. The temperature-dependent contributions to the changes in elastic moduli are each associated with an acoustic attenuation and exhibit a significant isotopic dependence. These temperature-dependent contributions are analysed to determine the critical temperatures and relaxation strengths as functions of composition. The role of phase transitions and stress-induced changes in short-range ordering is considered in interpreting the temperature-dependent changes in modulus and the attenuation. It is shown that the results are consistent with a relaxation due to stress-induced changes in the short-range ordering of the hydrogen interstitials, i.e. a Zener relaxation.