ELASTIC CONSTANT SOFTENING AND SHORT-RANGE ORDERING PHENOMENA IN METAL-HYDROGEN SYSTEMS

Abstract
Stress induced short-range order changes, which give rise to anelastic relaxation processes (Zener effects) have been found to occur in several metal-hydrogen systems, such as Pd-H, Pg-Ag-H, Nb-H and Zr-H. The main results obtained so far in various Me-H systems will be reviewed and the perspectives for further work examined. Particular attention will be paid to the Nb-H system where elastic constant softening effects in the α' phase have been observed on approaching the α'→β transition temperature. The composition dependent elastic moduli changes were shown to consist of a temperature independent and a temperature dependent part. The temperature independent changes were analyzed on the basis of contributions due to volume changes and to changes in the electronic structure of the alloy. The temperature dependent contributions to the elastic moduli changes were each associated with an acoustic attenuation and exhibited a significant isotopic dependence. It was shown that the results are consistent with a relaxation due to stress-induced changes in the short-range order of the H interstitials i.e. a Zener relaxation.