A Study of Hydrogen Mobility in a Glassy Zr₂Pd Using Internal Friction

Hydrogen-related internal friction peaks have been measured in amorphous Zr2Pd and Y64Fe36, covering a range of hydrogen concentrations up to 0.86 H/M for the former alloy. The internal friction peaks were found to be several times wider than a peak due to a single relaxing defect, and quite asymmetric with a long low temperature tail, as with previous measurements on hydrogen in amorphous metals. They showed a thermally activated relaxation time with a frequency prefactor and a range of activation energies indicative of a point defect source. The evidence is shown to strongly indicate a Snoek-type defect, consisting of single hydrogen atoms in interstitial sites with strongly elliptical or variable strain dipole tensors. The integral equation that gives the internal friction is inverted for the first time to yield the distribution of relaxation times. It is suggested that the main peak in the resulting distribution of activation energies results from hydrogen hopping through three-sided faces, primarily between tetrahedral sites. These sites are found to change at high concentration to have a lower strain dipole ellipticity and to become more well-defined. This is interpreted as signaling a change in the structure of the amorphous metal itself. In addition there appear to be a small number of other deep-well sites where hydrogen is initially trapped at low concentrations. These have a very broad distribution of activation energies ranging from the main peak down to below 0.1 eV. A model is proposed to account for this by hopping of hydrogen between larger sites through distorted four-sided faces in the metal lattice.