New aspects of mechanical relaxation by hydrogen in metals

After an overview over the relevant relaxation mechanisms, divided into short- and long-range hydrogen motion and processes coupled with matrix defects, some recent work is reviewed with emphasis on two aspects: (i) comparison of similar relaxation effects in different amorphous, quasicrystalline and crystalline structures from the viewpoint of short-range order. The Snoek-type mechanism is discussed for the case of polytetrahedral order, and some conclusions are pointed out with respect to differences and similarities in short-range order between amorphous and quasicrystalline phases; (ii) observations of new effects based on known mechanisms. Stress gradients in the submicrometer range can produce high-frequency Gorsky effects, either by transverse H diffusion in nanomechanical resonators or by intercrystalline diffusion in ultrafine-grained polycrystals. The latter effect was studied simultaneously together with the Zener-type hydrogen relaxation in CuZr2, which was advantageous for the identification, characterization and understanding of both effects.