Effect of hydrogen on the temperature-dependent activation volume and the strain rate sensitivity of structural steel, coarse-grained and nanocrystalline Nickel

For more than 150 years, it has been considered proven that hydrogen generally degrades the mechanical performance of metals. Nevertheless, there is no consensus on the exact mechanisms, how hydrogen affects plastic deformation. The strain rate sensitivity of a material results from a thermally activated contribution to the rate-determining deformation process, e.g. to dislocation slip or dislocation grain boundary interaction. In this study, the extent to which hydrogen affects thermally activated dislocation motion and hence the strain rate sensitivity was investigated. For this purpose, specimens were cathodically charged in situ, and subjected to nanoindentation. In addition, macro-tensile tests with strain rate jumps were performed varying the temperature into the cryogenic range, to inhibit effusion, but also to test the effect of hydrogen on the activation parameters of deformation. Hydrogen was shown to increase the strain rate sensitivity of f.c.c. nickel, whereas it is not affected for a structural steel with a b.c.c. lattice. The activation volume for plastic deformation in a direct comparison between nanocrystalline and coarse-grained f.c.c. nickel and the b.c.c. structural steel shows, that the rate-determining deformation mechanism appears to change for f.c.c. but not for the b.c.c. material.