Hydrogen-enhanced microplasticity of austenitic steels studied by means of internal friction

Hydrogen-induced damping at low temperatures is studied in austenitic steels Cr18–Ni16–Mn10, Cr25–Ni20 and Cr25–Ni40. The orientation dependence of the hydrogen-caused Snoek-like relaxation measured in single crystals gives evidence for the orthorhombic symmetry of the s–H complexes created by hydrogen and substitutional atoms in the γ solid solution. The observed splitting of the Snoek-like relaxation peak is analysed on the basis of the relaxation theory caused by non-cubic defects in the cubic matrix. It is shown that the increase in the chromium and nickel content changes the symmetry of the s–H complexes. Based on the analysis of the experimental data in Arrhenius coordinates and on the temperature of the Snoek-like peak caused by the local jumps of hydrogen atoms, one can suppose that chromium increases the enthalpy of the local migration of hydrogen atoms in s–H complexes whereas nickel decreases it. Hydrogen charging diminishes the critical strain above which internal friction depends on the strain and increases the damping in its strain-dependent range. The results are interpreted in terms of hydrogen-caused decrease in the start stress of the localised plastic deformation and an increased mobility of emitted dislocations, which is consistent with the hypothesis of the hydrogen-enhanced localised plasticity.