Hydrogen Effects on Strain Localization in FCC Metals at the Initial Stage of Plastic Deformation

Single crystals of pure copper, nickel and AISI 316L austenitic stainless steel were tensile tested in order to study hydrogen effects on the strain localization in form of slip bands appearing on the polished specimen surface. It was found that hydrogen increases markedly, up to 9 %, the stress, which corresponds to the Stage I of plastic deformation. Hydrogen changes markedly the dislocation slip appearance: from the homogeneously distributed single slip lines observed in hydrogen-free specimens to the localized, periodically ordered groups or bands of slip lines in the presence of hydrogen. Observations of the fine structure of the slip bands performed with AFM show that in the presence of hydrogen the slip offset spacing is reduced as compared to that in hydrogen-free specimens. Using positron annihilation spectroscopy it was found that the hydrogen-induced dislocation slip refinement is accompanied by the excessive generation of crystal lattice defects in the form of vacancy clusters. The obtained results for the studied FCC metals are compared and analyzed in terms of the mechanism of double-cross slip and the role of hydrogen in the excessive vacancy generation is discussed.