Dislocation transport of hydrogen in iron single crystals

Abstract

Iron single crystals with a proper combination of surface orientation and tensile axis were used to separately study the hydrogen transport rates by screw, edge and mixed dislocations, as a function of strain rate, lattice hydrogen concentration and temperature. Significant concurrent trapping was suppressed, except in one deliberate case, by selecting easy glide orientations. It was found that qualitatively the hydrogen flux transported by dislocations intimately reflects dislocation egression from the monitored crystal surface. Quantitatively, the hydrogen transport rate increased with decreasing strain rate; edge kinks appeared to possess the greatest capability of transporting hydrogen at the lowest strain rate (1.6 × 10 −8 s −1) employed. An insignificant effect of lattice hydrogen concentration on dislocation transport was observed, and was attributed to the kinetic nature of this transport process where equilibrium between hydrogen concentration on the dislocations and in the lattice is unlikely. A minimal temperature effect on dislocation transport rate was also observed within a narrow temperature range, from 12 to 78°C, consistent with the expectation that the extra thermal energy provided by increased temperature was too small to alter the interaction between hydrogen and mobile dislocations. This study demonstrated the need to establish proper experimental condition to discriminate and compare trapping and transport effects.