Plastic flow in Fe-Cr-Ni austenitic steel under the presence of solute H: A study via room temperature creep

A connection between the effects of solute hydrogen (H) on macroscopic flow stress and microscopic dislocation mobility has been a subject for understanding the plastic flow behavior of H-charged austenitic steels and other face-centered cubic (FCC) alloys. In this study, we try to solve this problem by examining the room temperature creep of a Fe-24Cr-19Ni-based Type310S austenitic stainless steel uniformly charged with 9000 at ppm solute H in a pressurized gaseous H 2 environment. Stress-dip test to decompose the flow stress into effective (thermal) and internal (athermal) stresses, as well as a brief analysis of dislocation structure development in deformed uncharged and H-charged samples by electron channeling contrast imaging and hardness measurement, were supplementally employed. It is emphasized that solute H atoms consistently act as short-range obstacles hindering the movement of dislocations and thereby causes significant solid solution-hardening. Nevertheless, two opposite H-effects to accelerate and retard the creep rate were macroscopically identified depending on whether the stress level applied to H-charged specimens is above or below the flow stress under non-charged condition. These newly found, seemingly contradicting phenomena were interpreted based on the stress-dependent change of the rate-controlling mechanisms predominating thermally activated dislocation motion. Primary rate-controlling obstacles are H atoms themselves when creep acceleration manifests at high stress, while H atoms, other alloying elements, and forest dislocations cooperatively work to retard creep under low stress. The potential model of dislocation motion under the presence of these multiple obstacle types is finally proposed.