Hydrogen concentration-induced stresses in an environmental TEM

Abstract

The hydrogen-enhanced localized plasticity (HELP) mechanism is a leading candidate among proposed hydrogen embrittlement (HE) mechanisms. Transmission electron microscopy (TEM) measurements of an increased dislocation mobility on exposure to hydrogen have provided the most direct evidence for the HELP mechanism. However, it has been hypothesized that the observed dislocation behavior is not a consequence of the interaction between hydrogen atoms and dislocations but a systematic error due to the nature of in situ TEM. Specifically, the electron beam in TEM microscopes can dissociate hydrogen, leading to a hydrogen fugacity much greater than the applied pressure. Such high fugacity will generate a large concentration gradient between the surface and interior of a TEM sample. It has been proposed that the observed dislocation mobility is due only to stresses arising from this concentration gradient rather than an effect of the interaction with hydrogen. Here we calculate the expected stresses for H/Fe and H/Ni systems fugacities expected in an in situ TEM. We show the stresses to be an order of magnitude too low to impact dislocation mobility in the H/Fe system, and that the concentration gradient-induced stresses dissipate quickly in both H/Fe and H/Ni systems---well before observation by TEM would occur.