Hydrogen Concentration-Induced Stresses in an Environmental TEM

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 upon exposure to hydrogen provide the most direct evidence for the HELP mechanism. However, 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 concentration-gradient stresses rather than an effect of the interaction with hydrogen. Here we calculate the expected stresses for H/Fe system to be an order of magnitude too low to impact dislocation mobility, and we show that the concentration gradient-induced stresses dissipate quickly in both H/Fe and H/Ni systems - well before observation by TEM would occur.