The presence of hydrogen can increase the mobility of dislocations during plastic deformation which results in highly localized plastic deformation and faster failure. These observations led to the notion of hydrogen enhanced localized plasticity (HELP) mechanism. In addition to the HELP mechanism, another most-cited mechanism, namely hydrogen enhanced decohesion (HEDE) also has been proposed. HEDE considers a reduction in the cohesive bond strength between the metal atoms in the presence of hydrogen. High concentrations of hydrogen and the associated decohesion events could
occur at a variety of locations, such as particle-matrix interfaces and grain boundaries.
Despite the several suggested mechanisms to describe HE, there is no universally accepted HE mechanism. This is owing to different observed indications which have been attributed to different mechanisms of HE in different studies. A generally recognized common feature is that some critical concentrations of hydrogen must build-up at potential flaws, for failure
to initiate. Thus, the partitioning of hydrogen inside the metal and its pattern of migration are of paramount importance for understanding the phenomena and designing alloys with improved HE behavior.