  Hydrogen (H) induced damage in metals has been a long-standing woe for many
industrial applications. One form of such damage is linked to H clustering, for
which the atomic origin remains contended, particularly for non-hydride forming
metals. In this work, we systematically studied H clustering behavior in bcc
metals represented by W, Fe, Mo, and Cr, combining first-principles
calculations, atomistic and Monte Carlo simulations. H clustering has been
shown to be energetically favorable, and can be strongly facilitated by
anisotropic stress field, dominated by the tensile component along one of the
<001> crystalline directions. We showed that the stress effect can be well
predicted by the continuum model based on H formation volume tensor, and that H
clustering is thermodynamically possible at edge dislocations, evidenced by
nanohydride formation at rather low levels of H concentration. Moreover,
anisotropy in the stress effect is well reflected in nanohydride morphology
around dislocations, with nanohydride growth occurring in the form of thin
platelet structures that maximize one <001> tension. In particular, the <001>
type edge dislocation, with the <001> tensile component maximized, has been
shown to be highly effective in facilitating H aggregation, thus expected to
play an important role in H clustering in bcc metals, in close agreement with
recent experimental observations. This work explicitly and quantitatively
clarifies the anisotropic nature of stress effect on H energetics and H
clustering behaviors, offering mechanistic insights critical towards
understanding H-induced damages in metals.
