Discrete dislocation plasticity HELPs understand hydrogen effects in bcc materials

In an attempt to bridge the gap between atomistic and continuum plasticity
simulations of hydrogen in iron, we present three dimensional discrete
dislocation plasticity simulations incorporating the hydrogen elastic stress
and a hydrogen dependent dislocation mobility law. The hydrogen induced stress
is incorporated following the formulation derived by Gu and El-Awady (2018)
which here we extend to a finite boundary value problem, a microcantilever
beam, via the superposition principle. The hydrogen dependent mobility law is
based on first principle calculations by Katzarov et al. (2017) and was found
to promote dislocation generation and enhance slip planarity at a bulk hydrogen
concentration of 0.1 appm; which is typical for bcc materials. The hydrogen
elastic stress produced the same behaviour, but only when the bulk
concentration was extremely high. In a microcantilever, hydrogen was found to
promote dislocation activity which lowered the flow stress and generated more
pronounced slip steps on the free surfaces. These observations are consistent
with the hydrogen enhanced localized plasticity (HELP) mechanism, and it is
concluded that both the hydrogen elastic stress and hydrogen increased
dislocation mobility are viable explanations for HELP. However it is the latter
that dominates at the low concentrations typically found in bcc metals.