  Hydrogen atoms absorbed by metals in the hydrogen-containing environments can
lead to the premature fracture of the metal components used in load-bearing
conditions. Since metals used in practice are mostly polycrystalline, grain
boundaries (GBs) can play an important role in hydrogen embrittlement of
metals. Here we show that the reaction of GB with lattice dislocations is a key
component in hydrogen embrittlement mechanism for polycrystalline metals. We
use atomistic modeling methods to investigate the mechanical response of GBs in
alpha-iron with various hydrogen concentrations. Analysis indicates that
dislocations impingement and emission on the GB cause the GB to locally
transform into an activated state with a more disordered atomistic structure,
and introduce a local stress concentration. The activation of the GB segregated
with hydrogen atoms can greatly facilitate decohesion of the GB. We show that
the hydrogen embrittlement model proposed here can give better explanation of
many experimental observations.
