  Liquid metal embrittlement (LME) is a common feature of systems in which a
low melting point liquid metal is in contact with another, higher melting
point, polycrystalline metal. While different systems exhibit different LME
fracture characteristics, the penetration of nanometer-thick liquid metal films
along the grain boundary is one of the hallmarks of the process. We employ EAM
potentials optimized for Al-Ga binary alloys in a series of MD simulations of
an Al bicrystal (with a $\Sigma 5 36.9 ^{\circ} /[010]$ symmetric tilt
boundary) in contact with liquid Ga with and without an applied stress. Our
simulations clarify the mechanism of LME and how it is affected by applied
stresses. The interplay of stress and penetrating Ga atoms leads to the
nucleation of a train of dislocations on the grain boundary below the liquid
groove root which climbs down the grain boundary at a nearly constant rate. The
dislocation climb mechanism and the Ga penetration are coupled. While the
dislocations do relax part of the applied stress, the residual stresses keep
the grain boundary open, thereby allowing more, fast Ga transport to the
penetration front (i.e., Ga layer thickening process). The coupled Ga transport
and ``dislocation climb'' is the key to the anomalously fast, time-independent
penetration of Ga along grain boundaries in Al. The simulations explain a wide
range of experimental observations of LME in Al-Ga the literature.
