  Nanoindentation is a convenient method to investigate the mechanical
properties of materials on small scales by utilizing low loads and small
indentation depths. However, the effect of grain boundaries (GB) on the
nanoindentation response remains unclear and needs to be studied by
investigating in detail the interactions between dislocations and GBs during
nanoindentation. In the present work, we employ a three-dimensional multiscale
modeling framework, which couples three-dimensional discrete dislocation
dynamics (DDD) with the Finite Element method (FEM) to investigate GB effects
on the nanoindentation behavior of an aluminum bicrystal. The interaction
between dislocations and GB is physically modeled in terms of a penetrable GB,
where piled-up dislocations can penetrate through the GB and dislocation debris
at GBs can emit full dislocations into grains. In the simulation, we confirmed
two experimentally observed phenomena, namely, pop-in events and the dependence
of indentation hardness on the distance from GB. Two pop-in events were
observed, of which the initial pop-in event is correlated with the activation
and multiplication of dislocations, while the GB pop-in event results from
dislocation transmission through the GB. By changing the distance between the
indenter and GB, the simulation shows that the indentation hardness increases
with decreasing GB-indenter distance. A quantitative model has been formulated
which relates the dependency of indentation hardness on indentation depth and
on GB-indenter distance to the back stress created by piled-up geometrically
necessary dislocations in the plastic zone and to the additional constraint
imposed by the GB on the plastic zone size.
