  We perform atomistic Monte Carlo simulations of bending a Lennard-Jones
single crystal in two dimensions. Dislocations nucleate only at the free
surface as there are no sources in the interior of the sample. When
dislocations reach sufficient density, they spontaneously coalesce to nucleate
grain boundaries, and the resulting microstructure depends strongly on the
initial crystal orientation of the sample. In initial yield, we find a reverse
size effect, in which larger samples show a higher scaled bending moment than
smaller samples for a given strain and strain rate. This effect is associated
with source-limited plasticity and high strain rate relative to dislocation
mobility, and the size effect in initial yield disappears when we scale the
data to account for strain rate effects. Once dislocations coalesce to form
grain boundaries, the size effect reverses and we find that smaller crystals
support a higher scaled bending moment than larger crystals. This finding is in
qualitative agreement with experimental results. Finally, we observe an
instability at the compressed crystal surface that suggests a novel mechanism
for the formation of a hillock structure. The hillock is formed when a high
angle grain boundary, after absorbing additional dislocations, becomes unstable
and folds to form a new crystal grain that protrudes from the free surface.
