Atomistic Simulation of Size Effects in Bending a Single Crystal

We perform atomistic Monte Carlo simulations of bending a Lennard-Jones single crystal in two dimensions. In examining initial yield, we find an apparent ``reverse'' size effect. However, when strain rate effects are taken into account, we demonstrate that the size effect disappears. Once geometrically necessary dislocations coalesce to form grain boundaries, we observe a size effect of the usual kind, e.g. smaller samples support a higher scaled bending moment than larger samples. We compare simulation results with recent experiments on bending of highly annealed nanowires [B. Wu et al, Nature Matls 4, 525, 2005.] Finally, we observe a topological instability in the evolution of a grain boundary intersecting a free surface under compressive stress. The grain boundary buckles and nucleates a protruding grain, suggesting a novel mechanism for the formation of a hillock on a compressed metal surface.