| Experimental studies of deformation of crystalline solids have demonstrated that the hardness of a material in the plastic, or inelastic, regime is inversely related to the size of the sample, for samples up to a few hundred microns in size. It has also been observed that a material's mechanical response to deformation depends on the structure and patterning of dislocations, or defects, in the material. The underlying mechanisms behind dislocation patterning and size effects in plasticity are not well understood.; To study these effects, atomistic Monte Carlo simulations of bending a Lennard-Jones single crystal in two dimensions are performed. In these simulations, when dislocations reach sufficient density, they coalesce to form grain boundaries. A reverse-size effect is demonstrated, which can be attributed to a shortage of dislocation sources and high effective strain rate. The effects of crystal orientation, temperature, and strain rate are explored, and a scaling relationship between size- and rate-effects is proposed. In one simulation, an instability in grain boundary evolution suggests a novel mechanism for hillock formation on metal surfaces. Simulations of pure compression are also performed on both single crystal and polycrystalline samples, and the results are compared to those involving bending.; An effective means of expanding the capabilities of simulation techniques is through multiscale algorithms. The performance of a coupled atomistics-continuum formulation has been tested on surface relaxation simulations of gold, as well as 2-d and 3-d simulations of gold nanowire under applied compression. The coupling algorithm involves communication between the coarse and fine scales via ghost atoms, and the energy contribution from bonds between real and ghost atoms is weighted such that the energy is minimized based on a consistency condition of homogeneous deformation. The algorithm was updated to allow the use of Embedded Atom Method (EAM) potentials, which offer a more realistic representation of material modeling. |
