Atomistic modeling of deformation at the nanoscale

Studies of two nanoscale systems with important engineering applications, Al-Si nanocomposites and Au nanoasperities, are investigated using molecular dynamics. The first system is studied in order to investigate improvements to nanocrystalline Al coating by introducing Si particles. In composite systems, the role of bimaterial interfaces can dominate material properties so the study begins with an examination of tensile strengths and toughness for various Al/Si interfaces and shows a general interface will have high strength and toughness. The study then turns to Al-Si composites with three major results: (i) the deformation and fracture mechanisms are different in the Al-Si composites relative to the all-Al materials with Al-Si plasticity and failure predominately localized at Al/Si interfaces, (ii) the Al-Si nanocomposites have a higher yield stress than the all-Al nanocrystals, consistent with recent experimental data, and (iii) with different failure modes, the tensile strengths of the Al-Si and all-Al materials are similar with the Al-Si strengths correalating well with the Al/Si interface strengths. These results show that Al-Si interfaces control the mechanical behavior in the nanocomposites and indicate that Al-Si nanocomposites can be engineered for enhanced hardness over all-Al nanocrystals.;An nanoasperities are perfect for studying the scaling of hardness at a length scale which current models fail to predict. Large-scale molecular dynamics model the compression of the smallest of surface asperities or nanopyramids to predict the magnitude and scaling of hardness H versus contact size ℓc. Three major results emerge: (i) regimes of near-power-law size scaling H ≈ ℓ-hc of the hardness exist, with etaMD ≈ 0.32 and etaexpt ≈ 0.75; (ii) unprecedented quantitative and qualitative agreement between MD and experiments is achieved, with HMD ∼ 4 GPa at ℓ c, = 36 nm and at Hexpt ∼ 2.5 nm, and (iii) an analytic model that incorporates the energy costs of forming energetically favorable defect structures to accommodate the deformation predicts the magnitude and scaling of the hardness in good agreement with the MD. The model predictions indicate a transition from a nucleation-dominated regime to dislocation-interaction-dominated regime at larger sizes, with a change in scaling to eta ≈ 0.5--0.7. These results provide a basic framework for predicting size-dependent plasticity of realistic engineered surfaces.