Atomistic and continuum modeling of the structure and mechanical properties of metal nanowires

We performed atomistic simulations to study the effects of free surfaces on the structure and mechanical properties of fcc gold nanowires. Gold <100> and <111> nanowires were created with initial atomic positions corresponding to the bulk fcc lattice. Because of the tensile surface stresses in the side surfaces, the nanowires created by this way are not in equilibrium and contract to equilibrium upon relaxation. While the surface stress causes wires of both orientations and all sizes to increasingly contract with decreasing cross-sectional area, when the cross-sectional area of a <100> nanowire is below a critical size, surface stress induces instability and structural changes in the nanowires. Depending on the potential used, the surface stress can induce phase transformation and reorientation (MEAM), or yielding and reorientation (EAM). A continuum model based on surface stress is used to explain the phase transformation, yielding and reorientation.; We performed uniaxial loading on the relaxed nanowires to study the elastic and plastic response. The effective Young's modulus of both the fcc gold <100> and <111> nanowires slightly increases with a decrease of cross-sectional area. The elastic properties of nanowires are influenced by free surfaces and edges and nanowires can be viewed as a heterogeneous, or composite, solid with elastic properties that vary with positions. Nonlinear elasticity manifests itself in the core of the nanowires through the contraction induced by the tensile surface stress.; Nanowires yield via the nucleation and propagation of {lcub}111{rcub}<112> partial dislocations. The magnitude of the tensile yield stress is much larger than that of the compressive yield stress for <100> nanowires. This asymmetry is due to the surface stresses and different slip systems active in tensile and compressive yielding. The critical RSS averaged over the core of the [100] nanowires just before tensile or compressive yielding does not change much with the wire size, however this critical RSS still has the loading condition (tension vs. compression) and orientation dependence, suggesting a lack of a universal Schmidt law criterion for yielding of nanowires. The critical RSS averaged over the core of nanowires can be roughly given by a surface stress based continuum model.