We review recent experimental data that indicate an exceptional strength of small metallic objects (nanopillars, nanowires and nanoparticles), exceeding the strength of their bulk counterparts by orders of magnitude. To understand the origins of this high strength, we performed a series of molecular dynamics simulations of the uniaxial compression of single- and bicrystalline gold nanopillars. Simulations were performed for different nanopillars crystallographic orientations, nanopillar cross-sections, misorientation parameters of grain boundaries, and over a range of temperatures using several different interatomic potentials. For some nanopillar orientations, the initially perfect samples exhibit buckling or shear instabilities that can be interpreted in terms of the fcc-hcp phase transition at high pressure and the stress dependence of the shear modulus. For the others, the yield stress was observed to be either a linear or parabolic function of temperature, depending on the choice of interatomic potential, nanopillar cross-section and/or nanopillar size. For example, the temperature dependence of the yield stress of the <111> oriented nanopillars was linear for those with hexagonal and square cross-sections, but parabolic for those with circular cross-sections. We suggest a simple yield nucleation criterion in which the nucleation of the first Shockley partial at the surface of a nanopillar occurs at a critical strain at the surface, which includes contributions from thermal vibration, elastic loading and thermal expansion. We demonstrate that the yield condition correctly describes the temperature dependence of the yield stress observed in the full set of computer simulations.

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