Effect of crystal orientation on the size effects of nano-scale fcc metals

The present work investigates the dominant mechanisms in the plasticity of nano-sized fcc metallic samples. Molecular dynamics simulations of nanopillar compression show that plasticity always starts with the nucleation of dislocations at the free surface, and the crystal orientation affects the subsequent microstructural evolution. The Schmid factor of leading and trailing partials plays a decisive role in leading to the twinning, or slip deformation. A significant difference is observed in the strength of pillars of the same size with different orientations. The power-law equation exponent is completely dependent on the crystal orientations, and a weak or no size effect is observed in the compression of [100]- and [110]-oriented nanopillars. The observed orientation based behaviour decreases by confining the free surface.

本研究针对纳米尺度面心立方（face-centered cubic, fcc）金属试样的塑性变形主导机制展开探究。通过纳米柱压缩的分子动力学（molecular dynamics, MD）模拟发现，塑性变形始终始于自由表面处的位错形核，且晶体取向会对后续微观结构演化过程产生显著影响。先导不全位错与尾随不全位错的施密特因子（Schmid factor），对孪生变形或滑移变形的触发起到决定性作用。相同尺寸但不同取向的纳米柱，其强度存在显著差异。幂律方程指数完全由晶体取向决定，且在[100]取向与[110]取向的纳米柱压缩试验中，仅观测到微弱的尺寸效应，或未发现尺寸效应。通过限制自由表面，可弱化实验中观测到的晶体取向相关行为差异。