Although deformation processes in submicron-sized metallic crystals are well documented, the direct observation of deformation mechanisms in crystals with dimensions below the sub-10-nm range is currently lacking. Here, through in situ high-resolution transmission electron microscopy (HRTEM) observations, we show that (1) in sharp contrast to what happens in bulk materials, in which plasticity is mediated by dislocation emission from Frank-Read sources and multiplication, partial dislocations emitted from free surfaces dominate the deformation of gold (Au) nanocrystals; (2) the crystallographic orientation (Schmid factor) is not the only factor in determining the deformation mechanism of nanometre-sized Au; and (3) the Au nanocrystal exhibits a phase transformation from a face-centered cubic to a body-centered tetragonal structure after failure. These findings provide direct experimental evidence for the vast amount of theoretical modelling on the deformation mechanisms of nanomaterials that have appeared in recent years.

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This letter addresses the issue of deformation mechanisms and mechanical tensile behavior of the twinned metal nanowires using atomistic simulations. Free surfaces are always the preferential dislocation nucleation sites in the initial inelastic deformation stage, while with further plastic deformation, twin boundary interfaces will act as sources of dislocations with the assistance of the newly formed defects. The smaller the twin boundary spacing, the higher the yielding stresses of the twinned nanowires. Twin boundaries, which serve both as obstacles to dislocation motion and dislocation sources, can lead to hardening effects and contribute to the tensile ductility. This work illustrates that the mechanical properties of metal nanowires could be controlled by tailoring internal growth twin structures. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2721367͔Due to the unique mechanical, electrical, and optical properties, materials with nanometer-sized structure have attracted a great deal of interest in the past few decades [1][2][3] Many researchers have demonstrated, through both experiments and analysis, that the structure and properties of nanowires can be quite different from those of bulk materials due to the effects of larger free surfaces. [4][5][6][7][8][9][10][11][12][13][14] Simulations reported in the previous literatures are generally defect-free and the sizes of the modeled wires are usually smaller than 6 nm. [6][7][8][9][10][11][12][13][14] Meanwhile experimentally, twins, a class of planar defects, are observed most often in single-crystal metal ͑copper, silver, and gold͒ nanowires with a ͓111͔ growth orientation. 15,16 Because twin boundaries ͑TBs͒ in nanowires will strongly affect the physical properties of nanowires, it is of great significance to investigate the details of twin structures and their roles in metal nanowires. Until now, there are still some key issues about twin related deformation which are not quite clear and need fundamental understanding. For example, what is the role of TBs in mechanical deformation? Does it act as grain boundaries or surfaces? In our recent work, the mechanical behavior of fivefold twinned nanowires is studied. 17 We found that the strengthening mechanism in the fivefold twinned nanowires is due to the TBs as obstacles to the dislocation motion. In this letter, we address the effect of TBs on inelastic deformation of the twinned nanowires using atomistic simulations.In this work we focus on twinned Cu nanowires with ͗111͘ growth orientation and nearly square cross section as indicated from the experimental observations. 15,16 The initial configuration of the twinned nanowires is constructed by repeating ⌺3 coherent twins in the ͗111͘ axis orientation. 18 Four samples are prepared for the simulations. The first one consists of two twins, the second consists of four twins, and the third contains five twins. The twin boundary spacings ͑TBSs͒ of the first, second, and third wires are 14 nm ͑sample I͒, 7 nm ͑sample II͒, and 5 nm ͑sample III͒, respect...