In Situ Visualization of Birth and Annihilation of Grain Boundaries in an Au Nanocrystal

The formation and vanishing processes of a low angle grain boundary (GB) in nanosized Au during tension and release of stress, respectively, were obsvered by in situ high resolution transmission electron microscopy. The nucleation of perfect dislocations led to the formation of a 15° low angle GB inside an Au nanocrystal upon off-axial tensile loading (coupled uniaxial tensile and bending stress). Strikingly, the dislocations were completely annihilated accompanied with the disappearance of the GB after the removal of external stress, indicating that plastic bending is recoverable in the nanocrystal. The back force and surface stress played important roles in such a pseudoelastic behavior. This transient GB dynamics cannot be captured in ex situ experimental investigations. Such pseudoelastic bending deformation in nanosized crystals will have an important impact on the designing of nanomechanical devices with ultrahigh bending capability.

Figure 1

Sequential TEM images of the tapered single nanocrystal Au under off-axial tensile stress. The arrows in (a) and (d) show the axial direction of the crystal.

Figure 2

HRTEM images showing the occurrence of a stacking fault (a) and twinning (b) induced by the surface nucleated partial dislocations. The annotations $M$ and $T$ stand for matrix and twin, respectively. (c) Fourier filtered image of the boxed area in (a) showing the alteration from fcc to hcp stacking ordering when a stacking fault is present. (d) Schematic drawing of slip directions inside a (111) plane. The black and red lines represent the Burgers vectors of full and partial dislocations, respectively.

Figure 3

(a) A low angle GB formed due to the full dislocation pileups. (b) Fourier filtered image of the enclosed area in (a) showing the detailed GB configuration. (c) The disappearance of the GB accompanied with the release of external stress. The white lines are guidance for the eyes, showing the relative misorientation angle between the crystals across the GB. The arrow points out the location where fracture occurs.