Chevron Defect at the Intersection of Grain Boundaries with Free Surfaces in Au

We have identified a new defect at the intersection between grain boundaries and surfaces in Au using atomic resolution transmission electron microscopy. At the junction line of $90°$ $⟨110⟩$ tilt grain boundaries of (110)-(001) orientation with the free surface, a small segment of the grain boundary, about 1 nm in length, dissociates into a triangular region with a chevronlike stacking disorder and a distorted hcp structure. The structure and stability of these defects are confirmed by atomistic simulations, and we point out the relationship with the one-dimensional incommensurate structure of the grain boundary.

Figure 1

Atomic resolution transmission electron micrographs of chevron defects at the intersection of the $90°$ $⟨110⟩$ tilt grain boundary with the free surface [(a),(b)]. The triangular region of dissociation is marked by dashed lines in (b). The boundaries of the defect are parallel to {111} planes in both grains. Inside the defect, there are stacking faults on alternate layers resulting in a local hcp stacking sequence. The Fourier diffractograms in (c) and (d) illustrate the difference between the undissociated boundary (c) and the chevron defect in the dissociated boundary (d).

Figure 2

(a) Atomistic simulation of relaxation of the $(110)/(001)$ grain boundary at the free surface. Local atomic energy is indicated by shading (white for lowest energy, black for highest energy at the grain boundary). For higher energies, the color saturates to black. Atoms located at the free surface have the highest energy. (b) Multislice phase contrast image simulation for imaging conditions used in this work.

Figure 3

Variation of defect energy with size. For each size, four different configurations were considered, and the solid line traces the lowest-energy values. The dashed line is a fit of the type $a{x}^2+bx$ to these data. The calculated curve is not continuous, because there is a discrete number of configurations, and its roughness is probably due to the fact that the defect intersects (and replaces) a nonperiodic boundary.