Step Coalescence by Collective Motion at an Incommensurate Grain Boundary

Using extended time series scanning transmission electron microscopy, we investigate structural fluctuations at an incommensurate grain boundary in Au. Atomic-resolution imaging reveals the coalescence of two interfacial steps, or disconnections, of different height via coordinated motion of atoms along close-packed directions. Numerical simulations uncover a transition pathway that involves constriction and expansion of a characteristic stacking fault often associated with grain boundaries in face-centered cubic materials. It is found that local atomic fluctuations by enhanced point defect diffusion may play a critical role in initiating this transition. Our results offer new insights into the collective motion of atoms underlying the lateral advance of steps that control the migration of faceted grain boundaries.

Figure 1

(a) Intensity average of several HAADF-STEM fast scan images showing a $\{001\}/\{110\}$ boundary containing two steps. Fivefold structural units are outlined in yellow. (b) Colored composite image of averaged intensities before and after a structural event to highlight the atomic displacements.

Figure 2

$[110]$ projections of the experimental (a) and simulated (d) initial boundary configuration with $1|1$ and $2|3$ steps, simulated saddle point configuration (e), and experimental (c) and simulated (f) final configuration with a $3|4$ step. All simulations are color-coded for atomic position along the $[110]$ direction, as shown in the perspective view of the initial configuration (b). Dotted lines indicate stacking faults and the color legend describes a $\frac{1}{2}[110]$ period.

Figure 3

Energy variation $\mathrm{\Delta }E$ of the Au film for 241 simulated atomic configurations along the transition path between initial ($x=0$) and final ($x=1$) observed structures. Insets show schematic views of the corresponding structure at end points and maximum total energy.

Figure 4

Simulated step configuration showing the displacement magnitude of atoms during the transition. Shaded atoms have moved by more than 50 pm. (Note that the few outliers visible in this figure are surface atoms whose positions are highly sensitive to small grain shifts associated with the coordinated shuffle.)