9R Structure at Grain Boundaries in Face Centered Cubic (FCC) Materials
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One type of dissociations at grain boundaries is to form a thin layer with the 9R structure in face centered cubic (FCC) materials, e.g. in Au (gold) [1]. The 9R stacking sequence, abc/bca/cab, can be formed by inserting an intrinsic stacking fault every third close-packed plane, suggesting that low SFE (stacking fault energy) is the main factor for 9R formation. The authors had shown the presence of a ~10 Å (see Figure 4741a) wide layer with stacking faults formed within every three close-packed planes by high resolution transmission electron microscopy (HRTEM) observations and atomistic simulations of facets in a gold 90° tilt boundary. Figure 4741a shows an example of facets in a gold 90° tilt boundary.

HRTEM micrograph showing the boundary between 90°

Figure 4741a. HRTEM micrograph showing the boundary between 90° related grains in an Au crystal. The boundary is stepped on facets. A modulation of the lattice fringe contrast, with a period of three {111} planes. [1]

Figure 4741b shows the dissociated region of the interface. The bending of the {111} fringes to the right of the interface. The position of the stacking faults in the dissociated region (9R) was identified by viewing the image at an inclined angle and sighting along the direction of the arrow. The white lines, inclined at an angle of 7°, indicate the position and approximate extent of the faults. Figure 4741c shows the simulated 9R region. Figure 4741c shows the configuration in the dissociated region is in the 9R stacking arrangement (abc/bca/cab).

dissociated region of the interface.

Figure 4741b. The dissociated region of the interface. [1]

Simulated 9R region

Figure 4741c. Simulated 9R region. [1]

The configuration in the dissociated region is in the 9R stacking arrangement (abc/bca/cab)

Figure 4741c The configuration in the dissociated region is in the 9R stacking arrangement (abc/bca/cab). [1]

 

[1] D. L. Medlin, S. M. Foiles, and D. A. Cohen, Acta Mater. 49, 3689 (2001).

 

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