Computed Structures of Twist Boundaries Compared with Tem Observations

The structure of (111) twist boundaries in f.c.c. metals is investigated by computer simulation. For coincidence orientations in the vicinity of the twin orientation a characteristic six-star pattern has been found, in agreement with TEM observations on dislocation networks in nearcoherent twin boundaries in Au. Dislocation networks in near-coherent twin boundaries in Au have recently been studied in TEM /1,2/. The most stable network in twin boundaries appears to be in the form of a six-star pattern, consisting of three parallel sets of Shockley partial dislocations with associated steps in the boundary. Fig. 1 shows an example of such a pattern, imaged under such a condition that the different segments of the six-star lying on adjacent (111) planes are clearly shown by a different contrast. In an effort to reproduce these patterns in a computer-simulation study, we have recently investigated the atomic configurations in (111) twist boundaries close to the twin misorientation, the results of which are described below. Method of computation The method of calculating the atomic positions in a twist boundary in f.c.c. metals has been described elsewhere /3/. Briefly, the simulated bicrystal consists of a block of atoms made up of 12 (111) lattice planes. Different starting configurations for the atoms in a given coincidence orientation can be employed; relaxation occurs under the influence of pair-wise forces which the atoms exert on each other as a result of the assumed interaction potential. Various potentials appropriate for Al have been employed in this investigation. Fixed boundary conditions were applied in the z-direction (perpendicular to the boundary), whereas periodic boundary conditions were used in directions perpendicular to z. The (111) planes are hereafter indicated by z = 1,2,...12.