0.3truein As mentioned earlier, GBS plays an important role in the deformation process (e.g. superplastic flow). In order to understand the mechanism involved in GBS, a microscopic knowledge of the atomic configurations and of the movements of individual atoms is necessary. GBS provides plastic shear displacements parallel to the boundary interfaces, occurring when the system is subject to external forces. The mobility of the GBS process through the application of shear displacements is easier to implement in a computer simulation than the direct application of external shear stress. Therefore, in this work GBS is simulated by applying constant shear displacements, each increment followed by a complete relaxation (energy minimization) of the boundary structure. In the computational crystal, periodic boundary conditions are applied in grain boundary plane (x- and z- directions). Since the CSL grain boundary structure studied in this work can be obtained by repeating the a CSL cell in x- and y-directions, the structure with a displacement of aCSL (aCSL is the lattice parameter of the CSL cell in x-direction, see Figure 2) is equivalent to the initial undisplaced structure under the periodic conditions described above. Therefore the total displacement in each case is limited to the value of aCSL for the given grain boundary. The increments (described in percentage of aCSL) are selected to be small enough to capture all the energy jumps.
To simulate the GBS process, one of the two grains that form the CSL structure (see Figure 3) is rigidly shifted with respect to the other, along x-direction of the interface, in steps equal to 2.5. After each increment the configuration is relaxed to its local equilibrium state and the grain boundary energy is computed. The grain boundary energy profile associated with the GBS process then provides the tool necessary to predict the grain boundary mobility. Figure 7 - Figure 10 give such results for four typical grain boundaries: a twin boundary (11), two boundaries with different orientations: (21), (14) and a high index boundary (35). In each case the top crystal (above the line) is moved to the right along x direction to simulate the GBS process.