The nature of faults in the dissociated 9R-phase at an incoherent twin boundary was considered. An analysis of the dislocation content of a 5-layer break in the normal ABC/BCA/CAB stacking sequence of 9R showed that this defect was associated with a secondary grain boundary dislocation with a Burgers vector of a/6[¯211]. Atomistic simulations of secondary grain boundary dislocations at the incoherent twin interface showed that the sign of the Burgers vector was important in determining the core structure of the dislocation. The resultant atomistic structures, and the 5-layer width of the fault, could be directly understood by analyzing the set of twinning dislocations that made up the boundary. Atomistic calculations showed further, in agreement with the experimental observations, that a shear parallel to the interface caused an increase in the fault-width that accompanied growth of the 9R-phase. The calculations and analysis showed that, for a suitable geometry, a positive secondary grain boundary dislocation resulted in a 5-layer separation between the pure edge dislocations. In the ideal Σ = 3 boundary, these dislocations occurred every 3 planes, terminating the periodic array of stacking faults that corresponded to the 9R phase. At the positive secondary grain boundary dislocation, the 5-layer separation between dislocations then produced a corresponding 5-layer separation between stacking faults. The position of the dislocations was sensitive to shear loading. Thus, as a bicrystal was sheared parallel to the interface, the array of dislocations moved so as to increase the width of interfacial dissociation. Under such loading, the positive secondary grain boundary dislocation then produced a wide 5-layer high region of local face-centered cubic stacking, whereas the negative secondary grain boundary dislocation would produce a local region of hexagonal close-packed stacking.

Effect of Grain Boundary Dislocations on 9R Stacking Errors at an Incoherent Twin Boundary in Copper. D.L.Medlin, S.M.Foiles, G.H.Campbell, C.B.Carter: Materials Science Forum, 1999, 294-296, 35-42