Atomic structure of (111) twist grain boundaries in f.c.c metals

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Abstract
In this paper we have studied the atomic structures of (111) twist boundaries and investigated the applicability of the structural unit model which has previously been established for tilt boundaries and (001) twist boundaries by Sutton and Vitek. The calculations were carried out using two different descriptions of interatomic forces. A pair potential for aluminium, for which the calculations were made at constant volume, and a many-body potential for gold, for which the calculations were performed at constant pressure. The atomic structures of all the boundaries studied were found to be very similar for both the descriptions of atomic interactions. This suggests that the principal features of the structure of (111) twist boundaries found in this study are common to all f.c.c. metals. At the same time it supports the conclusion that calculations employing pair potentials are fully capable of revealing the generic features of the structure of grain boundaries in metals. The results obtained here, indeed, show that structures of all the boundaries with misorientations between 0° and 21–79° (∊=21) are composed of units of the ideal lattice and/or the 1/6‘112’ stacking fault on (111) planes, and units of the ∊ = 21 boundary. Similarly, structures of boundaries with misorientations between 21–79° and 27–8° (X= 13), 27–8° and 38–21° (1=1) and 38–21° and 60° (Z = 3) can all be regarded as decomposed into units of the corresponding delimiting boundaries. Therefore we conclude that the atomic structure of (111) twist boundaries can well be understood in the framework of the structural unit model. A related aspect analysed here in detail is the dislocation content of these boundaries. This study shows both the general relation between dislocation content and atomic structure of the boundaries, which is an integral part of the structural unit model, and features specific to the dislocation networks present in the (111) twist boundaries. Furthermore, the dislocation content revealed by the atomistic calculations can be compared in several cases with transmission electron microscope (TEM) observations and the results are discussed in this context.