SIGNIFICANCE OF NONCENTRAL FORCES IN ATOMISTIC STUDIES OF GRAIN-BOUNDARIES IN BCC TRANSITION-METALS

The effects of non-central forces in atomistic studies of grain bounda ries in molybdenum and tungsten, the transition metals with half-fille d d-band, are investigated. For this purpose we have used two differen t types of potential which include different number of moments of the local density of electronic states when evaluating the total energy: t he central-force Finnis-Sinclair potentials which include the scalar s econd moment and the potentials constructed by Carlsson which include the fourth and the matrix second moments. The energy terms associated with these two moments represent non-central interactions and assure t hat the bcc-fcc structural energy difference is reproduced with good a ccuracy. For the three boundaries studied, the non-central forces have been found to be very important in determining the lowest energy stru ctures. In particular, the energy differences between multiple structu res depend on specific orientations and geometries of the atomic clust ers at and near the interface. On the other hand, central-force potent ials favour structures with atomic separations close to those found in the bulk with no regard to bond orientation. As a consequence the low est-energy structures predicted by the two potential schemes differ in details in both the local atomic relaxations and the magnitude of the rigid-body displacements of the grains, although many general feature s of the boundary structures remain the same, independent of the poten tials used. The calculations also show that it is not possible to iden tify the major non-central contribution with the fourth moment alone. Thus inclusion of both the matrix second moment and the fourth moment energy contributions is essential for an appropriate description of no n-central atomic interactions.