Atomic structures of grain boundaries in copper-bismuth alloys: Ab initio and empirical modelling

A remarkable phenomenon observed in the Cu-Bi system is the segregation-induced faceting of grain boundaries. It was found in previous studies that for grain misorientations corresponding to the usual twin in the fee lattice the facets containing a high concentration of bismuth are (111)/(11(1) over bar) boundaries which in pure copper correspond to the coherent twin boundaries. Combined atomistic modelling and high resolution electron microscopy of these facets revealed that an ordered Cu-Bi double-layer is formed in the boundary. In the present work we employ an ab initio full potential linear muffin tin orbital (FP-LMTO) method to study possible Cu-Bi compounds related to this grain boundary structure and show that this double-layer is a unique two-dimensional gain boundary structure. At the same time we further test the validity of the central-force many-body potentials which were used in previous studies by comparing the ab initio data with calculations performed using these potentials. This study demonstrates that the potentials can be used with confidence for Cu concentrations higher than 66 atom% when the Cu-Bi system exhibits metallic behaviour although alternate structures with very similar energies may be found using these potentials.