Low energy heterophase boundaries in the system silver/nickel and in other weakly bonded systems

Abstract Observation of the rotation during sintering of single crystal spheres on a single crystal nickel substrate reveals eight types of low energy heterophase boundaries in the Ag/Ni system. The crystallography of these boundaries is discussed against the background of known low-energy configurations in the systems Au/LiF, Au/NaCl, Au/KCl, Au/MgO, Cu/MgO, and Ag/NaCl, which exhibit comparatively weak interface bonding similar to Ag/Ni. Certain boundary configurations recur in systems of different lattice misfit and bonding type, and two configurations emerge as specially favoured: the “parallel cube” type and the “half-twin” configuration, where one crystal is bounded by a (111) or (211) twin plane. It is shown that all configurations which involve high index limiting planes can be reduced to these simple types if facetting of the boundary or twinning in one crystal is assumed to occur. A minimal set of “irreducible boundaries” is identified, from which all observed boundaries can be hypothetically composed by combination with facetting and/or twinning; the irreducible boundaries themselves have all been observed directly in sphere rotation experiments, and they cannot be decomposed into any simpler configurations. All irreducible boundaries can be described in terms of both the coincidence and the lock-in model, but strains up to 3.7% are required to bring about reasonable lock-in periodicities and coincidence cell sizes. Boundary energies do not correlate with the density of coincidence or lock-in sites. It is concluded that in weakly bonded systems, which have low driving force for relaxation at the boundaries, the character of the limiting planes of the adjoining crystals and the parallelism of relatively close-packed directions are at least as important as the geometrical fit (coincidence or lock-in) of these planes.