Atomic structures and macroscopic translation states for Σ = 5 (310)[00l] symmetrical tilt grain boundaries in body-centred cubic transition metals were calculated, within the framework of local density functional theory, by using total-energy and force calculations and an ab initio mixed-basis pseudopotential method. In the case of Mo and W, translation states of the optimized grain-boundary structures were found, with the neighbouring grains being displaced parallel to the [001] tilt axis. This yielded grain boundaries without mirror symmetry. In the case of Nb, a mirror-symmetry broken translation state was also found. However, this involved a smaller grain displacement and was almost energetically degenerate with a mirror-symmetry conserved translation state. Experimental distinction of this mirror-symmetry broken translation state, from a mirror-symmetrical one in a Nb bicrystal, was essentially impossible by means of high-resolution transmission electron microscopy. In the case of Ta, the translation state of the optimized grain-boundary structure was mirror-symmetrical. The ab initio results for Nb and Mo were in close agreement with corresponding experimental observations of Nb and Mo bicrystals; made using high-resolution transmission electron microscopy. This revealed a conserved and broken mirror symmetry for Nb and Mo, respectively. The ab initio results provided a reliable data-base for the evaluation and improvement of empirical interatomic interaction models which would be suitable for the large-scale atomistic simulation of defects in body-centred cubic transition metals.

Symmetrical Tilt Grain Boundaries in Body-Centred Cubic Transition Metals - an ab initio Local-Density-Functional study. T.Ochs, O.Beck, C.Elsässer, B.Meyer: Philosophical Magazine A, 2000, 80[2], 351-72