Boron in copper: a perfect misfit in the bulk and cohesion enhancer at a grain boundary

Our ab initio study suggests that boron segregation to the Sigma 5(310)[001] grain boundary should strengthen the boundary up to 1.5 ML coverage (15.24 at/nm^2). The maximal effect is observed at 0.5 ML and corresponds to boron atoms filling exclusively grain boundary interstices. In copper bulk, B causes significant distortion both in interstitial and regular lattice sites for which boron atoms are either too big or too small. The distortion is compensated to large extent when the interstitial and substitutional boron combine together to form a strongly bound dumbell. Our prediction is that bound boron impurities should appear in sizable proportion if not dominate in most experimental conditions. A large discrepancy between calculated heats of solution and experimental terminal solubility of B in Cu is found, indicating either a sound failure of the local density approximation or, more likely, strongly overestimated solubility limits in the existing B-Cu phase diagram.