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

TL;DR

This study uses ab initio calculations to show that boron segregation at copper grain boundaries can strengthen the material, with optimal effects at specific coverages, and reveals discrepancies in solubility predictions.

Contribution

It provides new insights into boron behavior at copper grain boundaries and bulk, highlighting the strengthening mechanism and solubility issues with existing models.

Findings

Boron segregation strengthens copper grain boundaries up to 1.5 ML coverage.

Maximum strengthening occurs at 0.5 ML coverage with boron filling boundary interstices.

Discrepancy between calculated heats of solution and experimental solubility suggests phase diagram inaccuracies.

Abstract

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.