  Alloying a material and hence segregating solutes to grain boundaries is one
way to tailor a material to the demands of its application. Direct observation
of solute segregation is necessary to understand how the interfacial properties
are altered. In this study, we investigate the atomic structure of a high angle
grain boundary both in pure copper and upon silver segregation by
aberration-corrected scanning transmission electron microscopy and
spectroscopy. We further correlate the experiments to atomistic simulations to
quantify the local solute excess and its impact on grain boundary properties.
We observe that the grain boundary structure remains intact upon silver
segregation and up to five different positions within a structural unit serve
as segregation sites. By combining the atomic resolution observation with
atomistic modelling, we are able to quantify the local silver concentration and
elucidate the underlying segregation mechanism.
