The fact that traces of the element Bi embrittle the normally ductile metal copper has been known for over a hundred years. There have been some previous attempts to explain this and similar phenomena in terms of a weakening of Cu-Cu bonds by withdrawal of elecrons by the impurity atoms segregated at grain boundaries. Another class of explanation refers to a possible 'stiffening' of the bonds around an impurity, thereby reducing the local ductility. We have made a series of calculations using density functional theory to study electronic structure, the strength of bonding across a grain boundary (Σ19a) and the local 'stiffness' of bonds.

Our findings cast doubt on the conventional explanations. Bi is a much larger atom than Cu. However, it has negligible effect on bond-stiffness in the direction required for dislocation mobility, on the (111) plane. Neither does it withdraw electrons from Cu-Cu bonds. The explanation instead is a simple size effect, which drives segregation to boundaries that have excess volume, where the Bi atoms `lever' the Cu atoms apart. We have repeated our calculations for the embrittling element Pb, also an oversized impurity,and find a very similar set of results.

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