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The Embrittlement of Cu by Bi: explanation based on first-principles calculations

Mike W. Finnis 1Tony Paxton 1Rainer Schweinfest 1,2

1. Queen's University Belfast (QUB), Stranmillis Road, Belfast BT9 5AH, United Kingdom
2. science and computing ag, Hagellocher Weg 71, Tübingen 72070, Germany

Abstract

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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Presentation: invited oral at E-MRS Fall Meeting 2004, Symposium H, by Mike W. Finnis
See On-line Journal of E-MRS Fall Meeting 2004

Submitted: 2004-06-30 09:36
Revised:   2009-06-08 12:55