Cu Grain Boundary Embrittlement by Liquid Hg: A Comparison between Experiment and ab-initio Modeling
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Cu Grain Boundary Embrittlement by Liquid Hg: A Comparison between Experiment and ab-initio Modeling Julien Colombeau1, Thierry Auger1, Duane Johnson2 and Linlin Wang2 1 MSSMat laboratory, UMR CNRS 8579, Ecole Central Paris, Chatenay-Malabry, France 2 Ames laboratory, Iowa State University, Ames, Iowa, USA ABSTRACT We have studied the LME phenomenon for the Cu/Hg couple, from an experimental and a computational point of view. We compared the LME behavior of standard oxygen free high conductivity (OFHC) copper with Grain Boundary Engineered (GBE) copper (containing a high fraction of special ∑3 GBs). Experimentally, we find that special ∑3 GBs in copper are less prone than general GB to LME by liquid mercury. In parallel, we have investigated the difference in LME induced fracture between the symmetric ∑3(111)[110]70.5° tilt GB and the symmetric ∑5(210)[100]36.87° tilt GB by ab-initio calculations. The Hg segregation trend has been evaluated for these 2 GBs. Ab-initio tensile tests on the ∑ 3(111) GB with and without segregated Hg atoms have been performed. Finally solid/liquid interfaces have been modeled using ab-initio molecular dynamics (AIMD) in order to calculate solid-liquid surface energies (γSL). Using a Griffith approach, we have evaluated the energy difference γGB - 2 γ SL. The LME mechanism in Cu/Hg is discussed. INTRODUCTION Liquid metal embrittlement (LME) is the phenomenon of reduction of the fracture resistance of a normally ductile solid metal (SM) when it is stressed in contact with a liquid metal (LM). In most cases, LME induces an intergranular brittle fracture. This implies that grain boundary (GB) embrittlement by a liquid metal is the main process to be understood. At the same time, there are several experimental observations of segregation of impurities at general GBs for some of the SM/LM couples that are prone to LME (1). In particular, high resolution transmission electron microscopy experiments of annealed samples clearly showed segregation of impurity atoms into general GBs (2). It is therefore natural to think that LM atoms segregation into SM GBs is an important microscopic aspect of the LME mechanism. Several studies have been carried out these past two decades, attempting to understand LME using ab-initio calculations. From such studies, several explanations have been given to explain the GB weakening once an impurity has segregated. Recently it was advocated that the weakening is due to an atomic size effect, in which the atomic bonds are weakened due to segregated atoms pushing away neighbor SM atoms (Bi in Cu or Ni for example) (3). Another point of view is based on the electronic charge transfer from SM/SM bonds to SM/impurity bonds with the net effect of weakening SM/SM bonds (4). Notwithstanding the embrittlement origin, the assumption made in these models is that the impurity is already present at the GB. This is not necessarily the case in the process of LME which is a fast process where only adsorption could occur. It is one of the goals of this paper to investigate whether o
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