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An ab initio Investigation on the Effects of Impurity in Aluminum Grain Boundary Guang-Hong Lu, Tomoyuki Tamura, Masao Kamiko, Masanori Kohyama1, Ryoichi Yamamoto Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 1 Special Division of Green Life Technology, AIST Kansai, 1-8-31 Midorigaoka, Ikedashi, Osaka 563-8577, Japan

ABSTRACT The electronic structure of AlΣ9 tilt grain boundary with segregated impurity atoms of Na, Ca, Si and S, respectively, has been investigated by an ab initio pseudopotential method. Na and Ca segregation causes the boundary to expand and the charge density to decrease significantly. There forms several weak bond regions. Si segregation increases the charge density between Si and the neighboring Al atom. There forms a stronger Al-Si bond that is a mixture of covalent and metallic character in the boundary. For S segregation, though there forms the stronger bond between Al and S atom, some Al-S bonds may become weaker than the former Al-Al bonds because of the charge density decrease. It is concluded that the mechanism of Na or Ca-promoted Al grain boundary embrittlement is one kind of ‘decohesion model’, that of Si is ‘bond mobility model’. It can’t be decided the embrittlement mechanism by S segregation is classified into ‘bond mobility model’ or ‘decohesion model’.

INTRODUCTION It seems that theoretically there are two kinds of mechanism of impurity-promoted embrittlement of metal grain boundaries. One is called ‘decohesion model’. In this model, the metal-impurity or metal-metal cohesion is weakened by impurity segregation in metal grain boundary and thus induces embrittlement. A typical one is presented by Losch[1], which indicates that the metal-metal bonds neighboring to the impurity-metal bonds are weakened by the embrittling element in group IV-VI. Supporting calculations are carried out by Briant and Messmer[2-3] on a nickel cluster with sulfur impurity, and a iron cluster with phosphors impurity, respectively. The other is ‘bond mobility model’ proposed by Haydock[4]. In this model segregated non-metallic impurities at the metal grain boundary bind with the neighboring metal atoms. This increases the covalent character of the bonds and reduces their mobility. Under stress, the bonds between grains cannot slide smoothly and thus stress concentration is generated. Goodwin et al. [5-6] carried out the supercell calculation based on the density functional theory (DFT) on the embrittlement by Ge and As impurities in the host Al[111], supporting ‘bond mobility model’. Recently it has been experimentally found that Na, Ca[7-8] and S[9] have the embrittling effect on Al alloys. As for Si, it is reported that the precipitated Si promotes the nucleation and growth of the voids of the Al conductor films for LSI use[10]. Therefore we carry out the calculation of the effects of these impurities on the Al grain boundary. Results show that it is of great interest to compare the effects of these impurities segregated on the Al boundary. R2.4.1