Aluminum grain boundary decohesion by dense sodium segregation

Despite numerous investigations, grain boundary (GB) embrittlement of metallic structural materials is a poorly understood fundamental phenomenon in materials science. One of the well-known examples is that minute traces of sodium induce an embrittlement in aluminum alloys that results in drastic failure and limits their applications. From first-principles density function theory calculations, we found that sodium atoms densely segregate and neighbor into the 5(012)[100] GB in aluminum with large segregation energies and that the GB strength drops to only one fifth of the strength of the clean Al GB. Gradual sodium segregation leads to not only a large GB expansion but also to the replacement of stronger Al-Al metallic bonds with the weaker Al-Na mixed ionic-metallic bonds and Na-Na metallic bonds. This result in a drastic GB decohesion that reduces the GB tensile strength dramatically until it approaches the strength of bulk sodium. Dense segregation of sodium forms a Na film along the GB and opens an easy channel for oxidation and corrosion along the GB, which further accelerates the intergranular embrittlement.