Sodium-induced embrittlement of an aluminum grain boundary

Primary aluminum produced industrially by electrolysis inevitably contains some sodium which is an undesired impurity element in aluminum alloys as it promotes intergranular fracture. However, the physical origins of Na-induced intergranular embrittlement in aluminum are still unclear. This work provides a comprehensive investigation of the nature of the Na-induced grain-boundary embrittlement in aluminum by means of first-principles calculations with the highly precise full-potential linearized augmented plane-wave method within the framework of the Rice-Wang thermodynamic model and within the method of ab initio tensile test. We introduce a free-surface slab model and determine the grain-boundary and free-surface energies, the most energetically favorable segregation site of Na along the Al grain boundary, its segregation energy to the Al grain boundary, and the possible fracture modes of the grain boundary with Na in the different sites and their corresponding fracture energies. We establish that Na has a large driving force (-0.84eV/atom) to segregate from Al bulk to the symmetrical grain-boundary core site, and its segregation significantly reduces grain-boundary strength. We show that the method using the Rice-Wang thermodynamic model and the method of ab initio tensile test are essentially equivalent and both confirmed that Na is a strong intergranular embrittler with a potency of +0.62eV/atom. Na segregation leads to grain-boundary expansion and a significant charge density decrease over the whole grain boundary. Analysis in terms of the relaxed atomic and electronic structures and bonding characters shows that the aluminum-sodium bond has ionic character and is weak in both grain-boundary and free-surface environments.