Using a first-principles computational tensile test, we show that the ideal tensile strength of an Al grain boundary ͑GB͒ is reduced with both Na and Ca GB segregation. We demonstrate that the fracture occurs in the GB interface, dominated by the break of the interfacial bonds. Experimentally, we further show that the presence of Na or Ca impurity, which causes intergranular fracture, reduces the ultimate tensile strength when embrittlement occurs. These results suggest that the Na/ Ca-induced intergranular embrittlement of an Al alloy originates mainly from the GB weakening due to the Na/ Ca segregation.

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