An article and a method for forming a single crystal casting are disclosed. The article includes a single crystal nickel-based superalloy having a composition including greater than about 80 ppm boron (B) and a substantially single crystal microstructure with at least one grain boundary. A creep rupture strength of the article is substantially maintained up to a mismatched grain boundary of about 40 degrees. The method for forming a single crystal casting includes positioning a mold on a cooling plate, the mold including a single crystal selector, providing a molten nickel-based superalloy composition in the mold, the molten composition including greater than about 80 ppm boron (B), cooling the molten composition with the cooling plate, and forming a unidirectional temperature gradient by withdrawing the mold from within a heat source to form the single crystal casting including a substantially single crystal microstructure having at least one grain boundary.