Investigation of grain-boundary structure-segregation relationship in an N18 nickel-based superalloy

Atom probe tomography and transmission electron microscopy were associated in order to characterize both structurally and chemically different general grain boundaries in a boron-doped N18 nickel-based superalloy. Boron, molybdenum and chromium segregation to the grain boundaries was demonstrated. The distribution of segregated atoms is shown to be nonuniform, both perpendicular to and in the plane of the grain boundaries. The Gibbsian interfacial excess was measured as a function of the misorientation angle and of the degree of twist-tilt character of the grain boundaries, leading to better knowledge of the structure-segregation rate relationship. For boron interfacial excess higher than a critical value, a thin chemically disordered film forms on each side of the grain boundaries. This film could explain the boron-induced mechanical reinforcement of the grain boundaries.