BORON AND CARBON IN NICKEL, IRON, AND NI3AL

Small amounts of boron and carbon in nickel, iron, and Ni3Al behave similarly except that carbon, when segregated into grain boundaries in Ni3Al and other nickel rich alloys, promotes rather than suppresses intergranular embrittlement. It is suggested that this stems from the absence of a stable carbide in the nickel system, as contrasted with the readiness to form carbides and borides in the Fe-C, Ni-B, and Fe-B systems. Excess carbon in nickel rich grain boundaries is thus expected instead to form graphite-like structures, so weakening the boundaries. This behaviour is attributed to a large electron concentration, leading to some occupation of antibonding spd hybrid states and a positive heat of solution.