Grain boundary faceting and abnormal grain growth in nickel

A correlation between grain boundary faceting and abnormal grain growth has been observed in recrystallized polycrystalline Ni at varying annealing te mperatures, with or without C added. Carburized Ni specimens deformed to 50 pet show faceted grain boundaries and abnormal grain growth when annealed at temperatures below 0.7 T-m, where T-m is the melting point of Ni in abso lute scale. When annealed at or above 0.7 T-m, the grain boundaries are smo othly curved and, therefore, have a rough structure, and normal grain growt h is observed. In the specimens annealed in vacuum without carburization, a ll grain boundaries are faceted at 0.55 T-m, and some of them become deface ted at higher temperatures. The specimens annealed in vacuum at temperature s between 0.55 and 0.95 T-m show abnormal grain growth. When the grain boun daries have a rough structure and are, therefore, nearly isotropic, normal grain growth is indeed expected, as shown by the simulation and analytical treatment. When all or a fraction of the grain boundaries are faceted, with the facet planes corresponding to the singular cusp directions in the vari ation of the boundary energy against the inclination angle, abnormal grain growth can occur either because some grain boundary junctions become immobi le due to a torque effect, or the growth occurs by a step mechanism.