When deformed Ni specimens are annealed in vacuum, abnormal grain growth (AGG) occurs at all temperatures between 0.55 Tm and 0.95 Tm, where Tm is the melting point of Ni. The addition of carbon to Ni suppresses the tendency for AGG. If Ni is carburized, no AGG occurs at temperatures above 0.7 Tm. The specimens showing pronounced AGG have grain boundaries which are faceted at large scales enough to be observable under an optical microscope and a transmission electron microscope. These observations show that AGG occurs when the grain boundaries are faceted. Grain boundary faceting indicates that grain boundary structure is ordered and its energy is anisotropic. It is possible that the grain boundary migration mechanism varies with the grain boundary structure and the migration rate of the faceted boundaries may vary non-linearly with the driving force. This may lead to AGG.
The AGG behavior of Ni-base alloys, Ren 41 and a model alloy (Alloy M) with a composition of Ni- 24 Co- 4 Al- 4 Ti- 5 Cr- 5 Mo by weight percent has been also investigated. In Ren 41, no AGG occurs during annealing above 1200℃ in Ar but decarburization at the specimen surface during annealing in air induces AGG. These results are similar to those observed in Ni. Alloy M showed no AGG during annealing at 1300℃ in Ar and the grain boundaries were smoothly curved as observed at the intergranular fracture surface. During annealing at 1200℃, however, AGG occurred and the grain boundaries were faceted. These results confirm that observations in Ni that when the grain boundaries are faceted at low temperatures, AGG occurs.
In Fe- 3% Si steel AGG occurs while producing small island grains behind the boundary of the growing grains. The three- and four-sides grains (as seen at cross section) often have convex shapes, indicating that they are also growing. These observations are consistent with the proposal that AGG in this alloy occurs by the penetration of grain boundaries and triple junction by the growing grains when the grain boundaries are anisotropic. Such penetrations may occur if the grain boundaries are pinned by precipitates as in this alloy.
In Ren 41, grain boundary serration has also been observed to be induced by the precipitation of plate-like $M_6C$ during heat-treatment at 1050℃-1070℃ and the grain boundaries were observed to migrate during the liquation of MC particles during the heat-treatment at 1250℃. The latter phenomenon is a type of chemically induced grain boundary or liquid film migration.
