The effects of annealing temperature and oxygen atmosphere on the grain growth of pure Ag have been investigated. The cold-compressed Ag specimens are annealed at various temperatures under oxygen atmosphere and vacuum, respectively.
The interior and surface of the specimen annealed at 800℃ under oxygen atmosphere show the abnormal grain growth(AGG), while those under vacuum show the normal grain growth(NGG). As annealing temperature increases, the coarsening behavior of the internal and surface grains gradually changes from the AGG to the NGG under both oxygen atmosphere and vacuum. In case of annealing under oxygen atmosphere, compared to annealing under vacuum, the AGG has been observed at higher temperature and the microstructure has a stronger tendency of the abnormal behavior.
The relationship between the grain coarsening behavior and the grain boundary structure is discussed to explain the experimental observations. The grain boundary faceting has been observed frequently in the specimen annealed under oxygen atmosphere and the dihedral angle distribution of the specimen annealed under oxygen is wider than that under vacuum. Also the surface microstructure under oxygen atmosphere has been observed to have thermal faceting at higher temperature. Thus the role of oxygen in Ag is to make surface and grain boundary structure ordered and to increase the anisotropy of surface and grain boundary energy. In polycrystalline materials, there are two types of grain boundaries. These are special boundaries having ordered atomic structure and random boundaries having disordered atomic structure. Impurity addition as well as temperature change causes the grain boundary phase transition. As the grain boundary phase transition occurs, the grain boundary mobility and diffusivity abruptly change. The occurrence of the AGG is thought to be related to the existence of special boundary. The special grain boundaries in the specimen annealed at low temperature or under oxygen atmosphere are supposed to have the non-linear mobility and anisotropic grain boundary energy. These special grain boundaries seem to trigger the evolution of the AGG in the pure Ag specimen.
However, the mechanism through which the special grain boundaries evolve the abnormal grain is not well understood yet. Also the motions of grain boundaries are coupled each other through the grain boundary triple junctions and many geometrical constrictions exist during the grain coarsening in polycrystalline materials. Thus it seems to be difficult to know how the special grain boundaries affect the grain coarsening behavior in polycrystalline materials and trigger the AGG.
