In part Ⅰ, the effects of deformation and grain boundary structure on grain growth in Cu of 99.9999% purity have been studied. Firstly, the grain growth occurring after the large deformed specimens are recrystallized has been investigated. Stagnant grain growth occurs when the specimens are compressed to 20% and annealed at 600℃, 10% at 700℃, and 5% at 800℃. However, when the specimens are compressed to more than these deformations and annealed at each temperature, abnormal grain growth (AGG) occurs. The results indicate that there is a critical grain size for AGG at each temperature and as annealing temperature increases the critical grain size for AGG increases. When annealed at various temperatures after the same deformation (35%, 50%, 65%), the number of the abnormally large grains increases with increase of annealing temperature, and finally their impinged grain size decreases. When annealed at the same temperature (600℃, 800℃, 950℃) after various deformations, the number of the abnormally large grains increases with increase of deformation amount, and their size decreases. When annealed at 800℃ after 65℃ or 75℃ compression or at 950℃ after 50% compression, double AGG occurs. Double AGG occurs in the condition that the impinged grain size after AGG is smaller than the critical grain size at each emperature and therefore readily occurs during annealing at high temperature after high deformation.
Secondly, the grain growth occurring in the specimens slightly deformed has been investigated. It is observed that when the specimens in stagnant grain growth conditions are slightly deformed, AGG readily occurs. If AGG is already occurring, the small deformation causes more grain to grow abnormally and sooner. The observations with transmission electron microscopy (TEM) show that the dislocation density within the grains after some growth in the deformed specimen is about same as that in the undeformed specimen. But the dislocation density at the grain boundaries of the deformed specimens remains high during the heat-treatment. Therefore, the main effect to small deformation is to increase the dislocatiion density at grain boundaries, which increases in effect their mobility at relatively small driving forces.
The results obtained from experimental observations have been discussed in views of two-dimensional nucleation theory for AGG of polycrystalline materials. The observations with TEM show that the grain boundaries in the specimens where AGG or stagnant grain growth occurs are faceted, indicating atomically singular structure and the fraction of the faceted grain boundary decreases with increase of annealing temperature. The results support the hypothesis that AGG occurs because the faceted singular grain boundary migrates either by two-dimensional nucleation of the boundary steps or by spiral growth on defects.
In part Ⅱ, the observations of island grains formed during annealing of pure Cu, their orientation determined by electron back scattered pattern, and their shrinkage upon further annealing are described and these results are discussed in terms of the dependence of the grain boundary migration on its structure. The island grains observer in the specimen where AGG occurs after prolonged annealing under stagnant grain growth condition have low angle misorientation relationship with abnormally large grains, indicating low grain boundry energy and mobility. In spite of their low energy and mobility, the island grains are unstable and eventually shrink. However, when AGG occurs very rapidly with fine matrix grains, even those grains with relatively high boundary energy and mobility are also trapped in the growing grains and shrink to disappear very rapidly.
