Mechanism of abnormal grain growth in ultrafine-grained nickel

Abstract

During annealing of ultrafine nickel powder compacts, abnormal grain growth (AGG) occurred with the formation of cube-shaped grains. The observed AGG is similar to the late-stage abnormal grain growth (LS-AGG) that is commonly observed in electrodeposited (ED) Ni sheets. The cube shape of the abnormal grains with {100} facets was identical to that of late-stage abnormal grains in ED Ni. As the temperature increased, the time to the appearance of abnormal grains decreased. After impingement of abnormal grains, little further grain growth occurred, indicating stagnant grain growth behavior. The formation of abnormal grains in the present study, however, cannot be explained by the previously suggested AGG mechanisms, i.e. the S phase wetting mechanism and initial texture mechanism, because no amorphous phase containing S was present and no texture developed in our samples. In contrast, the observed grain growth behavior can be well explained in terms of the coupling effect of the maximum driving force for grain growth and the critical driving force for appreciable migration of faceted boundaries. It is concluded that the AGG in ultrafine-grained Ni occurs via mixed migration mechanisms, i.e. diffusion and interface reaction-controlled, of faceted boundaries with respect to the driving force.