RELATIONSHIPS BETWEEN INTERFACE STRUCTURE AND SUPERPLASTIC DEFORMATION

Superplastic flow in polycrystals is achieved through atom by atom transport of matter from interfaces stressed in compression to those in tension. The rate of superplastic flow is therefore controlled by the rate of emission and absorption of atoms at interfaces and by the rate of grain boundary diffusion. The structure of the interface can influence both these processes. In recent years the understanding of the relationship between structure and superplastic behavior has evolved through a multifaceted approach that includes phenomenology, micromechanical modelling, critical experiments, and studies of interface structure in a variety of materials. On this basis, three types of structure dependent superplastic flow are explored in this paper. In Type I, boundary structure is such that the exchange of atoms at the interface is much faster than grain boundary diffusion. In this type boundaries generally have a high Σ structure. In Type II, boundaries contain a small amount of fluid which leads to enhanced diffusion at the interface provided that the crystal phase is at least somewhat soluble in the liquid. In Type III the interfaces are predominantly low angle boundaries. Interestingly, the last type produces the fastest rates of superplastic deformation and, therefore, is technologically most significant, but also is the least understood. We speculate that grain boundary dislocations shuttle between interfaces to produce Type III superplasticity