Critical review of mechanism of superplastic deformation in fine grained metallic materials

Abstract

Superplastic deformation is arguably the most dramatic and direct illustration of the influence of grain boundaries on the mechanical properties of fine grained metals. Stable tensile elongations in excess of 1000%can be attained under suitable conditions, and microstructural observations show that most of this strain is a direct result of grain boundary sliding, with only transient accommodation strains taking place within the grains. The aim of this paper is to provide a critical review of some of the mechanisms that have been proposed to explain this behaviour. The attributes of superplasticity with which models must be consistent are first summarised, and superplasticity is compared with diffusion creep, this being an obvious starting point for understanding the process. Various models from the literature are then described and compared critically with the experimental evidence and the theory of grain boundary structure. Models based on the climb of dislocations from the head of a pileup are shown to have serious shortcomings on both theoretical and experimental grounds. A model developed by the author based on the basic geometrical theory of grain boundary structure is described, and is shown to lead quite naturally to explanations for many of the attributes of superplastic deformation. Finally, the paper identifies areas in which further progress is required.