A Kinetic Study of the Emerging of Grains and Block of Grains from the Inner Volume to the Free Surface of a Cd-Zn Alloy Superplastically Deformed

A local measurement technique for the study of the kinetic processes of emerging of grains or blocks of grains from the inner volume to the free surface of superplastic materials during deformation is presented and used for the case of the Cd-Zn eutectic alloy deformed at room temperature. This technique could be used to evaluate the approximate time of fracture due to fissure or cavitation growth in samples under superplastic deformation. In principle, this technique will be useful for the development of physical procedures, which allows retarding the process of formation of low mismatch angle, , between neighboring grains, process which gives place to blocks of grains which dynamically behave as units under the shear stress action. For materials with nanocrystalline structures, such processes are expected to be higher than those of the case of microcrystalline materials. 1. Introduction Superplasticity is the ability of polycrystalline material to exhibit, in a generally isotropic way, very high tensile elongations prior to failure [1]. Such phenomena occur mainly by grain-boundary sliding (GBS), between individual grains, with size lower than 10 micrometers, which glide relative to each other with little or no change in shape during long plastic deformation [2, 3]. Also, there exists some experimental evidence for cooperative grain-boundary sliding during superplastic deformation [4–7]. Most of the early studies dealt with microduplex metallic alloys which make the grain growth very difficult, and the unabated scientific interest on superplasticity has from long ago broadened the scope to include intermetallic compounds and nanocrystalline materials [8]. This interest has arisen partly from a scientific viewpoint and partly from the increasing awareness that superplastic materials can be utilized in forming complex shapes in simple and inexpensive forming operations. There are two main advantages in utilizing superplastic materials for metal forming operations. First, large strains can be achieved without necking. Second, the stresses required for superplastic deformation are generally low [9]. Recently, a model which was validated against experimental observations concerning metals, alloys and ceramics of micrometer and submicrometer grain sizes, nanostructured materials, and intermetallics has been proposed to account for optimal superplasticity; their central assumption is that the rate controlling deformation process on GBS is confined to high-angle grain/interphase boundary regions that are essential for grain-boundary sliding developing to a