Characterization of ultra-fine grained materials produced by torsion straining

It is well established that superplasticity occurs by the process of grain boundary sliding and it requires a very small grain size, typically in the range of similar to 1-10 mu m It is anticipated that further reduction of grain size, especially below 1 mu m, will lead to superplasticity at high strain rates or low temperatures, which is clearly beneficial for superplastic forming processes. Several methods are available for reducing the grain size of polycrystalline samples based on subjecting the materials to intense plastic straining. Of these procedures, torsion straining under high pressure leads to small disks, with diameter of similar to 15 mm, which can be easily prepared for examination by transmission electron microscopy. This paper describes the synthesis and characterizations of three different materials (Al-3%Mg, Cu and Ni) prepared in a submicrometer condition using the torsion straining technique. Specimens were examined using high-resolution electron microscopy and it is demonstrated that the grain boundaries of these materials are in a high-energy and non-equilibrium configuration and contain regular and irregular arrangements of facets and steps. This paper describes the nature of the boundaries in these three materials and the significance of processing at different fractions of the melting temperatures.