Recent works in high strain rate superplasticity

High strain rate superplasticity (i.e., superplastic behavior at strain rates over 10(-2) s(-1)) has been observed in many metallic materials such as aluminum alloys and their matrix composites and it is associated with an ultra-fine grained structure of less than about 3 mu m. Experimental investigations showed that a maximum elongation was attained at a temperature close to the partial melting temperature in many high-strain-rate superplastic materials. Recently, a new model, which was considered from the viewpoint of the accommodation mechanism by an accommodation helper such as a liquid, was proposed in which superplasticity was critically controlled by the accommodation helper both to relax the stress concentration resulting from the sliding at grain boundaries and/or interfaces and to limit the build up of internal cavitation and subsequent failure. The strong supports far this hypothesis were provided with direct observations of segregation at interfaces or grain boundaries and the preferential melting of these enriched grain boundaries using an in situ TEM technique, and of the nucleation, growth and interlinkage of cavities during high-strain-rate superplastic flow. The critical conditions of the quantity and distribution of a liquid phase for optimizing superplastic deformation was investigated and then the origin of high strain rate superplasticity was considered for attaining the optimum design in microstructural control for the distribution of the accommodation helpers as well as the grain size refinement.