High-strain-rate superplasticity in metallic materials and the potential for ceramic materials

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. Its deformation mechanism appears to be different from that in conventional superplastic materials. Experimental investigations showed that a maximum elongation was attained at a temperature close to the partial melting temperature in many superplastic materials exhibiting high-strain-rate superplasticity. Recently, a new model, which was considered from the Viewpoint of the accommodation mechanism by an accommodation helper such as a liquid or glassy phase, 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 critical conditions of the quantity and distribution of a liquid phase for optimizing superplastic deformation was discussed and then applied to consider the possibility of attaining high-strain-rate superplasticity in ceramic materials.