Abstract
High-strain-rate superplasticity 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 3μm. This high-strain-rate superplasticity is defined in JIS H7007 by Japanese Standards Association as superplasticity which appears when the rate of strain is 10-2 s-1 or over. These strain rates are considerably higher than the typical forming rate used for the conventional superplastic materials, but rather close to the commercial hot working rates of 10-1 to 10-2 s-1. Its deformation mechanism appears to be different from that of conventional superplastic materials. A new model is considered from the viewpoint of the accommodation mechanism by an accommodation helper such as a liquid or glassy phase. The new mechanism is proposed in which superplasticity is 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. A few examples of the industrial applications of high-strain-rate superplasticity are demonstrated.
