Now and future 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. In order to obtain high strain rate superplasticity, it is required not only to attain ultra-fine grain size but also to relax the stress concentrations around second phase particles or reinforcements. A special accommodation process by an accommodation helper such as a liquid phase is required to attain superplasticity when the stress concentrations around second phase particles or reinforcements are not relaxed by diffusional flow and/or diffusion-controlled dislocation movement in a solid state, On the other hand, a special accommodation process is not required when the stress concentrations are sufficiently relaxed by the diffusion processes in a sold state. The critical strain rate is presented from this view, in a strain rate range below which the accommodation mechanism is diffusional flow and/or diffusion-controlled dislocation movement in a solid state, on the other hand, in a strain rate range above which a special accommodation mechanism is required.