Superplasticity hos been observed in many aluminum metal-matrix composites at extremely high strain rates (approximately 0.1–1 S−1). These materials generally exhibited a strain-rate sensitivity value of about 0.3 and a maximum elongation of about 300%. It is believed thot the presence ofa liquid phose, or in some cases a low-melting-point region, at the reinforcement/matrix interfaces is responsible for the phenomenon. This phenomenon is not observed in all reinforced composites, despite the fact thot they contain fine grain sizes. Thus, a fine matrix grain size is a necessary but insufficient condition for the high-strain-rate superplasticity.