Pergamon PII: S0020-7403(97)00056-8 Int. J. Mech. Sci. Vol. 40, Nos. 2 3, pp. 305-311, 1998 (Q 1997 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0020-7403/98 $19.00 + 0.130 HIGH-STRAIN-RATE SUPERPLASTICITY OF PARTICULATE REINFORCED ALUMINIUM MATRIX COMPOSITES K.C. CHAN and B.Q. HAN Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Hung Horn, Hong Kong Abstract In recent years, there have been a lot of research efforts in studying high-strain-rate superplasticity of aluminum matrix composites. However, the superplastic deformation behavior of the composites has not been fully understood. Superplastic deformation mechanism in aluminum matrix composites was considered to be grain boundary sliding and interfacial sliding. In this paper, the model proposed recently by the authors to explain the superplastic deformation of a particulate reinforced aluminium matrix composite at temperatures above and below its solidus temperatures is further developed. The theoretical predictions are in agreement with the published experimental findings. © 1997 Published by Elsevier Science Ltd. Keywords: high strain rates, metal matrix composites, superplasticity. NOTATION cr applied stress ao threshold stress ~ons strain rate due to grain boundary sliding ~l strain rate due to interfacial sliding ~t total strain rate G shear modulus Dr interfacial diffusivity Dgb grain boundary diffusivity 11 length of the pile-up in front of the reinforcement dm matrix grain size d, particle size of reinforcement Vr volume fraction of reinforcement h climb distance T absolute temperature T" solidus temperature b Burgers vector k Boltzmann's constant w binding energy between a solute atom and a boundary dislocation Co solute concentration corresponding to w = 0 2 distance between solute atoms on a dislocation line INTRODUCTION Discontinuously reinforced metal matrix composites (MMCs) have been successfully manufactured through powder metallurgy technology in the last decade. This kind of MMCs is attractive for many structural applications because of their high specific strength and modulus of elasticity. However, in general, these materials have relatively low room temperature ductility which means that to certain extent they are not easy to be shaped. Even at elevated temperatures, they normally show only limited tensile ductility. Recently, a number of researchers [1-7] have reported that some discontinuously reinforced aluminum MMCs could behave superplastically when tested at high strain rates under the right conditions. These findings are significant since one of the major shortcomings of conventional superplastic forming process is that the forming rate is too low. The discovery of superplastic capability of some aluminum-based MMCs at high strain rates thus gives these materials a better opportunity for industrial applications. Although, there are so many experiments on this kind of superplasticity, deformation mechanism on high-strain-rate superplasticity of MMCs has not been 305