Pergamon J. Mcch. Phys. Solids, Vol. 46. No. I, pp. I-28, 1998 86“ 1997 Elsevier Science Ltd Printed m Great Britain. All rights reserved PI1 : SOO22-5096(97)00038-4 0022-5096197 $17.00 + 0.00 MICROSTRUCTURAL FACTORS CONTROLLING THE STRENGTH AND DUCTILITY OF PARTICLE- REINFORCED METAL-MATRIX COMPOSITES J. LLORCA and C. GONZALEZ Department of Materials Science. Polytechnic University of Madrid, E.T.S. de lngenieros de Caminoa. 28040-Madrid, Spain (Rrcriwd 19 December 1996 ; in r~oi.sed fbrm 20 Mu>, 1997) ABSTRACT A micromechanical model is developed to simulate the mechanical response in tension of particle-reinforced metal-matrix composites. The microstructure of the composite is represented as a three-dimensional array of hexagonal prisms with one reinforcement at the centre of each prism. The shape, volume fraction and state (either intact or broken) of the reinforcement is independent for each cell, so the interaction among all these factors could be studied. The tensile response of the composite is determined from the behaviour of the intact and damaged cells, the fraction of damaged cells being calculated on the assumption that the reinforcement strength follows the Weibull statistics. The model is used to determine the microstructural factors which provide optimum behaviour from the point of view of the tensile strength and ductility. The analyses included the effect of the matrix and reinforcement properties, the reinforcement volume fraction. the interaction between reinforcements of different shape and the heterogeneous distribution of the reinforcements within the composite. c 1997 Elsevier Science Ltd. All rights reserved. Keywords: A. ductility, A. strengthening mechanisms, A. microstructures. A. fracture mechanisms, B. particulate reinforced material. I INTRODUCTION The reinforcement of metallic matrices (mainly aluminium and magnesium) with ceramic particles has given rise to a new group of composite materials. These particle- reinforced metal-matrix composites (PRC) use cheap ceramic reinforcements (nor- mally A1203 or SIC) and can be processed by standard techniques, factors which considerably reduce the manufacturing costs. The addition of the particles increases the stiffness and strength as well as wear and creep resistance, and the behaviour is nearly isotropic. This combination leads to an excellent balance between material price and mechanical properties, which is appealing for many industrial applications. From the mechanical behaviour standpoint, the main drawback of these materials is their very low ductility. This is shown in Fig. 1, where the composite tensile ductility (normalized by the ductility of the unreinforced alloy) is plotted as a function of the reinforcement volume fraction for a number of commercially available PRC (Lloyd. 1994; LLorca, 1994). Despite the experimental scatter, this figure shows that the ductility of these composites is significantly reduced as compared to the unreinforced