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I.

INTRODUCTION

IN most metallic materials, grain size influences mechanical properties, such as yield strength, ductility, fracture toughness, fatigue strength, and creep strength. In particle-reinforced metal matrix composites, the grain size is generally small compared to an unreinforced material of similar composition, and it has been shown that particle-stimulated nucleation (PSN), coupled with particle pinning during normal grain growth, is responsible for this small grain size. [~-1~ Accordingly, the influence of grain size on mechanical properties of these materials has received little attention, in part because of the perceived difficulty of producing substantial variations in grain size. Recrystallization studies in particle-reinforced metal matrix composites have shown that the nucleation potency of the reinforcement increases with increasing reinforcement size I5,61 and is more pronounced near reinforcement clusters. [8,91 These previous studies, however, have all been conducted using large deformations, where it would be expected that the nucleation density is high and the recrystallized grain size would be smaller than the interparticle spacing. Under these conditions, normal grain growth continues after complete recrystallization until the grain boundaries are pinned by the reinforcement. For a review of the various particlepinning models for normal grain growth, the reader is D.C. VAN AKEN, Associate Professor, is with the Department of Metallurgical Engineering, University of Missouri-Rolla, Rolla, MO 65401. P.E. KRAJEWSK1, Senior Research Engineer, Research and Development Center, General Motors Corporation, Warren, MI 48090. G.M. VYLETEL, Engineer, and J.E. ALLISON, Staff Scientist, are with the Ford Research Laboratory, Ford Motor Company, Dearborn, MI 48124. J.W. JONES, Professor, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Manuscript submitted October 3, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A

referred to the work of Nes et al. [lu and Hillert. [~21These models predict a limiting grain size that is defined by the volume fraction and size of the reinforcement. Monte Carlo simulations of recrystallization in materials containing particles have greatly enhanced the understanding of the statistical interactions between a moving boundary and the second-phase particles. [131As was first observed experimentally by Doherty and Martin, I14J these simulationst~31 showed that second-phase particles can either retard or are nonactive in the growth of new grains (or subcells), but the type of interaction depends upon the stored energy of the deformed matrix, e.g., stored dislocation density. At low (subcritical) stored energies, the second-phase particles pin boundaries and effectively inhibit the formation of new grains. Under these conditions, recrystallization remains incomplete or does not occur. At higher stored energies, the particles do not impede the recrystallization front and the final recrystallized grain size is inversely