It is a significant fact that the size of second-phase particles has an important effect on the macroscopic plastic work hardening behavior of metals and their alloys or metal-matrix composites. The classical plasticity theories cannot explain this size effect since their constitutive laws possess no internal material lengths. We use the theory of mechanism-based strain gradient (MSG) plasticity to investigate the particle size effect and find good agreements with the experiments of aluminum matrix reinforced by silicon carbide particles as well as with prior numerical studies by other strain gradient plasticity theories. It is shown that, at a fixed particle volume fraction, smaller particles give larger plastic work hardening of the composite than large particles do.
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