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By using the constitutive equation based on the mechanism-based strain gradient plasticity with finite element software, the yield strength, uniform elongation, and toughness of aluminum alloy 6063 with different grain sizes, different particle diameters and volume fractions were studied numerically. The toughness is defined as the product of yield strength and uniform elongation. The calculation results indicate that the grain refinement and particle refinement cannot substantially improve the uniform elongation but can increase the yield strength of Al alloy when the grain size is on the order of the micron and submicron scale. When the grain size less than 2 mu m, Al alloys usually exhibit high strength and low uniform elongation, and when the grain size greater than 5 mu m, the materials exhibit low strength and high elongation; in either case the toughness is low. However, in the grain size of several micrometers, the toughness of Al alloy is the highest. (C) 2010 Elsevier Ltd. All rights reserved.