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The effect of grain size on the SRS of the Al-Mg alloy (Al 5083) was examined using nanoindentation at different loading rates (50-50,000 (microN/s), and by uniaxial compression tests with strain rates between 10(exp-6) and 1 s(exp-1). The negative SRS was more pronounced in the NC powder than in the UFG material, while it was slightly positive in the conventional CG alloy. The negative SRS in the NC and UFG materials was attributed to pinning of dislocations by interstitial solute atoms as well as the small grain size, both of which restrict dislocation motion. Indentation at higher loading rates is expected to trigger dislocation motion by unlocking from solute atoms, increasing the density of mobile dislocations and causing greater plastic deformation for a given load. The negative SRS was more pronounced at lower loading rates during nanoindentation, suggesting that thermally activated grain boundary diffusion of solute atoms has an important role in pinning dislocations. The activation volumes of the NC and UFG materials suggest that overcoming the Peierls type barrier caused due to pinning by solute atoms is rate-controlling, while that of the CG material indicates that dislocation intersections are involved.