Quasi-static micro-indentation hardness studies of specimens of high-purity polycrystals and single crystals were performed at room temperature by using a Vickers diamond and a WC-Co sphere with a radius 200µm. The polycrystalline specimens were prepared by the heavy deformation of as-received specimens, followed by annealing at various temperatures and times. The resultant grain sizes ranged from 15 to 520µm. As-formed specimens with various grain sizes were found to have differing dislocation densities. The smaller the grain size, the higher was the dislocation density. The Vickers hardness for a given indenter load, as determined by using the projected contact areas of indentations, was found to increase with decreasing grain size. In the same way, the Meyer hardness

increased with decreasing grain size. It was argued that the observed increase in hardness of the polycrystals, with decreasing grain size, was due to the initial dislocation densities in the grains and was not due to the grain boundaries which, because of an absence of associated impurities, did not appear to act as dislocation barriers in this high-purity face-centered cubic metal. As the indentation hardness values of single crystals of differing orientations were very similar, any differences in the orientations of contiguous grains could not affect the hardness values of the polycrystals. Thus, for large single crystals (10mm x 10mm x 10mm) having the same dislocation densities as that of polycrystalline specimens of grain size 15µm, the indentation hardness values were quite similar.

The Influence of Grain Size on the Indentation Hardness of High-Purity Copper and Aluminium. Y.Y.Lim, M.M.Chaudhri: Philosophical Magazine A, 2002, 82[10], 2071-80