The influence of grain size on the indentation hardness of high-purity copper and aluminium

Quasistatic microindentation hardness studies of specimens of high-purity polycrystals and single crystals of copper and aluminium have been made at room temperature using a Vickers diamond and a tungsten carbide-cobalt sphere of radius 200mum. The polycrystalline specimens of copper were prepared by heavy deformation of as-received specimens followed by thermal annealing at different temperatures and for different times; the grain sizes produced were in the range 15-520 mum. The polycrystalline samples of the as-received aluminium had a grain size of 330 +/- 40 mum. As-formed specimens of copper of different grain sizes were found to have different dislocation densities; the smaller the grain size, the higher was the dislocation density. The Vickers hardness for a given indenter load, determined using the projected contact areas of indentations, was found to increase with decreasing grain size. Similarly. the Meyer hardness increased with decreasing grain size. It is argued that the observed increase in hardness of the polycrystals with decreasing grain size is due to the initial dislocation densities in the grains and not due to the grain boundaries which, because of the lack of impurities at them, do not appear to act as dislocation barriers in these high-purity fcc metals. Moreover, as the indentation hardness values of single crystals of copper of different orientations are very similar, any differences in the orientations of contiguous grains will not affect the hardness values of polycrystals. Thus for large single crystals of copper (10 mm x 10 mm x 10 mm), having the same dislocation density as that of the polycrystalline specimens of copper of grain size 15 +/- 7 mum, the indentation hardness values were quite similar. Experimental results from the indentation hardness tests, made using both Vickers and spherical indenters, on polycrystalline aluminium specimens and relatively large single crystals showed that there was insignificant influence of the grain size.