Microhardness of bulk and higher density nanocrystalline copper obtained by hot compaction

Nanocrystalline copper (Cu) samples with relatively higher density (>98.5%) and smaller grain size (20-50 nm), have been prepared by gas-condensation technique with subsequent in-situ hot compaction using a novel consolidation device. It is found that Vickers microhardness of all as-synthesized nanocrystalline copper samples compacted under the different pressure (1.0-3.5 GPa) at 423 K (0.315 T_m) are greater than those of coarse grained Cu (50 μm). No significant grain growth is observed. However, the hardness appears to increase with increasing grain size ranging from 20±0.12 nm to 24.8±0.12 nm, in contrast, the normal Hall-Petch relation between 20±0.15 nm and 50±0.15 nm under the same density are indicated. This complicated result is attributed to higher density, smaller grain size and internal microstrains of samples.