STRUCTURE AND HARDNESS OF NANOCRYSTALLINE SILVER

Nanocrystalline silver (Ag) was prepared by in situ compacting of ultr a-fine silver particles: The structures of as-compacted and annealed s pecimens were analysed by high-resolution transmission electron micros copy and scanning electron microscopy. Vickers microhardness was measu red on the specimens. The ultra-fine particles aggregate before compac tion. It is found that the nanocrystalline specimens are obtained by t he compaction of the aggregates. Microstructure inside the aggregates does not change as the compacting pressure increases from 0.25 to 2.00 GPa. The compacting pressure affects on the structure and density of the boundaries between the aggregates, i.e. the formation of the crack -type defects of about 1 mu m at the boundaries. Thermal stability of nanocrystalline Ag is significantly low; grain coarsening starts below 200 degrees C. However, a nanometre-sized layered structure forms in local regions upon annealing and is stable up to 800 degrees C. Vicker s microhardness of as-compacted specimens increases with increasing co mpacting pressure. The increase is attributed to the decrease of the n umber of crack-type defects. Vickers microhardness of nanocrystalline Ag begins to decrease due to grain coarsening upon annealing around 20 0 degrees C. The microhardness of nanocrystalline Ag deviates from the Hall-Fetch relation.