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.