ATOMIC-STRUCTURE AND THERMAL-STABILITY OF NANOCRYSTALLINE SILVER

Atomic structure of grain boundaries in nanocrystalline silver was investigated by high resolution electron microscopy. Nanocrystalline silver was prepared by compacting ultrafine particles produced by gas condensation technique at mechanical pressures of 250 MPa and 2 GPa in a vacuum of 10(-6) Pa. Isochronal annealing was performed in the temperature range from ambient temperature to 1073 K. Vickers micro-hardness was also measured for samples which were compacted from 250 MPa to 2 GPa and annealed isochronally increasing the temperature. Coherent twin boundary was dominant in as-compacted nanocrystalline silver. Voids from 3 to 5 nm in size were observed not in the grain but on the grain boundary especially at the grain boundary triple point. Microstructural difference between the specimens compacted at 500 MPa and 2 GPa was small. However, the surface defects of the order of 1-mu-m in size decreased with increasing compacting pressure. Vickers microhardness (H(v)) of as-compacted specimens increased with compacting pressure. The H(v) decreased rapidly with an increase in temperature to about 473 K. Grain coarsening started upon heating. The grain shape was mostly isotropic with a diameter of 20-50 nm and occasionally elongated 100-300 nm when twins were aligned. The average grain diameter of the specimen, however, remained below 1-mu-m even up to 1073 K.