Investigation of microstructure thermal evolution in nanocrystalline Cu

Abstract

The microstructure of nanocrystalline Cu prepared by compacting nanoparticles (50–60 nm in diameter) under high pressures has been studied by means of positron lifetime spectroscopy and X-ray diffraction. These nanoparticles were produced by two different methods. We found that there are order regions interior to the grains and disorder regions at the grain boundaries with a wide distribution of interatomic distances. The mean grain sizes of the nanocrystalline Cu samples decrease after being annealed at 900 °C and increase during aging at 180 °C, which are observed by X-ray diffraction, revealing that the atoms exchange between the two regions. The positron lifetime results clearly indicate that the vacancy clusters formed in the annealing process are unstable and decomposed at the aging time below 6 hours. In addition, the partially oxidized surfaces of the nanoparticles hinder grain growth when the samples age at 180 °C, and the vacancy clusters inside the disorder regions, which are related to Cu 2O, need longer aging time to decompose. The disorder regions remain after the heat treatment in this work, in spite of the grain growth, which will be good for the samples keeping the properties of nanocrystalline material.