The thermal stability of nanocrystalline Cu prepared by high energy ball milling

Full density nanocrystalline (NC) Cu with average grain size of 33 nm was prepared through high energy ball milling. Effects of annealing on microhardness and activation volumes (V*) were studied. The magnitude observed for these characteristic deformation parameters is very different from their course-grained (cg) counterpart. The much higher micro-hardness of as-prepared Cu sample of 1.7 GPa was not detected to decrease after annealing at 773 K for 1 h with corresponding small value of activation volumes of 22.6. A prominent decrease of microhardness was detected after higher temperature annealing with a rapidly increase of activation volumes. The considerably higher microstrain and impurities stemming from high energy ball milling should be responsible for the relatively higher thermal stability of NC Cu. During annealing process, the strain release process occurred prior to the grain growth process and the impurities hindered the grain coarsening process, therefore, the NC Cu has a relatively higher thermal stability. The present investigation demonstrates that the thermal properties of NC materials are determined by not only the grain size but also the microstructure of grain boundaries.