Thermal stability of nanocrystalline grains in Cu-W films

It has been hypothesized that introduction of proper solute atoms, which preferentially segregates to grain boundaries, can stabilize the nanoscale grain size at elevated temperatures. To explore this possibility, we synthesized pure Cu and Cu-3 at. % W nanocrystalline films using magnetron sputtering. Scanning electron microscopy and x-ray diffraction showed that while grains in pure Cu coarsen rapidly when annealed at 673 K, Cu-3% W maintains a stable microstructure when annealed at the same temperature, achieving grain size of 37 nm after 120 min annealing. Nanoindentation measurements gave results consistent with the grain size evolution: upon annealing at 673 K, the hardness of the pure Cu film decreases to 40% of its initial value after 20 min, while the Cu-3% W film retains more than 90% of its initial hardness even after 120 min. Atom probe analysis shows W segregation in the Cu-3% W film. Results of this study indicate that solute segregation can be used as a method to fabricate nanocrystalline materials that are stable against grain coarsening at elevated temperatures.