Zr alloying effect on the microstructure evolution and plastic deformation of nanostructured Cu thin films

The magnetron sputtering technique was employed to prepare Zr-alloyed Cu thin films with Zr addition ranging from 0 to 7.0 at.%. Microstructure evolution with Zr doping was characterized by using the transmission electron microscope and atom probe tomography. Plastic deformation characteristics (hardness, strain rate sensitivity and activation volume) of the Cu-Zr alloyed films were examined by using nanoindentation testing. Significant Zr doping effects on the microstructure were clearly uncovered that (i) the Zr segregated at grain boundaries and tuned the grain boundary structure. With increasing the Zr addition, CuZr amorphous particles at grain boundaries (3.0 at.% Zr) and even three dimensional CuZr amorphous grain boundary network (7.0 at.% Zr) were formed; (ii) the grains (size and morphology) and nanotwins were notably influenced by the Zr doping. In particular, the nanotwin thickness was reduced from ∼25 nm in the pure Cu film down to ∼5 nm in the Cu-Zr alloyed films. Accompanied with the microstructure evolution, the Cu-Zr thin films displayed hardness and strain rate sensitivity highly sensitive to the Zr doping, i.e., hardness increasing while strain rate sensitivity decreasing with raising the Zr addition. The strengthening and deformation mechanisms were discussed in terms of the microstructure-property relationship. Three regimes were divided within the studied Zr doping range: grain/nanotwin boundary-dominated strengthening mechanism in the regime I of pure Cu film, dislocation nucleation-controlled nanotwin softening mechanism in the regime II of Zr addition ≤3.0 at.%, and intergranular amorphous layer-mediated strengthening mechanism in the regime III of Zr addition up to 7.0 at.%. In addition, the reduction in strain rate sensitivity with Zr doping was quantitatively described by adopting a model that involves the thermally activated partials depinning mechanism based on the effective dislocation segment length.