Microstructural evolution, mechanical properties and deformation mechanisms of nanocrystalline Cu thin films alloyed with Zr

The hardness, tensile ductility and fatigue lifetime of nanostructured (NC) Cu-Zr alloyed thin films have been systematically measured at different Zr additions (0, 0.5, 2.0, 4.0, 8.0 at.%). Experimental results showed that the Cu-0.5 at.% Zr film exhibited the highest hardness, largest ductility and longest fatigue lifetime, which are increased by 120%, 80%, and above an order of magnitude, respectively, in comparison with its pure Cu counterpart. The simultaneous improvements of mechanical properties are rationalized with respect to microstructural evolutions related to Zr segregation at grain boundaries (GBs). Besides refinement in NC grains and enhancement in nanotwins, grain orientation was apparently modified by the GB Zr segregation. (1 1 0) grains were promoted and layer-like microstructure was formed with coexistence of three orientation of grains at the upper layer, facilitating Cu grain growth at room temperature. GB doping strengthening worked as an additional strengthening mechanism and mechanically driven grain growth became the predominant deformation mechanism and fatigue mechanism, which accounted for the optimal combination of mechanical properties achieved in the Cu-0.5 at.% Zr film. When the Zr content was >2.0 at.%, however, amorphous phases were formed at the GBs due to locally increased Zr concentration. The GB doping strengthening effect was concomitantly weakened and strength was gradually decreased. In addition, stress/strain localization occurred readily at the amorphous phase regions, triggering intergranular fracture and reducing ductility. During cyclic testing in the low stress range, GB amorphous phases strengthened rather impaired the GBs, which gave the Cu-Zr films a fatigue lifetime greater than that of the pure NC Cu film. The present results clearly indicate that, once solute content is suitably explored, the solute segregation at GBs in NC metals can not only retain NC grains in loading-free conditions, but also impart enhanced mechanical properties when exposed to applied stress.