Fig. 1. Schematic diagram of the fabricated NL Cu/Zr architectures. (a) Two homogeneous samples with an equivalent layer thickness of 10 or 100 nm. (b) Bimodal sample consists of alternating one 100 nm Cu-Zr bilayer and two 10 nm Cu-Zr bilayers. (c) Heterogeneous NL composite with one or several Cu-Zr bilayers as interlayers at each 100 nm Cu-Zr interface. HA, HB and HC denote three heterogeneous samples with one 2.5 nm, 5 nm and 10 nm Cu-Zr bilayer as interlayer, respectively, while HC2 and HC3 denote two heterogeneous samples with two and three 10 nm Cu-Zr bilayers as interlayer, respectively.

Fig. 2. XRD patterns of monolayer Cu and Zr films and the heterogeneous NL Cu/Zr samples with interlayer of one 2.5 nm (HA) and 5 nm (HB) Cu/Zr bilayer.

Fig. 3. The microstructures of the heterogeneous NL Cu/Zr architectures. Representative cross-sectional bright field TEM images of HA (a), HC (d), and the dark field TEM image of HC2 (g). HAADF-STEM images of HA (b) and HC (e). LAADF-STEM image of HC2 (h). Insets in (b), (e) and (h) are the corresponding EDS maps. The yellow dash lines in (b), (e) and (h) outline the Cu-Zr bilayers between the 100 nm Cu and Zr layers. (c), (f) and (i) are the corresponding selected area electron diffraction (SAED) patterns of (a), (d) and (g), respectively.

Fig. 4. Deformation morphologies of 100 nm Cu/Zr (a) and 10 nm Cu/Zr (c) composites after 50 g microindentation, and the corresponding cross-sectional SEM image of 100 nm Cu/Zr (b) and the HAADF-STEM image of 10 nm Cu/Zr (d) composites. The white lines in (a) and (c) denote where the cross-sectional SEM and STEM samples were taken, respectively. The blue arrows in (b) and (d) indicate the position of the indenter.

Fig. 5. HAADF-STEM image (a), and corresponding EDS map (b) of the local shear banding region of 10 nm Cu/Zr composite. (c) EDS line scanning profiles follow the path indicated by the white arrow in (b).

Fig. 6. Deformation morphologies of heterogeneous Cu/Zr composite samples (HA (a), HB (b), HC (c), HC2 (d), and HC3 (e)) and bimodal sample (f) after 50 g microindentation for 5 s holding time. The numerals in (a-c) indicate the number of the shear bands. The white lines in (a), (c), (e) and (f) designate where the cross-sectional STEM samples were taken.

Fig. 7. (a-d) Cross-sectional HAADF-STEM images of heterogeneous composites (samples HA, HC, HC3) and bimodal composites (d) after 50 g microindentation, respectively. (a1) EDS map of (a). (b1-d1) The corresponding EDS maps of the regions indicated by the white squared box in (b-d), respectively. The shear bands in each sample are designated by yellow arrows. The numerals in (a) and (b) indicate the number of the shear bands, which correspond to the shear bands numbered in Fig. 6(a) and (c), respectively. The blue arrows in (a-d) denote the position of the indenter.

Fig. 8. Schematic diagram of the shear band with geometric parameters, showing the interface kink angle (φ), the tilt angle of the shear band (α), and the width of the shear band (WSB).

Fig. 9. Strain of the 100 nm layers within the shear band of the heterogeneous (HA, HC and HC3) and bimodal composites after 50 g microindentation.

Fig. 10. The normal strain (εn) of the 100 nm Cu and Zr layers within the shear band from the sample bottom to the top for heterogeneous (HA, HC, HC3) and bimodal composites.

Fig. 11. Yield strength versus strain delocalization for different NL Cu/Zr composites. The strain delocalization is defined as 1/ε, where the ε denotes the average layer strain of the 100 nm layers inside the shear band. The yield strength of HA and 10 nm Cu/Zr composites [51] are calculated by hardness/3, the yield strength of HC, HC3, 100 nm Cu/Zr and bimodal composites are adopted from the micropillar compression experiments in our previous work [51,55]. The “l” and “N” denote the thickness and number of the thin bilayer, respectively.

Fig. 12. Schematic diagram of the deformation process for heterogeneous Cu/Zr composite related to the number of Cu-Zr bilayers. Dislocations first slip on the {111} plane in the 100 nm Cu layer and accumulate at the interface, resulting in stress concentration at the interface near the 100 nm Cu layer (a, b). Two deformation modes associated with the number of Cu-Zr bilayers, i.e., multiple shear bands (b) and single shear band (e). The initiation of shear band induced by grain boundary (GB) sliding and grain rotation in Cu-Zr bilayer with various numbers (c, f).