Fig. 1. Tribometer test and calculation of plastic shear strain (PSS). (a) Coefficient of friction (COF)-time curve recorded by the instrument (COF is estimated to be 0.6). (b) Wear track from the top view. The directions along the sliding direction, normal to the sliding surface, and perpendicular to the sliding direction are defined as SD, ND, and TD, respectively. (c) Relationship between the equivalent PSS (ε) and distance from the scratch surface after 10 cycles of shear deformation. Scale bars in (c) are 200 nm.

Fig. 2. Microstructure of the Cu-50 at.% Cr binary alloy before deformation. (a) Scanning electron microscopy back scattered electrons (SEM-BSE) image and EDS mapping showing Cr dendritic grains distributed in the Cu matrix. (b) HAADF-STEM image and EDS mapping showing profuse Cu NPs in a Cr grain. (c) 3D atomic map showing the Cu/Cr interface highlighted with 15 at.% Cu isoconcentration surface. (d) Compositional change across the Cu/Cr interface.

Fig. 3. Orientation relationship between Cu and Cr. (a) Inverse pole figure (IPF) of the Cu-50 at.% Cr binary alloy. Inset comes from the enlarged, yellow-boxed area. (b, d) HR-TEM images of two Cu/Cr interfaces. (c, e) Fast Fourier transform (FFT) images of corresponding locations in (b) and (d), showing N-W and K-S relations, respectively. The Cu/Cr interfaces are roughly outlined by yellow dashed lines. Notably, there is a slight overlap between the Cu particle and the surrounding Cr matrix in (b).

Fig. 4. Microstructures after 10 cycles of severe shear deformation. (a) ABF-STEM image. Inset red boxes show the estimated PSS. (b) HAADF-STEM image and EDS mapping. (c) HAADF-STEM image of the yellow-boxed area in (a) and EDS mapping. (d) Grain sizes evolution of Cu and Cr phases as a function of the distance from the scratch surface. (e, h) Combined images of phases and HAADF-STEM images of the blue-boxed area in (c) and orange-boxed area in (a), respectively, acquired by precession electron diffraction. (f, i) IPFs of areas (e) and (h), respectively. (g, j) Misorientation along lines 1 and 2 in (f) and lines 3 and 4 in (i), respectively.

Fig. 5. Defects in Cu and Cr grains in the top-most layers after 10 cycles of severe shear deformation. (a) BF-TEM image showing the formation of twin lamellae and stacking faults in a Cu grain. (b) HAADF-STEM image of the white-boxed area in (a). (c) ABF-STEM image of a Cr lamella (outlined by the white dashed line). (d) HAADF-STEM image of the white-boxed area in (c). Dislocations are denoted by “┴”. (e) BF-TEM image showing many low-angle GBs in Cr and two Cu NPs at Cr GBs. All Cr grains outlined by cyan dashed lines in (e) were aligned close to the [001] zone axis. Misorientation angles between two adjacent Cr grains are labeled on GBs.

Fig. 6. Microstructure of the Cu/Cr binary alloy after half cycle severe shear deformation. (a) HAADF-STEM image showing the formation of profuse dislocations and sub-grains (denoted by white arrows) in the Cu matrix. (b) EDS mapping. (c) Annular bright-field (ABF) STEM image showing a gradient dislocation density distribution in the Cr grain in (a). (d) ABF-STEM of the enlarged white boxed area in (c). (e) Enlarged yellow-boxed area in (d). (f) HAADF-STEM image and EDS mapping of the enlarged white-box in (a). (g, h) High-magnification HAADF-STEM images showing black contrast areas (denoted by orange arrows) on the side of the Cu grain by tilting the Cr and Cu grains to [113] and [111] zone axes, respectively.

Fig. 7. Evolution of Cr lamellae in the Cu matrix after 10 cycles of severe shear deformation. (a) HAADF-STEM image showing the formation of many separated Cr lamellae (denoted by cyan arrows) and particles (denoted by red arrows) in the top surface of the Cu matrix. (b) EDS line scan along line 1 in (a). (c) BF-TEM image of the enlarged white box in (a) showing grain refinement of the Cr lamella. GBs in the Cr lamella and the surrounding Cu matrix are roughly outlined by cyan and orange dashed lines, respectively. The Cu/Cr interface is outlined by a white dashed line. (d) HR-TEM image of the white-boxed area in (c). (e) BF-TEM image of a Cr lamella. (f) FFT images of different locations in (e) verify that the Cr lamella is a single crystal. (g) HR-TEM images of locations 1 and 3 in (e) further verify the formation of the single-crystal Cr lamella. (h) ABF-TEM image showing the formation of two single-crystal Cr lamellae in the Cu matrix. (i) Variation of aspect ratio and length of the Cr lamella with the distance below the scratch surface.

Fig. 8. Formation of Cr lamellae in the Cu matrix after 10 cycles of severe shear deformation. (a) BF-TEM image showing the formation of Cr lamellae in the Cu matrix in the ND-TD sample. The image was acquired from ～400 nm below the scratch surface. (b) Enlarged white boxed area in (a) showing formation of LAGBs in a Cr grain. (c) HR-TEM image of the white-boxed area in (b) showing lattice rotation on both sides of the LAGBs. α and β are tilt angles of the TEM holder. (d) FFT image of (c). (e) HR-TEM image of the green-boxed area in (b). (f) Enlarged yellow-boxed area in (e). (g) Enlarged white-boxed area in (e) showing profuse dislocations along the LAGB. Dislocations are denoted by “┴”. (h) HAADF-STEM image showing the formation of many Cr lamellae at the top-surface (～100 nm below) of a Cr block. (i) Schematic illustration showing the formation process of Cr lamellae in the Cu matrix.

Fig. 9. Alloying analysis by atom probe tomography (APT) after 10 cycles of shear deformation. (a) APT reconstruction of two Cu NPs in the Cr. (b) 2-D contour plot highlighting the Cu concentration variation from a 2 nm (top) and 10 nm (bottom) slice. (c) Compositional change along the white arrow in (b). (d) 3-D isoconcentration surfaces of the Cu NP 2 in (a). 3 at.% (purple) and 15 at.% (yellow) Cu isoconcentration surfaces are displayed here to delineate the boundaries between Cu and Cr. (e, h) APT reconstruction of Cr nanograins in Cu. (f, i) 2-D contour plot highlighting the Cr concentration variation of Cr nanograins in (e) and (h), respectively. (g) 2-D contour plot highlighting the O concentration variation. (j-l) Compositional change along with white arrows 2-4, respectively. Notably, (a), (e), and (h) come from 50 nm, ～120 nm, and ～50 nm below the scratch surface, respectively.

Fig. 10. Refinement of Cu NPs in the Cr matrix after 10 cycles of severe shear deformation. (a) HR-TEM image of a Cu particle in the Cr matrix (～2.9 μm below the scratch surface). The Cu/Cr interface is roughly outlined by yellow dashed lines. (b) FFT image of (a) showing the Cu particle maintains the N-W relation with the surrounding Cu matrix. (c) Enlarged white-boxed area in (a) showing the formation of a Cu domain in the original Cu NP. (d) Inverse FFT image of the selected diffraction spots [denoted by red circles in (b)] showing the formation of many Cu domains in the original Cu particle. (e) Enlarged white-boxed area in (d).

Fig. 11. Oxidization of various Cu/Cr interfaces simulated by Ab initio method. (a) Stability of interstitial O (Oi) at the coherent (Model A) and incoherent (Model B, C) Cu/Cr interfaces (see also Fig. S11) and in the pure bulk Cu and Cr. (b, c) Atomic charge distribution at the Cu/Cr interfaces (b) and in the presence of Oi (within 3 ? from Oi) (c). The configurations of Model B and C chosen to show charge distribution in panel c are the ones with lower energy in panel a.