Shear-Induced Mixing Governs Codeformation of Crystalline-Amorphous Nanolaminates

Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding, and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu–100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with noncrystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite.

Figure 1

(a) SEM view of $\mathrm{CuZr}/\mathrm{Cu}$ multilayers after Vickers indentation with a load of 10 g. (b) Cross-sectional view of shear bands beneath an indent. (c) Bright field TEM image of an area containing shear bands. The yellow lines mark a region investigated by APT [the data in (e) were taken from a slightly different region in the same sample). (d) Bright field TEM image of an APT tip with corresponding CuZr and Cu nanobeam diffraction patterns. The arrows point to the shear band region. The TEM image of the APT tip provides crystallographic information of the sheared Cu layer. (e) APT reconstructed volume from the exact APT tip shown in (d).

Figure 2

(a) Two different 3D Cu isoconcentration surface views and a schematic of a portion of the same APT measurement presented in Fig. 1 containing a shear band. Also shown are several regions of interest (ROIs) above and within the shear band (ROI 1 and ROI 2) and outside of the shear band (ROI 3), which are selected for further chemical analysis; their 1D-concentration profiles are shown in (b), (c), and (d), respectively.

Figure 3

HRTEM observations of shear bands in 100 nm CuZr–10 nm Cu multilayers subjected to different shear strains. (a) HRTEM image of an area containing several cross-phase shear bands with high shear strain. (b) HRTEM image together with NBD patterns along the pathway of a selected shear band (spots 2–7) and from a region in the amorphous layer (spot 1). The sizes of the circles indicate the diameter of the diffraction aperture. Some of the NBD patterns show both an inner halo ring (amorphous) and some weak discrete diffraction spots (crystalline) outside of the inner halo ring (see small arrows, NBD spots 3,4,6). (c) HRTEM image of the area containing a shear band with a shear strain $\sim 1.0$. (d) Inverse FFT image using only the $\{111\}$ reflections in the strain localized region indicated in (c).

Figure 4

Schematic illustration of the proposed mechanism explaining mechanical alloying induced by the crossing of the $\mathrm{CuZr}/\mathrm{Cu}$ interface by shear dislocations: Nanolaminate deformation proceeds by dislocation slip in the crystalline Cu that coincides with shear bands in the amorphous CuZr layers. Cu atoms are dragged from the crystalline layers across the heterointerfaces when dislocations release their shear into the CuZr phase. Also, complete crystalline Cu nanoclusters are displaced into the amorphous CuZr phase. Both effects lead to massive chemical intermixing between the layers in the 2–3 nm narrow shear band regions.