Forced chemical mixing of immiscible Ag-Cu heterointerfaces using high-pressure torsion

Forced chemical mixing in nanostructured Ag${}_{60}$Cu${}_{40}$ eutectic alloys during severe plastic deformation by high-pressure torsion (HPT) was quantitatively studied using x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. Nearly complete chemical homogenization of the original lamellar structure with a wavelength of $\approx$ 165 nm was achieved after a shear strain of $\approx$ 350. The chemical mixing is accompanied by extensive grain refinement leading to nanocrystalline grains with average sizes of $\approx$ 42 nm. A Monte Carlo computer simulation model, which attributes mixing to dislocation glide, shows reasonable agreement with the experimental results. The model also shows that the characteristic strain for chemical homogenization scales linearly with the length scale of the system $L$, and not with the square of the length scale $L^2$, as would be expected for Fickian diffusion.

Figure 1

As-cast Ag${}_{60}$Cu${}_{40}$ eutectic alloy: (a) bright-field TEM micrograph and corresponding SAED pattern, (b) STEM/EDX elemental mapping across several lamellae, and (c) dark-field TEM image.

Figure 2

X-ray diffractograms of Ag${}_{60}$Cu${}_{40}$ eutectic alloy in the as-cast state and after different number of HPT cycles.

Figure 3

DSC results of Ag${}_{60}$Cu${}_{40}$ eutectic alloy after different number of HPT cycles: (a) DSC scans, and (b) total stored enthalpy plotted as a function of strain.

Figure 4

Transmission electron micrographs of a disk processed by HPT for 10 cycles: (a) STEM overview, (b) STEM/EDX elemental mapping of the marked area in (a), and (c) dark-field image and corresponding SAED pattern.

Figure 5

(a) STEM overview from the center of a disk processed by HPT for 10 cycles and corresponding SAED pattern, and (b) bright-field image of the marked area in (a).

Figure 6

SRO parameter as a function of shear strain for four eutectic wavelengths of the Ag-Cu bilayer sample as determined from MC simulation (solid lines). The as-cast curve corresponds to the experimental sample, consisting of $\lambda ={\lambda }_0=$ 724 atomic layers with 30$\%$ Cu in a contiguous layer. The dashed line on curve marked 30 nm/165 nm is an extrapolation of the data assuming exponential mixing. Inset shows the SRO parameter plotted versus normalized strain. Symbols represent experimental data from Fig. 3b; they were converted to SRO parameters assuming a regular solution model.

Figure 7

(a) The simulated concentration profile of solute atoms across an interface is plotted for strains $\gamma$ of 1, 5, and 20. The data points (symbols) are fitted with an error function (lines). The inset shows the interface width, obtained from the fitting parameter $d$, as a function of the square root of the applied strain. (b) Number of A-B bonds plotted as a function of the interface width for shear mixing and diffusive mixing.

Figure 8

Cross sectional views of atoms, subjected to shear mixing (top) or diffusive mixing (bottom). Shear mixing leads to predominantly low frequency roughening of the interfaces compared to the atomic scale disorder introduced by diffusive mixing. Grayscale indicates distance from the cut plane.