Anatomizing deformation mechanisms in nanocrystalline Pd$_{90}$Au$_{10}$

We utilized synchrotron-based in-situ diffraction and dominant shear deformation to identify, dissect, and quantify the relevant deformation mechanisms in nanocrystalline $\mathrm{Pd}_{90}\mathrm{Au}_{10}$ in the limiting case of grain sizes at or below 10 nm. We could identify lattice and grain boundary elasticity, shear shuffling operating in the core region of grain boundaries, stress driven grain boundary migration, and dislocation shear along lattice planes to contribute, however, with significantly different and nontrivial stress-dependent shares to overall deformation. Regarding lattice elasticity, we find that Hookean linear elasticity prevailed up to the maximal stress value of $\approx$ 1.6 GPa. Shear shuffling that propagates strain at/along grain boundaries increases progressively with increasing load to carry about two thirds of the overall strain in the regime of macroplasticity. Stress driven grain boundary migration requires overcoming a threshold stress slightly below the yield stress of $\approx$ 1.4 GPa and contributes a share of $\approx$ 10% to overall strain. Appreciable dislocation activity begins at a stress value of $\approx$ 0.9 GPa to then increase and eventually propagate a maximal share of $\approx$ 15% to overall strain. In the stress regime below 0.9 GPa, which is characterized by a markedly decreasing tangent modulus, shear shuffling and lattice- and grain boundary elasticity operate exclusively. The material response in this regime seems indicative of nonlinear viscous behavior rather than being correlated with work- or strain hardening as observed in conventional fcc metals.