  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.
