  We studied solid solution effects on the mechanical properties of
nanocrystalline (NC) $\mathrm{Pd}_{\mathrm{100-x}} \mathrm{Au}_{\mathrm{x}}$
alloys ($0 \leq \mathrm{x} < 50 \mathrm{at.\%}$) at the low end of the
nanoscale. Concentration has been used as control parameter to tune material
properties (elastic moduli, Burgers vector, stacking fault energies) at
basically unaltered microstructure (grain size $D\approx 10\mathrm{ nm}$). In
stark contrast to coarse grained fcc alloys, we observe solid solution
softening for increasing Au-content. The available predictions from models and
theories taking explicitly into account the effect of the nanoscale
microstructure on the concentration-dependent shear strength have been
disproved without exception. As a consequence, it is implied that dislocation
activity contributes only marginally to strength. In fact, we find a linear
correlation between shear strength and shear modulus which quantitatively
agrees with the universal behavior of metallic glasses discovered by Johnson
and Samwer [W.L. Johnson and K. Samwer, PRL 95, 195501 (2005)].
