Plastic yielding in nanocrystalline Pd-Au alloys mimics universal behavior of metallic glasses

Abstract

We studied solid solution effects on the mechanical properties of nanocrystalline (NC) ${\mathrm{Pd}}_{100\ensuremath{-}\mathrm{x}}{\mathrm{Au}}_{\mathrm{x}}$ alloys $(0\ensuremath{\le}x<50\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{at}.%)$ 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 $D\ensuremath{\approx}10\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$). 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 [Phys. Rev. Lett. 95, 195501 (2005)].