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

TL;DR

This study reveals that nanocrystalline Pd-Au alloys exhibit plastic yielding behavior similar to metallic glasses, with solid solution softening and a shear strength correlation that challenges traditional dislocation-based models.

Contribution

It demonstrates that dislocation activity is minimal in nanocrystalline Pd-Au alloys and that their shear strength correlates with shear modulus, mimicking metallic glass behavior.

Findings

Solid solution softening observed with increasing Au content.

Dislocation-based models do not predict the observed behavior.

Shear strength linearly correlates with shear modulus, akin to metallic glasses.

Abstract

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)].