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) Pd100−xAux alloys (0 ≤ x < 50 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 ≈ 10 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)].

Predicting yield strengths of noble metal high entropy alloys

“…For the Noble metal systems studied here, where all elements crystallize in an fcc structure at low temperature, and where all elements aside from Ni are non-magnetic, we expect Vegard's law to work reasonably well. Deviations to Vegard's law for many binary solid solution alloys within this family of alloys appear small in a range of experiments and first-principles studies [10,11,12,13,14,15].…”

Anatomizing deformation mechanisms in nanocrystalline Pd 90 Au 10

“…The total elastic strain, ε elastic , related to the sample length measured along the loading direction, is then given by ε elastic = (1 − χ)ε x + χε GB , where ε x is the elastic strain due to crystal elasticity, and ε GB is the strain contribution of the myriads of GBs occupying a length share χ. Consequently, (1 − χ) represents the length share of crystallites along the loading direction. The parameter χ is directly connected with the grain size D vol and can be computed if the width σ of the grain size distribution 6 and the structural width of the GBs, δ, are known [33,35].…”

“…2 represents the crystal elastic share to overall deformation and the second term accounts for the GB share. With σ = 1.5 (see Fig 8(a) and 8(b)) and δ = 0.76 ± 0.01 nm [33], the evolution of the share of crystal elasticity as a function of increasing deformation can be computed and is shown in Fig. 4(b).…”

“…4(a) and note that this value represents the effective Youngs's modulus of the aggregate of crystallites, which must be slightly larger than the true Reuss lower bound of the aggregate. We recently determined the effective (high frequency) Young's modulus of GBs by means of ultrasound measurements on unloaded samples to 61 ± 8 GPa [33]. However, under an applied load, we cannot rule out, even at the onset of deformation, configurational changes to occur in the core region of GBs that should give rise to an increase of E GB .…”

Abnormal grain growth mediated by fractal boundary migration at the nanoscale

“…To answer this question, we focus on the salient features of these specimens: (i) their extremely low intracrystalline defect density 22 , 23 , (ii) their statistically isotropic and homogeneous microstructure 23 , 24 , and (iii) their unusually small average grain size of ∼10 nm 25 . The first property argues against boundary migration driven by excess energy stored within grain volumes, as occurs during recrystallization.…”