Anatomizing deformation mechanisms in nanocrystalline Pd 90 Au 10

Abstract

We utilized synchrotron-based in-situ diffraction and dominant shear deformation to identify, dissect, and quantify the relevant deformation mechanisms in nanocrystalline Pd 90 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 ≈ 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 ≈ 1.4 GPa and contributes a share of ≈ 10 % to overall strain. Appreciable dislocation activity begins at a stress value of ≈ 0.9 GPa to then increase and eventually propagate a maximal share of ≈ 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.

Revealing Grain Boundary Sliding from Textures of a Deformed Nanocrystalline Pd–Au Alloy

“…Instead, finite element analysis was employed using Abaqus to generate load–displacement curves that best approximated the experimental curves. More details on SCS testing are given in [ 5 , 11 , 12 ].…”

“… Experimentally measured intensity profile along the Debye–Scherrer rings ( a ) for the (111) ( b ), (200) ( c ), and (220) reflections ( d ) at a compressive strain of 0.27 applied to the gauge section in a load direction normalized with the intensity of the undeformed sample (taken from [ 5 ]). Overlaid are the simulated intensity profiles.…”

“…For nanocrystalline (nc) materials, grain-boundary-mediated deformation is considered a cooperative shearing process in which several grains are involved [ 4 ]. Another approach is to assign the grain boundaries to an amorphous structure in which shear shuffling or shear transformation takes place [ 5 ]. By applying general terminology, this process is simply called grain boundary sliding (GBS) here.…”

“…Recently, a new GBS model was presented and applied successfully on the texture evolution of a Pd–10 atom % Au nc alloy [ 7 ]. That model is applied in the present work using experimental results published recently on the same alloy [ 5 ] for a different strain path. By comparing experimental and simulated texture results, the amount of GBS was estimated.…”

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.…”