Jochen Lohmiller

Deformation-induced grain growth and twinning in nanocrystalline palladium thin films

Summary The microstructure and mechanical properties of nanocrystalline Pd films prepared by magnetron sputtering have been investigated as a function of strain. The films were deposited onto polyimide substrates and tested in tensile mode. In order to follow the deformation processes in the material, several samples were strained to defined straining states, up to a maximum engineering strain of 10%, and prepared for post-mortem analysis. The nanocrystalline structure was investigated by quantitative automated crystal orientation mapping (ACOM) in a transmission electron microscope (TEM), identifying grain growth and twinning/detwinning resulting from dislocation activity as two of the mechanisms contributing to the macroscopic deformation. Depending on the initial twin density, the samples behaved differently. For low initial twin densities, an increasing twin density was found during straining. On the other hand, starting from a higher twin density, the twins were depleted with increasing strain. The findings from ACOM-TEM were confirmed by results from molecular dynamics (MD) simulations and from conventional and in-situ synchrotron X-ray diffraction (CXRD, SXRD) experiments.

The effect of solute segregation on strain localization in nanocrystalline thin films: Dislocation glide vs. grain-boundary mediated plasticity

Deformation mechanisms of nanocrystalline Au and AuCu thin films on compliant substrates were investigated by synchrotron-based in situ tensile testing and Automated Crystal Orientation Mapping using transmission electron microscopy. The results demonstrate that intragranular dislocation plasticity, inferred from evolution of deformation texture, is responsible for the formation of periodic and ordered shear bands in AuCu films. In contrast, pure Au films deform homogeneously without shear band formation and without evolution of deformation texture. Cu solutes are deemed to pin grain boundaries thereby enforce dislocation glide, while in pure Au, plasticity is carried by grain boundary shear and grain boundary migration.

Anatomizing deformation mechanisms in nanocrystalline Pd90Au10

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.

Temperature-dependent strain localization and texture evolution of highly nanotwinned Cu

Strong differences in the plastic strain and texture development in high purity nanotwinned copper foils deformed to failure in tension at ambient and liquid nitrogen (LN) temperatures have been observed. High energy microdiffraction patterns for the room temperature (RT) sample showed two distinct deformation regions relative to its fracture surface: d   500 μm; while for the sample deformed at 77 K, three distinct regions were observed: d   1550 μm. The localized plastic deformation in the RT tensile test and the increased ductile deformation at LN temperature are discussed in terms of the dualistic nanotwins plus micro-scale columnar grain structure.