Synchrotron-based in situ tensile testing was used to study the dominant deformation mechanisms of nanocrystalline Pd thin films on a compliant substrate. An x-ray diffraction peak profile analysis reveals an (hkl) independent deformation induced peak asymmetry. It is argued that the asymmetry is caused by a broad distribution of elastic strains among individual grains and the complexity of accommodation processes. The reversal of peak asymmetry manifests the transition from heterogeneous microplasticity to dislocation-based macroplasticity. Independently, stress-driven grain boundary migration is active.

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SummaryThe 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.

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