S. Brandstetter

On the Microstructure of Nanoporous Gold: An X-ray Diffraction Study

The evolution of the grain structure, internal strain, and the lattice misorientations of nanoporous gold during dealloying of bulk (3D) Ag-Au alloy samples was studied by various in situ and ex situ X-ray diffraction techniques including powder and Laue diffraction. The experiments reveal that the dealloying process preserves the original crystallographic structure but leads to a small spread in orientations within individual grains. Initially, most grains develop in-plane tensile stresses, which are partly released during further dealloying. Simultaneously, the feature size of the developing nanoporous structure increases with increasing dealloying time. Finally, microdiffraction experiments on dealloyed micron-sized nanoporous pillars reveal significant surface damage introduced by focused ion beam milling.

Internal and effective stresses in nanocrystalline electrodeposited Ni

Stress reduction experiments performed during tensile deformation of nanocrystalline electrodeposited Ni demonstrate high values for the effective and the internal stress as compared to coarse grained metals and evidence the existence of a negative creep. The results are interpreted in terms of a thermally activated dislocation mechanism where propagation is hindered by pinning at grain boundaries.

Following peak profiles during elastic and plastic deformation : A synchrotron-based technique

Understanding the elastic and plastic deformation properties of nanostructured metals requires the development of in situ testing methods that can follow the footprints of the deformation mechanism(s) during mechanical testing. Here we present an in situ synchrotron x-ray-diffraction technique which allows the measurement of diffraction profiles continuously during mechanical testing, providing an in situ peak profile analysis capability. The in situ approach is achieved thanks to the development of a microstrip detector allowing the instantaneous measurement of the diffraction pattern over a 2θ range of 60°. This in situ technique allows for the first time a comparison of the footprints of the plastic deformation mechanism during loading and after unloading. The measurements are performed on several types of freestanding dog bones, covering sample thicknesses down to the submicron range.

In situ observation of rapid reactions in nanoscale Ni–Al multilayer foils using synchrotron radiation

The observation of rapid reactions in nanoscale multilayers present challenges that require sophisticated analysis methods. We present high-resolution in situ x-ray diffraction analysis of reactions in nanoscale foils of Ni0.9V0.1–Al using the Mythen II solid-state microstrip detector system at the Material Science beamline of the Swiss Light Source Synchrotron at Paul Scherrer Institute in Villigen, Switzerland. The results reveal the temperature evolution corresponding to the rapid formation of NiAl intermetallic phase, vanadium segregation and formation of stresses during cooling, determined at high temporal (0.125 ms) and angular (0.004°) resolution over a full angular range of 120°.

Temperature-dependent residual broadening of x-ray diffraction spectra in nanocrystalline plasticity

In situ x-ray diffraction peak profile analysis at room temperature has shown that peak broadening during plastic deformation is reversible upon unloading for nanocrystalline metals, demonstrating the lack of a developing permanent dislocation network. In this letter, we show that the peak broadening is not reversible when similar load-unload cycles are performed at 180 K. However, by then warming the sample to 300 K, peak broadening recovers to a great extent and all subsequent plastic deformation load∕unload cycles are characterized again by a reversible peak broadening. The temperature-dependent residual peak broadening provides explicit evidence of a thermal component in the nanocrystalline deformation mechanism.

Evidence of internal Bauschinger test in nanocomposite wires during in situ macroscopic tensile cycling under synchrotron beam

In situ multiple tensile load-unload cycles under synchrotron radiation are performed on nanocomposite Cu∕Nb wires. The phase specific lattice strains and peak widths demonstrate the dynamics of the load-sharing mechanism where the fine Cu channels and the Nb nanotubes store elastic energy, leading to a continuous buildup of internal stress. The in situ technique reveals the details of the macroscopically observed Bauschinger effect.