S. Brandstetter

Bio: S. Brandstetter is an academic researcher from Centre national de la recherche scientifique. The author has contributed to research in topics: Nanocrystalline material & Dislocation. The author has an hindex of 10, co-authored 10 publications receiving 366 citations. Previous affiliations of S. Brandstetter include University of Grenoble & Paul Scherrer Institute.

On the microstructure of nanoporous gold: an x-ray diffraction study

Abstract: The evolution of the grain structure, internal stresses, and the lattice misorientations of nanoporous gold (npAu) 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. Furthermore, most grains develop in-plane tensile stress. The feature size of the developing nanoporous structure increases with increasing dealloying time.

Internal and effective stresses in nanocrystalline electrodeposited Ni

Abstract: 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

Abstract: 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.

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

Abstract: 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

Abstract: 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.

Disclinations, dislocations and continuous defects: a reappraisal

Abstract: Disclinations were first observed in mesomorphic phases. They were later found relevant to a number of ill-ordered condensed-matter media involving continuous symmetries or frustrated order. Disclinations also appear in polycrystals at the edges of grain boundaries; but they are of limited interest in solid single crystals, where they can move only by diffusion climb and, owing to their large elastic stresses, mostly appear in close pairs of opposite signs. The relaxation mechanisms associated with a disclination in its creation, motion, and change of shape involve an interplay with continuous or quantized dislocations and/or continuous disclinations. These are attached to the disclinations or are akin to Nye's dislocation densities, which are particularly well suited for consideration here. The notion of an extended Volterra process is introduced, which takes these relaxation processes into account and covers different situations where this interplay takes place. These concepts are illustrated by a variety of applications in amorphous solids, mesomorphic phases, and frustrated media in their curved habit space. These often involve disclination networks with specific node conditions. The powerful topological theory of line defects considers only defects stable against any change of boundary conditions or relaxation processes compatible with the structure considered. It can be seen as a simplified case of the approach considered here, particularly suited for media of high plasticity or/and complex structures. It cannot analyze the dynamical properties of defects nor the elastic constants involved in their static properties; topological stability cannot guarantee energetic stability, and sometimes cannot distinguish finer details of the structure of defects.