S. Van Petegem

Crystal rotation in Cu single crystal micropillars : In situ Laue and electron backscatter diffraction

In situ microdiffraction experiments were conducted on focused ion beam machined single crystal Cu pillars oriented for double slip. During deformation, the crystal undergoes lattice rotation on both the primary and critical slip system. In spite of the initial homogeneous microstructure of the Cu pillar, rotation sets in already at yield and is more important at the top of the pillar than at the bottom, demonstrating the inhomogeneous stress state during a microcompression experiment. The rotation results are confirmed by electron backscatter diffraction measurements.

Defect structure in micropillars using X-ray microdiffraction

White beam x-ray microdiffraction is used to investigate the microstructure of micron-sized Si, Au, and Al pillars fabricated by focused ion beam (FIB) machining. Comparison with a Laue pattern obtained from a Si pillar made by reactive ion etching reveals that the FIB damages the Si structure. The Laue reflections obtained from the metallic pillars fabricated by FIB show continuous and discontinuous streakings, demonstrating the presence of strain gradients.

Free volume in nanostructured Ni

Abstract The free volume in nanostructured Ni, synthesized by different techniques, was measured by electron microscopy and positron lifetime spectroscopy. Even in the fully dense samples 1–2 nm sized pores as well as smaller nanovoids were observed indicating that these nanovoids are an intrinsic property of the microstructure of nanostructured materials.

Plasticity of multiscale nanofilamentary Cu/Nb composite wires during in situ neutron diffraction: Codeformation and size effect

In situ neutron diffraction was performed on Cu∕Nb nanocomposite wires composed of a multiscale Cu matrix embedding Nb nanofilaments with a diameter of 267nm and spacing of 45nm. The evolution of elastic strains and peak profiles versus applied stress evidenced the codeformation behavior with different elastic-plastic regimes: the Cu matrix exhibit size effect in the finest channels while the Nb nanowhiskers remain elastic up to the macroscopic failure, with a strong load transfer from the Cu matrix onto the Nb filaments. The measured yield stress in the finest Cu channels is in agreement with calculations based on a single dislocation regime.

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.

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.

A strong micropillar containing a low angle grain boundary

In situ white beam Laue diffraction experiments on focused ion beam machined single crystal 10μm Ni pillars are performed to explore the relation between the initial and evolving microstructures and the subsequent flow response. The pillar has a flow stress that is at least in the upper bound of the scatter of the flow stresses obtained for similar Ni pillars. Detailed analysis of the Laue pattern suggests that the strength is in part due to the low angle grain boundary acting as a dislocation barrier.

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.

Footprints of deformation mechanisms during in situ x-ray diffraction: Nanocrystalline and ultrafine grained Ni

In situ x-ray diffraction demonstrates that in ultrafine grained Ni synthesized by high pressure torsion with a coherent scattering domain size of 80nm, a dislocation network is still built up during tensile deformation whereas this is not the case for electrodeposited nanocrystalline metals with a coherent scattering domain size of 30nm. Simultaneously, the technique shows for the first time important differences in macroscopic stress accommodation during plastic deformation between the nanocrystalline and ultrafine grained Ni, such as the origin of the reduction in flow stress.