3D-Laue micro diffraction to characterize fatigue damage in bi-crystalline micro cantilevers

Laue microdiffraction (µLaue) – a synchrotron based technique applying sub-micron sized (0.5x0.5µm²), polychromatic x-ray beams – has proven to shed unprecedented light onto the deformation behaviour of small volumes. So far, µLaue was used in situ as well as for 2D mapping to understand the tensile, compression and fatigue behaviour of metals. 3D-µLaue – also called “Differential Aperture X-ray microscopy” [1] – extend the capabilities of µLaue to achieve submicron 3D spatial characterization. An absorbing wire is moved between the sample surface and the detector. It is used to partially shadow the Laue pattern in order to reconstruct depth-resolved Laue patterns (see Fig.1). The achieved depth-resolution is in the order of 0.5µm³. In recent years we have adopted the 3D approach and coupled it with an in situ deformation rig. In this work we present the in situ characterization of low cycle fatigue damage in focussed ion beam (FIB) milled, micron sized copper single and bi-crystalline cantilevers (5x5x25µm³). Each bi-crystalline sample contains one single grain boundary located at the neutral plane. The experiments were performed at BM32 of the European Synchrotron (ESRF), using a combination of a micro straining rig and 3D-µLaue. From the measurements the lattice orientation, deviatoric strain tensor and the density of unpaired dislocations are extracted with sub-micron resolution. This is done at several different deformation stages, i.e. after ¼, ½, ¾ and full cycle. Please click Additional Files below to see the full abstract

Full elastic strain tensor determination at the phase scale in a powder metallurgy nickel-based superalloy using X-ray Laue microdiffraction

Laue microdiffraction is used to determine the full elastic strain tensor of the γ and γ′ phases in grains of a nickel-based superalloy with a coarse-grained microstructure. A `rainbow' filter and an energy dispersive point detector are employed to measure the energy of Bragg reflections. For the two techniques, an uncertainty of ±2.5 × 10−3 Å is obtained for the undetermined crystal lattice parameter. Our measurements show that the filter method provides better confidence, energy resolution, accuracy and acquisition time. The sensitivity of each method with respect to the γ–γ′ lattice mismatch is demonstrated with measurements in samples with average precipitate sizes of 200 and 2000 nm. For the 200 nm precipitate size, the lattice mismatch is less than 2 × 10−3 Å and the dilatational strains are close to ±1.5 × 10−3 depending on the considered phase. For the 2000 nm precipitate size, the lattice mismatch is close to 8 × 10−3 Å and almost no elastic strain occurs in the microstructure

Influence of the U3O7 domain structure on cracking during the oxidation of UO2

International audienceCracking is observed when a UO2 single crystal is oxidised in air. Previous studies led to the hypothesis that cracking occurs once a critical depth of U3O7 oxidised layer is reached. We present some μ-Laue X-ray diffraction results, which evidence that the U3O7 layer, grown by topotaxy on UO2, is made of domains with different crystalline orientations. This observation was used to perform a modelling of oxidation coupling chemical and mechanical parameters, which showed that the domain patterning induces stress localisation. This result is discussed in comparison with stress localisation observed in thin layer deposited on a substrate and used to propose an interpretation of UO2 oxidation and cracking

Lattice strain and tilt mapping in stressed Ge microstructures using X-ray Laue micro-diffraction and rainbow-filtering

Micro-Laue diffraction and simultaneous rainbow-filtered micro-diffraction were used to measure accurately the full strain tensor and the lattice orientation distribution at the sub-micron scale in highly strained, suspended Ge micro-devices. A numerical approach to obtain the full strain tensor from the deviatoric strain measurement alone is also demonstrated and used for faster full strain mapping. We performed the measurements in a series of micro-devices under either uniaxial or biaxial stress and found an excellent agreement with numerical simulations. This shows the superior potential of Laue micro-diffraction for the investigation of highly strained micro-devices.Comment: 28 pages, 10 figure

In situ bending of an Au nanowire monitored by micro Laue diffraction

International audienceThis article reports on the first successful combination of micro Laue (µLaue) diffraction with an atomic force microscope for in situ nanomechanical tests of individual nanostructures. In situ three-point bending on self-suspended gold nanowires was performed on the BM32 beamline at the ESRF using a specially designed atomic force microscope. During the bending process of the self-suspended wire, the evolution of µLaue diffraction patterns was monitored, allowing for extraction of the bending angle of the nanowire. This bending compares well with finite element analysis taking into account elastic constant bulk values and geometric nonlinearities. This novel experimental setup opens promising perspectives for studying mechanical properties at the nanoscale

EBSD-assisted Laue microdiffraction for microstrain analysis

The X-ray Laue microdiffraction (mLaue) technique has been establishing itself as a reliable means for microstrain analysis for the past few decades. One problem with this technique is that when the crystal size is significantly smaller than the probed volume and when the diffracting crystals are closely oriented, a large number of individual mLaue patterns are superimposed in a complex way on the recorded diffraction images. In that case, because of the difficulty of isolating unambiguously a single-grain mLaue pattern, a reliable analysis of strains is tedious manually and hardly achievable with current automated methods. This issue is even more severe for low-symmetry crystals or when highenergy X-rays are used, since each single-crystal mLaue pattern already contains a large number of spots. This paper proposes overcoming this challenge through the development of a combined approach coupling mLaue and electron backscatter diffraction (EBSD). The capabilities of this 'EBSD-assisted mLaue' automated method are illustrated on a monoclinic zirconia-based specimen and mLaue diffraction patterns are analysed with the crystal orientation input from EBSD. The obtained results are statistically reliable, reproducible and provide a physical insight into the micromechanical characteristics of the material

Huge local elastic strains in bulk nanostructured pure zirconia materials

From the liquid state to room temperature, two successive solid-state phase transitions occur in pure zirconia. It is well-known that the last one (tetragonal to monoclinic) is martensitic and induces large volume variations and shear strains. Elastic and inelastic behaviors of zirconia-based materials are strongly influenced by this transition and the associated strain fields that it induces. Knowledge of strain and stress at the crystal scale is thus a crucial point. Using fully dense pure zirconia polycrystals obtained by a fuse casting process, we have determined at a sub-micrometric scale, by X-ray Laue microdiffraction, the strains map at room temperature in as-cast specimens and after a post elaboration high temperature thermal treatment. We observed that the fluctuation of deviatoric elastic strain is huge, the standard deviation of normal component being in the range of 1–2%. The heat treatment tends to even further increase this range of fluctuation, despite the development of a multiscale crack network formed during the cooling. Correspondingly, the associated stress level is also huge. It lies in the 5 GPa range with stress gradient amounting 1 GPa μm−1.This work was done in the frame of the ASZTECH research program funded by the ANR (ANR-12-RMNP-0007). We acknowledge the ESRF and the French Collaborating Research Group (F-CRG) for provision of synchrotron radiation facilities beamtimes and beamline staff for their assistance. The authors are thankful to I. Cabodi and O. Bories (Saint- Gobain CREE) for the supply of the bulk zirconia-based materials

Evidence of 3D strain gradients associated with tin whisker growth

We have used Differential Aperture X-ray Microscopy (DAXM) to measure grain orientations and deviatoric elastic strains in 3D around a tin whisker. The results show strain gradients through the depth of the tin coating, revealing a higher strain deeper in the Sn layer. These higher strains are explained by the volume change occurring during growth of the intermetallic phase Cu6Sn5 at the interface between the Cu substrate and the Sn coating and at grain boundaries between Sn grains