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

Evaluation of grain-average stress tensor in a tensile-deformed Al–Mn polycrystal by high-energy X-ray diffraction

International audienceThree-dimensional X-ray diffraction was applied to characterize the strain/stress evolution in individual grains of an Al-0.3 wt% Mn polycrystal deformed in situ at a synchrotron source. Methodological aspects concerning the calibration of the geometrical setup and the evaluation of the strain/stress tensors are discussed. A two-step calibration method separately treating the detector and the rotation axis allows one to determine the centre-of-mass position and crystallographic orientation of grains with standard errors of about 1.5 mm and 0.02 degrees, respectively. Numerical simulations indicate that the error of normal strain components (about 1 x 10(-4)) is mainly caused by calibration errors, while the error of shear components (about 0.5 x 10(-4)) is largely influenced by counting statistics and random spot-centre errors due to detector distortion. The importance of monitoring the beam energy is emphasized

Optimal polyhedral description of 3D polycrystals: method and application to statistical and synchrotron X-ray diffraction data

International audienceA methodology is presented for optimal polyhedral description of 3D polycrystals from experimental properties. This is achieved by determining, by optimization, appropriate attributes of the seeds of Laguerre tessellations. The resulting tessellations are optimal in the sense that no further improvements are possible using convex geometries. The optimization of Laguerre tessellation combines a new, computationally-efficient algorithm for updating tessellations between iterations to a generic optimization algorithm. The method is applied to different types of experimental data, either statistical, such as grain size distributions, or grain-based, as provided by synchrotron X-ray diffraction experiments. It is then shown how the tessellations can be meshed for finite-element simulations. The new method opens the way to more systematic and quantitative analyses of microstructural effects on properties. The presented algorithms are implemented and distributed in the free (open-source) software package Neper

On the calibration of high-energy X-ray diffraction setups. II. Assessing the rotation axis and residual strains

International audienceThe calibration of high-energy X-ray diffraction setups using an area detector and a rotation axis is discussed. The characterization of the tilt and spatial distortions of an area detector was discussed in part one of this series [Borbely, Renversade, Kenesei & Wright (2014). J. Appl. Cryst. 47, 1042-1053]. Part II links the detector frame to the laboratory frame comprising an additional rotation axis and introduces a general diffractometer equation accounting for all sources of misalignment. Additionally, an independent high-accuracy method for the evaluation of the crystallographic orientation and cell parameters of the undeformed reference crystal is presented. Setup misalignments are mainly described in terms of a residual strain tensor, considered as a quality label of the diffractometer. The method is exemplified using data sets acquired at beamlines ID11 (European Synchrotron Radiation Facility) and 1-ID (Advanced Photon Source) on Al and W single crystals, respectively. The results show that the residual strain tensor is mainly determined by the detector spatial distortion, and values as small as 1-2 x 10(-4) can be practically achieved. (C) 2014 International Union of Crystallograph

On the calibration of high-energy X-ray diffraction setups. I. Assessing tilt and spatial distortion of the area detector

International audienceThe geometry of high-energy X-ray diffraction setups using an area detector and a rotation axis is analysed. The present paper (part 1) describes the methodology for determining continuously varying spatial distortions and tilt of the area detector based on the reference diffraction rings of a certified powder. Analytical expressions describing the degeneration of Debye rings into ellipses are presented and a robust calibration procedure is introduced. It is emphasized that accurate detector calibration requires the introduction of spatial distortion into the equation describing the tilt. The method is applied to data sets measured at the Advanced Photon Source and at the European Synchrotron Radiation Facility using detectors with different physical characteristics, the GE 41RT flat-panel and the FReLoN4M detector, respectively. The spatial distortion of the detectors is compared with regard to their use in structural and strain tensor analysis, a subject treated in part 2 of the calibration work [Borbely, Renversade & Kenesei (2014). J. Appl. Cryst. Submitted]

Microtomographic assessment of damage in P91 and E911 steels after long-term creep

International audienceTwo flat hollow cylinders made of martensitic 9 wt.% Cr steels were creep deformed under in-service conditions typical of steam pipes at fossil-fuel fired power plants. Damage in the tubes was assessed through synchrotron X-ray microtomography by evaluating the shape, size and spatial-distribution of voids. The analysis of the size distribution of non-coalesced voids suggested that void growth is controlled by the plasticity constrained diffusional mechanism, a hypothesis verified by micromechanical simulations. A much higher void density was found in steel grade P91 compared to E911

Damage fluctuations in creep deformed copper studied with synchrotron X-ray microtomography

International audienceDamage localization during power law creep of copper has been investigated in situ with synchrotron X-ray microtomography. The analysis of the area fraction of cavities corresponding to a given material slice has revealed that damage localization begins relatively early at half of the creep lifetime. The amplitude of the maximum fluctuation shows parabolic behavior as a function of mean void volume fraction. Existing models of damage evolution underpredict the amount of real damage and overpredict power-law creep life-time

Comparison between diffraction contrast tomography and high-energy diffraction microscopy on a slightly deformed aluminium alloy

International audienceThe grain structure of an Al-0.3 wt% Mn alloy deformed to 1% strain was reconstructed using diffraction contrast tomography (DCT) and high-energy diffraction microscopy (HEDM). 14 equally spaced HEDM layers were acquired and their exact location within the DCT volume was determined using a generic algorithm minimizing a function of the local disorientations between the two data sets. The microstructures were then compared in terms of the mean crystal orientations and shapes of the grains. The comparison shows that DCT can detect subgrain boundaries with disorientations as low as 1 degrees and that HEDM and DCT grain boundaries are on average 4 mm apart from each other. The results are important for studies targeting the determination of grain volume. For the case of a polycrystal with an average grain size of about 100 mm, a relative deviation of about <= 10% was found between the two techniques