Validity of Crystal Plasticity Models Near Grain Boundaries: Contribution of Elastic Strain Measurements at Micron Scale

Synchrotron Laue microdiffraction and digital image correlation measurements were coupled to track the elastic strain field (or stress field) and the total strain field near a general grain boundary in a bent bicrystal. A 316L stainless steel bicrystal was deformed in situ into the elasto-plastic regime using a four-point bending setup. The test was then simulated using finite elements with a crystal plasticity model comprising internal variables (dislocation densities on discrete slip systems). The predictions of the model are compared with both the total strain field and the elastic strain field obtained experimentally. While activated slip systems and total strains are reasonably well predicted, elastic strains appear overestimated next to the grain boundary. This suggests that conventional crystal plasticity models need improvement to correctly model stresses at grain boundaries

On the Accuracy of Elastic Strain Field Measurements by Laue Microdiffraction and High-Resolution EBSD: a Cross-Validation Experiment

Determining the accuracy of elastic strain measurements in plastically deformed alloys is an experimental challenge. To develop a novel cross-validation procedure, a controlled elasto-plastic strain gradient was created in a stainless steel single crystal by four point bending deformation. The corresponding elastic strain field was probed, with an intragranular spatial resolution, in-situ by Laue microdiffraction and ex-situ by High Resolution EBSD. Good agreement is found for the two independent measurements and the predictions of a mechanical model, at plastic strains below 0.5 %. The accuracy of the measurements is estimated at 3.2 × 10 − 4

Accuracy assessment of crystal orientation indexations by EBSD

International audienceAccuracy and uncertainty analyses are essential for every measurement technology. In crystal orientation indexation by electron backscatter diffraction (EBSD), a series of accuracy estimations have been made for the Hough transform and dictionary indexation methods. The mean angular deviation is a standard parameter to indicate orientation accuracy, but this criterion is indirect and closely related to the accuracy of the projection center coordinates. Precise known orientation relationships are necessary to evaluate orientation accuracy without the ground truth. The current work uses the natural crystal twins and hardware orientation relationships to assess the orientation accuracy directly. The accuracy level for different EBSD analysis methods is compared through four experimental data sets of varying pattern definitions and noise levels. It is found that the full pattern match (FPM) algorithms improve the accuracy as compared to Hough indexation, and the gain varies greatly between 14% for fast acquisitions and 20 times for high-quality patterns. Depending on the resolution and quality of diffraction patterns, FPM results in an accuracy of crystal orientation between 0.04°and 0.9°. Comparing the two FPM variants, matching the gradients of diffraction patterns performs better in the case of high-to-median quality acquisitions while matching the pattern itself is more accurate for more noisy and low-definition patterns

Improved EBSD indexation accuracy by considering energy distribution of diffraction patterns

International audienceRegistering experimental and simulated electron diffraction patterns is increasingly used for advanced electron backscatter diffraction indexation (EBSD) analysis, yet the accuracy of registration is limited by several effects not accounted for in pattern simulation, such as the Kikuchi band asymmetry, gray level reversal, optical distortion and non-uniform electron energy. Though some of these phenomena can be simulated by Monte Carlo method and dynamical simulation, the computation is highly demanding and their effects on EBSD calibration are seldom analyzed. Here a simple weighting of energy for simulated diffraction pattern is proposed based on several master patterns calculated beforehand , to effectively account for the electron energy distribution. Integrated digital image correlation is alternatively applied to quantify the electron energy field and calibrate geometry parameters. A metric of the accuracy of indexed crystal orientation is proposed based on a large-area high-definition experimental EBSD acquisition on a (100)-face single crystal Si wafer. The consideration of inhomogeneous energy distribution reduces the crystal orientation discrepancy by 0.128°for the Si dataset

Enhanced EBSD calibration accuracy based on gradients of diffraction patterns

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Improved high-resolution EBSD analyses by correcting radial distortion of electron diffraction patterns

International audienceRegistering experimental and simulated electron diffraction patterns is increasingly used for advanced electron backscatter diffraction indexation (EBSD) analysis, yet the accuracy of registration is limited by several effects not accounted for in pattern simulation, such as the Kikuchi band (K-band) asymmetry, gray level reversal and (mainly radial) optical distortion. Radial distortion parameters have previously been measured with chessboardtype standard samples. Simulated patterns have been adopted to demonstrate the necessity of optical distortion removal in EBSD analyses. However there still lacks an efficient and precise radial distortion assessment and correction method. Here a simple radial distortion model, including barrel and pincushion distortions, is proposed to rectify the diffraction patterns during EBSD analyses. The correlation between experimental pattern and the simulated master pattern permits to index the diffraction pattern and assess the radial distortion simultaneously. The method is applied to three high-definition experimental electron diffraction datasets acquired with different cameras. The radial distortion parameter is identified with a relative uncertainty below 4%. The consideration of radial distortion improves the correlation between experimental and simulated patterns. Gray level profiles of the K-bands are analyzed to verify the correctness of image registration. The current method provides a fast, economic yet precise correction of the radial distortion for advanced EBSD analyses

Humidity‐Induced Degradation Processes of Halide Perovskites Unveiled by Correlative Analytical Electron Microscopy

International audienceAbstract Improving the stability of lead halide perovskite solar cells (PSCs) for industrialization is currently a major challenge. It is shown that moisture induces changes in global PSC performance, altering the nature of the absorber through phase transition or segregation. Understanding how the material evolves in a wet environment is crucial for optimizing device performance and stability. Here, the chemical and structural evolution of state‐of‐the‐art hybrid perovskite thin‐film Cs 0.05 (MA 0.15 FA 0.85 ) 0.95 Pb(I 0.84 Br 0.16 ) 3 (CsMAFA) is investigated after aging under controlled humidity with analytical characterization techniques. The analysis is performed at different scales through Photoluminescence, X‐ray Diffraction Spectroscopy, Cathodoluminescence, Selected Area Electron Diffraction, and Energy Dispersive X‐ray Spectroscopy. From the analysis of the degradation products from the perovskite layer and by the correlation of their optical and chemical properties at a microscopic level, different phases such as lead–iodide (PbI 2 ), inorganic mixed halide CsPb(I 0.9 Br 0.1 ) 3 and lead‐rich CsPb 2 (I 0.74 Br 0.26 ) 5 perovskite are evidenced. These phases demonstrate a high degree of crystallinity that induces unique geometrical shapes and drastically affects the optoelectronic properties of the thin film. By identifying the precise nature of these specific species, the multi‐scale approach provides insights into the degradation mechanisms of hybrid perovskite materials, which can be used to improve PSC stability

Validity of Crystal Plasticity Models Near Grain Boundaries: Contribution of Elastic Strain Measurements at Micron Scale

International audienceSynchrotron Laue microdiffraction and digital image correlation measurements were coupled to track the elastic strain field (or stress field) and the total strain field near a general grain boundary in a bent bicrystal. A 316L stainless steel bicrystal was deformed in situ into the elasto-plastic regime using a four-point bending setup. The test was then simulated using finite elements with a crystal plasticity model comprising internal variables (dislocation densities on discrete slip systems). The predictions of the model are compared with both the total strain field and the elastic strain field obtained experimentally. While activated slip systems and total strains are reasonably well predicted, elastic strains appear overestimated next to the grain boundary. This suggests that conventional crystal plasticity models need improvement to correctly model stresses at grain boundaries

Microstructural Evolution of Neutron Irradiated Stainless Steels

Materials of the core internals of Pressurized Water Reactors (austenitic stainless steels) are submitted to neutron irradiation. Some baffle/former bolt cracking were detected by non-destructive examinations and removed for metallurgical examination. The most likely cause to cracking was deduced to be Irradiation Assisted Stress Corrosion Cracking. To understand the ageing mechanisms associated to irradiation and propose life predictions of the component, irradiation damage is investigated experimentally by Transmission Electron Microscopy for two representative austenitic stainless steels, SA 304L and CW 316. The microstructure of these austenitic stainless steels constitutive of internals structures of PWR is characterized by TEM and compared to the microstructure obtained after irradiation for high doses up to 40 dpa at 320°C in experimental OSIRIS (mixed flux spectrum) and BOR-60 (fast breeders) reactors and at 300°C in experimental SM-2 (mixed flux spectrum) reactor. Only minor differences were detected