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

Direct measurement of local constitutive relations, at the micrometre scale, in bulk metallic alloys

Multiscale models involving crystal plasticity are essential to predict the elastoplastic behavior of structural materials with respect to their microstructure. However, those models are often limited by a poor knowledge of the local constitutive behavior. This article reports a method to measure the mechanical behavior directly, at the micrometre scale, in bulk crystalline materials. Local strain and stress states were evaluated at the surface of a bent stainless steel crystal by combining total strain measurements – performed with the digital image correlation technique on optical images – with elastic strain measurements obtained by Laue microdiffraction. A local constitutive relation was measured, in an efficient nondestructive way, without the need for full-field simulations. The method was validated by a comparison between the measured local behavior and the macroscopic behavior of the single crystal

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

3D Characterisation of microstructure and elastic strain in polycrystals by Laue microdiffraction

La caractérisation des contraintes internes présentes dans les matériaux de structure ou fonctionnels est primordiale pour une optimisation de leurs propriétés et de leur tenue en service. Ce travail de thèse est une contribution au développement d'une technique de microscopie par diffraction des rayons X, appelée "Differential Aperture X-ray Microscopy", (DAXM, permettant la caractérisation 3D et non-destructive de la microstructure de matériaux cristallins et des contraintes internes présentes dans le matériau. Cette technique est basée sur l'utilisation du rayonnement synchrotron; nous avons utilisé la ligne CRG BM32 du synchrotron européen ESRF. Le faisceau de rayons incident est polychromatique (5-25keV) et fortement focalisé (section sub-micrométrique). En raison la pénétration du faisceau dans le matériau, qui est typiquement de quelques dizaines de microns, l'image de diffraction collectée est une superposition des diagrammes de Laue de tous les cristaux situés sur le trajet du faisceau incident. La DAXM utilise, en supplément de la microdiffraction Laue "classique", un masque mobile (ici un fin fil de tungstène) qui absorbe une partie des faisceaux diffractés. L'analyse de l'évolution des niveaux de gris des pixels de l'image en fonction de la position du masque permet non seulement de reconstruire la microstructure du matériau en profondeur mais aussi d'accéder à la distribution 3D des déformations élastiques (et des contraintes associées). L'un des avantages de la DAXM est sa résolution spatiale, de l'ordre du micromètre, qui permet d'envisager l'analyse des concentrations de contraintes dans les matériaux polycristallins, dans le cadre des approches micromécaniques expérimentales.Le travail mené dans cette thèse avait pour but d'améliorer le dispositif expérimental existant,de mettre en place la formulation théorique du problème, et de développer les outils numériques permettant le traitement des données.Du point de vue expérimental, nous avons notamment développé une machine d'essai mécanique in-situ (flexion 4-points) adaptée à la ligne BM32, et nous proposons un masque multi-fil qui devrait permettre de réduire significativement la durée de l'acquisition des données.Nous avons établi les équations de triangulation reliant la position des pixels du détecteur,la position du fil, et la profondeur de la source le long du faisceau incident. On montre ainsi que la reconstruction 3D nécessite une procédure de dérivation des niveaux de gris; nous nous sommes limités dans ce travail à une dérivation par différence finie d'ordre 1, qui reste sensible au bruit d'image. Ces équations font apparaître la nécessité de déterminer la géométrie du montage avec grande précision. On propose pour cela l'utilisation de la fluorescence de l'échantillon. On adjoint aux équations géométriques une description mathématique simplifiée de l'atténuation du faisceau par l'échantillon, prenant en compte un coefficient d'absorption unique. Le modèle de calibration est testé sur plusieurs matériaux, avec de très bons résultats.La capacité de la DAXM à reconstruire une microstructure est testée sur des échantillons modèles pour lesquels la géométrie 3D de la microstructure est parfaitement connue : empilement de fin fils de GaN sur un substrat, et plan de macle dans un polycristal d'acier inoxydable (316L). On montre que la résolution de la DAXM est variable d'un pixel à l'autre du détecteur; la microstructure peut cependant être reconstruite avec une précision de l'ordre du micromètre.La DAXM est ensuite testée sur un échantillon d'UO2 implanté d'ions Kr, créant une couche de surface d'épaisseur micrométrique fortement déformée (collaboration CEA-Cadarache). On observe que la méthode de reconstruction proposée produit d'importants artefacts, qui sont dus à la transmission variable des faisceaux diffractés dans le masque. Nous mettons en place la formulation permettant de prendre en compte cet effet.The characterization of the internal stresses present in structural or functional materials is essential for an optimization of their properties and their durability in service. This thesis work is a contribution to the development of the so-called '' Differential Aperture X-ray Microscopy'' (DAXM) technique, allowing 3D and non-destructive characterization of the microstructure of crystalline materials and internal stresses. This technique makes use of synchrotron radiation; we used the beamline CRG BM32 of the European synchrotron ESRF. The polychromatic and highly focused incident beam penetrates the sample, and the collected diffraction image is a superimposition of the Laue diagrams of all the crystals located along the path of the incident beam. The DAXM uses, in addition to the "conventional" Laue microdiffraction technique, a moving mask that absorbs part of the diffracted beams. The analysis of the evolution of the gray levels of the image pixels as a function of the position of the mask makes it possible not only to reconstruct the microstructure of the material at depth but also to access the 3D distribution of the elastic deformations (and associated stress). One of the advantages of the DAXM is its spatial resolution, of the order of a micrometer, which makes it possible to envisage the analysis of stress concentrations in polycrystalline materials, within the framework of experimental micromechanical approaches.The work carried out in this thesis was aimed at improving the existing experimental system,to put in place the theoretical formulation of the problem, and to develop the numerical tools allowing the processing of the data.From an experimental point of view, we have developed an in-situ mechanical test device (4-point bending) adapted to BM32, and we propose a multi-wire mask to significantly reduce the data acquisition time.We have established the geometric equations of the problem. It is thus shown that the 3D reconstruction requires a gray scale derivation procedure. This work is limited to the use of a finite difference derivation method of order 1, which remains sensitive to image noise. These equations show the need to determine the geometry of the setup with great precision. For this purpose, the use of the fluorescence of the sample is proposed, coupled with a simplified description of the beam attenuation by the sample taking into account only a single absorption coefficient. The calibration model is tested on several materials, with very good results.The capacity of the DAXM to reconstruct a microstructure is tested on model samples for which the 3D geometry of the microstructure is perfectly known: a stack of GaN wires on a substrate, and a twin plane in a stainless steel polycrystal. It is shown that the resolution of the DAXM is variable from one pixel to the other of the detector; the microstructure can however be reconstructed with an accuracy of the order of one micrometer.The DAXM is then tested on a sample of UO2 implanted by Kr ions, creating a highly deformed surface layer with micrometric thickness (collaboration with CEA-Cadarache). It is found that the proposed reconstruction method is affected by the variable transmission of the diffracted beams in the mask. We propose a formulation that takes this effect into account

Caractérisation 3D de la microstructure et des déformations élastiques des polycristaux par microdiffractiodiffraction Laue

The characterization of the internal stresses present in structural or functional materials is essential for an optimization of their properties and their durability in service. This thesis work is a contribution to the development of the so-called '' Differential Aperture X-ray Microscopy'' (DAXM) technique, allowing 3D and non-destructive characterization of the microstructure of crystalline materials and internal stresses. This technique makes use of synchrotron radiation; we used the beamline CRG BM32 of the European synchrotron ESRF. The polychromatic and highly focused incident beam penetrates the sample, and the collected diffraction image is a superimposition of the Laue diagrams of all the crystals located along the path of the incident beam. The DAXM uses, in addition to the "conventional" Laue microdiffraction technique, a moving mask that absorbs part of the diffracted beams. The analysis of the evolution of the gray levels of the image pixels as a function of the position of the mask makes it possible not only to reconstruct the microstructure of the material at depth but also to access the 3D distribution of the elastic deformations (and associated stress). One of the advantages of the DAXM is its spatial resolution, of the order of a micrometer, which makes it possible to envisage the analysis of stress concentrations in polycrystalline materials, within the framework of experimental micromechanical approaches.The work carried out in this thesis was aimed at improving the existing experimental system,to put in place the theoretical formulation of the problem, and to develop the numerical tools allowing the processing of the data.From an experimental point of view, we have developed an in-situ mechanical test device (4-point bending) adapted to BM32, and we propose a multi-wire mask to significantly reduce the data acquisition time.We have established the geometric equations of the problem. It is thus shown that the 3D reconstruction requires a gray scale derivation procedure. This work is limited to the use of a finite difference derivation method of order 1, which remains sensitive to image noise. These equations show the need to determine the geometry of the setup with great precision. For this purpose, the use of the fluorescence of the sample is proposed, coupled with a simplified description of the beam attenuation by the sample taking into account only a single absorption coefficient. The calibration model is tested on several materials, with very good results.The capacity of the DAXM to reconstruct a microstructure is tested on model samples for which the 3D geometry of the microstructure is perfectly known: a stack of GaN wires on a substrate, and a twin plane in a stainless steel polycrystal. It is shown that the resolution of the DAXM is variable from one pixel to the other of the detector; the microstructure can however be reconstructed with an accuracy of the order of one micrometer.The DAXM is then tested on a sample of UO2 implanted by Kr ions, creating a highly deformed surface layer with micrometric thickness (collaboration with CEA-Cadarache). It is found that the proposed reconstruction method is affected by the variable transmission of the diffracted beams in the mask. We propose a formulation that takes this effect into account.La caractérisation des contraintes internes présentes dans les matériaux de structure ou fonctionnels est primordiale pour une optimisation de leurs propriétés et de leur tenue en service. Ce travail de thèse est une contribution au développement d'une technique de microscopie par diffraction des rayons X, appelée "Differential Aperture X-ray Microscopy", (DAXM, permettant la caractérisation 3D et non-destructive de la microstructure de matériaux cristallins et des contraintes internes présentes dans le matériau. Cette technique est basée sur l'utilisation du rayonnement synchrotron; nous avons utilisé la ligne CRG BM32 du synchrotron européen ESRF. Le faisceau de rayons incident est polychromatique (5-25keV) et fortement focalisé (section sub-micrométrique). En raison la pénétration du faisceau dans le matériau, qui est typiquement de quelques dizaines de microns, l'image de diffraction collectée est une superposition des diagrammes de Laue de tous les cristaux situés sur le trajet du faisceau incident. La DAXM utilise, en supplément de la microdiffraction Laue "classique", un masque mobile (ici un fin fil de tungstène) qui absorbe une partie des faisceaux diffractés. L'analyse de l'évolution des niveaux de gris des pixels de l'image en fonction de la position du masque permet non seulement de reconstruire la microstructure du matériau en profondeur mais aussi d'accéder à la distribution 3D des déformations élastiques (et des contraintes associées). L'un des avantages de la DAXM est sa résolution spatiale, de l'ordre du micromètre, qui permet d'envisager l'analyse des concentrations de contraintes dans les matériaux polycristallins, dans le cadre des approches micromécaniques expérimentales.Le travail mené dans cette thèse avait pour but d'améliorer le dispositif expérimental existant,de mettre en place la formulation théorique du problème, et de développer les outils numériques permettant le traitement des données.Du point de vue expérimental, nous avons notamment développé une machine d'essai mécanique in-situ (flexion 4-points) adaptée à la ligne BM32, et nous proposons un masque multi-fil qui devrait permettre de réduire significativement la durée de l'acquisition des données.Nous avons établi les équations de triangulation reliant la position des pixels du détecteur,la position du fil, et la profondeur de la source le long du faisceau incident. On montre ainsi que la reconstruction 3D nécessite une procédure de dérivation des niveaux de gris; nous nous sommes limités dans ce travail à une dérivation par différence finie d'ordre 1, qui reste sensible au bruit d'image. Ces équations font apparaître la nécessité de déterminer la géométrie du montage avec grande précision. On propose pour cela l'utilisation de la fluorescence de l'échantillon. On adjoint aux équations géométriques une description mathématique simplifiée de l'atténuation du faisceau par l'échantillon, prenant en compte un coefficient d'absorption unique. Le modèle de calibration est testé sur plusieurs matériaux, avec de très bons résultats.La capacité de la DAXM à reconstruire une microstructure est testée sur des échantillons modèles pour lesquels la géométrie 3D de la microstructure est parfaitement connue : empilement de fin fils de GaN sur un substrat, et plan de macle dans un polycristal d'acier inoxydable (316L). On montre que la résolution de la DAXM est variable d'un pixel à l'autre du détecteur; la microstructure peut cependant être reconstruite avec une précision de l'ordre du micromètre.La DAXM est ensuite testée sur un échantillon d'UO2 implanté d'ions Kr, créant une couche de surface d'épaisseur micrométrique fortement déformée (collaboration CEA-Cadarache). On observe que la méthode de reconstruction proposée produit d'importants artefacts, qui sont dus à la transmission variable des faisceaux diffractés dans le masque. Nous mettons en place la formulation permettant de prendre en compte cet effet

Tachyarrhythmias in Congenital Heart Diseases: From Ion Channels to Catheter Ablation

Major advances in pediatric cardiology in recent decades, especially surgical techniques, have resulted in an increasing number of patients with congenital heart disease (CHD) surviving to adulthood. This has generated new challenges, particularly with regards to the late onset of complex arrhythmias. Abnormal anatomy, surgical scarring, chronic hypoxemia, hemodynamic compromise, neuro-hormonal abnormalities, and genetic factors can all contribute to creating a unique substrate for arrhythmia development. This review attempts to synthesize the current state of knowledge spanning the spectrum from underlying mechanisms of arrhythmias in patients with congenital heart disease to current ablative strategies. We discuss existing knowledge gaps and highlight important areas for future research

Full Characterization of Water Transport Properties in Polyetherketoneketone (PEKK)

Polyetherketoneketone (PEKK) is of increasing interest for the manufacture of composites in the aeronautical field. It is essential to evaluate the PEKK absorption of fluids, especially water, with which it may come into contact during its processing and use. In this work, we provide for the first time water transport parameters such as water diffusivity and solubility for PEKK in amorphous and semicrystalline states using water immersion as well as dynamic vapor sorption (DVS) experiments. Water diffusion is modeled using Fick’s second law with variable boundary conditions, taking into account possible relative humidity variation during the DVS experiments. Based on similar characterizations of PEEK, we discuss the fact that PEKK absorbs more water than PEEK in terms of polarity

Determination of deviatoric elastic strain and lattice orientation by applying digital image correlation to Laue microdiffraction images: The enhanced Laue-DIC method

A new method of determining the deviatoric elastic strain and lattice orientation from Laue microdiffraction images is presented. Standard data treatment methods can suffer from the difficulty of precisely pinpointing the positions of diffraction peaks on two-dimensional Laue images. In a previous article, digital image correlation (DIC) was introduced for the treatment of Laue images, leading to the so-called Laue-DIC method. This performed better than the standard method in terms of the deviatoric elastic strain increment and relative rotation from one lattice to another, particularly when the shape of the Laue spots departs from regular ellipsoids. The present work intends to push forward the Laue-DIC method, aiming to determine the deviatoric elastic strain and lattice orientation, as well as the calibration parameters. The performance of this new method, named enhanced Laue-DIC, is assessed by modeling the spot displacements and accounting for random fluctuations relevant for typical experimental conditions. When the enhanced Laue-DIC method is applied to the case of an in situ deformed Si crystal, the obtained standard deviation of local stress is of the order of 1-2 MPa, while the calibration parameters are optimized to high accuracy

Embrittlement of polybutylene terephthalate induced by injection molding

International audienceEmbrittlement caused by injection molding was investigated on a commercial grade of Polybutylene terephthalate (PBT). The effect of temperature and residence time in the barrel on mechanical properties was studied. Results were discussed from macromolecular analysis by Gel Permeation Chromatography and crystallinity measurements by differential scanning calorimetry. It was shown that embrittlement exclusively occurs by a chain scission process in those conditions. Oxygen is shown to play a negligible role in degradation while thermolysis seems to be the main source of degradation. It was here described by a first simple kinetic model based on the beta abstraction of hydrogen at the vicinity of ester groups. A processing window was thus determined using this model and might help practitioners for processing PBT parts