The electric vehicle transportation industry relies on advanced battery systems with high-voltage wiring harnesses composed of copper and aluminum conductors. Joining these dissimilar metals using conventional fusion welding is challenging due to the formation of brittle intermetallic compounds and differences in their thermal and metallurgical properties. Friction stir welding (FSW) has emerged as an effective technique for manufacturing Al–Cu joints in electric vehicle battery assemblies. In this study, dissimilar lap joints between AA6063 aluminum alloy (top) and commercially pure copper (bottom) were FSWed. A carbide-coated crown probe with a scroll-featured shoulder was used to promote material flow and suppress tunnel defects. Macrostructural, microstructural, and mechanical characterizations were conducted using optical microscopy, SEM–EDS- EBSD, and microhardness testing. Distinct particle-rich zones containing dispersed Cu particles within an Al matrix were observed along upwards flow tracks indicative of complex material transport within the stir zone. The welded interface contained primarily θ-Al₂Cu and γ-Al₄Cu₉ intermetallics (IMCs). Microhardness mapping showed pronounced softening in the aluminum stir zone (∼100 HV0.1) and heat-affected zone (∼60 HV0.1), contrasted with localized hardening near Cu-rich intermetallic compounds (∼130 HV0.1), relative to base material hardness values (∼70 HV0.1 for Al and ∼80 HV0.1 for Cu). EBSD analysis focused on the hook in the AS confirmed significant grain refinement due to dynamic recrystallization in addition to a partially-retained texture. Overall, the findings provide insights into the mechanisms involved in the material flow during the Cu hook formation in Al–Cu bi-metallic FSW joints, providing guidance for reliable bi-metallic connections in electric vehicle battery systems.

Al-Mg alloys possess a good resistance to solidification cracking during fusion-based manufacturing despite their wide freezing range. Proper understanding of Mg solute segregation during the solidification of Al alloys is essential to model solidification paths and predict solidification crack formation. Based upon cast and welded specimens of AA5754-H111 alloy, mass-balance analysis applied on EDS maps shows divergences compared to theoretical Scheil solidification paths. Those were associated to variable partition coefficients during the completion of solidification. The methodology effectiveness in tracking the partition coefficient throughout solidification progression is demonstrated.

In this work, we combine complementary diagnostic methods to characterise thoroughly crystalline silicon with a view to its application to photovoltaic (PV) solar cells: structural and chemical analysis, and charge transport investigations. On one hand, secondary ion mass spectrometry (SIMS) and Micro Fourier transform infrared spectroscopy (mu FTIR) are used to detect chemical impurities and their spatial distribution. On the other hand, electron backscatter diffraction (EBSD) reveals grain structure up to the presence of sub-grains. In addition, surface photovoltage (SPV) is used to measure minority carrier lifetime whereas photoluminescence (PL) is used to provide maps that show the variation of the non-radiative recombination activity on the analysed areas. The method consists of taking benefit of complementary characterisation techniques at different scales to investigate structural defects, location and form of impurities as well as the related electrical properties in silicon for photovoltaic applications. This study shows that impurities concentrate at defect sites: sub-grains and twin boundaries depending on their crystalline quality. Concerning the charge transport characterisation, these areas act as recombination centers, decreasing the electrical properties. Noticeably, the oxygen is found to be present in interstitial position in the bulk whereas at the level of sub-grains, it is in precipitate form. Combining these techniques enhances our understanding of the impact of impurities in conjunction with the presence of structural defects at several scales on photovoltaic performance.

Performing in situ scanning electron microscope (SEM) tests is an interesting way to visualise strain heterogeneities under mechanical loading. An essential step before performing the tests is to define the acquisition conditions. The aim of this paper is to propose a classification of the acquisition conditions that are most important for the accuracy of strain measurements using digital image correlation (DIC) in in situ SEM tests. More than 200 image pairs were acquired using a field emission gun SEM. The influence of different acquisition conditions was investigated: acceleration voltage, probe current, working distance, magnification, number of integrated images, image resolution, integration and number of integrated images, scan speed, contrast, brightness and exposure time of the sample in a given area. The methodology implemented in this work is an interesting tool for detecting scan line shift, drift distortion, spatial distortion and rastering artefacts. It allows the optimization of SEM acquisition conditions for strain measurements. Finally, optimal acquisition conditions for in situ testing are proposed and used to perform a tensile test on pure copper. The main factors highlighted include the size of the subset used in the DIC, the beam stabilisation time before image acquisition and the size of the images, which play a significant role in the results. It is recommended to apply the methodology to each device to optimise the acquisition conditions.

For all fabrication processes of the photovoltaic (PV) industry based on silicon, grain boundaries, dislocations, and impurity contamination control during solidification remains a major challenge to improve the electrical properties. In particular, carbon (C) is a major deleterious impurity for solar cells. The combination of X-ray radiography and Bragg diffraction imaging (topography) achieved in situ during silicon solidification allowed us to characterise the dynamics of the growth mechanisms involved in the formation of the grain structure and of defects, related to the presence of C. Ex situ techniques were used to characterise the grain structure and for a more precise analysis of the defects and their associated distortion fields. In the presence of C, it is shown that the resulting grain structure is constituted by a higher proportion of high-order and incoherent twin boundaries compared with the case of pure samples. Crystal distortion is characterised at the grain scale level and at a lower scale, both in situ and after cooling-down. The highest distortion at the grain scale corresponds to the position of the high order twin boundaries and is accentuated during cooling-down following solidification. Locally distorted regions and sub-grains are distributed all over the samples. They are observed in situ during the solidification from various seeds containing C (mono-crystals, industrial ribbons and multi-crystalline samples) and are retrieved after solidification. A model implying the presence of SiC precipitates at the solid-liquid interface is proposed to explain the formation during solidification of these sub-grains and of the associated local distortion.

Understanding the mechanisms at the microstructure scale is of great importance for modeling the behavior of materials at different scales. To this end, digital image correlation (DIC) is an effective measurement method for evaluating the strains generated by various loading conditions. The objective of this paper is to describe the experimental setup and the use of high resolution digital image correlation (HRDIC) during in situ Scanning Electron Microscope (SEM) tests in order to provide a coupling between polycrystalline modeling and experiment in the near future. The HRDIC technique is used to evaluate the tensile behavior of a pure copper polycrystal at room temperature. Several magnitudes are investigated in order to discuss the representativeness of the results with respect to the macroscopic scale. The selected image correlation parameters are discussed regarding the ability of the technique to define inter-and intra-granular strain heterogeneities. Finally, based on EBSD ana-lyzes, the impact of grain orientation on the mechanical behavior is discussed. The Schmid factor, calculated from a macroscopic stress, appears to be the determining factor concerning the orientation of the location bands. On the other hand, it is not sufficient to define the mean strains in the grains.

The influence of oxidizing and nitriding parameters on the nitrogen concentration of grain-oriented electrical steels preliminary to the development of the final Goss texture was explored.Results show that the nitrogen enrichment is driven by a ferrite to austenite transformation during thermochemical treatments.Such a transformation is promoted by (i) a redistribution of ferrite-forming elements close to the surface during oxidizing prior to nitriding, (ii) the oxygen content within the oxide layer prior to nitriding, (iii) the temperature of oxidizing and nitriding, and (iv) the nitrogen enrichment during nitriding.Optimization of the nitrogen content, and thus the precipitation kinetics of alloying elements nitrides (e.g.inhibitors) required for the development of the final Goss texture can be controlled by an optimization of the oxide layer growth, the temperatures and gas mixture of nitriding.

In this work, the Yb:YAG laser beam welding of the magnesium alloy AM60 was studied. A laser power of 2 kW and a welding speed of 3.5 m / min give a different welding quality than that obtained by CO2 laser with the same parameters. The metallurgical characterization, by optical microscopy, showed the formation of four distinct zones : base metal (BM), heat affected zone (HAZ), the partially fusion zone (PFZ) and the fusion zone (FZ), due to the thermal effect produced by the laser welding thermal cycle. Their dimensions are quantified. The microstructural examination using scanning electron microscopy showed the presence of fine dendritic structure in the FZ although the use of electron dispersive spectroscopy analysis confirm that an eutectic Mg17Al12 phase are surrounded by α-Mg solid solution in the HAZ. Electron backscattered diffraction technique revealed an important grain refinement in FZ and considerable twining phenomena in HAZ, but no texture. X-ray diffraction technique has been used, full width at half maximum of diffraction peaks is measured; it also confirmed the grain refinement in FZ in comparison to BM and HAZ. Both microhardness and tensile proprieties of the complete weld joint are similar to those of the BM.

To control the final grain structure and the density of structural crystalline defects in silicon (Si) ingots is still a main issue for Si used in photovoltaic solar cells. It concerns both innovative and conventional fabrication processes. Due to the dynamic essence of the phenomena and to the coupling of mechanisms at different scales, the post-mortem study of the solidified ingots gives limited results. In the past years, we developed an original system named GaTSBI for Growth at high Temperature observed by Synchrotron Beam Imaging, to investigate in situ the mechanisms involved during solidification. X-ray radiography and X-ray Bragg diffraction imaging (topography) are combined and implemented together with the running of a high temperature (up to 2073 K) solidification furnace. The experiments are conducted at the European Synchrotron Radiation Facility (ESRF). Both imaging techniques provide in situ and real time information during growth on the morphology and kinetics of the solid/liquid (S/L) interface, as well as on the deformation of the crystal structure and on the dynamics of structural defects including dislocations. Essential features of twinning, grain nucleation, competition, strain building, and dislocations during Si solidification are characterized and allow a deeper understanding of the fundamental mechanisms of its growth.

Directional solidification of a cast mono silicon seed and of a float-zone (FZ) silicon seed was performed and the grain and defect structures of the seeds as well as of the regrown parts are analyzed. In situ X-ray diffraction imaging enabled the observation of the dislocation arrangements. During the heating process, in the FZ seed, mobile dislocations glide on {111} planes, whereas in the cast mono seed dislocations are arranged in a mainly immobile cellular structure. Ex situ grain orientation mappings reveal the presence of subgrains with misorientations up to 3° in the regrown part of the cast mono-seeded sample, which are not observed in the regrown part of the FZ-seeded sample. Subgrain boundaries characterized by misorientations around the [001] growth axis propagate roughly along the growth axis and increase their misorientation by merging with new subgrain boundaries appearing in their vicinity. Although the first inception of subgrain formation cannot be revealed, the comparison of the dislocation arrangements in the two seeds strongly suggests an influence of the latter on subgrain formation. In the regrown part, interactions between subgrain boundaries and twin boundaries show that they can follow Σ3{111} and Σ9{221} grain boundaries or cross Σ3{111} grain boundaries. Whether Σ3{111} GBs are crossed or not depends among other things on the orientation of the grains on either side of the twin. It demonstrates that the grain orientation relationship and not only the grain boundary character play an important role in the subgrain structure evolution and redistribution in a multicrystalline silicon ingot.

This work is dedicated to the grain structure formation in silicon ingots with a particular focus on the crystal structure strain building and its implication in new grain nucleation process. The implied mechanisms are investigated by advanced in situ X-ray imaging techniques during silicon directional solidification. It is shown that the grain structure formation is mainly driven by Σ3 <111> twin nucleation. Grain competition phenomena occurring during the growth process lead to the creation of higher order twin boundaries, localised strained areas and associated crystal structure deformation. On the one hand, it is demonstrated that local strain building can be directly related to the characteristics of the twin boundaries created during silicon growth due to grain competition. On the other hand, space restriction due to competition during growth can be at the origin of local strain building as well. Finally, the accumulation of all these factors generating strain is responsible for spontaneous new grain nucleation. When occurring, both grain nucleation and subsequent grain structure reorganisation contribute to lower the strain in the growing ingot. It is demonstrated as well that the local distribution of the strained areas created during silicon growth is retrieved after cooling down, from melting temperature to room temperature, on top of an additional larger scale deformation of the sample due to the cooling down only.

In this research, Al-Fe-Cr quasicrystal (QC) reinforced Al-based metal matrix composites were in-situ manufactured by using selective laser melting (SLM) from the powder mixture. The parametrical optimization based on our previous work was performed with focus on laser scanning speed. From the optimized parameters, an almost dense (99.7%) free-crack sample was fabricated with an ultra-fine microstructure. A phase transition from decagonal QC Al65Cu25Fe10Cr5 to icosahedral QC Al91Fe4Cr5 could be observed as laser scanning speed decreases. Differential scanning calorimetry curves show that the QC phase is quiet stable until 500 °C. And then, the effects of annealing temperature on the microstructural and mechanical properties were determined. The results indicate that the recrystallization and growth behavior of α-Al grains could be prevented by QC particle during annealing. Furthermore, the growth of QC particle, which tends to form a porous structure, leads an improvement of Young modulus and decline of ductility.

Fe-Ni-Si soft magnetic parts, using Ni coated high silicon steel powder, were manufactured by selective laser melting process. The type of defect changes from porosity to cracks and the relative density increases, from 50% to 99%, with the decreasing laser scanning speed. The microstructural analyses indicate that the low laser scanning speed fully melted the nickel coating and high-silicon steel core. The EBSD study showed that the separated island and lamellar mesostructures appeared on the top and side view respectively. Moreover, no apparent texture were observed. The magnetization saturation of SLM processed sample decreased, as the laser scanning speed was increased. Consequently, the magnetic properties of SLM processed Fe-Ni-Si alloy also showed anisotropic feature in building and scanning directions, which can be attributed to their different mesostructure.

This paper deals with the measurement of residual stresses in cementite after gas-nitriding of a 33CrMoV12-9 steel. During nitriding, precipitation of nanometric alloying elements nitrides and cementite at grain boundaries occurs leading to an increase of superficial hardness and providing compressive residual stresses in the surface layer. The stress state in the ferritic matrix has generally been measured to characterize the mechanical behaviour of the nitrided case while the other phases are not taken into account. In order to better understand the mechanical behaviour (e.g. fatigue life and localization of cracks initiation) of heterogeneous material such as in case of nitrided surfaces, the nature (sign, level) of residual stresses (or pseudo-macro-stresses) of the present phases can be calculated from measurements using X-ray diffraction to select the considered phase. Due to a low volume fraction of cementite through a nitrided case, an approach based on X-ray and electron backscattered diffractions (XRD and EBSD respectively) is proposed to perform stress measurements in cementite. An optimization of the surface preparation (by mechanical and/or chemical polishing techniques) prior to EBSD analysis was performed in order to minimize deformation induced by surface preparation. Pseudo-macro-stresses were calculated in tempered martensite and cementite. Results are compared to local residual stress measurements carried out by a cross-correlation method using EBSD patterns.

This work is dedicated to the advanced in situ X-ray imaging and complementary ex situ investigations of the growth mechanisms when silicon solidifies on a monocrystalline seed oriented 〈110〉 in the solidification direction. It aims at deepening the fundamental understanding of the phenomena that occur throughout silicon crystal growth with a particular focus on mechanisms of formation of defects detrimental for photovoltaic applications. Namely, grain nucleation, grain boundary formation and evolution, grain competition, twining occurrence, dislocation generation and interaction with structural defects are explored and analysed. Nucleation of twin crystals preferentially occurs on {111} facets at the edge of the sample where solid – liquid – vapor triple point lines exist in interaction also with the crucible as well as, at grain boundary grooves at the solid – liquid interface (solid – solid – liquid triple lines), where two grains are in competition, either on the {111} facets of the groove or in the groove. Enhanced undercooling and/or stress accumulation levels are found to act as driving forces for grain nucleation. Additionally, it is demonstrated that twin formation has the property to relax stresses stored in the crystal during the growth process. However, grains formed initially in twin position can undergo severe distortion when they are in direct competition or when they are squeezed in – between grains. Moreover, we show by X-ray Bragg diffraction imaging that on the one hand, coherent Σ3 〈111〉 grain boundaries efficiently block the propagation of growth dislocations during the solidification process, while on the other hand, dislocations are emitted at the level of incoherent and/or asymmetric Σ27a 〈110〉 at the encounter with either Σ3 〈111〉 or Σ9 〈110〉 grain boundaries. Indeed, grain boundaries that deviate from the ideal coincidence orientation act as dislocation sources that spread inside the surrounding crystals.

Considering the need to reduce waste production and greenhouse emissions and still keeping high energy efficiency various 4th generation nuclear energy systems have been proposed As far as graphite-moderated reactors are concerned (future high temperature fast or thermal reactors) one of the key issues is the large volumes of irradiated graphite encountered With the objective to reduce volume of waste in the HTR concept it is very important to be able to separate the fuel from low level activity graphite representing a large volume The separated TRISO particles can then be reprocessed for waste separation or disposed off in geological repository In addition preparation of acid-GICs from the separated graphite may constitute a way to recycle this wasteWe used HTR-type compact fuel with ZrO2 TRISO particles to test two separation methods low (H2SO4 + H2O2) and high (H2SO4 + HNO3) temperature acid treatments In both cases the TRISO separation was complete but some TRISO layers oxidized at high temperature At low temperature the desegregation of graphite grains is facilitated by intercalation of sulfuric acid between the graphene layers The acid-GIC obtained consists of pure phases of high quality suggesting their potential industrial recycling (C) 2010 Published by Elsevier BV

Considering the need to reduce waste production and greenhouse emissions by still keeping high energy efficiency, various 4 generation nuclear energy systems have been proposed. As far as graphite moderated reactors are concerned, one of the key issues is the large volumes of irradiated graphite encountered (1770 m for fuel elements and 840 m for reflector elements during the lifetime (60 years) of a single reactor module [1]). With the objective to reduce volume of waste in the HTR concept, it is very important to be able to separate the fuel from low level activity graphite. This requires to separate TRISO particles from the graphite matrix with the sine qua non condition to not break TRISO particles in case of future embedding of particles in a matrix for disposal. According to National Regulatory Systems, in case of limited graphite waste production or of short duration HTR projects (e.g. in Germany), direct disposal without separation is acceptable. Nevertheless, in case of large scale deployment of HTR technology, such approach is not economical and sustainable. Previous attempts in graphite management (furnace, fluidised bed and laser incinerations and encapsulation matrices) dealt with graphite matrix only. These are the reasons why we studied the management of irradiated compact-type fuel element. We simulated the presence of fuel in the particles by using ZrO2 kernels. Compacts with ZrO2 TRISO particles were manufactured by AREVA NP. Two original methods have been studied. First, we tested high pressure jet to erode graphite and clean TRISO particles. Best erosion rate reached about 0.18 kg/h for a single nose ending. Examination of treated graphite showed a mixture of undamaged TRISO particles, particles that have lost the outer pyrolytic carbon layer and ZrO2 kernels. Secondly, we studied the thermal shock method by immerging successively graphite into liquid nitrogen and hot water to cause fracturing of the compact. This produced particles and graphite fragments with diameter ranging from several centimetres to less than 500 μm. This relatively simple and economic method may potentially be considered as a pre-treatment step and be coupled with other method(s) before reprocessing and recycling for example.

Considering the need to reduce waste production and greenhouse emissions by still keeping high energy efficiency, various 4(th) generation nuclear energy systems have been proposed. As far as graphite moderated reactors are concerned, one of the key issues is the large volumes of irradiated graphite encountered (1770 m(3) for fuel elements and 840 m(3) for reflector elements during the lifetime (60 years) of a single reactor module [1]). With the objective to reduce volume of waste in the HTR concept, it is very important to be able to separate the fuel from low level activity graphite. This requires to separate TRISO particles from the graphite matrix with the sine qua non condition to not break TRfSO particles in case of future embedding of particles in a matrix for disposal.According to National Regulatory Systems, in case of limited graphite waste production or of short duration HTR projects (e.g. in Germany), direct disposal without separation is acceptable. Nevertheless, in case or large scale deployment of HTR technology, such approach is not economical and sustainable. Previous attempts in graphite management (furnace, fluidised bed and laser incinerations and encapsulation matrices) dealt with graphite matrix only. These are the reasons why we studied the management of irradiated compact-type fuel element. We simulated the presence of fuel in the particles by using ZrO(2) kernels. Compacts with ZrO(2) TRISO particles were manufactured by AREVA NP.Two original methods have been studied. First, we tested high pressure jet to erode graphite and clean TRISO particles. Best erosion rate reached about 0.18 kg/h for a single nose ending. Examination of treated graphite showed a mixture of undamaged TRISO particles, particles that have lost the outer pyrolytic carbon layer and ZrO(2) kernels. Secondly, we studied the thermal shock method by immerging successively graphite into liquid nitrogen and hot water to cause fracturing of the compact. This produced particles and graphite fragments with diameter ranging from several centimetres to less than 500 mu m. This relatively simple and economic method may potentially be considered as a pretreatment step and be coupled with other method(s) before reprocessing and recycling for example.

Dans un souci de reduction du volume de dechets nucleaires et de revalorisation des matieres combustibles, une strategie de gestion du combustible des reacteurs a haute temperature (HTR) est developpee dans cette etude. La reduction de volume passe par la separation des particules TRISO hautement radioactives et du graphite faiblement radioactif (les deux etant reunis dans un assemblage de combustible appele compact) tandis que le recyclage total necessite la separation du coeur de la particule, valorisable, et de sa gangue, dechet ultime. Les methodes de separation doivent preserver l'integrite des TRISO afin d'empecher la fuite des radioelements. Ainsi, le traitement de choc thermique entre l'azote liquide et l'eau chaude permet une division partielle des compacts mais ne permet de recuperer que peu de particules. L'erosion du graphite par jet d'eau a haute pression presente le risque de fracturer les particules. La combustion totale du carbone libere toutes les billes. Le traitement des compacts par les ultrasons dans l'eau erode le graphite en fonction de l'intensite de travail, des direction et distance d'attaque, de la temperature et du gaz de saturation, nettoyant les particules. L'attaque acide des compacts par un melange H2O2 + H2SO4 provoque l'intercalation du graphite par l'acide, faisant gonfler la structure et liberant ainsi les billes intactes. Les TRISO d'une part et leurs gangues d'autre part ont ensuite ete vitrifiees par frittage de maniere a obtenir une forte densite, jusqu'a un taux de 25% vol. Enfin, la lixiviation des composites dans l'eau ultrapure a 90°C montre de fortes proprietes de confinement.

For various countries, the direct disposal of high level nuclear fuel wastes is a key option for the backend of the fuel cycle. For HTR/VHTR reactors this is assumed for the introductory phase of this reactor system. However, closed fuel cycles or a separation of spent coated-particles from the graphite moderator and specific treatment, conditioning and disposal of these waste streams are also possible. In the European Community project “RAPHAEL”, fuel waste performance is going to be studied in depth, including post-irradiation fuel characterization, analysis of the stability and failure mechanism of coatings and of fuel kernels and overall performance of waste packages with compact fuel and/or only with fuel particles in geological disposal environments. Different confinement matrices for separated fuel particles (vitrification, SiC, ZrO2) have been adapted to limit release of radionuclides into groundwater at low temperatures over geological time spans. The investigations are limited to Low-Enriched Uranium (LEU) fuel with uranium oxide and uranium oxycarbide kernels that will allow higher burn-up, but may be more susceptible to leaching.