Three-dimensional full-field X-ray orientation microscopy

International audienceA previously introduced mathematical framework for full-field X-ray orientation microscopy is for the first time applied to experimental near-field diffraction data acquired from a polycrystalline sample. Grain by grain tomographic reconstructions using convex optimization and prior knowledge are carried out in a six-dimensional representation of position-orientation space, used for modelling the inverse problem of X-ray orientation imaging. From the 6D reconstruction output we derive 3D orientation maps, which are then assembled into a common sample volume. The obtained 3D orientation map is compared to an EBSD surface map and local misorientations, as well as remaining discrepancies in grain boundary positions are quantified. The new approach replaces the single orientation reconstruction scheme behind X-ray diffraction contrast tomography and extends the applicability of this diffraction imaging technique to material micro-structures exhibiting sub-grains and/or intra-granular orientation spreads of up to a few degrees. As demonstrated on textured sub-regions of the sample, the new framework can be extended to operate on experimental raw data, thereby bypassing the concept of orientation indexation based on diffraction spot peak positions. This new method enables fast, three-dimensional characterization with isotropic spatial resolution, suitable for time-lapse observations of grain microstructures evolving as a function of applied strain or temperature

Uniaxial compression of calcite single crystals at room temperature: insights into twinning activation and development

E-twinning is a common plastic deformation mechanism in calcite deformed at low temperature. Strain rate, temperature and confining pressure have negligible effects on twinning activation which is mainly dependent on differential stress. The critical resolved shear stress (CRSS) required for twinning activation is dependent on grain size and strain hardening. This CRSS value may obey the Hall–Petch relation, but due to sparse experimental data its actual evolution with grain size and strain still remains a matter of debate. In order to provide additional constraints on twinning activation and development, new mechanical tests were carried out at room temperature on unconfined single crystals of calcite, with different sizes and crystallographic orientations. Uniaxial deformation was performed at a controlled displacement rate, while the sample surface was monitored using optical microscopy and a high-resolution CCD (charge-coupled device) camera. The retrieved macroscopic stress–strain behavior of the crystals was correlated with the surface observations of the deformation process. Results show (1) the onset of crystal plasticity with the activation of the first isolated mechanical twins during the strain hardening stage, and (2) the densification and thickening of twin lamellae during the steady-state flow stress stage. Such thickening of twin lamellae at room temperature emphasizes that calcite twin morphology is not controlled solely by temperature. The different values for the CRSS obtained for the activation of isolated twins and for the onset of twin densification and thickening raises questions regarding the appropriate value to be considered when using calcite twin data for stress inversion purposes.</p

Transcriptome 3′end organization by PCF11 links alternative polyadenylation to formation and neuronal differentiation of neuroblastoma

In gene regulation, diversification at the transcriptome 3′end is linked to differentiation and dedifferentiation. Here, the authors discover extensive transcriptome 3′end-alterations in neuroblastoma, regulated by PCF11, and provide an interactive data repository of transcriptome-wide alternative polyadenylation

Effect of the p

The degradation behaviour of two InGaAs/AlGaAs laser structures differing only in the Zn doping concentration of the p+ GaAs contact layer has been compared. The ageing tests used in this comparison are performed on lasers with Anti-Reflection (AR) coatings on both facets, so as to increase the carrier density in the quantum well and the gradual degradation rate. This kind of ageing test has been discussed in a previous paper, where a possible effect of Zn diffusion was suspected. The defects generated during ageing are studied by Low Temperature (80 K)—Spectrally Resolved Cathodo-Luminescence (LT-SRCL), Cathodo-Luminescence Imaging (CLI) and Transmission Electron Microscopy (TEM). The impact of the acceptor concentration in the contact layer is thus clarified

ELIMINATION OF DISLOCATIONS IN GaAs SINGLE CRYSTALS

Pour plusieurs applications technologiques, des cristaux semiconducteurs de grande taille sont demandés. Divers types de défauts (ponctuels et étendus) sont présents dans ces cristaux. Dans le cas du GaAs, on sait que la présence de dislocations diminue fortement la durée de vie des lasers Ga1-xAlxAs/GaAs. De plus, de récentes études montrent que les propriétés électroniques des circuits intégrés au GaAs sont influencées par l'existence de dislocations. Leur présence dans les cristaux synthétisés par la méthode LEC (Liquid Encapsulated Czochralski) est essentiellement due aux contraintes thermiques existant pendant la croissance. Dans ce papier, nous présentons des résultats expérimentaux montrant qu'il est possible de réduire fortement (un facteur 10 au moins) la densité de dislocations, soit en diminuant le niveau de contraintes thermiques (technique Kyropoulos sous encapsulation liquide), soit en ajoutant des impuretés isoélectroniques (Indium) dans le bain liquide.Large size semiconductor crystals are being requested in several device-oriented technologies. These crystals exhibit various defects (point and extended defects). Indeed, it is well-known in the case of GaAs that the presence of dislocations strongly decreases the life-time of Ga1-xAlxAs/ GaAs lasers. Moreover, it appears now, that the dislocations have a detrimental effect on GaAs integrated circuits. Their presence in LEC (Liquid Encapsulated Czochralski) crystals are mainly due to the thermal stresses occurring during the growth. We present in this paper some experimental results showing that it is possible to strongly reduce (by a factor 10, at least) the dislocation density either by decreasing the thermal stresses level (Liquid Encapsulated Kyropoulos Technique) or by adding an isoelectronic impurity (indium to the melt)

Comparison of microstructure features and mechanical properties for additive manufactured and wrought nickel alloys 625

International audienceThe microstructure characteristics as well as the mechanical properties of an Inconel 625 alloy obtained by three processes: forging, SLM and LMD, are investigated. For the last two processes, known as "additive manufacturing", the influence of printing parameters is considered as well as the role of possible heat treatments. First, microstructural analyses (SEM and EBSD) underline the presence of columnar dendrites with a very heterogeneous grain size for additive manufactured as-built materials. The microstructures appear highly textured, particularly for SLM ones which are also often finer than the ones obtained by LMD. Heat treatments and particularly a 1h-1100 • C annealing is proven to improve the printed parts microstructure and to avoid a drastic decrease in terms of ductility, particularly for LMD parts. The LMD process with controlled laser power, coupled with appropriate heat treatment, finally produces materials with both microstructures and tensile mechanical properties close to or better than those of the wrought alloy

Deformation of aluminum in situ SEM and full field measurements by digital image correlation: evidence of concomitant crystal slip and grain boundary sliding

Mechanical testing in situ scanning electron microscopy (SEM) has become a standard technique for multiscale micromechanical investigation of polycrystalline materials. Direct observation of developing strain heterogeneities allows identification of the active mechanisms and quantification of their respective contributions to the overall strain. We developed a novel experimental setup for thermomechanical testing in situ SEM, especially suited to full strain field measurements. These are based on digital image correlation (DIC), from the sample scale to the scales of the aggregate and the single grain. We present results obtained during simple compression, at controlled displacement rates and at temperatures up to 400°C, of nearly pure polycrystalline aluminum exhibiting randomly oriented coarse grains (ca. 300 m in size). Electron microlithography was applied to produce specific surface marking patterns appropriate for the different scales of interest. Full surface strain fields were obtained by digital image correlation (DIC) analysis. The localization patterns evidenced dominant crystal slip plasticity, but also substantial simultaneous and continuous activity of grain boundary sliding (GBS), the contribution of which increased with temperature. We therefore advocate that experiments such as these here presented are necessary to go beyond a description in terms of deformation mechanism maps, which attribute deformation to a single mechanism

A synchrotron transmission X-ray microscopy study on precipitate evolution during solid-state thermal cycling of a stainless steel

International audienceDuring additive manufacturing of stainless steels, sub-micron sized oxide (i.e., MnSiO3 , SiO2 , and CrMn2O4) and non-oxide (i.e., sulfide, in particular MnS, and possibly carbides, phosphides and nitrides) precipitates form during solidification. But do they evolve during the subsequent solid-state thermal cycling (SSTC) until the end of the printing process? A recent study on subjecting thin-film lamellae extracted from an additively manufactured stainless steel to heating-cooling treatments inside a transmission electron microscope (TEM) confirmed that precipitate composition can indeed evolve during SSTC. However, that study could not provide any conclusive evidence on precipitate volume fraction, density, and size evolution. In this work, we have quantified these changes using a novel experimental procedure combining (i) micropillar extraction from an additively manufactured stainless steel, (ii) subjecting them to different SSTC (including annealing) inside a TEM, (iii) performing synchrotron transmission X-ray microscopy to identify precipitates, and (iv) using a machine learning model to segment precipitates and quantify precipitate volume fraction, density, and size. Comparing these quantities before and after each SSTC/annealing sequence reveals that new oxides nucleated during rapid SSTC with high maximum temperature. However, during slow SSTC with high maximum temperature and annealing, precipitates dissolve because of oxygen evaporation during SSTC inside the TEM. A new empirical relationship correlating precipitate sizes and cooling rates is proposed. It is in good agreement with data collected from conventional casting, directed energy deposition, and powder bed fusion processes