Evolution of the dendritic morphology with the solidification velocity in rapidly solidified Al- 4.5wt.%Cu droplets

International audienceThe microstructure morphology of Al-4.5wt.%Cu droplets formed by the Impulse Atomization technique is investigated. Three-dimensional reconstructions by synchrotron X-ray micro-tomography of several droplets reveal different morphologies in droplets of similar diameter and produced in the same batch. Moreover, microstructural features also indicate that the development of the dendrite arms occurs in some droplets along crystallographic axes instead of the usual directions observed in conventional casting for the same alloy. It has been observed that such an unusual growth direction of the dendrites is directly related to the solidification velocity. We underpin these results by carrying out comparisons with a solidification model. Predictions are used to discuss the change of dendrite growth direction, as well as the existence of a dendrite growth direction range for a given type of droplets. In addition, the effect of the droplet size and the cooling gas on the dendrite growth direction range observed experimentally is also investigated by using the model. 1. Introduction Rapid solidification techniques have been developed as they enable to obtain a wide variety of structures which cannot be formed under conventional solidification processes [1]. They differ by the way to form the liquid as a strip or a droplet and by the method of heat extraction. Atomization techniques are used to make metallic powders which are used for making a desired object by pressing or by sintering [2]. The liquid metal generated as a stream breaks up into droplets by Rayleigh-Plateau instability, which subsequently solidify in a much colder medium. In the Impulse Atomization (IA) technique the liquid is pushed through a nozzle plate to form the liquid streams [3]. In order to deepen the understanding of the microstructure formation in the droplets, synchrotron X-ray micro-tomography was carried out at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). Three-dimensional reconstructions of a large number of droplets were obtained, enabling the inner microstructure of the droplets to be statistically analysed for the first time. In a previous paper, we showed that four distinct morphologies could be identified in droplets of the same size and from the same batch [4]. Such a range of morphologies can be linked to a range of solidification velocities for the droplets. Indeed, while Rappaz and co-workers highlighted the <100

Dendrite growth morphologies in rapidly solidified Al-4.5wt.%Cu droplets

International audienceThe impulse atomization process developed at the University of Alberta (Canada) enables metallic powders to be solidified with controlled process parameters and improved properties. In order to investigate the microstructure morphologies in droplets of Al- 4.5wt.%Cu alloys, three-dimensional reconstructions of several droplets are obtained by using synchrotron X-ray micro-tomography, allowing a visualization of the inner microstructure in three dimensions. The analysis of the reconstructed volumes reveals that a wide range of morphology, from highly branched to "finger-bundle", can be obtained for different droplets of similar diameter and produced in the same batch. Unexpectedly for this alloy, microstructural features also indicate that the development of the dendrite arms (primary and of higher orders) occurs in most droplets along crystallographic axes, instead of the usual directions observed in conventional casting technologies

Simultaneous X-ray radiography and diffraction topography imaging applied to silicon for defect analysis during melting and crystallization

International audienceSynopsis A setup for simultaneous, time-resolved X-ray radiography and diffraction topography imaging is presented. It is used to study defect generation and growth mechanisms during heating, solidification and cooling of a silicon crystal. Abstract One of the key issues to be resolved to improve the performance of silicon solar cells is to reduce crystalline defect formation and propagation during the growth process fabrication step. For this purpose, the generation of structural defects such as grain boundaries and dislocations in silicon must be understood and characterised. We combine in situ X-ray diffraction imaging, historically named topography, with radiography imaging to analyse the development of crystal defects before, during and after crystallisation. Two individual indirect detector systems are implemented to record simultaneously the crystal structure (topographs) and the solid-liquid morphology evolution (radiographs) at high temperature. This allows for a complete synchronisation of the images and for an increased image acquisition rate compared to previous studies that used X-ray sensitive films to record the topographs. The experiments are performed with X-ray synchrotron radiation at beamline ID19 at the European Synchrotron Radiation Facility (ESRF). We present in situ observations of the heating, melting, solidification and holding stages of silicon samples to demonstrate that with the upgraded setup detailed investigations of time-dependent phenomena are now possible. The motion of dislocations is recorded during the entire experiment, so that their interaction with grain boundaries and their multiplication through the activation of Frank-Read sources can be observed. Moreover, the capability to record with two camera-based detectors allows for the study of the relationship between strain distribution, twinning and nucleation events. In conclusion, the simultaneous recording of topographs and radiographs has great potential for further detailed investigations of the interaction and generation of grains and defects that influence the growth process and the final crystalline structure in silicon and other crystalline materials

On the Deformation of Dendrites During Directional Solidification of a Nickel-Based Superalloy

International audienc

Investigation of subgrains in directionally solidified cast mono-seeded silicon and their interactions with twin boundaries

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

On the twinning impact on the grain structure formation of multi-crystalline silicon for photovoltaic applications during directional solidification

Grain orientation and competition during growth has been analyzed in directionally solidified multi-crystalline silicon samples. In situ and real-time characterization of the evolution of the grain structure during growth has been performed using synchrotron X-ray imaging techniques (radiography and topography). In addition, Electron Backscattered Diffraction has been used to reveal the crystalline orientations of the grains and the twin relationships. New grains formed during growth have two main origins: random nucleation and twinning. It is demonstrated that the solidified samples are dominated by P3 twin boundaries showing that twinning on {111} facets is the dominant phenomenon. Moreover, thanks to the in situ characterization of the growth, it is shown that twins nucleate on {111} facets located at the sides of the sample and at grain boundary grooves. The occurrence of multiple P3 twins during growth prevents the initial grains from developing all along the sample, and twin boundaries with higher order coincidence site lattices can form at the encounter of two grains in twin position. The grain competition phenomenon following nucleation and twinning acts as a grain selection mechanism leading to the final grain structure

X-ray Based in Situ Investigation of Silicon Growth Mechanism Dynamics—Application to Grain and Defect Formation

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

Regulation of Gene Expression in Plants through miRNA Inactivation

Eukaryotic organisms possess a complex RNA-directed gene expression regulatory network allowing the production of unique gene expression patterns. A recent addition to the repertoire of RNA-based gene regulation is miRNA target decoys, endogenous RNA that can negatively regulate miRNA activity. miRNA decoys have been shown to be a valuable tool for understanding the function of several miRNA families in plants and invertebrates. Engineering and precise manipulation of an endogenous RNA regulatory network through modification of miRNA activity also affords a significant opportunity to achieve a desired outcome of enhanced plant development or response to environmental stresses. Here we report that expression of miRNA decoys as single or heteromeric non-cleavable microRNA (miRNA) sites embedded in either non-protein-coding or within the 3′ untranslated region of protein-coding transcripts can regulate the expression of one or more miRNA targets. By altering the sequence of the miRNA decoy sites, we were able to attenuate miRNA inactivation, which allowed for fine regulation of native miRNA targets and the production of a desirable range of plant phenotypes. Thus, our results demonstrate miRNA decoys are a flexible and robust tool, not only for studying miRNA function, but also for targeted engineering of gene expression in plants. Computational analysis of the Arabidopsis transcriptome revealed a number of potential miRNA decoys, suggesting that endogenous decoys may have an important role in natural modulation of expression in plants

Nurses' perceptions of aids and obstacles to the provision of optimal end of life care in ICU

Contains fulltext : 172380.pdf (publisher's version ) (Open Access

On the interest of microgravity experimentation for studying convective effects during the directional solidification of metal alloys

International audienceUnder terrestrial conditions, solidification processes are often affected by gravity effects, which can significantly influence the final characteristics of the grown solid. The low-gravity environment of space offers a unique and efficient way to eliminate these effects, providing valuable benchmark data for the validation of models and numerical simulations. Moreover, a comparative study of solidification experiments on earth and in low-gravity conditions can significantly enlighten gravity effects. The aim of this paper is to give a survey of solidification experiments conducted in low-gravity environment on metal alloys, with advanced post-mortem analysis and eventually by in situ and real-time characterization. (C) 2016 Academie des sciences. Published by Elsevier Masson SAS