Spectroscopie mécanique de la relaxation des contraintes d'interface dans les composites à matrice métallique

Metal matrix composites are known for their excellent specific mechanical properties. Nevertheless, because of the thermal expansion mismatch between the matrix and the reinforcements, thermal stresses arise at the interfaces of such materials. In order to relax these stresses, dislocations are emitted from the interface and propagate in the matrix, which can influence the mechanical behavior of the composite material. Mechanical spectroscopy, which characterizes the ability of a solid to dissipate energy under an external excitation, was used to study the relaxation mechanisms occurring at the interface of MMCs. Indeed, the mobile matrix dislocations near the interface are the main sources of damping, making this technique very sensitive to these mechanisms. Composites studied here were processed by gas-pressure infiltration of the reinforcement preform by the molten metal. The processed composites were based on aluminum, magnesium and Mg-2%Si alloy, which is known for its high damping level. They were reinforced by short misorientated alumina fibers or long and aligned carbon and silicon carbide fibers. In the case of long fibers, two types of orientation were obtained: parallel or perpendicular to the composite axis. In order to modify the interface morphology, a heat treatment was carried out on aluminium matrix composites reinforced by alumina fibers during the infiltration process. By increasing the contact time between the molten matrix and the fibers, alumina crystals were formed on the surface of the fibers, whose size increased with time. The specific elastic moduli of the processed composites were clearly superior to those of matrix alone and their value agreed well with the existing theoretical models. Composites were submitted to thermal cycles from 120 K to 500 K and the internal friction and dynamical modulus were measured as a function of temperature. It was shown that the behavior of these two parameters, the large maximum at low temperatures and the modulus anomaly in the case of magnesium composites was driven by the motion of the dislocations activated in the matrix in order to relax thermal stresses. The internal friction was also characterized by a transient contribution, depending on the heating or cooling rate dT/dt and the excitation frequency ω. By using a model developed by Mayencourt and al., it was possible to determine two parameters C1 and C2 which were sensitive respectively to the mobile dislocation density relaxing the thermal stresses and the interface strength. The difference in the behavior of these two parameters as a function of temperature tended to show the great potential of the magnesium matrix composites. Indeed, the interface strength was decreasing at low temperature, allowing a better toughness whereas it was increasing at high temperature, improving the creep resistance. Finally, in the case of the aluminum matrix composites reinforced by short fibers with alumina crystals at the interface, it was observed that the elastic modulus was decreasing as crystal size increased. However, when the crystals exceeded a certain size, they were acting as pinning points between matrix and fibers, resulting in the increase of the modulus almost to its initial value. By determining the parameter C1 from the transient damping, it was shown that this parameter followed the same trend as the elastic modulus, which made it a good probe for the interface quality

The certification of the mass fraction of carbon in cementite grains in a Fe-C matrix: IRMM-471

The report describes the production and certification of IRMM-471, a reference material certified for the carbon mass fraction of its cementite (Fe3C) grains. The Fe3C grains are dispersed within an iron pearlite matrix and present an average grain diameter between 20 µm and 50 μm. IRMM-471 has been produced and certified in order to be used as calibrant in electron probe micro-analyser (EPMA) for carbon determination in iron and steel products.JRC.D.2-Standards for Innovation and sustainable Developmen

Different Mi-2 Complexes for Various Developmental Functions in Caenorhabditis elegans

Biochemical purifications from mammalian cells and Xenopus oocytes revealed that vertebrate Mi-2 proteins reside in multisubunit NuRD (Nucleosome Remodeling and Deacetylase) complexes. Since all NuRD subunits are highly conserved in the genomes of C. elegans and Drosophila, it was suggested that NuRD complexes also exist in invertebrates. Recently, a novel dMec complex, composed of dMi-2 and dMEP-1 was identified in Drosophila. The genome of C. elegans encodes two highly homologous Mi-2 orthologues, LET-418 and CHD-3. Here we demonstrate that these proteins define at least three different protein complexes, two distinct NuRD complexes and one MEC complex. The two canonical NuRD complexes share the same core subunits HDA-1/HDAC, LIN-53/RbAp and LIN-40/MTA, but differ in their Mi-2 orthologues LET-418 or CHD-3. LET-418 but not CHD-3, interacts with the Krüppel-like protein MEP-1 in a distinct complex, the MEC complex. Based on microarrays analyses, we propose that MEC constitutes an important LET-418 containing regulatory complex during C. elegans embryonic and early larval development. It is required for the repression of germline potential in somatic cells and acts when blastomeres are still dividing and differentiating. The two NuRD complexes may not be important for the early development, but may act later during postembryonic development. Altogether, our data suggest a considerable complexity in the composition, the developmental function and the tissue-specificity of the different C. elegans Mi-2 complexes

Differing Requirements for RAD51 and DMC1 in Meiotic Pairing of Centromeres and Chromosome Arms in Arabidopsis thaliana

During meiosis homologous chromosomes pair, recombine, and synapse, thus ensuring accurate chromosome segregation and the halving of ploidy necessary for gametogenesis. The processes permitting a chromosome to pair only with its homologue are not fully understood, but successful pairing of homologous chromosomes is tightly linked to recombination. In Arabidopsis thaliana, meiotic prophase of rad51, xrcc3, and rad51C mutants appears normal up to the zygotene/pachytene stage, after which the genome fragments, leading to sterility. To better understand the relationship between recombination and chromosome pairing, we have analysed meiotic chromosome pairing in these and in dmc1 mutant lines. Our data show a differing requirement for these proteins in pairing of centromeric regions and chromosome arms. No homologous pairing of mid-arm or distal regions was observed in rad51, xrcc3, and rad51C mutants. However, homologous centromeres do pair in these mutants and we show that this does depend upon recombination, principally on DMC1. This centromere pairing extends well beyond the heterochromatic centromere region and, surprisingly, does not require XRCC3 and RAD51C. In addition to clarifying and bringing the roles of centromeres in meiotic synapsis to the fore, this analysis thus separates the roles in meiotic synapsis of DMC1 and RAD51 and the meiotic RAD51 paralogs, XRCC3 and RAD51C, with respect to different chromosome domains

Comparison between Centrifugal Sedimentation and Dynamic Light Scattering for Nanoparticle Sizing

The Institute for Reference Materials and Measurements (IRMM) has produced and distributes a series of reference materials with certified particle size. IRMM has the intention to broaden the range of these certified particle size reference materials to the nanoparticles range. To perform nanoparticle sizing, a variety of instruments exists, each based on different physical principles. The certification process has started with an evaluation of two of the most common nanoparticle sizing techniques: centrifugal sedimentation and dynamic light scattering. The main difference between these techniques is the resolution for multi-modal particle size distributions. due to the separation process occurring in the disc centrifuge, centrifugal sedimentation has an excellent resolution and is able to resolve modes with a diameter rratio as low as 1.1 whereas dynamic light scattering is not able to resolve modes with a diameter ratio lower than 4. However, dynamic light scattering is based on first principles and calibration is not needed as for centrifugal sedimentation for which the sedimentation velocity must be calibrated. This paper will present the results of the comparison tests, illustrating the advantages and drawbacks associated with each technique and it will show how these two techniques can complement each other in the certification process.JRC.D.2-Reference material

Measurement of the Size of Spherical Nanoparticles by Means of Atomic Force Microscopy

Several techniques are nowadays available to determine the size distribution of nanoparticulate matter. Among these techniques, atomic force microscopy (AFM) is especially valuable because it can provide three-dimensional information on the shape of individual nanoparticles. This paper describes a new method to determine the size distribution of a population of spherical nanoparticles deposited on a hard substrate. The method is based on the acquisition and analysis of topographical AFM images. The size of individual nanoparticles is obtained by fitting the topographical region associated with the nanoparticle with a sphere. Tests on model systems based on nanoparticle reference materials consisting of polystyrene (PS) latex suspensions show promising results. The measured mean particle size is larger than the reference value, but this is a predictable effect of the AFM tip shape. Tests on a bi-modal mixture of two PS latex reference materials show the impact of the quality of the dispersion of the nanoparticles on the results obtained with the new technique.JRC.D.2-Reference material

Comparison of Dynamic Light Scattering and Centrifugal Sedimentation for Nanoparticle Sizing

To support the efforts of increasing confidence in the comparability of measurements in the field of nanotechnology, the Institute of Reference Materials and Measurements (IRMM) intends to produce reference materials with certified particle size in the nanometer range. Several techniques, which are based on different physical principles, have been developed to perform nanoparticle sizing. As a starting point in the certification of nanoparticle reference materials, IRMM is evaluating the differences between results obtained with two of the most commonly used nanoparticle sizing techniques: dynamic light scattering (DLS) and centrifugal sedimentation. The main difference between these techniques is their power to resolve multi-modal particle size distributions. Due to the separation process occurring in the disc centrifuge, centrifugal sedimentation has an excellent resolution and is able to resolve modes with a diameter ratio as low as 1.1 whereas dynamic light scattering is not able to resolve modes with a diameter lower than 4. However, dynamic light scattering is based on first principles and calibration is not needed as for centrifugal sedimentation, for which the sedimentation velocity must be calibrated. This paper will present experimental results, illustrating the advantages and drawbacks associated with each technique and it will show how these two techniques can complement each other in the certification process.JRC.D.2-Reference material

Measurement of the Size of Spherical Nanoparticles by Means of Atomic Force Microscopy

Several techniques are nowadays available to determine the size distribution of nanoparticulate matter. Among these techniques, atomic force microscopy (AFM) is especially valuable because it can provide three-dimensional information on the shape of individual nanoparticles. This paper describes a new method to determine the size distribution of a population of spherical nanoparticles deposited on a hard substrate. The method is based on the acquisition and analysis of topographical AFM images. The size of individual nanoparticles is obtained by fitting the topographical region associated with the nanoparticle with a sphere. Tests on model systems based on nanoparticle reference materials consisting of polystyrene (PS) latex suspensions show promising results. The measured mean particle size is larger than the reference value, but this is a predictable effect of the AFM tip shape. Tests on a bi-modal mixture of two PS latex reference materials show the impact of the quality of the dispersion of the nanoparticles on the results obtained with the new technique.JRC.D.2-Reference material