Unveiling the impact of the effective particles distribution on strengthening mechanisms: A multiscale characterization of Mg+Y2O3 nanocomposites

International audienceMost models used to account for the hardening of nanocomposites only consider a global volume fraction of particles which is a simplified indicator that overlooks the particles size and spatial distribution. The current study aims at quantifying the effect of the real experimental particles spatial and size distribution on the strengthening of a magnesium based nanocomposites reinforced with Y 2 O 3 particles processed by Friction Stir Processing (FSP). X-ray tomographic 3-D images allowed to identify the best FSP parameters for the optimum nanocomposite. A detailed analysis indicates that the observed hardening is mainly due to Orowan strengthening and the generation of geometrically necessary dislocations (GND) due to thermal expansion coefficients (CTE) mismatch between magnesium and Y 2 O 3 particles. A multiscale characterization coupling 3D X-ray laboratory, synchrotron nanoholotomography and transmission electron microscopy (TEM) has been used to investigate particles size and spatial distribution over four orders of magnitude in length scales. Two dedicated micromechanical models for the two strengthening mechanisms are applied on the experimental particle fields taking into account the real particles size and spatial distribution, and compared to classical models based on average data. This required to develop a micromechanical model for CTE mismatch hardening contribution. This analysis reveals that the contribution from CTE mismatch is decreased by a factor two when taking into account the real distribution of particles instead of an average volume fraction

Pore morphology of polar firn around closure revealed by X-ray tomography

Understanding the slow densification process of polar firn into ice is essential in order to constrain the age difference between the ice matrix and entrapped gases. The progressive microstructure evolution of the firn column with depth leads to pore closure and gas entrapment. Air transport models in the firn usually include a closed porosity profile based on available data. Pycnometry or melting–refreezing techniques have been used to obtain the ratio of closed to total porosity and air content in closed pores, respectively. X-ray-computed tomography is complementary to these methods, as it enables one to obtain the full pore network in 3-D. This study takes advantage of this nondestructive technique to discuss the morphological evolution of pores on four different Antarctic sites. The computation of refined geometrical parameters for the very cold polar sites Dome C and Lock In (the two Antarctic plateau sites studied here) provides new information that could be used in further studies. The comparison of these two sites shows a more tortuous pore network at Lock In than at Dome C, which should result in older gas ages in deep firn at Lock In. A comprehensive estimation of the different errors related to X-ray tomography and to the sample variability has been performed. The procedure described here may be used as a guideline for further experimental characterization of firn samples. We show that the closed-to-total porosity ratio, which is classically used for the detection of pore closure, is strongly affected by the sample size, the image reconstruction, and spatial heterogeneities. In this work, we introduce an alternative parameter, the connectivity index, which is practically independent of sample size and image acquisition conditions, and that accurately predicts the close-off depth and density. Its strength also lies in its simple computation, without any assumption of the pore status (open or close). The close-off prediction is obtained for Dome C and Lock In, without any further numerical simulations on images (e.g., by permeability or diffusivity calculations).</p

Particle-induced morphological modification of Al alloy equiaxed dendrites revealed by sub-second in situ microtomography

The study of dendritic growth is a challenging topic at the heart of intense research in material science. Understanding such processes is of prime importance as it helps predicting the final microstructure governing material properties. In the specific case of the design of metal-matrix nanocomposites (MMNCs), the addition of nano-sized particles inside the metallic melt increases the complexity as their influence on the growth morphology of dendrites is not yet fully understood. In the present experimental study, we use in situ X-ray tomography imaging with very high temporal resolution (0.35 s per 3D image) coupled with in situ ultrasonic melt homogenisation to record, in 3D and real time, the free growth at high cooling rates (~2 K.s-1) of equiaxed dendrites in an AA6082 alloy containing Y2O3 nanoparticles. The careful 3D analysis of the dendrite morphologies as well as their solidification dynamics reveals that in the case of well-dispersed particles, dendrite equiaxed growth occurs through complex hyper-branched morphologies. Such behaviour is believed to arise from particle-induced modification of the solidification processes at the origin of multiple splitting, branching and curving mechanisms of the dendrite arms. These results shed light on long-standing empirical and modelling statements and open new ways for direct investigation of equiaxed growth in metallic alloys and composites.European Commission in the 7th Framework Program (contract FP7-NMP3-LA-2012-280421) ExoMet Project by the European Space Agency and by the individual partner organizations; ESRF-MA1876 long term project

A Model for the Development of the Rhizobial and Arbuscular Mycorrhizal Symbioses in Legumes and Its Use to Understand the Roles of Ethylene in the Establishment of these two Symbioses

We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis–outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants

Structural properties of solid foams

International audienc

Sintered hollow spheres: Random stacking behaviour under uniaxial tensile loading

International audienceThe behaviour of stainless steel hollow sphere foams under uniaxial tensile loading is characterized both at the macroscopic scale and at the scale of the individual cell. Three foams made by stacking and sintering of stainless steel hollow spheres were investigated. The cell radius size was kept constant, as was the neck size between spheres, while the density was varied by changes to the shell thickness. Two failure modes were observed under tensile loading: neck failure and shell-tearing