Understanding the Interdependence of Penetration Depth and Deformation on Nanoindentation of Nanoporous Silver

International audienceA silver-based nanoporous material was produced by dealloying (selective chemical etching) of an Ag 38.75 Cu 38.75 Si 22.5 crystalline alloy. Composed of connected ligaments, this material was imaged using a scanning electron microscope (SEM) and focused ion-beam (FIB) scanning electron microscope tomography. Its mechanical behavior was evaluated using nanoindentation and found to be heterogeneous, with density variation over a length scale of a few tens of nanometers, similar to the indent size. This technique proved relevant to the investigation of a material's mechanical strength, as well as to how its behavior related to the material's microstructure. The hardness is recorded as a function of the indent depth and a phenomenological description based on strain gradient and densification kinetic was proposed to describe the resultant depth dependence

Nanoindentation: A powerful tool to explore the wide chemical space of high entropy alloys

High entropy alloys (HEA) are multi-component alloys, without any minor or major elements (i.e. : all elements are very concentrated) and they form a unique solid solution. It was proven that, especially for the system Co-Cr-Fe-Mn-Ni, they exist for a very wide range of composition [1]. This opens the opportunity of multi-properties optimization, like cost, density and mechanical resistance. However, to take advantage of this opportunity, accelerated mechanical testing tools are required. Please click Additional Files below to see the full abstract

Fuel cell electrodes from organometallic Pt precursors: an easy atmospheric plasma approach

An organometallic powder (platinum (II) acetylacetonate) is decomposed in the post-discharge of an atmospheric RF plasma torch to deposit Pt nanoparticles on carbon black supports. The resulting nanohybrid materials are characterized by FEG-SEM and XPS techniques to highlight their high content in Pt, their oxidation degree, and the dispersion of the Pt nanoparticles on the substrate. ICP-MS and electrochemical characterizations in a single fuel cell (cyclic voltammetry, dynamic polarization curves) are also performed on electrodes realized by treating the powder mixture overlaid on gas diffusion layers. The comparison of the catalytic activity and the Pt loading with commercially available electrodes shows the great potential of this simple innovative, fast, and robust deposition method

Cyclic plasticity and fatigue damage of CrMnFeCoNi high entropy alloy fabricated by laser powder-bed fusion

The CrMnFeCoNi high-entropy alloy is highly printable and holds great potential for structural applications. However, no significant discussions on cyclic plasticity and fatigue damage in previous studies. This study provides significant insights into the link between print processes, solidification microstructure, cyclic plasticity and fatigue damage evolution in the alloy fabricated by laser powder bed fusion. Thermodynamics-based predictions (validated by scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDX)) showed that Cr, Co and Fe partition to the core of the solidification cells, whilst Mn and Ni to the cell boundaries in all considered print parameters. Both dislocation slip and deformation twinning were found to be responsible for plastic deformation under monotonic loading. However, the former was found to be the single dominant mechanism for cyclic plasticity. The surface finish helped to substantially delay the crack initiation and cause lack-of-fusion porosity to be the main source of crack initiation. Most significantly, the scan strategies significantly affect grain arrangements and grain dimensions, leading to noticeable effects on fatigue crack propagation; in particular, the highest resistance crack propagation was seen in the meander scan strategy with 0° rotation thanks to the most columnar grains and the smallest spacing of grain boundaries along the crack propagation path

Synthèse et caractérisation d'un acier ODS préparé par un procédé inspiré du broyage réactif - Etude de l'influence des conditions de broyage et recuit.

In the framework of ODS (Oxide Dispersion Strengthened) steels development for application as core materials in future nuclear reactors, a new process based on a reactive-inspired ball-milling, which consists in using YFe3 and Fe2O3 instead of Y2O3, and the influence of synthesis conditions on nano-oxides features were studied. To do so, ODS steels were prepared by ball-milling and annealing and then multi-scale characterizations were done after each step of the process. Atom probe tomography and small angle neutron scattering were used. It was shown that reactive-inspired ball-milling process was efficient for ODS steel synthesis, although it does not modify nano-oxides features. Broadly speaking, the nature of milling reactants has no impact on nano-oxides formation. Moreover it was proven that nano-oxides nucleation can start during ball-milling and afterwards it continues during annealing with a very fast kinetic. The features of nano-oxides formed in this way can be controlled through variations of ball-milling parameters (intensity, temperature and atmosphere) and annealing parameters (duration and temperature). Finally, it was shown that controlling ball-milling conditions was necessary to control the ODS steel final state.Dans le cadre du développement des aciers ODS (Oxide Dispersion Strengthened) pour le cœur des réacteurs nucléaires du futur, ce travail s'est focalisé sur l'étude d'une nouvelle voie de synthèse inspirée du broyage réactif, consistant à remplacer le réactif Y2O3 par YFe3 et Fe2O3, et sur l'influence des conditions de synthèse sur les caractéristiques des nano-oxydes. Pour cela, des aciers ODS ont été synthétisés par broyage et recuit et des caractérisations multi-échelles ont été effectuées après chaque étape de la synthèse. En particulier, on a utilisé la sonde atomique tomographique et la diffusion des neutrons aux petits angles. On a ainsi montré que le procédé inspiré du broyage réactif permet la synthèse d'un acier ODS mais qu'il ne modifie pas les caractéristiques des nano-oxydes. Plus généralement, la forme des réactifs de broyage n'a pas d'influence sur la formation des nano-oxydes. Ensuite, on a mis en évidence que la germination des nano-oxydes pouvait commencer au cours du broyage avant de se poursuivre avec une cinétique très rapide au cours du recuit. Les caractéristiques des nano-oxydes ainsi formés peuvent être contrôlées en faisant varier les paramètres de broyage (intensité, température et atmosphère) et de recuit (durée et température). Enfin, on a montré qu'il était nécessaire de contrôler les conditions de broyage pour maîtriser l'état final d'un acier ODS

Precipitation and Hardness of Carbonitrides in a CrMnFeCoNi High Entropy Alloy

International audienceIn order to explore precipitation hardening, carbonitrides precipitates are formed within a CrMnFeCoNi high entropy alloy (HEA). The matrix and the precipitates are independently characterized by electron probe microanalysis, atom probe tomography, transmission electron microscopy, and nanoindentation. The matrix is a face-centered cubic multi-component non-equimolar solid-solution, that is, Cr depleted, which is stable and interestingly as hard as its equimolar counterpart. The precipitates are stable chromium carbonitrides with a micron-scale platelets morphology. They exhibit a hexagonal structure, are semi-coherent with the matrixm and are harder than the matrix

Vers une amélioration du recyclage des métaux et alliages métalliques

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