An Experimental Assessment of the α + α’ Miscibility Gap in Fe-Cr

International audiencePhase separation in the Fe-Cr system has been studied experimentally. The upper limit of the miscibility gap and phase separation kinetics during the early stages of decomposition was studied using Vickers micro-hardness tests and atom probe tomography. The results indicate that the upper limit of the miscibility gap in current thermodynamic descriptions is overestimated, but it is necessary to further analyze the transition across the upper limit of the miscibility gap before drawing conclusions. This work demonstrates the efficient combination of micro-hardness measurements and atom probe tomography to investigate the miscibility gap in the Fe-Cr system

Microstructure à fine échelle d alliages à propriétés de magnétorésistance géante (relation avec les propriétés magnétiques. Cas de Rubans Cu80FexNi20-x (x=5,10,15 at%))

Un matériau magnétorésistif voit sa résistance électrique varier lorsqu un champ magnétique lui est appliqué. Le but de cette étude est de relier les différents paramètres structuraux aux propriétés magnétiques et de magnétorésistance de systèmes granulaires de Cu80FexNi20-x (at%) élaborés sous forme de rubans. L influence de la composition de l alliage est d abord présenté. Puis, la microstructure d un ruban de Cu80Fe10Ni10(at%), présentant le maximum de magnétorésistance, a été précisément caractérisée de l échelle microscopique à l échelle atomique. Cet échantillon est constitué de précipités fcc riches en fer et en nickel cohérents avec la matrice riche en cuivre. L étude des propriétés magnétiques de ce ruban a permis de montrer que des interactions magnétiques sont présentes qui diminuent la MRG. Les paramètres structuraux (distributions de taille et de composition, fraction volumique et densité de précipités, largeur des interfaces ) ont été corrélés aux propriétés magnétiques.A magnetoresistive material is a material whose electrical restistance varies when a magnetic fiel is applied. The aim of this study is to link the different structural parameters with magnetic and giant magnetoresistance properties of granular Cu80FexNi20-x (at%) ribbons. First part, the effect of the composition os presented. Then, the microstructure of a Cu80Fe10Ni10 (at%) ribbon, presenting the highest magnetoresistance, was characterized precisely from a micrometer scale dow, to an atomic scale. The sample consists of fcc (Fe,Ni)-rich precipitates coherent with the matrix rich in copper. The study og the magnetic properties of this ribbon has shown that magnetic interactions are present that reduce MRG. Structural data (size and composition distributions, precipitates volume fractions and density, interface width...) were correlated with magnetic properties.ROUEN-BU Sciences (764512102) / SudocROUEN-BU Sciences Madrillet (765752101) / SudocSudocFranceF

An Experimental Assessment of the α + α’ Miscibility Gap in Fe-Cr

International audiencePhase separation in the Fe-Cr system has been studied experimentally. The upper limit of the miscibility gap and phase separation kinetics during the early stages of decomposition was studied using Vickers micro-hardness tests and atom probe tomography. The results indicate that the upper limit of the miscibility gap in current thermodynamic descriptions is overestimated, but it is necessary to further analyze the transition across the upper limit of the miscibility gap before drawing conclusions. This work demonstrates the efficient combination of micro-hardness measurements and atom probe tomography to investigate the miscibility gap in the Fe-Cr system

An Experimental Assessment of the α + α’ Miscibility Gap in Fe-Cr

International audiencePhase separation in the Fe-Cr system has been studied experimentally. The upper limit of the miscibility gap and phase separation kinetics during the early stages of decomposition was studied using Vickers micro-hardness tests and atom probe tomography. The results indicate that the upper limit of the miscibility gap in current thermodynamic descriptions is overestimated, but it is necessary to further analyze the transition across the upper limit of the miscibility gap before drawing conclusions. This work demonstrates the efficient combination of micro-hardness measurements and atom probe tomography to investigate the miscibility gap in the Fe-Cr system

Mechanism of Si3N4 precipitation in nitrided Fe-Si alloys: A novel example of particle-stimulated-nucleation

International audienceAtom probe tomography (APT) was employed to investigate the precipitation process of Si3N4 particles after plasma assisted nitriding and subsequent annealing of Fe-Si alloy. The nitriding was done using 15N2 medium to enable quantitative analysis of Si and N by APT. Al, as a minor alloying element, is found to play an important role in the precipitation sequence. It is shown that Al forms Al-N clusters before Si3N4 precipitation takes place, and provides nucleation sites for the precipitation of Si3N4. Composition measurements show that silicon nitrides are enriched in Al, Ti and Mn as compared to the matrix

Mechanism of Si3N4 precipitation in nitrided Fe-Si alloys: A novel example of particle-stimulated-nucleation

Atom probe tomography (APT) was employed to investigate the precipitation process of Si3N4 particles after plasma assisted nitriding and subsequent annealing of Fe-Si alloy. The nitriding was done using 15N2 medium to enable quantitative analysis of Si and N by APT. Al, as a minor alloying element, is found to play an important role in the precipitation sequence. It is shown that Al forms Al-N clusters before Si3N4 precipitation takes place, and provides nucleation sites for the precipitation of Si3N4. Composition measurements show that silicon nitrides are enriched in Al, Ti and Mn as compared to the matrix.Design des Alliages Métalliques pour Allègement des Structure

Phase-field modelling of spinodal decomposition during ageing and heating

International audienceDespite the tremendous success of phase-field (PF) modelling in predicting many of the experimentally observed microstructures in solids, additional progress is required in order to apply it to predict microstructure evolution in real alloy systems. One way to achieve this is to couple thermodynamic and kinetic databases with PF model. In this work, we present phase-field simulations of spinodal decomposition in Fe-Cr alloy during thermal ageing and anisothermal heating. In the PF method, the local free energy is directly constructed using the CALPHAD method. During isothermal ageing, the morphology of decomposed phases consisted in an interconnected irregular shape for short ageing times, and a further ageing caused the change to a droplet like shape of the decomposed Cr-rich phase. The influence of heating rate on phase transformations is then simulated and compared with experimental results obtained by differential thermal analysis, carried out with heating rates in the range 0.5 °C.min-1 to 15 °C.min-1. The simulation results show that heating rate strongly influences the microstructure morphology

Phase-field modelling of spinodal decomposition during ageing and heating

International audienceDespite the tremendous success of phase-field (PF) modelling in predicting many of the experimentally observed microstructures in solids, additional progress is required in order to apply it to predict microstructure evolution in real alloy systems. One way to achieve this is to couple thermodynamic and kinetic databases with PF model. In this work, we present phase-field simulations of spinodal decomposition in Fe-Cr alloy during thermal ageing and anisothermal heating. In the PF method, the local free energy is directly constructed using the CALPHAD method. During isothermal ageing, the morphology of decomposed phases consisted in an interconnected irregular shape for short ageing times, and a further ageing caused the change to a droplet like shape of the decomposed Cr-rich phase. The influence of heating rate on phase transformations is then simulated and compared with experimental results obtained by differential thermal analysis, carried out with heating rates in the range 0.5 °C.min-1 to 15 °C.min-1. The simulation results show that heating rate strongly influences the microstructure morphology

Study by Differential Thermal Analysis of Reverse Spinodal Transformation in 15-5 PH Alloy

International audienceAlloy 15-5 PH is a stainless steel with 15 wt.% Cr and 5 wt.% Ni that is precipitation hardened by addition of Cu. In its semi-finished state, this alloy consists in Cu-supersaturated soft martensite; its high specific properties come from a final tempering consisting in a heating to 550-600°C, holding for 4 hours, and then air cooling. This treatment leads to nanometric Cu precipitation that hardens the material and to transformation of some martensite to reverted austenite which is then stable and provides ductility. While a' embrittlement of such steels is known to occur at temperature in the range 450-520°C, it has been reported that they can be sensitive to the same phenomenon after long term ageing at temperature as low as 300°C, with a significant loss of ductility and an increase of the ductile-to-brittle transition temperature. Atom probe studies showed that this degradation is related to demixtion of martensite into Fe-rich and Cr-rich phases. Depending on the ageing temperature, demixtion can proceed through a nucleation and growth precipitation or by spinodal decomposition of the martensitic matrix. The present study reports differential thermal analyses (DTA) performed upon heating samples of material held at various temperatures (290-525°C) for various times (410 h to 8500 h) that have been characterized by atom probe. A clear DTA signal is obtained upon the reverse spinodal transformation that is further found to depend on ageing conditions

Influence of structural parameters on magnetoresistive properties of CuFeNi melt spun ribbons

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