Sophie Rivoirard

Catalytic effect of additives on the hydrogen absorption properties of nano-crystalline MgH2(X) composites

Hydrogen sorption properties of MgH2–X nanocomposite powders (X=V, Nb, Ti, TiCN) are conditioned by the milling step. The respective roles of the additives and the milling time have been dissociated. The study reveals details not only on the hydrogen sorption process but also on the particle size reduction, distribution and agglomeration, that depend on the nature of the additives. In addition to the effect related to the extra-elements, the milling time has to be optimised to lead to the best particles size reduction without any agglomeration of the grains.

Dilatation measurements for the study of the α/γ transformation in pure iron in high magnetic fields

Magnetic field processing is a new promising tool for the structural and functional control of materials. A significant potential exists for tailoring microstructures and impacting kinetics of phase transformation in steels. A high magnetic field modifies the Gibbs free energy. As a result, the phase diagram is shifted upwards so that the Ac1 and Ac3temperatures increase as the magnetic field is increased. In this work, a new device for the heat treatment and in situ control of the transformation is described. For the first time, a dilatation measurement is used to study the shift of the ferrite/austenite equilibrium in high magnetic field up to 16 T and to quantify the ferrite concentration during the transformation. Experimental results for the transformations in pure iron are presented. Comparisons are made with the expected values based on the Weiss molecular field model near the Curie point.

HDDR process of NdFeB with an excess of intergranular Nd-rich phase under magnetic field

Abstract High coercive isotropic NdFeB powders can be obtained using the hydrogenation-disproportionation-desorption-recombination (HDDR) process. In order to produce anisotropic coercive powders, a static magnetic field of 7 T has been applied during the recombination stage, using an NdFeB alloy, with an excess of intergranular Nd/Cu eutectic. In parallel, the behaviour of the Nd/Cu eutectic has been studied by in-situ neutron diffraction experiments: under hydrogen, the Nd/Cu eutectic is solid in all temperature ranges, but under vacuum, hydrogen desorbs and the Nd/Cu becomes liquid. To induce a rotation of the magnetic Nd 2 Fe 14 B crystallites, it is necessary to reach a pronounced desorption of hydrogen. Hence, we have been able to produce anisotropic NdFeB material under a magnetic field by increasing the holding time during the recombination stage of the HDDR process.

Localisation of Zr in Nd-Fe-B alloys

Abstract X-ray absorption fine structure measurements and thermomagnetic measurements were performed in order to localise zirconium in Nd–Fe–B-based alloys. These experiments show that most of the introduced Zr (0.7–2 at%) is ordered as the ZrB 2 structure which is well evidenced by annealing at 1000°C. Zr addition also induces the formation of the Nd 2 Fe 17 phase at the expense of the Nd 2 Fe 14 B phase.

Kinetic effects of magnetic field on the γ/α interface controlled reaction in iron

The effect of magnetic field on the austenite (γ) to ferrite (α) transformation kinetics is studied in iron by means of dilatation measurements under magnetic field. In the frame of the Johnson–Mehl–Avrami-Kolmogorov analysis and considering a spherical growth mode for the ferrite grains, the Avrami exponent and the activation energy for the interface mobility are calculated. These parameters are found to be weakly influenced by the application of a 16 T magnetic field. This indicates that they are related mainly to the total amount of energy available for the transformation whatever its origin (whether magnetic or chemical). Whereas the magnetic field strongly affects the α/γ thermodynamic equilibrium through the Gibbs free energy of phases, it has a non noticeable influence on the transformation kinetics itself.

High temperature dilatation measurements by in situ laser interferometry under high magnetic field

This article contains a description of a laser interferometer coupled with a furnace for in situ dilatation measurements under high magnetic field. The apparatus fits an 18 T superconducting magnet with a 32 mm diameter room temperature bore. This optical method was chosen for dilatation measurements because it is not perturbated by the magnetic field. The measured sample can be heated up to 1500 K under a controlled atmosphere, and heating and cooling rates can be varied within a range of 10 K/s. The resolution is below 50 nm. As an example of using in situ dilatation measurements to follow phase transformations, the increase in the ferrite/austenite transformation temperature in pure iron in a magnetic field up to 16 T was experimentally evidenced.

Texturing Nd–Fe–B magnets under high magnetic field

An original approach is explored in the preparation of anisotropic hard magnetic alloys. This constitutes a proof of principle toward the preparation of anisotropic bonded magnets. Nd–Fe–B ribbons (50% Nd2Fe14B+50% Nd–Cu alloy), constituted of Nd2Fe14B grains embedded in a Nd–Cu eutectic matrix, were annealed under an applied magnetic field up to 16 T at various temperatures above the Nd–Cu melting temperature. The grain orientation mechanism is described in terms of a competition between the aligning magnetic field torque acting on the solid magnetic grains and the friction counter torque at the interface between the grains and the liquid matrix. The large temperature effect on the orientation behavior is attributed to the associated increase in the liquid phase volume fraction.

Thermodynamic analysis using experimental magnetization data of the austenite/ferrite phase transformation in Fe?xNi alloys (x = 0, 2, 4?wt%) in a strong magnetic field

The effect of a strong magnetic field on the transformation temperature from ferrite to austenite and from austenite to ferrite is examined in Fe–xNi alloys (x = 0, 2, 4 wt%). Alloy magnetization measurements are used to calculate the magnetic contribution to the driving force and to account thermodynamically for the field dependence of the transformation temperatures. The predicted shift of the transformation temperatures is compared with experimental dilatometry measurements of transformation temperatures, up to 16 T. The use of experimental magnetization measurements is found to be an accurate alternative to the Weiss molecular field theory for the prediction of the magnetization of ferrite close to its Curie point.

In situ characterization of phase transformations in a magnetic field in Fe-Ni alloys

We have investigated the effect of magnetic field on the austenite(γ)-ferrite(α) equilibrium in Fe-xNi alloys with x = 0, 2, 4 wt%. The α→γ and γ→α transformations have been followed as a function of applied magnetic field by a laser dilatometer installed in a 16T superconducting magnet. In addition, magnetic measurements at high temperature have been used to follow the magnetic behavior of each alloy composition during a complete heat treatment. We observe a shift of the phase diagram to higher temperature as the magnetic field is increased. We also find that the α-phase is either in the paramagnetic or in the ferromagnetic state as the transformation proceeds, depending on the amount of Ni. This results in an increase of the transformation temperature which is respectively proportional to the magnetic field if ferrite is formed in the ferromagnetic state and proportional to the square of the magnetic field if ferrite is paramagnetic.

Rheological study of hot-forged NdFeB and related permanent magnet properties

Abstract For the first time, forging at very high strain rates ( e >100 s −1 ) has been used successfully to produce magnets directly from bulk Nd–Fe–B–Cu alloy. This is achieved by reducing significantly the size of the Nd 2 Fe 14 B crystallites, which leads to a coercivity of 796 kA m −1 and by developing a Nd 2 Fe 14 B c -axis fibre texture along the forging direction, which leads to a remanence of 1 T, giving a final energy product close to 200 kJ m −3 . Correlations between the rheological behaviour and the permanent magnet properties of the alloy are demonstrated and four critical parameters have been identified: (1) an increase in the strain rate improves coercivity but reduces extrinsic magnetic anisotropy; (2) an increase in the strain promotes a microstructure with higher levels of coercivity and texture; (3) when the overall viscosity of the sample decreases, the anisotropy increases for a given deformation rate; and (4) high temperatures enhance the coercivity for very fast deformation rates. Through mechanical tests, it has been shown that both the level of anisotropy and the maximum stress at various temperatures can be closely correlated with the different microstructures obtained below and above 1273 K. A brittle to ductile transition in the mechanical behaviour of the alloy has been observed. Grain refinement as well as alignment mechanisms are discussed.