Magnetic Properties of NdFeB Alloys Obtained by Gas Atomization Technique.

摘要

Since Sagawa reported in [1] on the excellent hard magnetic properties of NdFeB permanent magnets, the utilization of anisotropic sintered NdFeB magnets in large motor and generator applications has grown spectacularly. Such hard magnetic material consists of a major tetragonal intermetallic compound (Nd₂Fe₁₄B), with excellent intrinsic hard magnetic properties, and a minor Nd-rich phase. Obviously, the magnetic behavior depends strongly on the microstructure of the magnet material, which in turn is determined by the production process. Microstructural factors affecting the hard magnetic character include the mean and the standard deviation of the grain size distribution, the orientation degree of the grains, the distribution of the Nd-rich phase, and the presence of a residual amount of other secondary phases (e.g. α-Fe, NdFe₄B₄, and other borides). All kind of defects in the microstructure, especially soft magnetic phases, can easily deteriorate the hard magnetic properties. It has been reported a slope change in the curve of magnetization as a function of temperature at around 150 K due to a spin reorientation transition (SRT) [2]. Such effect has been carefully analyzed for sintered magnets, since it is essential for the design of sensors, magnetic apparatus or magnetomechanical devices for cryogenic applications [3]. Alternative production routes of NdFeB magnets result in new microstructures whose impact on magnetic properties has to be carefully studied. Inert gas atomization is one of the novel processes under evaluation to produce NdFeB powders [4]. This technique consists on breaking a liquid metal stream into droplets by means of a high velocity inert gas flow. These droplets become spherical particles after solidification. The small size of the droplets, typically in the microns range, and the high velocity of the gas enable a fast heat transfer between both, resulting in high cooling rates and fine microstructures. Using this technique, we have produced several NdFeB alloys. After splitting the as-atomized powders in different size fractions by sieving, their microstructure and magnetic properties have been studied. In this work, we report the magnetic properties as a function of temperature, between 1.8 and 400 K, and of particle size. Fig. 1 shows the characteristic microstructure of a single gas atomized NdFeB particle, whose main constituents are Nd₂Fe₁₄B grains of a few microns in size. The Inverse Pole Figure (IPF) demonstrates the random crystallographic texture of the material. The cooling rate of gas atomized particles increases when the particle size is reduced. As a result, larger particles exhibit higher microsegregation and, hence, the precipitation of soft magnetic α-Fe phase. On the other hand, smaller particles display finer microstructures. As for the coercive field, it was observed that it increases significantly when the particle size is reduced, reflecting a higher difficulty for reverse domain nucleation (less surface defects, finer grain size, lower volume fraction of secondary soft magnetic phases, etc.). Fig. 2 shows the temperature dependency of the saturation magnetization, M_s(T). The anomaly, i.e. a slope change of the M_s(T) curve, observed around 150 K could be ascribed to the spin-reorentation transition (SRT) mentioned before, which has been reported to occur in the same temperature range when the magnetic field is applied parallel and perpendicularly to the sample direction [2]. In contrast with the measurements performed in single crystals and anisotropic sintered magnets, isotropic gas atomized powders exhibit an increment of saturation magnetization below the split tilt temperature of the SRT. This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 720838 (NEOHIRE project).