Abstract
The nanoanalytical high resolution TEM/STEM investigation of the intergranular grain boundary phase of anisotropic sintered and rapidly quenched heavy rare earth-free Nd-Fe-B magnet materials revealed a difference in composition for grain boundaries parallel (large Fe-content) and perpendicular (low Fe content) to the alignment direction. This behaviour vanishes in magnets with a high degree of misorientation. The numerical finite element micromagnetic simulations are based on the anisotropic compositional behaviour of GBs and show a decrease of the coercive field with an increasing thickness of the grain boundary layer. The magnetization reversal and expansion of reversed magnetic domains primarily start as Bloch domain wall at grain boundaries parallel to thec-axis and secondly as Néel domain wall perpendicular to thec-axis into the adjacent hard magnetic grains. The increasing misalignment of grains leads to the loss of the anisotropic compositional behaviour and therefore to an averaged value of the grain boundary composition. In this case the simulations show an increase of the coercive field compared to the anisotropic magnet. The calculated coercive field values of the investigated magnet samples are in the order ofμ0HcJ=1.8 T–2.1 Tfor a mean grain boundary thickness of 4 nm, which agrees perfectly with the experimental data.
Highlights
Sintered and rapidly quenched Nd-Fe-B magnets with a high coercive field and energy density product are widely used for large scale applications, such as in wind turbines and electric powered automotive devices
The easy axis of magnetization [001] of the grains is illustrated with arrows and was measured via selected area electron diffraction (SAED) patterns and Fourier Transformation (FFT) of high resolution TEM/STEM (HRTEM) images, as shown in the magnified image of Figure 5(a)
The grain boundary junction (GBj) were identified with SAED and FFT of HRTEM images as the cubic c-(Pr,Nd)2O3 phase (a = 1.108 nm, Ia3) [23]
Summary
Sintered and rapidly quenched Nd-Fe-B magnets with a high coercive field and energy density product are widely used for large scale applications, such as in wind turbines and electric powered automotive devices. During the past 3 decades a large number of microstructural investigations of rare earth (RE) permanent magnets have revealed a detailed knowledge of the influence of the complex, multiphase microstructure, especially near grain boundaries on the coercivity of the magnets [1, 2]. Analytical electron microscopy studies have shown that the Nd-rich grain boundary phase with about 10 nm thickness limits the coercivity of Nd-Fe-B magnets [3]. It is fairly well known that the coercive field is increased by the addition of small amounts of dopant elements, such as Al [4], Ga [5], and Cu [6], which influence the wetting behaviour [7] of the liquid phase during sintering or hot working process and influences the magnetic decoupling of the grains. The replacement of the Ndrich intergranular phase by new dopant-containing phases improves the corrosion resistance of the magnet
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