We revisited the microstructure of a commercial Nd-Fe-B sintered magnet using scanning electron microscopy (SEM) and aberration corrected scanning transmission electron microscopy (STEM) in order to ...clarify the microstructure feature that is relevant to the coercivity. Two types of thin Nd-rich grain boundary (GB) phases were found: one is crystalline and the other is amorphous. The crystalline Nd-rich GB phase form at the flat surface of c-plane of the Nd2Fe14B grains, which contains a higher amount of Nd compared to the amorphous GB phase that form on the grain boundaries nearly perpendicular to the c-planes. This indicates that the intergranular exchange coupling in sintered Nd-Fe-B magnets is anisotropic. Correlative SEM and STEM analysis of triple junction Nd-rich phases revealed five types of Nd-rich phases, i.e., fcc-NdO, hcp-Nd2O3, NdFe4B4, metallic Nd-rich phases with the fcc structure and the Ia3¯ structure. The metallic Nd-rich phases form at the sharp edges of Nd-rich oxide and boride grains, from which thin Nd-rich phases is infiltrated along grain boundaries to form the thin GB phase. Misaligned Nd2Fe14B grains are often observed in direct contact with the Nd2O3 grains.
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We discuss the mechanism of the coercivity enhancement by the grain boundary diffusion process (GBDP) using Dy vapor based on detailed microstructural characterizations. Scanning electron microscopy ...and electron probe microanalysis showed that a (Nd,Dy)2Fe14B shell formed in the outer region of Nd2Fe14B grains while its thickness decreased from the surface to the center of a cube-shaped sample. Atom probe tomography showed that the Dy content at grain boundaries (GBs) was close to that in the (Nd,Dy)2Fe14B shell. High-temperature annealing (at 900°C) of a GB diffusion processed magnet led to the disappearance of the GB layers, which resulted in a substantial reduction in coercivity. This suggests that both the (Nd,Dy)2Fe14B shell and the Nd-rich GB phase layer are required microstructural features for the coercivity enhancement by the GBDP.
The compositions of grain boundaries (GBs) and other interfaces surrounding Nd2Fe14B grains in commercial Nd–Fe–B sintered magnets have been investigated by laser-assisted three-dimensional atom ...probe to understand the mechanism of the coercivity enhancement by post-sinter annealing. While only a slight segregation of Nd and Pr to the GBs was confirmed in the as-sintered sample, a thin Nd-rich amorphous phase layer was observed along the GBs with Cu segregation to the interfaces in the annealed sample. The segregation of Cu to NdOx/Nd2Fe14B interfaces was also found, suggesting that the Nd2Fe14B grains are enveloped by a Cu-enriched layer after the annealing. The concentration of Fe+Co in the thin GB layer was found to be as high as 65at.%, and a model amorphous film processed by sputtering with the same composition as the thin GB layer was found to be ferromagnetic. Ferromagnetic behavior of the thin GB layer suggested that Nd2Fe14B grains are magnetically coupled. The coercivity mechanism of the sintered magnets is discussed based on these new findings.
Micromagnetic simulations of magnetization reversals of Nd–Fe–B anisotropic magnets with exchange-coupled grains demonstrate that the local demagnetization factor decreases as the grain size is ...reduced. This explains the higher coercivity and the lower temperature dependence of coercivity in magnets with smaller grain sizes. When a fraction of Nd2Fe14B grains become a single domain state for average grain sizes <1μm, a higher magnetic field is needed to magnetize them, giving rise to a two-step initial magnetization curve.
Magnetic refrigeration (MR) is a key technique for hydrogen liquefaction. Although the MR has ideally higher performance than the conventional gas compression technique around the hydrogen ...liquefaction temperature, the lack of MR materials with high magnetic entropy change in a wide temperature range required for the hydrogen liquefaction is a bottle-neck for practical applications of MR cooling systems. Here, we show a series of materials with a giant magnetocaloric effect (MCE) in magnetic entropy change (-∆S
> 0.2 J cm
K
) in the Er(Ho)Co
-based compounds, suitable for operation in the full temperature range required for hydrogen liquefaction (20-77 K). We also demonstrate that the giant MCE becomes reversible, enabling sustainable use of the MR materials, by eliminating the magneto-structural phase transition that leads to deterioration of the MCE. This discovery can lead to the application of Er(Ho)Co
-based alloys for the hydrogen liquefaction using MR cooling technology for the future green fuel society.
We have characterized the microstructures of as-sintered and optimally post-sinter annealed Nd-rich Ga-doped Nd–Fe–B magnets by scanning electron microscopy (SEM) and aberration-corrected scanning ...transmission electron microscopy (STEM). While the Nd2Fe14B grains in the as-sintered sample with a coercivity of 0.99T are in direct contact with each other, those in the optimally annealed sample with a coercivity of 1.8T are completely enveloped by typically 10-nm-thick Nd-rich phase that contains little Fe. This strongly suggests that the Nd2Fe14B grains in the optimally annealed Nd-rich Ga-doped Nd–Fe–B magnets are exchange decoupled in contrast to those in the commercial sintered magnets.
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A newly developed Mg–Sn–Zn–Al based alloy shows substantial strengthening by artificial aging. A Na-doped Mg–5.4Sn–4.2Zn–2.0Al–0.2Mn–0.1Na (TZAM5420−0.1Na) (wt.%) alloy exhibited a ...significant increase in yield strength from 243 to 347MPa by a T6 treatment due to the uniform dispersion of nanoscale precipitates by aging. The trace addition of Na causes the formation of Sn–Na co-clusters in the early stage of aging, which provides heterogeneous nucleation sites for Mg2Sn precipitates. However, Na strongly segregates at grain boundaries and this degrades the ductility significantly. To overcome this problem, we developed a Na-free Mg–6.6Sn–5.9Zn–2.0Al–0.2Mn (TZAM6620) alloy, in which nano-scale MgZn2 precipitates are uniformly dispersed by double aging. Pre-aging caused the formation of Zn-rich Guinier Preston zones, which acted as heterogeneous nucleation sites for the MgZn2 precipitates. The double-aged TZAM6620 alloy exhibited a very high yield strength of 370MPa with large elongation of 14%.
We have investigated the microstructures of age hardened Mg–2.0Gd–1.2Y–
xZn–0.2Zr (
x
=
0, 0.3, and 1.0) (at.%) alloys to understand the remarkable age-hardening and unusual plastic elongation ...behavior. The age-hardening of the alloys occurs through the sequential precipitations of β′ and β
1 phases. The β
1 phase heterogeneously nucleates at the interface of the β′ phase, and relaxes strain fields around the β′. Although the addition of Zn degrades the age-hardening response, it causes the discontinuous precipitation of a 14H-type long-period stacking (LPS) phase at grain boundaries as well as within grains in the over-aged condition, which enhances the maximum tensile elongation. The composition of the β
1 phase was determined to be Mg–23.3
at.% RE–9.7
at.% Zn–2.0
at.% Zr (RE: rare-earth, Gd and Y), whereas that of the LPS is Mg–5.6
at.% RE–1.8
at.% Zn–1.0
at.% Zr.
The grain boundary diffusion process using an Nd70Cu30 eutectic alloy has been applied to hot-deformed anisotropic Nd–Fe–B magnets, resulting in a substantial enhancement of coercivity, from 1.5 T to ...2.3 T, at the expense of remanence. Scanning electron microscopy showed that the areal fraction of an Nd-rich intergranular phase increased from 10% to 37%. The intergranular phase of the hot-deformed magnet initially contained ∼55 at.% ferromagnetic element, while it diminished to an undetectable level after the process. Microscale eutectic solidification of Nd/NdCu as well as a fine lamellae structure of Nd70(Co,Cu)30/Nd were observed in the intergranular phase. Micromagnetic simulations indicated that the reduction of the magnetization in the intergranular phases leads to the enhancement of coercivity in agreement with the experimental observation.
To understand the mechanism of the increase in coercivity caused by post-sinter annealing of Nd–Fe–B-based magnets, we have investigated the microstructures of commercial sintered magnets by ...high-resolution scanning electron microscopy, transmission electron microscopy and atom probe tomography. Continuous thin layers of a Nd-rich amorphous phase were found along the grain boundaries in the post-sinter annealed sample, the chemical composition of which was determined to be Nd
30Fe
45Cu
24.1B
0.9. A fine Cu-enriched shell was also confirmed in the Nd-rich phase grain, suggesting the Nd
2Fe
14B grains are completely enveloped by the Cu- and Nd-enriched layers. Furthermore, a lamellar microstructure of the Cu-enriched phase was confirmed in some Nd-rich phase grains. The mechanism of the coercivity increase caused by post-sinter annealing is discussed based on these characterization results.
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