• Journal of Magnetics
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• v.15 no.4
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• pp.155-158
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• 2010

The HDDR (hydrogenation, disproportionation, desorption, and recombination) process can be used as an effective way of converting a no coercivity Nd-Fe-B ingot material, with a coarse $Nd_2Fe_{14}B$ grain structure, to a highly coercive one with a fine grain structure. Careful control of the HDDR process can lead to an anisotropic powder with good $Nd_2Fe_{14}B$ grain texture; the most critical step for inducing texture is disproportionation. The critical conditions (hydrogen pressure and temperature) for the disproportionation reaction of fully hydrogenated $Nd_{12.5}Fe_{81.1-(x+y)}B_{6.4}Ga_xNb_y$ (x = 0 or 0.3, y = 0 or 0.2) alloys, in different atmospheres of pure hydrogen and a mixed gas of hydrogen and argon, was investigated with TPA (thermopiezic analyser). From this, the hydrogen pressure-temperature diagram showing the critical conditions was established. The critical disproportionation temperature of the fully hydrogenated $Nd_{12.5}Fe_{81.1-(x+y)}B_{6.4}Ga_xNb_y$ alloys was slightly increased as the hydrogen pressure decreased in both pure hydrogen and mixed gas. The critical disproportionation temperature of the hydrogenated alloys was higher in the mixed gas than in pure hydrogen. Addition of Ga and Nb increased the critical disproportionation temperature of the fully hydrogenated Nd-Fe-B alloys.

• Proceedings of the KAIS Fall Conference
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• 2005.05a
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• pp.67-70
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• 2005

[ $a-Fe/Nd_2Fe_{14}B$ ]기 이상나노복합합금(two-phase nanocompositie alloy)의 자기적 특성 향상을 위하여, $Nd_9Fe_{84}B_7$ 조성의 합금을 급속 응고법과 등온열처리를 이용하여 제조한 후, 공정 조건의 변화에 따른 상, 미세조직, 자기특성의 변화 등을 조사하였다. 주어진 Nd-Fe-B 삼원계 합금을 35 m/s 이상에서 급랭 응고하였을 때 완전한 비정질 합금을 얻을 수 있었으며, 최적 자기특성을 얻기 위한 적정 열처리 구역은 $700^{\circ}C$/10 - 15분, 725$^{\circ}C$/7 - 12분, $750^{\circ}C$/5 - 7분으로 조사되었다. 이 구역들에서 열처리된 합금들은 모두 매끄러운 감자곡선(demagnetization curve)을 나타내, a-Fe와 $Nd_2Fe_{14}B$사이의 교환상호결합이 잘 일어나고 있음을 알 수 있었으며, 이때 a-Fe와 $Nd_2Fe_{14}B$의 평균 입자 크기는 각각 약 10nm와 40$\~$60 nm였다. 한편 상 전개의 차이에도 불구하고 열처리 후에는 모든 합금들이 $60\%$ 이상의 $M_r/M_s$ 비율을 나타내 열처리 조건의 변화에 관계없이 교환상호작용은 일어나고 있음을 알 수 있었다.

• Journal of Magnetics
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• v.17 no.3
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• pp.185-189
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• 2012

The coercivity of near single domain size $Nd_2Fe_{14}B$-type particles prepared by ball milling of HDDR-treated $Nd_{12.5}Fe_{80.6}B_{6.4}Ga_{0.3}Nb_{0.2}$ alloy was investigated. The feasibility of a surface nitrogenation for improving the coercivity stability of the fine $Nd_2Fe_{14}B$-type particles was also studied. The near single domain size $Nd_2Fe_{14}B$-type particles had a high coercivity of over 9 kOe. However, the coercivity radically deteriorated as the temperature increased in air (< 2 kOe at $200^{\circ}C$). This coercivity reduction was attributed to the soft magnetic phases, ${\alpha}$-Fe and $Fe_3B$, which formed on the surface of the fine particle due to oxidation. Surface nitrogenation of the fine particles significantly improved the stability of their coercivity. The improvement in coercivity stability was attributed to the formation of a thin nitrogenated layer on the surface of the fine $Nd_2Fe_{14}B$-type particles, which enhanced the anisotropy field and gave improved resistance to oxidation (dissociation).

• Journal of the Korean Magnetics Society
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• v.5 no.1
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• pp.34-37
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• 1995

$Nd_{x}Fe_{77}B_{23-x}(x=4,\;6,\;8,\;11,\;13)$ alloys were prepared by rapid quenching technique. They were crystallized at higher than $650^{\circ}C$ and subsequently the magnetic properties were investigated The melt-spun alloy of x=8 consisted of nearly single metastable compound $Nd_{2}Fe_{23}B_{3}$ and showed the soft magnetic property. In $8{\leq}x{\leq}13$ alloys, the coercivities were increased according to the increase of Nd content but the remanences decreased. In $4{\leq}x{\leq}6$ alloys, the coercivities and the remanences were increased by lowering Nd content. In x=4 alloy, the ratio of soft to hard magnetic phase was 70:30 and the energy product was $96kJ/m^{3}(12MG.Oe)$.

• Journal of Magnetics
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• v.7 no.4
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• pp.127-131
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• 2002

HDDR characteristics and magnetic properties of $Nd_{15}{(Fe_{1-x}Co_{x})}_{77}B_{8}$(x=0-0.6) alloys were investigated. The effect of applying magnetic field during the recombination step on the anisotropic nature of the HDDR-treated material was also examined. The $Nd_{15}{(Fe_{1-x}Co_{x})}_{77}B_{8}$ phase in the Nd-Fe-B alloys with high Co-substitution alloy had remarkably enhanced phase stability. The $Nd_{15}{(Fe_{1-x}Co_{x})}_{77}B_{8}$(x=0-0.6) alloys with high Co-substitution could be HDDR-treated successfully by only using high pressure hydrogen. However, these alloys had no appreciable coercivity. The poor coercivity of the HDDR-treated $Nd_{15}{(Fe_{1-x}Co_{x})}_{77}B_{8}$(x=0-0.6) alloys with high Co-substitution was attributed to the $Nd{(Fe,Co)}_2$ phase in the alloys. The magnetic filed applied during the recombination step had little effect on the anisotropic nature of the HDDR-treated powder.

• Journal of the Korean Magnetics Society
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• v.12 no.1
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• pp.34-40
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• 2002

In order to improve the microstructure of the strip cast Nd-Fe-B alloys that are frequently used for production of high energy sintered magnets, influence of various strip casting conditions on the microstructure and phase formation and distribution were investigated. Nd-Fe-B strips consisting of microstructures suitable for preparation of high energy sintered magnets could be obtained when the wheel speed was below 5 m/s. The compositional limit that can avoid the crystallization of free iron in the as-cast state was estimated to be Nd$\_$14/Fe$\_$79/B$\_$7/. Regardless of the compositional variation, <001> preferred orientation of Nd$_2$Fe$\_$14/B normal to the strip surface was always occurred below 5 m/s, which would eventually enhance the grain alignment during pressing the powder under a magnetic field. While the coercivity of the strip cast alloys increased with the increase of the wheel speed, mainly due to the refinement of Nd$_2$Fe$\_$14/B grains, it decreased with the reduction of Nd content in the alloy composition as the formation of free iron increased.

• Proceedings of the Korean Magnestics Society Conference
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• 2000.09a
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• pp.393-394
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• 2000

The magnetic alloy of $Nd_{2}Fe_{14}B$-type was synthesized by a mechano-chemical method, including the processes of spray-drying, de-binding, milling, $H_{2}$- and Ca-reduction and washing. The very fine $Nd_{2}Fe_{14}B$ particles smaller than $1\mu\textrm{m}$ could be achieved without the inevitable pulverization step in conventional processes.

• Transactions of Materials Processing
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• v.31 no.3
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• pp.124-128
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• 2022

NdFeB magnets have been positioned as the core materials in advanced technologies such as MRI (magnetic resonance imaging), FA (factory automation system), robot, motors, and so on based on the highest magnetic properties. To effectively improve the refined microstructure, the plastic deformation has been known as the good alternatives by the recrystallization. However, it has been regarded as being impossible because of the few slip systems in the RE-Fe-B magnets at room temperature. The purpose of this study was to investigate the possibility of control of grain refinement and magnetic anisotropy of NdFeB alloy powder by the severe plastic deformation. The NdFeB magnet powder was fabricated by gas atomization process, and the powder was pre-compacted at high temperature. The pre-compacted billets were deformed by HPT (high pressure torsion), and then the deformed billets were observed microstructure and magnetic properties. After the HPT process at room temperature, the grain size decreased with increasing because of the melted Nd-rich phase, and the anisotropy of Nd₂Fe₁₄B phase was formed after the HPT process.

• Journal of the Korean Magnetics Society
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• v.5 no.5
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• pp.412-415
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• 1995

By adding 1wt.% $HfB_{2}$ into $Nd_{3-4}Fe_{77-78.5}B_{18.5}$ alloys, the grain growth of $Fe_{3}B/Nd_{2}Fe_{14}B$ composite phases during annealing was found to be hindered by 40~50%. It is proposed that the addition of $HfB_{2}$ leads to the formation of fine dispersoids of $HfB_{2}$ in the $Fe_{3}B/Nd_{2}Fe_{14}B$ composite magnet. The maximum energy product($(B.H)_{max}$) as well as intrinsic coercivity($_{i}H_{c}$) for the $Nd_{3}Fe_{78.5}B_{18.5}\;+\;1wt.%\;HfB_{2}$ alloy were enhanced by more than 25% $(B.H)_{max}=10\;MGOe,\;_{i}H_{c}=2.5\;kOe)$ due to the addition of $HfB_{2}$ while remanent magntization($B_{r}$) was reduced slightly. Itis deduced that the formation of fine dispersoids will also play a role of magnetic domain pinning.

• Proceedings of the Korean Magnestics Society Conference
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• 1996.05a
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• pp.58-59
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• 1996