Hae-Woong Kwon

Effect of Grain Size and Die-Upset Temperature on Texture in Die-Upset Nd-Fe-B Magnet

Hot-pressed Nd13.5Fe77Si1B8 alloy compacts with various grain sizes ranging from approximately 50 nm to 200 nm were prepared using melt-spun material. The compacts were then subjected to die-upsetting. The effect of grain size in the hot-pressed compact on the texture in die-upset magnet was investigated. The effect of die-upset temperature on the texture in die-upset magnet was also investigated. Texture in the die-upset Nd13.5Fe77Si1B8 alloy samples decreased with increasing the grain size in the hot-pressed compact before the die-upsetting. This dependence of texture on the initial grain size could be explained by the unique texture formation mechanism of a stress-induced preferential grain growth via dissolution and precipitation. Texture of the die-upset sample was also influenced by the die-upsetting temperature. The samples die-upset at 700degC and 900degC had poorer texture compared to the samples die-upset at the temperature range of 750degC-850degC. The poor texture in the sample die-upset at lower temperature of 700degC was attributed to the less dissolution of the unfavorable Nd2Fe14B grains and low fluidity of the liquid Nd-rich grain boundary phase. The poor texture in the sample die-upset at higher temperature of 900degC was explained by the reduced elastic modulus and the excessive grain growth of the Nd2Fe14B grains.

Texture Studies on Die-Upset Nd-Lean Nd-Fe-(Cu)-Ga-B Alloy

Melt-spun Nd-lean Nd_xFe_93.5-xGa_0.5B₆ (x = 9, 10.5 and 12) alloys were die-upset, and the Nd₂Fe₁₄ B grain texture in the die-upset samples were investigated. The effect of Cu-addition on the texture of the die-upset Nd-lean Nd_xFe_93.5-xGa_0.5B₆ (x = 9, 10.5<i>,</i> and 12) alloys was studied, and the texture formation mechanism operating in these alloys was investigated. The die-upset Nd-lean Nd_xFe_91.5-xCu₂Ga_0.5B₆ (x = 9 and 10.5) alloys showed negligible texture even with Cu-addition. However, the Cu-added Nd_xFe_91.5-xCu₂Ga_0.5B₆ (x = 12) alloy showed an appreciable texture, and the texture increased with increasing the die-upset temperature. The Cu-containing Nd_xFe_91.5-xCu₂ Ga_0.5B₆ (x = 12) alloy developed the texture by a basal plane slip deformation during die-upsetting.

Preparation of Nd2Fe14B Single Domain Particles from Nd-Fe-B Alloy Ingot Using a Combination of HDDR and Mechanical Milling

This study examined the feasibility of the combining HDDR-process (hydrogenation, disproportionation, desorption and recombination) with mechanical milling to prepare single domain Nd₂Fe 14 B particles from a Nd-Fe-B alloy ingot. The Nd 15 Fe 77 B? alloy was HDDR-treated and then subjected to a roller-milling. In the HDDRtreated Nd 15 Fe 77 B? alloy, very small Nd₂Fe 14 B grains comparable to their critical single domain size (0.3 ㎛) were observed. These fine individual grains were separated successfully along the grain boundaries by a rollermilling. The separated Nd₂Fe 14 B grains were found to be single domain particles. These results suggest that single domain particles of the Nd₂Fe 14 B phase can be prepared from a Nd-Fe-B ingot alloy by combining a HDDR-process with mechanical milling.

Effect of the dehydrogenation speed and Nd content on the microstructure and magnetic properties of HDDR processed Nd-Fe-B magnets

The effect of Nd content and dehydrogenation speed on the microstructure and magnetic properties of hydrogenation-disproportionation-desorption-recombination (HDDR) processed Nd-Fe-B magnetic powders was studied. The NdxB6.4Ga0.3Nb0.2Febal (x=12.5–13.5, at.%) mold casting alloys were subjected to HDDR process after homogenization heat treatment. During desorption-recombination stage, dehydrogenation speed and time were systematically changed to control the speed of the desorption-recombination reaction. The higher Nd content resulted in better magnetic properties of the HDDR powder, and this was attributed to the thicker and more uniform Nd-rich phase at grain boundaries. It was also confirmed that the slow dehydrogenation speed could maximize the effect of high Nd content on the magnetic properties of HDDR powder. At the optimized dehydrogenation speed, the coercivity and remanence was 15.3 kOe and 13.0 kG, respectively, at 12.9 at.% Nd content, which resulted in a (BH)max of 36.8 MGOe.

Residual Hydrogen in Nd-Fe-B HDDR Powder and Its Effect on Coercivity of Hot-Pressed Compact

Residual hydrogen in the HDDR-treated Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2 powder and its effect on the coercivity of consolidated compact of the powder were investigated. Hydrogen desorption of the HDDR powder was studied by vacuum gauge and TPA. Compaction of the HDDR powder was performed by hot-pressing technique. The Nd_12.5Fe_80.6B_6.4Ga_0.3Nb_0.2 HDDR powder contained significant amount of residual hydrogen (approximately 1520 ppm). Coercivity of the hot-pressed compact was radically reduced when the compaction was performed at the temperature above 650 °C. The radical coercivity reduction in the compact hot-pressed above 650 °C was attributed to the presence of the soft magnetic phases, α-Fe and Fe₂B formed via disproportionation of the Nd₂Fe₁₄BH_x. The residual hydrogen may have contributed for the formation of Nd₂Fe₁₄BH_x.

Enhancement of coercivity in sintered Nd-Fe-B magnets by grain-boundary diffusion of electrodeposited Cu-Nd Alloys

We report an enhancement in the coercivity of sintered Dy free Nd-Fe-B magnets from 11.84 to 14.26 kOe by the grain-boundary diffusion of electrochemically deposited Cu-Nd. In the optimized electrochemical deposition and heat treatment conditions, a distinct Nd-rich grain-boundary phase was observed after the diffusion process; distributions of each element was carefully mapped by scanning electron microscopy equipped with backscattered electron detector. X-ray diffraction patterns indicated that Nd2Fe14B was oxidized by the inward diffusion of oxygen, which might be formed during the electrodeposition of Cu-Nd, forming antiferromagnetic Fe2O3 that might degrade the overall coercivity. A mechanism underlying the enhancement of coercivity is basically the same as the well-known proposed mechanism, distribution of a thin Nd-rich phase by grain-boundary diffusion process. In this study, electrochemical deposition process has been extensively investigated, and then the process was demonstrated to be successful and economically useful method to improve coercivity of the magnet.

The Influence of Dehydrogenation Speed on the Microstructure and Magnetic Properties of Nd-Fe-B Magnets Prepared by HDDR Process

The influence regarding the dehydrogenation speed, at the desorption-recombination state during the hydrogenation-disproportionation-desorption-recombination (HDDR) process, on the microstructure and magnetic properties of Nd-Fe-B magnetic powders has been studied. Strip cast Nd-Fe-B-based alloys were subjected to the HDDR process after the homogenization heat treatment. During the desorption-recombination stage, both the pumping speed and time of hydrogen were systematically changed in order to control the speed of the desorption-recombination reaction. The magnetic properties of HDDR powders were improved as the pumping speed of hydrogen at the desorption-recombination stage was decreased. The lower pumping speed resulted in a smaller grain size and higher DoA. The coercivity and the remanence of the 200-300 μm sized HDDR powder increased from 12.7 to 14.6 kOe and from 8.9 to 10.0 kG, respectively. In addition, the remanence was further increased to 11.8 kG by milling the powders down to about 25-90 μm, resulting in (BH) max of 28.8 MGOe.

Study on Oxidation and Coercivity of Nd 2 Fe 14 B Compound Crystal

Oxidation of the compound crystal and its effect on the coercivity of the fine crystal particles were investigated. Oxidation kinetics of the compound crystal was investigated using an excessively grown grains in the alloy ingot. Oxidation of the compound crystal occurred by dissociation of the phase into multi-phase mixture of -Fe, , and Nd oxides. Oxidation rate of the compound crystal showed no dependence on the crystallographic direction. The oxidation reaction was modeled according to simple linear relationship. Activation energy for the oxidation of compound crystal was calculated to be approximately 26.8 kJ/mol. Fine crystal particles in near single domain size was prepared by ball milling of the HDDR-treated alloy, and these particles were used for investigating the effect of oxidation on the coercvity. The near single domain size crystal particles () had high coercivity over 9 kOe. However, the coercivity was radically reduced as the temperature increased in air (). This radical coercivity reduction was attributed to the soft magnetic phases, -Fe and , which were formed on the surface of the fine particles due to the oxidation.

Kinetic study of the hydrogen-assisted disproportionation and recombination of Sm3(Fe, Co, V)29-type magnetic alloy

Abstract The kinetics of hydrogen-assisted disproportionation and recombination of the Sm 9 Fe 65 Co 20 V 6 alloy was investigated by means of isothermal TPA (thermopiezic analysis) and isothermal TMA (thermomagnetic analysis). The reaction rate of the disproportionation followed the parabolic rate law, and the activation energy for the reaction was ∼216.5 kJ/mol. The recombination reaction rate was fitted to the linear relationship, and the activation energy for the reaction was ∼241.0 kJ/mol.

Effect of Grain Boundary Modification on the Microstructure and Magnetic Properties of HDDR-treated Nd-Fe-B Powders

The microstructure and magnetic properties of HDDR–treated powders after grain boundary diffusion process (GBDP) with Nd–Cu alloy at different temperatures have been studied. The variation of GBDP temperature had multifaceted influences on the HDDR–treated powders involving the microstructure, phase composition and magnetic performance. An enhanced coercivity of 16.9 kOe was obtained after GBDP at 700 ℃, due to the modified grain boundary with fine and continuous Nd–rich phase. However, GBDP at lower or higher temperature resulted in poor magnetic properties because of insufficient microstructural modification. Especially, the residual hydrogen induced phenomenon during GBDP strongly depended on the GBDP temperature.