Nd-rich Grain Research Articles

Transmission electron microscopy studies of sintered Nd-Fe-B magnets prepared by cyclic sintering

A cyclic sintering process was demonstrated to achieve higher magnetic and mechanical properties due to homogeneous distribution of the Nd-rich phase during sintering. Densification greater than 98% of the theoretical density was achieved by the cyclic sintering of the Nd-Fe-B powder. In this study, transmission electron microscopy was used to evaluate the microstructural changes of the cyclically sintered Nd-Fe-B magnets. The Nd-rich triple junction phase in the cyclic sintered sample was h-Nd2O3 and the Nd-rich grain boundary phase was a mixture of h-Nd2O3 and h-Nd amorphous thin layers. The coercivity and bending strength increased from 1666.8 to 1828.5 kA/m and from 301.8 to 520 MPa, respectively.

Microstructure optimization to achieve high coercivity in anisotropic Nd–Fe–B thin films

A high coercivity of 2.35MAm−1 (29.5kOe) is demonstrated using perpendicular anisotropy Nd–Fe–B films. Single layer columnar grown Nd–Fe–B thin films with a coercivity comparable with that of sintered magnets (1.11MAm−1, 14.0kOe) were used as the starting film. The coercivity was doubled by annealing films capped with Nd, Nd/Cu, and Nd/Ag layers. Transmission electron microscopy indicated that the Nd2Fe14B columnar grains were isolated by a Nd-rich grain boundary phase formed by diffusion of the capped layers into the grain boundaries of the polycrystalline Nd–Fe–B films. The temperature dependence of coercivity was lower than that of commercial sintered magnets. The coercivity of the Nd/Cu diffusion processed sample at 473K was 0.979MAm−1 (12.3kOe), which is higher than that of Nd10Dy4Fe80B6 sintered magnets.

Tensile deformation behavior of Nd–Fe–B alloys

The tensile deformation behavior of Nd–Fe–B alloys was examined. A maximum elongation of 84% could be obtained in the alloys under optimum conditions. The tensile deformation of the alloys was found to be based on grain boundary sliding accommodated by an interface-controlled solution–precipitation process along a Nd-rich grain boundary phase. The mechanism for the texture formation in Nd–Fe–B alloys is also discussed based on the obtained results.

Effect of Dy on the microstructural and magnetic properties of an Nd-Fe-B strip-cast alloy

This paper examines the effect of Dy on the microstructural and magnetic properties of an (Nd32.57−x,Dyx)Febal.B0.97 (wt%; x=4.89, 6.51, 8.14, 9.77) strip-cast alloy. The average interlamellar spacing is reduced, and the grain uniformity and (00l) grain alignment in the direction perpendicular to the plane of the strip-cast alloy are enhanced as the Dy content is increased. Furthermore, the formation of & ga-Fe is increasingly suppressed as the Dy content is increased. These microstructural changes can be explained by an increase in the effective cooling rate of the melted alloy, which is caused by an increase in the solidus temperature of the (Nd,Dy)2Fe14B phase resulting from the substitution of Dy with Nd. The appearance of DyNdO3 precipitates in the grain boundaries also contributes to the grain refinement. Therefore, the improvement in the magnetic properties afforded by the addition of Dy depends on the large anisotropy field of Dy2Fe14B; it also depends on the grain refinement and alignment caused by the increased effective cooling rate and by the DyNdO3 precipitates that form along the Nd-rich grain boundaries.

Microstructural evolution of triple junction and grain boundary phases of a Nd-Fe-B sintered magnet by post-sintering annealing

The microstructural evolution of Nd-rich grain boundary phases (GBP) in connection with triple junction phases (TJP) during post-sintering annealing (PSA) was investigated. The Cu-rich TJP in the as-sintered sample was mostly a fcc-NdO phase but the GBP were a mixture of h-Nd2O3 and Nd phases. The fcc-NdO of the TJP in the as-sintered state gradually transformed to h-Nd2O3 during the first and the second PSA steps. However, it transformed to a C-Nd2O3 phase as both a massive form such as TJP and a thin GBP after the modified second PSA step. This suggests that the mechanism for the formation of metastable C-Nd2O3 may not be solely the interface energy. In contrast, the mixture of h-Nd2O3 and Nd of the GBP in the as-sintered state gradually transformed to C-Nd2O3 which is embedded in the amorphous matrix as the PSA goes from the first to second or modified second PSA step. The formation of the C-Nd2O3 GBP with an amorphous phase is the main factor for increasing the coercivity (from 21.8 to 30.4 kOe) after the second or modified second PSA step.

In situ dynamic observation and Mössbauer effect of microstructure of sintered Nd16Fe69Co8B7 magnet

In this paper the in situ dynamic observation of microstructure characterization of sintered Nd16Fe69Co8B7 magnet was carried out by using a high voltage electron microscope. At room temperature a number of spherical Nd2O3 inclusions are found to be embeded in some Nd2 (Fe,Co)14B grains. When temperature reaches to 400 °C, a few of small precipitates occur in the matrix, the precipitates grow up and are needle-like. With the temperature increasing to 500 °C, precipitates occur in the matrix. In addition, the Nd-rich phase and other undetermined phase are found in the grain boundary with increasing of temperature, remarkable change is not found for the Nd-rich grain boundary phase. When temperature reaches to 700 °C, a structure transition seems to take place in the undetermined grain boundary phase. Co underlying take up j2 crystal increase exchange function. Its result lead to increasement of Curie temperature TC. The j2 site has very strong face anisotropy, Co atom underlyings replacing Fe strengthens anisotropy. When Co content increases, the tetragonal phase Nd2 (Fe1−xCox)B uniaxial anisotropy is attenuated, besides as Co content increase, soft magnetism Nd(Co, Fe) appears. Therefore, as Co content increases, coercivity rapidly descended.

Microstructure of Nd-Rich Grain Boundary Phase in Die-Upset Nd-Fe-Co-Ga-B Magnet

Microstructure of Nd-Rich Grain Boundary Phase in Die-Upset Nd-Fe-Co-Ga-B Magnet

Coercivity enhancement of hydrogenation–disproportionation–desorption–recombination processed Nd–Fe–B powders by the diffusion of Nd–Cu eutectic alloys

We report the coercivity enhancement of hydrogenation–disproportionation–desorption–recombination processed Nd–Fe–B nanocrystalline powders from 1321 to 1552 kA m−1 (19.5 kOe) by the grain boundary diffusion of Nd–Cu melts with hyper-eutectic compositions. A distinct Nd-rich grain boundary phase was observed after the diffusion process by scanning electron microscopy and transmission electron microscopy; however, atom probe tomography indicated that the change in composition of the grain boundary phase was not significant. The mechanism of the coercivity enhancement is discussed based on these results.

Grain boundary structure and chemistry of Dy-diffusion processed Nd–Fe–B sintered magnets

We have investigated the microstructure of grain boundary diffusion processed Nd–Fe–B sintered magnets by high resolution scanning electron microscopy, high resolution transmission electron microscopy, and laser assisted atom probe to understand the mechanism of coercivity enhancement. After the vapor coating of Dy to the sintered magnets, the sample were annealed to let it diffuse in the sample through the grain boundary diffusion. The composition of the outer region of the Nd2Fe14B grains changed to (Nd,Dy)2Fe14B due to the substitution of Dy for Nd. Excess Nd ejected from the Nd2Fe14B grains led to the formation of continuous Nd-rich grain boundaries.

Optimisation of the processing of Nd–Fe–B with dysprosium addition

Magnets based on Nd16Fe76B8 with DyH3 additions have been sintered at different temperatures. The magnetic properties, microstructure and compositions have been examined. Details of progress of Dy entering the hard matrix phase (ϕ) is observed and an evolution mode of the relevant sintering process is proposed, observed and reported in the present work. Hydrogen absorption and desorption of the magnets were examined by HDTA. The optimum sintering temperature for these magnets has been established. Microstructural and EDX examinations revealed that a relatively Dy-rich phase formed in the Nd-rich grain boundary phase and this slowed down the substitution of Nd by Dy in the Nd2Fe14B matrix phase, possibly enhancing local coercivity without excessive attenuation of the remanence. Hydrogen Decrepitation (HD) occurred as sintered RE–Fe–B magnets were exposed to hydrogen at room temperatures.

Increased coercivity in sintered Nd–Fe–B magnets with NdF 3 additions and the related grain boundary phase

Enhanced coercivity was achieved by adding NdF 3 to both Dy-free and Dy-containing Nd–Fe–B sintered magnets with little decrease in remanence. NdF 3 reacts with the Nd-rich grain boundary phase and a new rare-earth oxyfluoride (ROF) phase with ordered face-centered cubic structure is formed. Dy was detected in the ROF phase in the Dy-containing magnet. This suggests that the increase of coercivity can be related to the ordered grain boundary phase as well as to Dy segregation.

Consolidation of hydrogenation–disproportionation–desorption–recombination processed Nd–Fe–B magnets by spark plasma sintering

We have successfully consolidated hydrogenation–disproportionation–desorption–recombination (HDDR) processed Nd–Fe–Co–Zr–B–Ga powder by spark plasma sintering (SPS). The field compacted samples were sintered at different temperatures ( T S) from 550 to 600 °C with compressive pressure of 80 MPa for 20 min. Microstructural investigations by transmission electron microscopy indicated that the sintered specimen exhibits Nd 2Fe 14B grains of ~300 nm with Nd-rich grain boundary phase. The optimum magnetic properties of B r: 1.22 T, H c: 928 kA/m, B H c: 600 kA/m, (BH) max: 210 kJ/m 3 were obtained in the sample sintered at 550 °C. The strategy for further improving the coercivity and remanence is discussed based on the microstructure-property relationships.

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.

Grain Boundary Phase Formation and Magnetic Properties of NdFeB/Nd Multilayered Films

Two kinds of NdFeB singlelayered and NdFeB/Nd multilayered films were prepared on each Mo substrate using a three-dimensional sputtering system. The singlelayered films annealed at 1073 K consisted of Nd2Fe14B main phase together with soft magnetic α-Fe phase as an impurity and only showed poor magnetic properties. However, for the multilayered films, the Nd-rich grain boundary phase among the Nd2Fe14B particles was more effectively formed in a similar manner as conventional NdFeB sintered magnets, so that the α-Fe impurity phase was completely disappeared and the numbers of surface defects on the main phase particles were decreased. The resultant multilayered film magnets of [NdFeB (650 nm)/Nd (10 nm)]4 gave high HcJ values around 1.58 MA/m for Br ≈1.21 T and (BH)max ≈279 kJ/m3, respectively.

The role of Cu addition in the coercivity enhancement of sintered Nd-Fe-B permanent magnets

To understand the mechanism of the coercivity enhancement by a trace addition of Cu in Nd-Fe-B sintered magnets, we investigated the microstructure difference between Cu-doped and Cu-free alloys using high resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), and laser assisted three dimensional atom probe (LA-3DAP). From a serial sectioning back scattered electron (BSE) images of the Nd-rich phase obtained by an integration of the focused ion beam (FIB) and HRSEM technique, it was found that Cu addition leads to a continuous formation of Nd-rich thin layers along the grain boundaries. 3DAP analysis has shown that a thin Cu-rich layer with a thickness of approximately 2 nm is present at the interface between the Nd2Fe14B and Nd-rich phase grains.

Effect of post-sinter annealing on the coercivity and microstructure of Nd–Fe–B permanent magnets

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.

Grain Interface Modification and Magnetic Properties of Nd–Fe–B Sintered Magnets

Effective grain interface modification was conducted to increase the coercivity of Nd–Fe–B sintered magnets without degrading their magnetic properties seriously. The surface of magnet pieces (3.0×3.0×2.8 mm3) was uniformly coated with a Dy or Tb metal film, and then each metal was introduced inside the magnets through the Nd-rich grain boundary regions for enriching them on the surface around the Nd2Fe14B grain particles. The coercivities of the Dy- and Tb-metal-treated magnets (HcJ = 1.45 and 1.81 MA/m, respectively) were considerably improved compared with that of the untreated magnet (HcJ = 0.98 MA/m), suggesting that the aligned magnetic domain structure was effectively stabilized by such a Dy- or Tb-enriched interface around the Nd2Fe14B grain particles.

種々の希土類化合物を用いて粒界拡散処理したＮｄ‐Ｆｅ‐Ｂ系焼結磁石の保磁力

In order to clarify the origin of coercivity enhancement of Nd-Fe-B sintered magnets by the Grain BoundaryDiffusion Process (GBDP), microstructural observations were performed and the coercivities of magnets processed with various rare-earth oxides were investigated. Microstructural analysis using FE-EPMA revealed that Dy supplied from the magnet surface diffused into whole magnets with a thickness of 2 mm. At the center of a magnet, the Dy distribution width around a grain boundary is almost comparable to the width of the Nd-rich grain boundary phase.When rare-earth (R) oxides whose R provide higher anisotropy in R2Fe14B than that of Nd2Fe14B, are used for GBDP, the magnets showed higher coercivity than that of the control, and vice versa. Although the diffusion depth of Dy into the Nd2Fe14B grains was unclear at the center of the magnet, the relationship between the kinds of rare-earth elements used for GBDP and the resultant coercivities strongly suggests that the rare-earth elements supplied from the surface of the magnets and absorbed along the grain boundary phase may diffuse into very thin portions of the Nd2Fe14B grain surface.

Microstructural studies and micromagnetic analysis of nanocrystalline NdFeCoMB (M = Ga, Ge) melt-spun ribbon

The effects of Ge and Ga additions on the microstructure and coercivity of melt-spun ribbons of Nd13.5 Fe(74.9−z) Co5.5 Mz B6 (z=0.0 to 0.54; M=Ge, Ga) alloys have been investigated. For the first time, a detailed microstructural study of nanometer-sized Nd-rich grain boundary phase enriched with small amounts of Ga and Ge was carried out using a three-dimensional atom probe (3DAP) technique. Ga partitions to thin uniform Nd-rich grain boundary phase, causing effective isolation of Nd2Fe14B grains. As a result, the coercivity increased. On the other hand, Ge caused the formation of Ge- and Nd-rich precipitates, resulting in the formation of dispersoids along the grain boundaries. Due to the poor isolation of the Nd2Fe14B grains, the coercivity decreased.

Ｎｄ‐Ｄｙ‐Ｆｅ‐Ｂ系焼結磁石の強磁場中熱処理と保磁力

The effect of a high magnetic field on coercivity was investigated in the annealing process for sintered Nd- Fe-B magnets containing Dy 10 wt.%. In a sample containing Cu 0.13 wt.%, a coercivity value Hc of 36 kOe was obtained for an annealing temperature Ta of 475°C under an annealing field Ha of 140 kOe. This value of Hc is 20% larger than that for the control sample (Ta = 475°C and Ha = 0). In samples without Cu, Hc values were considerably smaller (< 27 kOe) and no such enhancement of the coercivity by the magnetic fields was observed. The DSC curves exhibited a marked endothermic reaction just below 500°C only for samples containing Cu. This temperature is attributed to a eutectic point of Nd70Cu30. A possibility of field-induced alignment of Nd-rich grains in a matrix of Nd-Cu liquid is discussed; this would be closely related to the coercivity enhancement.