Nd-rich Grain Research Articles

Effects of Ag on the magnetic and mechanical properties of sintered NdFeB permanent magnets

Abstract This paper investigates the effects of Ag on the magnetic and mechanical properties of sintered NdFeB permanent magnets. The microstructure of the NdFeB permanent magnets was characterised using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) analysis. 0.1 wt% of Ag was added to the overall NdFeB composition to study the effects of Ag on the microstructural changes. The NdFeB and NdFeB + Ag magnets exhibit a two-phase structure with the main Nd2Fe14B phase and the Nd-rich grain boundary phase. It is found that Ag is enriched in the Nd-rich grain boundary phase and tends to alloy with the minor element, Cu. An enhanced coercivity (Hcj) is obtained in the NdFeB + Ag whilst the remanence (Br) and maximum magnetic energy product ((BH)max) decrease due to the addition of 0.1 wt% Ag. Impact resistance tests were carried out to examine the impact toughness of NdFeB and NdFeB + Ag magnets. It is shown that the magnets still exhibit a brittle fracture behaviour and no significant increase in the impact resistance of the NdFeB + Ag magnet is presented.

Coercivity enhancement of Nd-Fe-B sintered magnet by grain boundary diffusion process using Pr-Tb-Cu-Al alloys

Abstract Effect of Tb content and diffusion parameters (diffusion temperature and time) on boundary structure and magnetic properties of Nd-Fe-B sintered magnet was systematically studied by grain boundary diffusion process (GBDP) using Pr70−xTbxCu15Al15 (x = 5, 10, 15, 20, atom fraction) alloys. When x increases from 5 at% to 15 at%, coercivities of diffused magnets increase from the original 11.9 kOe to 22.4 kOe for 5 at% magnet (PTCA-5) and 23.5 kOe for 15 at% magnet (PTCA-15), with a slight decrease in remanence. With x increasing to 20 at% (PTCA-20), coercivity of diffused magnet is only 0.6 kOe higher than that of PTCA-15 magnet, with sharp decrease in remanence. So, Tb content in Pr-Tb-Cu-Al alloy should be limited. And when diffusion temperature and time increase from 600 °C and 2 h to 950 °C and 8 h, coercivities of diffused magnet linearly increase first, and then reach stable. The enhancement mechanisms for magnetic properties contribute to magnetic hardening by Tb-rich shells, magnetic isolation by continuous Nd-rich grain boundary phases and reduced lattice misfit between Nd-rich phases and Nd2Fe14B grains. Furthermore, distribution of Tb-rich shells along diffusion length can be optimized by adjusting Tb content in diffusion sources and diffusion parameters, contributing to coercivity enhancement of magnet.

Microstructure and magnetic properties of Nd-Fe-B sintered magnet by diffusing Pr-Cu-Al and Pr-Tb-Cu-Al alloys

Abstract Grain boundary diffusion process (GBDP) with low melting-point alloy ribbons, namely Pr70Cu15Al15 (PCA), Pr65Tb5Cu15Al15 (PTCA-5) and Pr50Tb20Cu15Al15 (PTCA-20, at%), was applied to Nd-Fe-B sintered magnet (52 M) as thick as 6 mm. The coercivities of diffused magnets by PCA, PTCA-5 and PTCA-20 increase significantly to 19.6 kOe, 24.1 kOe and 24.3 kOe, respectively, from original 14.7 kOe. Microstructural investigations indicate that the coercivity enhancement of PCA magnet is mainly attributed to magnetic isolation by continuous Nd-rich grain boundary phases (GBP). However, the coercivity enhancement of PTCA-20 magnet is mainly owing to magnetic hardening by Tb-rich shells (TRS), and that of PTCA-5 magnet results from a combined strengthening of magnetic isolation by GBP and magnetic hardening by TRS. Meanwhile, the coercivity of PTCA-5 magnet is as high as that of PTCA-20 magnet, but the former consumes Tb only 0.60 wt%, lower than 1.23 wt% for the latter.

Monitoring of the hydrogen decrepitation process by acoustic emission

Acoustic Emission Testing (AT) was applied to monitor the Hydrogen Decrepitation (HD) process for two different NdFeB-type materials: sintered NdFeB magnet and annealed NdFeB ingot alloy. Acoustic signals of clearly different characteristics were acquired for these two materials, indicating different course of the HD process. Acoustic waves of high signal strength and high peak amplitude were generated for the sintered NdFeB magnet whereas acoustic signals of low energy and low intensity were registered for the annealed NdFeB alloy. These findings were supported by real-time visual observation of the HD process. The sintered NdFeB magnet decrepitates rapidly and its particles spatter for a longer distance comparing to the ingot NdFeB alloy. Different behavior of the investigated materials can be explained in relation to their structural properties and manufacturing history. Fine microstructure, homogenously distributed thin layer of the Nd-rich grain boundary phase and stress accumulated after fabrication process can be responsible for more intense characteristic of the HD process in case of the sintered NdFeB magnets.

The sintering mechanism of fully dense and highly coercive Nd-Fe-B magnets from the recycled HDDR powders reprocessed by spark plasma sintering

The goal of this study was to justify the reprocessing of recycled HDDR Nd-Fe-B powders with spark plasma sintering (SPS) and to investigate the dependence of the final magnetic properties on SPS and thermal treatment. The initial recycled HDDR powder of the composition Nd13.4Dy0.6Fe78.6B6.1Nb0.4Al0.7 with 4760 ppm O2 content, coercivity (HCi) = 830 kA/m, and large particle size distribution <50–600 μm, was treated using the SPS parameters as follows: T = 650–850 °C for 1–5 min and 50 MPa pressure. The post SPS thermal treatment was performed at 750 °C for 15 min in vacuum. The optimal SPS conditions at 750 °C for 1 min, yielded fully dense magnets with the coercivity HCi = 1060 kA/m, which was boosted to 1160 kA/m after the post-SPS thermal treatment. The grain growth was obvious in samples SPS-ed above 800 °C and subsequently, the HCi was reduced. The SPS reprocessing beyond 850 °C was found to be detrimental to the overall magnetic properties due to the formation of bimodal grain size distribution origination from the abnormal grain growth (700–2600 nm). The redistribution of Nd-rich grain boundary phase between the Nd2Fe14B matrix grains in thermally treated magnets and the relaxation of the internal stresses induced via SPS are the possible reasons for the HCi surpassing the HCi of the starting recycled HDDR powder. It was shown that the SPS consolidation technique is suitable for producing fully dense nanograined bulk Nd-Fe-B magnets, with coercivities even exceeding the initial HDDR-powder and is one of the most suitable routes for revitalizing the Nd-Fe-B scrap magnets.

Origin of the coercivity difference in Nd-Fe-B sintered magnets processed from hydrogenation-disproportionation-desorption- recombination powder and jet-milled powder

Microstructures of Ga-doped ultra-fine-grained (UFG) Nd-Fe-B magnets processed from press-less sintering of He-jet-milled powder and hydrogenation-disproportionation-desorption-recombination (HDDR) powder have been investigated to understand the large coercivity difference in the two magnets with nearly the same grain size. While the sintered magnet processed from the jet-milled powder showed a very high coercivity of 2.14 T, the sintered magnet made from HDDR powder showed a low coercivity of only 1.31 T regardless of the similar grain size. Microstructure observations by scanning electron microscopy (SEM) and scanning transmission electron microscope (STEM) combined with energy-dispersive X-ray (EDS) analysis revealed that the Nd-rich grain boundary (GB) phase in the magnet produced from HDDR powder exhibits thickness of 1–2 nm with large variations in their chemical compositions, while the Nd-rich GB phase in the magnet made from He-jet-milled powder displays larger thickness variations (1–7 nm) along with significant increase of nonferromagnetic elements (Nd, Pr, Cu, Ga) in the GB phase after optimal annealing. The possible reasons for these differences in GB phases are discussed based on quantitative microstructure characterization results.

Anisotropic local hardening in hot-deformed Nd-Fe-B permanent magnets

The diffusion of low-melting Nd-Cu alloys is very effective to increase coercivity Hc in hot-deformed Nd-Fe-B permanent magnets without the use of heavy rare earth and to study the local hardening mechanism, especially the role of the Nd-rich grain boundary on the magnetic decoupling of the Nd-Fe-B grains on the nanoscale. In this study, we found that for a Nd-Cu diffusion parallel to the texture axis the increase in Hc is higher than for a diffusion perpendicular to it and strongly depends on the diffusion depth whereas remanence develops in an inverse manner. We note the following three observations to explain This behavior results from: a) a higher overall Nd and Cu concentration for the parallel diffusion revealed by global energy dispersive X-ray (EDX) maps leading to a distinct change in the broadness of the interaction domains visualized by Kerr microscopy, b) a higher degree of misalignment of the Nd2Fe14B grains observed by electron backscattered diffraction (EBSD), and c) a more effective local hardening on the macroscopic scale governed by dipolar and exchange interactions as modeled by micromagnetic simulations. The misalignment and the incorporation of Nd and Cu also lead to a volume expansion of the magnet of around 0.6–0.8% as proven by in-situ thermo-optical measurements (TOM).

Improved thermal stability of TbF3-coated sintered Nd–Fe–B magnets by electrophoretic deposition

Using electrophoretic deposition (EPD) method, the impact of TbF3 diffusion on the coercivity, microstructure and thermal stability of sintered Nd–Fe–B magnets with different rare earth (RE) content was investigated. In the diffused magnets with the RE content of 34 wt.%, the maximum coercivity about 28.12 kOe with less than 1.44 wt.% Tb was achieved, the coercivity temperature coefficient (β) was improved to -0.50 %/°C from -0.58 %/°C within the temperature interval 25-160 °C, and the maximum operating temperature further increased to about 160 °C. It suggested that TbF3 diffused magnets had much superior thermal stability than the annealed samples. This was attributed to the formation of the Tb-rich (Nd, Tb)2Fe14B phase in the outer region of the matrix grains and the improved Nd-rich grain boundary phase after TbF3 diffusion.

Post-sinter annealing influences on coercivity of multi-main-phase Nd-Ce-Fe-B magnets

Post-sinter annealing (PSA) plays an essential role on enhancing coercivity of the single-main-phase (SMP) Nd2Fe14B-based magnets due to the formation of continuous Nd-rich grain boundary (GB) layers to decouple the adjacent grains. Here we investigated the PSA effects on coercivity of the multi-main-phase (MMP) [(Nd,Pr)0.55Ce0.45]30.5FebalM1.0B1.0 (wt.%) magnet prepared by sintering the mixture of Ce-free and Ce-containing 2:14:1 powders, which has shown superior magnetic properties to the SMP one at the same average composition. The annealing temperature dependent investigation showed that the coercivity can be enhanced when annealing at low temperatures, but becomes even lower than that of the as-sintered state when annealing at higher temperatures, despite that continuous RE-rich GB layers are formed after annealing. When raising the annealing temperature, the RE gradients within the 2:14:1 phase grains are reduced to reach the ‘‘close to equilibrium (or SMP)’’ state due to the further elemental inter-diffusions among the Ce-lean and Ce-rich 2:14:1 grains, which weakens the magnetic impedance effect among them. These findings suggest a unique clue to enhance coercivity of the MMP RE-Fe-B magnets by PSA, i.e. the competitive contributions between the formation of continuous GB layers and the gradual chemical homogenization should be balanced.

Magnetic properties and magnetization behaviors of die-upset anisotropic (CeNd)-Fe-B multiphase magnets

The anisotropic hot deformed magnets consisting of Nd-rich and Ce-rich 2:14:1 main phases were prepared using dual alloy process. After heat treatment, crystallographic alignment was improved and compositional heterogeneity reduced. Coercivity and remanence descended with increasing Ce-Fe-B addition, and then were improved after diffusing heat treatment compared with magnets without heat treatment. Moreover, the magnetization behaviors of magnets were discussed and the coercivity mechanism was controlled by inhomogeneity of the domain wall pinning. The interaction domains were observed and the moment of Ce-rich grain was prior to reversal by comparison with Nd-rich grain in an applied field due to the lower magnetocrystalline anisotropy.

Effect of crushing methods on morphology and magnetic properties of anisotropic NdFeB powders

Crushing is a critical factor for the anisotropic NdFeB powders with high magnetic properties. However, the traditional crushing methods of disk-milling (DM) or impact crushing (IC), are difficult to obtain high-performance powders. Herein, a new method of preparing anisotropic powders with high performance was reported, which was through crushing the hot-deformed magnets perpendicular to c-axis direction (shear crushing, SC). The microstructure and alignment of the magnets were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results indicated that the cracks mainly extended along the Nd-rich grain boundary phase and/or the binding interface of ribbons. It not only could protect the magnetic phase from being damaged, but also could keep the grains integrated. Therefore, shear crushing was an effective method for the preparation of high-performance anisotropic powders and then to fabricate the bonded magnets with Br=9.0 kGs, Hci=10.48 kOe and (BH)max=17.7 MGOe.

Microstructure and Magnetic Properties of NdFeB Sintered Magnets Diffusion-Treated with Cu/Al Mixed Dyco Alloy-Powder

AbstractWe investigated the microstructural and magnetic property changes of DyCo, Cu + DyCo, and Al + DyCo diffusion-treated NdFeB sintered magnets. The coercivity of all diffusion treated magnet was increased at 880ºC of 1stpost annealing(PA), by 6.1 kOe in Cu and 7.0 kOe in Al mixed DyCo coated magnets, whereas this increment was found to be relatively low (3.9 kOe) in the magnet coated with DyCo only. The diffusivity and diffusion depth of Dy were increased in those magnets which were treated with Cu or Al mixed DyCo, mainly due to comparatively easy diffusion path provided by Cu and Al because of their solubility with Ndrich grain boundary phase. The formation of Cu/Al-rich grain boundary phase might have enhanced the diffusivity of Dy-atoms. Moreover, relatively a large number of Dy atoms reached into the magnet and mostly segregated at the interface of Nd2Fe14B and grain boundary phases covering Nd2Fe14B grains so that the core-shell type structures were developed. The formation of highly anisotropic (Nd, Dy)2Fe14B phase layer, which acted as the shell in the core-shell type structure so as to prevent the reverse domain movement, was the cause of enhancing the coercivity of diffusion treated NdFeB magnets. Segregation of cobalt in Nd-rich TJP followed by the formation of Co-rich phase was beneficial for the coercivity enhancement, resulting in the stabilization of the metastable c-Nd2O3phase.

Effects of Al/Cu co-doping on crystal structure and chemical composition of Nd-rich phases in Nd-Fe-B sintered magnet

We investigated the effects of Al/Cu co-doping on the magnetic and microstructural properties of a Nd-Fe-B sintered magnet. In particular, the structural and compositional changes of Cu-dissolved Nd-rich (Cu-rich) triple junction phase (TJP) and grain boundary phase (GBP) experienced upon Al/Cu co-doping were intensively analyzed. Al was selectively dissolved into the Culow-rich phases (B phase, 3–15 at.% Cu) other than the Cuhigh-rich phases (A phase, 20–45 at.% Cu). The B phase with an h-Nd2O3 structure was transformed to the C phase (Al-dissolved Culow-rich phases) with a c-Nd2O3 structure upon Al dissolution. In addition, the continuity of C GBP was improved and the Fe content of C GBP was reduced (25 → 5 at.% Fe). As a result, the coercivity (Hc) of the Al/Cu co-doped magnet was higher than that of a Cu-doped magnet. The critical role played by Al in improving the magnetic properties and microstructure of the Al/Cu co-doped magnet was elucidated. The detailed mechanism for the Hc enhancement of the Al/Cu co-doped magnet was discussed based on the results.

Corrosion evaluation for recycled Nd-Fe-B sintered magnets

The chemical stability is an important concern for Nd-Fe-B sintered magnets with respect to their practical applications. In our present study, recycled Nd-Fe-B sintered magnets with excellent magnetic properties have been fabricated from waste Nd-Fe-B sintered magnets as well as their corrosion behavior were investigated and compared with the original waste magnets. We find both magnets to exhibit approximately similar corrosion processes, corrosion rates, and magnetic properties. Moreover, potentiodynamic polarization curves and electrochemical impedance spectroscopy results reveal that both magnets exhibit similar corrosion thermodynamics and kinetics. For comparison, the recycled magnets have a slightly higher rare earth content and more Nd-rich grain boundary phase in their microstructure compared with those of the original waste magnets, but with a negligible influence to the corrosion resistance of the magnets. We conclude that the recycling process has no adverse influence on the performance of these magnets, thus demonstrating the applicability of the recycled Nd-Fe-B sintered magnets.

Coercivity enhancement of hot-deformed Nd-Fe-B magnet by grain boundary diffusion process using the reaction of NdHx and Cu nanopowders

In this study, the coercivity enhancement of hot-deformed Nd-Fe-B magnet by grain boundary diffusion process (GBDP) has been studied with a new method. The mixed powders of NdHx and Cu were used as a diffusion source instead of the Nd-Cu eutectic alloy powders in order to simplify the GBDP. Before GBDP, reactions of NdHx and Cu powders were examined by XRD patterns. The Nd-Cu eutectic reaction in the mixed powders was confirmed after heat treatment at temperature higher than 550 °C. In the GBDP, the amount of diffused Nd-Cu alloy increased with increasing heat treatment temperature. However, when the same amount of Nd-Cu is diffused, the coercivity increment of the heat-treated magnet at low temperature is higher than that of the heat-treated magnet at high temperature. It is observed that higher temperature can induce grain growth and composition change of the Nd-rich grain boundary phase. The large coercivity increment of approximately 8 kOe was achieved after GBDP at 600 °C for 1 h, where the diffused Nd-Cu was 5.85 wt%.

Enhanced coercivity thermal stability realized in Nd–Fe–B thin films diffusion-processed by Nd–Co alloys

A proposed Nd2Fe14B-core/Nd2(Fe, Co)14B-shell microstructure was realized by diffusion-processing textured Nd14Fe77B9 single-layer film with Nd100−xCox (x=10, 20 and 40) alloys to improve the coercivity thermal stability. The ambient coercivity was increased from around 1T in single-layer film to nearly 2T in diffusion-processed films, which was due to the Nd-rich grain boundaries as seen from transmission electron microscopy (TEM) images. The coercivity thermal stability was improved by the core/shell microstructure because Nd-rich grain boundaries provided the high ambient coercivity and Co-rich shell provided the improved coercivity stability.

Structure and chemical compositions of the grain boundary phase in Nd-Fe-B sintered magnets

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.

Anisotropic diffusion mechanism in grain boundary diffusion processed Nd–Fe–B sintered magnet

We investigated the anisotropic diffusion mechanism of Dy in a DyH2 dip-coated magnet in terms of both the crystal orientation of the Nd2Fe14B phase and the aligned direction of the magnet. A Dy-rich shell was formed preferentially at the interface, which is parallel to the [001] axis (c-axis) of the Nd2Fe14B crystal, during the grain boundary diffusion process (GBDP) because lattice diffusion of Dy perpendicular to the c-axis of the Nd2Fe14B crystal is much easier than that parallel to the c-axis. In contrast, the grain boundary diffusion depth in the direction perpendicular to the aligned direction (c-axis) of the magnet was much shorter (∼100 μm) than that in the direction parallel to the aligned direction (∼250 μm). Anisotropic grain boundary diffusion depending on the aligned direction of the magnet was not the result of the anisotropic Nd-rich grain boundary phase distribution but of anisotropic lattice diffusion depending on the Nd2Fe14B crystal orientation. Increasing the ratio of the surface areas perpendicular and parallel to the aligned direction (c-axis) of the magnet is expected to further improve the coercivity of the GBDP magnet.

3-D laser confocal microscopy study of the oxidation of NdFeB magnets in atmospheric conditions

Neodymium iron boron (NdFeB) magnets are used in a number of important applications, such as generators in gearless wind turbines, motors in electric vehicles and electronic goods (e.g.— computer hard disk drives, HDD). Hydrogen can be used as a processing gas to separate and recycle scrap sintered Nd-Fe-B magnets from end-of-life products to form a powder suitable for recycling. However, the magnets are likely to have been exposed to atmospheric conditions prior to processing, and any oxidation could lead to activation problems for the hydrogen decrepitation reaction. Many previous studies on the oxidation of NdFeB magnets have been performed at elevated temperatures; however, few studies have been formed under atmospheric conditions.In this paper a combination of 3-D laser confocal microscopy and Raman spectroscopy have been used to assess the composition, morphology and rate of oxidation/corrosion on scrap sintered NdFeB magnets. Confocal microscopy has been employed to measure the growth of surface reaction products at room temperature, immediately after exposure to air. The results showed that there was a significant height increase at the triple junctions of the Nd-rich grain boundaries. Using Raman spectroscopy, the product was shown to consist of Nd2O3 and formed only on the Nd-rich triple junctions. The diffusion coefficient of the triple junction reaction product growth at 20°C was determined to be approximately 4×10−13cm2/sec. This value is several orders of magnitude larger than values derived from the diffusion controlled oxide growth observations at elevated temperatures in the literature. This indicates that the growth of the room temperature oxidation products are likely defect enhanced processes at the NdFeB triple junctions.

Effect of WS2/Al co-doping on microstructural and magnetic properties of Nd−Fe−B sintered magnets

The effect of WS2 (0–1.2 wt.%) and Al (0.3 wt.%) doping on microstructural and magnetic properties of Nd2Fe14B sintered magnets was investigated. Grain growth in WS2-doped magnets was inhibited by the grain boundary (GB) pinning resulting in increased magnet coercivity. Al co-doping (0.3 wt.%) produced continuous Nd-rich grain boundary phases (GBPs), in which fine WFeB precipitates were distributed selectively at the grain boundaries due to the higher fluidity of the liquid phases. 0.6 wt.% WS2/Al co-doped magnets are characterized by the optimum magnetic properties because of the smaller grain sizes and formation of continuous Nd-rich GB phases. When the WS2 content exceeds 0.6 wt.%, magnetic properties of the resulting magnets deteriorate due to the reduction in magnet density and relatively large WFeB precipitates formed inside the main phase and at the GBs.