Grain Boundary Diffusion Process Research Articles

Effect of Heat Treatment Time on Dy–Cu Alloy Diffusion Process in Dy-Containing Commercial Nd–Fe–B Sintered Magnets

In this paper, the grain boundary diffusion process (GBDP) using a Dy70Cu30 (at.%) alloy as the diffusion source was performed in a commercial sintered Nd–Fe–B magnet, and the effect of heat treatment time on the microstructure and magnetic properties of the magnet was investigated in detail. For the processed magnets heat-treated at 860 °C, as heat treatment time increased, the coercivity and the depth of (Nd,Dy)2Fe14B core–shell structure increased first and then decreased. However, when the heat treatment time was more than 2 h, the diffusion path of Dy from the Dy-rich shell phase into the Nd2Fe14B grains was revealed, and a nearly homogeneous (Nd,Dy)2Fe14B phase was formed, which brought on the decrease in both the depth of visible core–shell structure and the coercivity of Nd–Fe–B magnet.

Grain boundary diffusion of different rare earth elements in Nd-Fe-B sintered magnets by experiment and FEM simulation

In the present work, we explore the influence of a surface-bulk coercivity gradient in Nd-Fe-B magnets produced by the Grain Boundary Diffusion Process (GBDP) on the overall coercivity. In our systematic and comprehensive study we diffused four different rare earth elements (Dy, Tb, Ce and Gd) in two different kinds of commercial Nd-Fe-B magnets, one very Dy-lean and one Dy-rich. By means of cutting the magnets into thin slices we obtain lateral coercivity profiles, from which diffusion constants are extracted. We find that in both magnets Tb diffuses significantly faster than Dy. The diffusion is generally slower in the Dy-lean magnet, which is attributed to the different chemistry and a smaller grain size. Ce diffuses slightly slower than Dy and the overall coercivity decrease is similar for Ce and Gd. With scanning electron microscopy it is revealed that, contrary to the magnets diffused with the heavy rare earths, the microstructure in the magnets treated with Ce show no (Nd,Ce)-Fe-B shells in the surface regions. While not of practical importance this allows some interesting insights into the metallurgy of (Nd,Ce)-Fe-B system. High-resolution scanning transmission electron microscopy coupled with electron probe microanalysis show the nano-scale distribution of Tb around the grain boundaries located in the bulk of the magnet. Finally, a simple model for the magnetization reversal in grain boundary diffusion processed gradient Nd-Fe-B magnets was developed and implemented into a FEM software. Our calculated demagnetization curves correspond very well for the Dy and Tb samples, but deviate significantly for Ce and Gd.

Grain Growth Behavior of Inconel 625 Superalloy

The grain growth (GG) behavior of Inconel 625 superalloy was studied in the temperature range of 900—1250 °C and holding time range of 10—80 min. Microstructures of the alloy were characterized by optical metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Grains grew obviously with either increasing temperature or extending holding time at temperatures above 1050 °C. However, at temperatures lower than 1050 °C, the GG was sluggish due to the pinning effect of carbide particles on grain boundary (GB). Threshold temperature for transition from mixed grain structure to uniform one was considered to be around 1100 °C. Once the temperatures surpassed 1200 °C, an instant increase in the grain size occurred showing no dependence on holding time. TEM analysis showed that the dominant second phase formed heterogeneously on the GB was M6C, which significantly impeded grain growth. On the basis of experimental data, the mathematical model of GG was established, which can describe GG behavior of Inconel 625 alloy during solution treatment (ST) at 1100—1250 °C. The activation energy for GG of Inconel 625 alloy was 207. 3 kJ, which suggested that the GG of Inconel 625 alloy was controlled by the process of GB diffusion.

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%.

Grain boundary diffusion in nanocrystalline Nd-Fe-B permanent magnets with low-melting eutectics

In order to combine the good thermal stability of nanocrystalline Nd-Fe-B magnets with the efficient grain boundary diffusion process (GBDP), low-melting eutectics have been mixed with Nd-Fe-B melt-spun ribbons, hot-compacted and subsequently die-upset. Transmission electron microscopy (TEM) analysis revealed the formation of 5–10 nm thick Dy-shells and a crystallography dependent diffusion into the individual solid grains on the nanoscale. Subsequent annealing at 600 °C leads to an enhanced diffusion but also some undesired Nd-O phases and induced grain growth depending on the annealing time, degree of deformation, strain rate and position within the sample. An optimized distribution of the low-melting eutectic was realized by milling the precursor powder and by using ternary alloys with reduced melting points. As a consequence a much higher effective increase in coercivity per wt%Dy was obtained compared to a homogeneous Dy-distribution which demonstrates the validity of this new approach.

High-coercivity Nd-Fe-B magnets obtained with the electrophoretic deposition of submicron TbF3 followed by the grain-boundary diffusion process

Using a grain-boundary diffusion process (GBDP) involving the electrophoretic deposition (EPD) of submicron TbF3 powder, we substantially increased the coercivity of sintered Nd-Fe-B permanent magnets. The experiments used magnets with low heavy-rare-earth (HRE) content (HRE = 1.2 wt%) and a coercivity of 790 kA/m (at 75 °C). After experiencing optimized conditions at 875 °C for 10 h and subsequent annealing at 500 °C for 1 h, the coercivity was increased to 1536 kA/m (at 75 °C). This value is 1.94 times higher than that for a sintered magnet, without post-sintering heat treatment. Furthermore, a vibration test revealed satisfactory adhesion of the TbF3 powder to the surface of the magnet with no detected reduction in coercivity. Using field emission gun scanning electron microscopy (FEG-SEM) with an energy dispersive spectroscope (EDS), we confirmed the formation of various secondary intergranular phases and the core-shell-type microstructure, which increases the coercivity. The Tb content in the magnet, exposed to the EPD-based GBDP, was controlled by inductively coupled plasma optical electron spectroscopy (ICP-OES). The additional Tb detected in the magnet due to the described technology was only 0.12 wt%.

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.

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 <TEX>$700^{\circ}C$</TEX>, 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.

Lanthanide (Nd, Gd) compounds with garnet and monazite structures. Powders synthesis by “wet” chemistry to sintering ceramics by Spark Plasma Sintering

Complex oxide Y2.5Nd0.5Al5O12 with garnet structure and phosphates NdPO4 and GdPO4 with monazite structure were obtained by using precipitation methods. Ceramics Y2.5Nd0.5Al5O12 and NdPO4 were processed by Spark Plasma Sintering (SPS). Relative density more 98%, sintering time did not exceed 8 min, sintering temperature 1330–1390 °C. Leaching rates of elements from ceramics were 10−6–10−7 g/(cm2 d). The process of ceramics sintering has two-stage character: the first step of sintering-compaction process is related to the plastic flow of the material, the second step–to the process of grain boundary diffusion and grain growth.

Large-scale micromagnetic simulation of Nd-Fe-B sintered magnets with Dy-rich shell structures

Large-scale micromagnetic simulations have been performed using the energy minimization method on a model with structural features similar to those of Dy grain boundary diffusion (GBD)-processed sintered magnets. Coercivity increases as a linear function of the anisotropy field of the Dy-rich shell, which is independent of Dy composition in the core as long as the shell thickness is greater than about 15 nm. This result shows that the Dy contained in the initial sintered magnets prior to the GBD process is not essential for enhancing coercivity. Magnetization reversal patterns indicate that coercivity is strongly influenced by domain wall pinning at the grain boundary. This observation is found to be consistent with the one-dimensional pinning theory.

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.

Effect of oxygen content of Nd–Fe–B sintered magnet on grain boundary diffusion process of DyH2 dip-coating

We investigated the effect of oxygen content on the microstructural and magnetic properties of a DyH2 dip-coated Nd–Fe–B sintered magnet. When the magnet had a low oxygen content (1500 ppm), the volume and size of the rare-earth-rich oxide (Nd–Dy–O) phase was reduced, and a uniform and continuous thin Nd-rich grain boundary phase (GBP) was well developed. The grain boundary diffusion depth of Dy increased from 200 to 350 μm with decreasing oxygen content from ∼3000 to 1500 ppm. The coercivity of the low-oxygen magnet increased from 19.98 to 23.59 kOe after grain boundary diffusion process (GBDP) while the remanence reduction was minimized. The formation of an fcc-NdOx Nd-rich phase in the high-oxygen magnet hindered the formation of a Nd-rich triple-junction phase and GBP. In contrast, a metallic dhcp-Nd phase, which was closely related to coercivity enhancement after GBDP, was formed in the low-oxygen magnet.

Enhancement of Coercivity of Nd-Fe-B Ultrafine Powders Comparable With Single-Domain Size by the Grain Boundary Diffusion Process

Nd-Fe-B sintered magnets are widely used in many applications. However, for the use in motors for hybrid electric vehicles (HEV), the development of Dy-free or Dy-lean Nd-Fe-B sintered magnets with high coercivity is desirable. Decreasing grain size of the Nd 2 Fe 14 B phase is an established method for increasing coercivity of Nd-Fe-B sintered magnets. We decreased the size of jet-milled powder to around 1 mm by helium (He) jet-milling in Dy-free Nd-Fe-B sintered magnets and obtained high coercivity (μ 0 H cJ ) of 2 T with (BH) max of around 400 kJm−3 [1]. We also succeeded further decrease in the size to 0.3 μm by combining hydrogenation-disproportionation-desorption-recombination (HDDR), hydrogen decrepitation (HD) and He jet-milling [2, 3]. However, the effect of annealing on magnetic properties of the ultrafine powders was not clear. On the other hand, the grain boundary diffusion (GBD) process [4-6] is well known as an effective method for the enhancement of coercivity. However, the effect for the ultrafine powders has not been investigated. Therefore, in this investigation, we investigated the effects of annealing and GBD process on coercivity of the Nd-Fe-B ultrafine jet-milled powders prepared by our developed process which combined HDDR, HD and He jet-milling.

Methods of compacting nanostructured tungsten–cobalt alloys from Nanopowders obtained by plasma chemical synthesis

The paper summarizes the experience of using traditional and modern methods of sintering WC–Co nanopowders obtained by plasma chemical synthesis. A comparative analysis of structure formation processes occurring at sintering WC–Co nanopowders in a quasi-steady and high-speed heating is carried out. It is shown that the basic regularities of structure evolution during sintering are rather general in nature and are similar both to conventional vacuum sintering and to high-energy compaction methods. It was established that, under high-speed heating conditions, a significant contribution to the acceleration of the sintering of nanopowder materials at low temperatures is made not only by a small grain size but also the process of grainboundary diffusion. It is shown that the samples of hard alloys sintered from tungsten carbide nanopowders produced by plasma chemical synthesis have significantly higher hardness and fracture toughness than those produced using conventional synthesis techniques and compacting. Using the technology of plasma chemical synthesis and spark plasma sintering, nanostructured WC and WC–Co samples with significantly better mechanical properties (hardness, fracture toughness) than those of conventional fine materials were obtained.

Coercivity enhancement of a Nd–Fe–B sintered magnet by diffusion of Nd70Cu30 alloy under pressure

Application of pressure during grain boundary diffusion process (GBDP) using Nd 70 Cu 30 eutectic alloy enhances the coercivity greatly from 1367 kA/m in the annealed state to 1660 kA/m. The uniformity of coercivity distribution was improved due to the large effective diffusion depth under pressure. Continuous grain boundary phase was observed both on the surface and in the center of the magnet after GBDP under pressure. Mechanism of coercivity enhancement due to micro cracks is discussed based on the observation.

Simultaneous application of Dy–X (X = F or H) powder doping and dip-coating processes to Nd–Fe–B sintered magnets

We investigated the microstructural and magnetic property changes of Nd–Fe–B sintered magnets with the application of simultaneous processing via powder doping and dip-coating, using DyF3 and/or DyH2. The simultaneous use of DyF3 powder doping and DyH2 dip-coating has a synergistic effect of improving the magnet’s coercivity (Hc) while minimizing the reduction of remanence (Br). DyF3 powder doping suppresses the formation of the rare-earth segregated oxide phase (RE-rich phase, Dy–Nd–O or Nd–O), dramatically improving the grain boundary diffusion depth of Dy (from 250 to 600μm) during the grain boundary diffusion process (GBDP). The RE-rich phase in Nd–Fe–B magnets is a major obstacle for the grain boundary diffusion of Dy because of the high surface energy of the RE-rich phase and the consumption by the non-stoichiometric RE-rich phase of diffused Dy to form a stoichiometric (Nd,Dy)2O3 phase. The slight reduction of the relative density of the as-sintered DyF3-doped magnet (from 98.1% to 97.5%) may have also helped to improve the grain boundary diffusion depth of Dy during the GBDP when the magnets were dip-coated with DyH2.

Faceted shell structure in grain boundary diffusion-processed sintered Nd–Fe–B magnets

Dysprosium enriched shell structure formed by the grain boundary diffusion process (GBDP) of a sintered Nd–Fe–B magnet was characterized by using scanning electron microscopy, electron back-scattered diffraction and transmission electron microscopy. Faceted core–shell interfaces with an abrupt change in Dy concentration suggest the Dy-rich shells are formed by the solidification of the liquid phase during cooling from the GBDP temperature. The Nd-rich phases are almost free from Dy, and their quantity near the surface of a bulk sample is much higher than that in the center, indicating that a higher fraction of a liquid phase exists near the surface during processing at 900°C. These microstructural features are explained on the basis of the phase equilibrium between Nd and Nd2Fe14B at the processing temperature and subsequent cooling. Based on the results, we discuss the coercivity enhancement by the GBDP using Dy vapor.

High-coercivity hot-deformed Nd–Fe–B permanent magnets processed by Nd–Cu eutectic diffusion under expansion constraint

The application of expansion constraint during the grain-boundary diffusion process using Nd70Cu30 eutectic alloy minimizes the remanence loss for the coercivity enhancement by optimizing the volume fraction of the Nd(Cu)-rich non-ferromagnetic intergranular layer. The diffusion-processed sample exhibits μ0Hc ∼ 2 T, μ0Mr = 1.36 T and (BH)max = 358 kJ m−3 at room temperature. Because of the low-temperature coefficient of coercivity, the magnet showed (BH)max ∼ 191 kJ m−3 at 200 °C, which is superior to that of Dy-containing high-coercivity sintered magnets.

Effects of accommodated grain boundary sliding on triple junction nanovoid nucleation in nanocrystalline materials

A theoretical model is suggested to investigate the effect of grain boundary (GB) sliding with GB diffusion accommodation on triple junction nanovoids nucleation in deformed nanocrystalline materials. Within our description, at high strain rate, nanovoids are nucleated due to GB sliding which is non-accommodated at comparatively low temperature and effectively accommodated by GB diffusion process at elevated temperature. In this paper, the corresponding energy characteristics of the pile-up of GB dislocations caused by GB sliding and GB diffusion are calculated, respectively. Then, the nucleation of nanovoid at triple junctions is studied and the equilibrium radius of nanovoid is obtained. Calculation results show that: the nucleation and the equilibrium radius of the triple junction nanovoid both depend significantly on the shear stress, the length of the pile-up and the GB structures; the accommodated GB sliding results in the suppression of nanovoid generation, and thereby promotes the ductility deformation behavior in a nanocrystalline material.

Effect of DyF3 on the corrosion behavior of hot‐pressed Nd–Fe–B permanent magnets

The grain boundary diffusion process (GBDP) with DyF3 is a promising method to obtain excellent magnetic properties in Nd–Fe–B permanent magnets. Compared to conventionally prepared (Dy,Nd)–Fe–B magnets the overall content of the precious heavy rare earth Dy is reduced. Thus, the GBDP with DyF3 is highly attractive to cut down costs and save resources. However, it is not well known how the addition of DyF3 affects the corrosion resistance of Nd–Fe–B permanent magnets. In this work, the anodic polarization behavior of hot‐pressed Nd–Fe–B magnets with and without DyF3 additions was investigated in electrolytes of different pH values. The results were compared with measurements on a commercially sintered Nd–Fe–B permanent magnet. In the polarization studies the only observed significant impact of DyF3 additions is an increased passive current density in alkaline media. The pitting susceptibility in NaOH solutions containing chloride ions is not affected by DyF3. Also in contrast to expectations, first free corrosion studies indicated no significant impact of the DyF3 additions on the dissolution rate.