Intergranular Phase Research Articles

Comparative study of Pr/PrFe/PrAl grain boundary diffusion processed Nd–Ce–Fe–B sintered magnets with high Ce substitution: Evolutionary microstructure and magnetic responses

Owing to high natural abundance, surplus stockpile and low price of Ce, the Nd–Ce–Fe–B magnet possesses outstanding advantages as a novel permanent alternative with reduced critical rare earth (RE). However, the wide application of Nd–Ce–Fe–B magnet with high Ce substitution is still impeded by the low-coercivity issue. Here we report a comparative study of Pr/PrFe/PrAl grain boundary diffusion processed (GBDP) Nd–Ce–Fe–B sintered magnets to unravel the evolutionary microstructure and magnetic responses towards higher coercivity. Via Pr diffusion-induced microstructural engineering, continuous REFe2 and RE-rich/Fe-lean intergranular phases coexist at a long diffusion depth, with concurrent formation of thick Pr-rich shell surrounding RE2Fe14B grains. The resultant magnetic performance with Hcj = 14.4 kOe, Br = 13.2 kG, and (BH)max = 41.0 MGOe reaches the highest level reported so far upon high Ce substitution of 40 wt%, revealing that Pr GBDP successfully manages to overcome the coercivity-remanence trade-off. For PrFe GBDP, significant quantities of Fe are redistributed to inevitably form Ce-rich REFe2 precipitates at the outmost diffusion surface, which retard the diffusion efficiency and limit the depth of continuous Pr-rich GBs/shells compared to pure Pr diffusion. For PrAl GBDP, extra Al induces ferromagnetic RE–Fe–Al phase to replace REFe2 phase at a deeper diffusion depth, which limits the coercivity increment. Further micromagnetic simulations coincide with the experimental results, revealing the priority of regulating non-ferromagnetic GB phase towards high-coercivity Nd–Ce–Fe–B. Above findings not only deepen our understanding on distinct metallurgical behaviors of Pr/PrFe/PrAl diffusion, but also delight future work on developing Ce-rich Nd–Ce–Fe–B magnets.

Impact of PTFE particle size in designing BaTiO3 dielectric composites under the cold sintering process

The Cold Sintering Process (CSP) can provide opportunities to fabricate high-performance BaTiO3 dielectric composites with polymer materials that are typically difficult to impossible to co-process under a conventional sintering process. Therefore, we investigated the preparation process of BaTiO3 sintered body by CSP and integrated a well-dispersed intergranular polymer phase. In this study, we focused on preparing BaTiO3 and Polytetrafluoroethylene (PTFE) composites. We considered the importance of the particle size of the PTFE phase, and correlated the impact on the composite dielectric properties. Through fitting a general-mixing-law to the dielectric properties as a function of volume fraction, we could deduce more homogeneous composites obtained in using the 200 nm PTFE powders. In addition, the temperature dependent dielectric properties and field dependent conductivity of the composites was investigated. It was found that with the good dispersion of the PTFE can suppress the leakage current density in the dielectric composites.

High-coercivity SmFe12-based anisotropic sintered magnets by Cu addition

We achieved a record-high coercivity of 1.40 T in a SmFe12-based ThMn12-type anisotropic sintered magnet. This magnet was prepared by sintering jet-milled powder made from strip-cast alloys with a nominal composition of Sm8Fe73.5Ti8V8Al2Cu0.5, using the conventional process for anisotropic sintered magnet production. Detailed microstructural investigations conducted using scanning electron microscopy and transmission electron microscopy revealed that the formation of Fe-lean intergranular phases (with < 20 at.% of Fe) surrounding the fine SmFe12 phase grains resulted in the high coercivity. Nanobeam electron diffraction patterns showed that these Fe-lean intergranular phases (IGPs) exhibiting excellent wettability in this magnet are mainly composed of two phases: a hexagonal close-packed Mg-type Sm-based phase and a primitive cubic CsCl-type SmCu-based phase, which co-exist in the same intergranular region. Such Fe-lean IGPs minimize the intergranular exchange coupling in the SmFe12 main phase, which contributes to the high coercivity, as revealed by magneto-optical Kerr effect microscopy. Overall, this study provides detailed microstructure-coercivity correlations and insights for the future development of high-performance SmFe12-based sintered magnets.

Significant coercivity enhancement of Ga-doped sintered Nd-Fe-B magnet by grain boundary diffusion perpendicular to the c-axis

Grain boundary diffusion (GBD) process is the most effective approach for enhancing coercivity of Nd-Fe-B magnets with minimal consumption of heavy rare earth (HRE). However, understanding of GBD mechanism is still limited, especially on the anisotropic diffusion behavior within sintered magnets. Grain boundary (GB) phase is the key factor in determining the diffusion process, which acts as diffusion channel for HRE atoms. The effects of doping Ga on the grain boundary microstructure and diffusion behavior of Tb in the sintered Nd-Fe-B magnet were investigated. Sintered magnets with different Ga content (0.15 wt%, 0.45 wt%) were prepared. It should be noted that Tb was diffused perpendicular to the easy axis (c-axis) of the magnets. The coercivity increased more significantly to 2.802 T (1.006 T higher than that of the post-sinter annealed state) in magnets with high Ga content than the low Ga content magnets (2.291 T, 0.669 T higher than the post-sintered annealed state). Such a huge coercivity increment almost reached the same value by diffusing Tb along the easy axis reported by other researchers. Chemical compositions and microstructure of the GB phase were found to be key factors in influencing the HRE diffusion process. A considerable amount of Nd6Fe13Ga phase was observed near triple junctions after post-sinter annealing in the high Ga content magnets, which markedly decreased Fe concentration in the intergranular phase. In addition, amorphous intergranular metallic Cu-rich phases provided smooth diffusion channels for HRE atoms. (Nd1-x, Tbx)2Fe14B shell was much more obvious in the high Ga content magnets. Ga dopping induced novel modifications in diffusion behavior of HRE atoms and strongly influenced the magnetic properties of permanent magnets. The present approach provide new path for the manufacture of high performance magnets, which is interesting both for technical applications and fundamental studies.

Phase transformation kinetics and grain orientation of multi-cation doped (Yb, Sm) α/β-SiAlON ceramic tool materials

The densification behavior, phase transformation kinetics, and grain orientation of multi-cation doped (Yb, Sm) α/β-SiAlON ceramic tool materials prepared via spark plasma sintering (SPS) were studied. The results indicated that the (Yb, Sm) α/β-SiAlON ceramics were rapidly densified at 1540 °C. Various intergranular phases were formed due to the doping of rare earth oxides with different ionic radii. β-SiAlON precipitated ahead of α-SiAlON and elongated rapidly above 1600 °C. The β-SiAlON grains were preferred to orient in the direction perpendicular to the SPS pressure due to the dissolution/precipitation and grain rotation mechanism. The tendency of orientation decreased with the growth of β-SiAlON grains and the increase of liquid phase. The orientation of β-SiAlON grains resulted in anisotropic mechanical properties of (Yb, Sm) α/β-SiAlON ceramics, which fracture toughness measured in the perpendicular plane (⊥) was higher than that in the parallel plane (//).

Ultrahigh coercivity and excellent thermal stability of hot-deformed Nd-Fe-B magnets by the diffusion of (Pr,Nd)-Dy-Cu-Al

The effects of the addition of (Pr0.25Nd0.75)10Dy80Cu5Al5 alloy powders on the magnetic properties and microstructure of the hot-deformed Nd-Fe-B magnets were investigated in this work. Through the addition of (Pr0.25Nd0.75)10Dy80Cu5Al5, an ultrahigh coercivity of 3.14 T was obtained in a 6-mm-thick hot-deformed magnet, and the temperature coefficient of coercivity and remanence were improved from −0.475%/℃ and −0.145%/℃ to −0.383%/℃ and −0.110%/℃ at 20 −180 ℃, respectively, showing the excellent thermal stability. Microstructure observations indicated that the formation of thick nonferromagnetic intergranular phase, the reduction of grain size and aspect ratio and the formation of the (Nd,Pr,Dy)2Fe14B main phase were achieved after the addition of (Pr0.25Nd0.75)10Dy80Cu5Al5, which were the main reasons for the enhancement of coercivity and thermal stability.

Microstructure and properties of aluminum nitride ceramics prepared by aqueous gelcasting method with nontoxic curdlan as gelling agent

Aluminum nitride ceramics were prepared by aqueous gelcasting method and pressureless sintering technique in N2 atmosphere using Y2O3 as sintering additives with nontoxic curdlan as gel system. The solidification mechanism of curdlan was studied. The effects of curdlan content and solid content on the microstructure, relative density and flexural strength of green bodies were investigated. The influences of Y2O3 content and sintering soaking time on the microstructure and properties of sintered bodies were also studied. The results show that, as the temperature increases to 80 °C, the ceramic powders solidify through three-dimensional gel networks of curdlan during gelling process. The green bodies can be successfully fabricated through aqueous gelcasting method with modified powder as original materials. Suitable curdlan content and solid content contribute to fabricating green body with uniform microstructures and high flexural strength. The relative density and flexural strength of sintered bodies enhance as the Y2O3 content and soaking time increase. The flexural strength and thermal conductivity are about 107.5∼172.3 MPa and 75.2∼112.5 W/(m·K), respectively. The sintered body with 4 wt% Y2O3 soaking for 3 h exhibits the highest thermal conductivity because of appropriate relative density, uniform microstructure and reasonable intergranular phase distribution. The mechanical property and thermal conductivity of sintered bodies can be improved by optimizing the gelcasting process parameter, Y2O3 content, and soaking time. The nontoxic gelling system will have wide application for aqueous gelcasting ceramic with complex shape.

Crystallographic studies of the intergranular phases in sintered PrNd23Ce7Gd1.5FebalB0.93Cu0.35M1 magnet with commercial N40M magnetic properties

In this work, we focus on the modification of the Cu element in commercial N40M magnets toward the composition and structure of the grain boundary phase. The magnetic properties and microstructure of Ce-magnets with low and high Cu content were investigated. The Ce-magnet with high Cu-content has higher coercivity and a more continuous and homogenous grain boundary phase. The grain boundary phase’s structural shift is found to be the source of the coercivity change. The transmission electron microscopy study shows that the PrNd23Ce7Gd1.5FebalB0.93Cu0.35M1 magnet contains fcc and hcp type (Ce, Nd)-oxide phases, each with a different O concentration. The formation of the superlattice has been examined, and the superlattice spots show that the Cu-rich phases have an ordered structure.

Microstructure, thermostability and tribological behavior of composite CoCrFeNiTix high-entropy alloy coatings fabricated by laser cladding

CoCrFeNiTix (x = 0,0.25,0.5,0.75,1.0) high-entropy alloy (HEA) coatings were prepared on 45 steel by laser cladding, to explore the microstructural evolution, thermostability and the high-temperature tribological behavior of the coating. Ti-doping promotes more intergranular hard BCC phase and TiO2 intermetallics to precipitate from the ductile matrix of FCC phase, resulting in an increase in the averaged microhardness. The favorable thermostability of the heterogeneous microstructure and the oxide layer induced by friction endow the laser cladded HEA coating with excellent dry sliding wear-resistance in a wide temperature range from room temperature to 800 °C. Taking the place of the adhesive wear at room temperature, the oxidative wear becomes dominant in the wear process when the temperature exceeds 200 °C, contributing to the decrease in the wear rate before 600 °C. The formation of spherical particles on the wear track, as well as the oxide layer at elevated temperature, are revealed from the perspective of thermodynamic. It is found that the protective oxide layer is susceptive to delaminate due to the synergetic effect of strength reduction, internal stress accumulation and the destruction of the bonding, which explains the rise of the wear rate from its lower value of 1.84 × 10−5 mm3/(N·m) at 600 °C to 9.64 × 10−5 mm3/(N·m) at 800 °C.

Improvement in tensile strength of GH3536-TiB2 composites fabricated by powder metallurgy

To further improve the tensile strength of the GH3536 alloy (Hastelloy X), the GH3536-1 wt%TiB2 composites were prepared successfully by powder metallurgy. The results show that numerous M3B2 and TiC particles precipitated in the GH3536-TiB2 composite system. Interestingly, M3B2 particles were distributed along the grain boundaries in the composites, while TiC particles were in-situ synthesized along the prior particle boundaries of the spherical GH3536 powders. In addition, the grain size of the composite matrix was decreased to 6.7 μm compared with that of the GH3536 alloy (9.5 μm) since the grain growth being hindered by numerous intergranular phases, which further caused the increase of twin boundary proportion (from 26.5% in the GH3536 alloy to 38.8% in the composites). Consequently, the yield strength of the prepared composites was improved to 361 MPa and 298 MPa, respectively, from 240 MPa and 200 MPa, representing a roughly 50% improvement over the GH3536 alloy at 700 °C and 815 °C. The improvement in high-temperature strength is mainly ascribed to grain refinement strengthening, twin strengthening, hetero-deformation-induced hardening, and the additional grain boundary strengthening effect induced by secondary phases.

Microstructure regulation and optimizing magnetic properties of (La, Ce, Pr)17Fe78B6 alloy via short-range grain boundary diffusion

Increasing the utilization ratio of La and Ce in rare earth permanent magnets is an effective measure to solve the balance utilization of rare earth resources. In this work, nanocrystalline melt spun [(Ce0.7La0.3)0.3Pr0.7]17Fe78B6 powder and low melting point PrTbCu alloy were used as the initial material and the diffusion source for short-range grain boundary diffusion (SRGBD) to improve the coercivity. High coercivity of up to 18.21 kOe can be achieved, enhancing by 108.82% compared with the initial powder, and meanwhile, the remanence was only slightly reduced by 2.13%. After that, nanocrystalline bulk magnets with excellent magnetic performance were produced through spark plasma sintering (SPS). TEM observation reveals that the bcc-RE2O3 intergranular phase that induced by short-range grain boundary diffusion is distributed between adjacent matrix phase grains, which is conductive to the improvement of magnetic isolation effect. EDS results show that the Tb-rich shell with high magnetocrystalline anisotropy field has formed. Moreover, it is found that Tb diffuses into the 2:14:1 phase more easily than Pr element during the initial stage of diffusion. Enhanced magnetic isolation effect and the formed Tb-rich shell in grain surface contributed to high coercivity.

Grain boundary engineering in Nd-based ThMn12 magnets and their nitrides: A comprehensive study of challenges and limitations

Grain boundaries in rare earth permanent magnets are of major importance to optimize coercivity and densification during sintering. In this work, grain boundary engineering of Nd-based ThMn12 magnets and their nitrides was systematically investigated. First, the parent compound with excess Nd (5, 20 and 40 at%) was studied. It is shown that a paramagnetic dhcp-Nd grain boundary is formed with excess of Nd above 20 at%. This intergranular phase promotes the liquid-phase sintering process at ∼ 700 °C and a bulk nanostructured Nd1.2(Fe,Mo)12 sample with a relative density of 95 % using SPS was obtained. By further doping with Cu, the grain boundary phase properties were tuned by lowering its melting point below 500 °C. Next, the focus is on the nitrides which have a coercivity of ∼ 0.6 T. During the nitrogenation process, the dhcp-Nd grain boundary phase reacts with nitrogen to form a paramagnetic NdN phase, which does not influence coercivity. However, NdN inhibits liquid-phase sintering of the ThMn12 magnets, due to its high melting point (> 1500 °C), limiting the relative density to 65 % after SPS. Thus, a proper grain boundary for nitrides is yet to be found but this work elucidates in detail the specific challenges of grain boundary engineering in Nd-based ThMn12 magnets, non-nitrided and nitrided.

A Critical Review of High-Temperature Tribology and Cutting Performance of Cermet and Ceramic Tool Materials

Cermet materials exhibit advanced mechanical and tribological properties, and are widely used for tribology, elevated temperature, and machining applications due to their unique amalgamation of hardness, strength, and toughness. This paper presents a comprehensive overview of various cermet systems and recent advances in high-temperature tribology and cutting performance of cermet and ceramic tool materials. It outlines microstructural properties, such as lessening grain sizes, obtaining extended grains, lowering grain boundary phase content, amorphous grain boundary phases crystallizing, inter-granular phase strengthening, and managing crack propagation path. Additionally, surface processing or surface modifications, such as surface texturing, appropriate roughness, or coating technique, can optimize the ceramic and cermet tribological performances. The purpose of this study is to present some guidelines for the design of ceramics and cermets with reduced friction and wear and increased cutting performance. The current research progress concerning tribological properties and surface texturing of cutting tool inserts is critically identified. Lubrication techniques are required in commercial applications to increase the lifetime of cutting tools used in harsh conditions. Liquid lubricants are still commonly utilized in relative motion; however, they have the limitations of not working in extreme settings, such as high-temperature environments. As a result, global research is presently underway to produce new solid lubricants for use in a variety of such conditions. This review also provides a quick outline of current research on this topic.

Heterogeneous bimodal microstructure and its formation mechanism of Mg-Gd-Y-Zn-Zr alloy during isothermal compression

Heterogeneous bimodal microstructure and its formation mechanism of Mg-Gd-Y-Zn-Zr alloy during isothermal compression at 450 °C and 0.001 s−1 was investigated in detail. Based on microstructure analysis, Mg-Gd-Y-Zn-Zr alloys with bimodal grain size distribution and bimodal texture distribution were obtained by uniaxial isothermal compression. There were more heavily bimodal levels in grain size and texture at true strain greater than 0.8. There was a significant inhomogeneous plastic deformation between and within the grains in Mg-Gd-Y-Zn-Zr alloy during the deformation, which is attributed to the difference the grain orientation and a large number of intergranular block-shaped LPSO phases. Further studies revealed that particle-stimulated nucleation (PSN) caused by a large intergranular LPSO phases and continuous dynamic recrystallization (CDRX) developed layer by layer toward the grain interior were the main grain refinement mechanisms. This PSN and CDRX mechanism contributed to the fine grains in bimodal grain size distribution and the random texture in bimodal texture distribution. However, the original coarse grains with basal orientation were preserved during deformation due to smaller plastic deformation and energy storage. Meanwhile, grain boundary segregation and intragranular lamellar LPSO precipitate promoted CDRX by retarding DDRX. In addition, the grain boundary pinning effect dominated by grain boundary segregation and intragranular lamellar LPSO precipitate hindered the coarsening of fine DRXed grains, which was the ultimate guarantee for the formation of bimodal microstructure. Ultimately, inhomogeneous plastic deformation, DRX, and grain boundary pinning effect were demonstrated as the necessary conditions for the formation of heterogeneous bimodal microstructure during thermal deformation of Mg-Gd-Y-Zn-Zr alloy.

Controlling dynamic recrystallization via modified LPSO phase morphology and distribution in Mg-Gd-Y-Zn-Zr alloy

Featured initial microstructures of Mg-11Gd-4Y-2Zn-0.5Zr alloy (wt%) were obtained by adjusting temperatures of solid solution and cooling methods, including island intergranular 18R and 14H LPSO phases with low-density stacking faults, differentially spaced lamellar intragranular 14H-LPSO phases, and network intergranular 18R-LPSO phases with high-density intragranular stacking faults. Effects of these featured LPSO phases and stacking faults on dynamic recrystallization (DRX) behavior were investigated via hot compression. Promoted DRX behavior via particle stimulated nucleation (PSN) is introduced by coexisting intergranular island 18R and 14H LPSO phases and intragranular wide spacing lamellar 14H-LPSO phases, contributing the highest DRX fraction of 42.6%. Conversely, it is found that DRX behavior with network intergranular 18R-LPSO phases and dense intragranular stacking fault is considerably inhibited with the lowest fraction of 22.8%. That is, the restricted DRX due to dislocations pinning by stacking faults overwhelms the enhanced DRX behavior via PSN of island intergranular 18R and 14H LPSO phases. Specially, compared with dense intragranular lamellar 14H-LPSO phases, high-density stacking faults exert a larger inhibition effect on DRX behavior.

Effect of HfSi2‐induced intergranular liquid phase on densification behavior of ZrB2

AbstractThe densification behavior and mechanical properties of ZrB2‐based composites were investigated. The results demonstrated that the fully dense ZrB2‐based composites could be obtained at lower sintering temperature (1600°C) and pressure (30 MPa) when the content of HfSi2 was above 20 vol.%. The as‐sintered composite was a special core–shell structure, with ZrB2 as the core and (Zr, Hf)B2 solid solution as the shell. The core–shell structure resulted from the diffusion of Hf atom into the boride matrix, which could accelerate the densification. In addition, the intergranular liquid phase induced by the HfSi2 addition filled the micropores of the composites effectively during the sintering. When the content of HfSi2 increased to 20 vol.%, its compressive strength, hardness, and fracture toughness all reached the maximum values, which were 1617 MPa, 15.99 GPa, and 2.44 MPa m1/2, respectively.

Microstructure and magnetic property optimization for Co-rich dual-main-phase Nd-(Fe, Co)-B sintered magnet

High-Co-containing RE-(Fe, Co)-B (RE, rare earth) sintered magnets by dual-main-phase route (DMP, sintering the mixture of Co-free and Co-containing RE2Fe14B powders) were developed in this work, which effectively suppresses the formation of Co-rich intergranular phases and significantly increases the Co-substitution level in Nd-Fe-B magnets. Magnetic properties of Br = 13.74 kGs, Hcj = 13.09 kOe, and (BH)max = 44.32 MGOe were obtained even when 35 at. % Co substitutes for Fe in the dual-main-phase magnets. Phase components analysis indicated that the typical RE(Fe, Co)2 and RE(Fe, Co)4B were detected until the relative concentration of Co up to 35 at. %, which are commonly conceived as harmful to magnetic properties. Microstructural analysis revealed the distribution of each intergranular phase at grain boundaries and the inhomogeneous distribution of RE in the core and shell of the grain. Compared to the addition of Co through alloying, introducing Co by mixing RE-(Fe, Co)-B and RE-Fe-B allows a metallurgical reaction between Co-rich intergranular phases and 2:14:1 matrix phase, accompanied by dramatical elemental interdiffusion during the sintering, thus the formation of Co-rich intergranular phases can be suppressed to a certain extent. These efforts provide another possibility to obtain Nd-Fe-B magnets with higher Co content and achieve better thermal stability.

Induction brazing of NdFeB magnet with PVD-deposited AgCu alloy

The NdFeB magnets service in high-power electric-mechanical conversion devices sometimes endure high stress and high-temperature impact, and adhesion of these magnets using glue cannot ensure the reliability. In this study, the NdFeB was induction brazed with physical vapor deposition (PVD)-deposited Ag72Cu28 alloy layer, then the microstructure, mechanical properties and magnetic properties of the brazed joints were characterized, and effects of the filler metal thickness and the induction heating time were analyzed. The results show that the PVD coating can effectively prevent the surface oxidation of the NdFeB. The AgCu filler metal penetrates into the NdFeB and is partly mixed with the liquefied Nd-rich intergranular phase of the NdFeB, forming a good metallurgical bonding. The thickness of the penetration layer increases obviously with increase of the heating time and filler metal thickness. The bonding strength of the NdFeB brazed joint increases firstly and then decreases with increasing heating time, and a peak bending strength of 196.94 MPa was obtained. Fracture of the brazed joint occurs inside the NdFeB substrate close to the penetration layer. After a further annealing, the coercivity of the brazed NdFeB sample is a little higher than that of the un-brazed sample, and the squareness decreases by only a little. Properties of the brazed joints might be further improved by optimizing the amount of filler metal and the induction heating parameters.

High-resistivity anisotropic hot-deformed Nd-Fe-B magnets prepared from DyF3 electrophoretic deposited powders

The electrical resistivities of anisotropic Nd-Fe-B-based magnets were successfully increased in both parallel and perpendicular directions to the c-axis by hot-deforming Nd-Fe-B melt-spun flakes that were coated with DyF3 using electrophoretic deposition. Consequently, the operating temperature under high-frequency AC magnetic field was reduced by 20 °C because of the reduction of eddy current loss. The resistive layer formed at the interface of original ribbons in the hot-deformed magnet was found to be NdF3 rather than DyF3. Transmission electron microscopy and atom probe tomography revealed that liquid Nd-rich intergranular phase reduces DyF3 to form NdF3 at the ribbon interfaces during hot-deformation at elevated temperature. Excess Dy diffuses into the original ribbon flakes through grain boundaries to form a Dy-rich shell in Nd2Fe14B grains, giving rise to a high coercivity of 1.8 T. This study demonstrates an alternative way to solve the thermal demagnetization problem of Nd-Fe-B-based permanent magnets during operation in electric motors via the resistivity increase of the magnet.

Elevated corrosion resistance and improved magnetic properties of sintered Nd-Fe-B magnets via the infiltration of DyCr film

The low coercivity at high temperature and poor corrosion resistance limit the application field of Nd-Fe-B magnets. In this work, a DyCr film was prepared on the surface of Nd-Fe-B magnet through magnetron sputtering, and then subjected to grain boundary diffusion (GBD) to improve magnetic properties and enhance corrosion resistance. The results show that the residual Cr-rich film exists on the surface of NdFeB magnets, acting as a protective coating to enhance the corrosion resistance, which is evidenced by the weight loss experiment and scanning electron microscopy (SEM) image. Moreover, Cr element in the grain boundary phase increase the electrochemical potential from − 0.985 to − 0.733 V, which is also beneficial in improving corrosion resistance. Transmission electron microscope (TEM) analysis confirmed that the crystal structure of intergranular phase remained unchanged after diffusion, i.e. BCC-RE2O3 (RE=Rare Earth) phase. The coercivity was improved significantly, obtaining an increment of 19.4% comparing to that of initial magnet, which is attributed to the formed Dy-rich shell with high magnetocrystalline anisotropy field and the improved distribution of grain boundary phase. This suggests that it is feasible to fabricate high coercivity magnets and strong corrosion resistance only through grain boundary diffusion, which is expected to further shorten the processing flow of Nd-Fe-B magnets.