Recent progress of grain boundary diffusion process for hot-deformed Nd-Fe-B magnets

Grain boundary diffusion process(GBDP)was first proposed for sintered Nd-Fe-B magnets to achieve the high utilization efficiency of heavy rare earth elements.Recent success of fabricating high performance nanocomposite magnets by GBDP indicates that this method also exerts huge applicable potential on hot-deformed Nd-Fe-B magnets.In this review,the development and magnetic property enhancement mechanisms of different diffusion methods proposed on hot-deformed magnets were thoroughly elucidated.Moreover,the improve room for further property enhancement and the accompanying problems of GBDP on hot-deformed magnets are also discussed in this article.

Enhancement mechanism of maximum energy product in hot-deformed Nd-Fe-B magnets by addition of ferromagnetic substances

It has been proved that the maximum magnetic energy product(BH)maxof hot-deformed Nd-Fe-B magnets can be effectively improved by adding substances with either high melting point or high saturation magnetization.In this work,we selected a ferromagnetic nano Fe_(55)Ni_(28)Co_(17) alloy powders with both high melting point and high saturation magnetization as the dopant to improve the(BH)max.By the addition of 1 wt% FeNiCo,the remanence increases by over 5%(from 1.36 to 1.44 T),resulting in a significant enhancement of(BH)maxfrom 355 to 396 kJ/m^(3)(from 44.6 to 49.7 MGOe).Microstructure observations reveal that the texture of grain alignment is improved,the concentrations of ferro magnetic elements(Fe,Ni,and Co) in the main phase and intergranular phase are increased so that the magnetization behavior of hot-deformed magnet changes(more easily shows reversible magnetization behaviors),which are the reasons why the high maximum energy product of hot-deformed magnet,is obtained.

Effect of Nb doping on microstructure and magnetic properties of hot-deformed Nd-Fe-B magnets with Nd-Cu eutectic diffusion

To restrict grain growth in coarse grain regions caused by the diffusion of Nd-Cu eutectic alloys,the Nb element was introduced into the precursor alloy to regulate the microstructure of melt-spun powder and die-upset magnets.The magnetic properties and thermal stability of die-upset magnets were appreciably improved through the addition of Nb.For the Nb-doped diffusion die-upset magnet,the grains inside the ribbons were refined and the coarse non-oriented surface crystallite got suppressed on the interface of ribbons during the hot-deformation process to form the anisotropic magnet.Moreover,Nd gathers at the intergranular phases,which is considered to enforce domain wall pinning force.The Nb-modified microstructure is advantageous to thermal stability and coercivity enhancement.High-resolution transmission electron microscopy images revealed that the Nb element gathered on the grain boundary and triple grain boundary to form c-Nb and h-Nb Fe B to hinder the grain growth during the hot-deformation process,which led to direct enhancement in the coercivity.Furthermore,the c-Nb and h-Nb Fe B are nonmagnetic phases that strengthened the magnetic isolation.However,the h-Nb Fe B precipitated from the hard magnetic phase and formed crystal defects which led to remanence deterioration.

Effect of Pr-Ga dual-alloys diffusion on magnetic properties and thermal stability of hot-deformed PrNd-Fe-B magnets

To investigate the influence of the addition of Pr-Ga alloys on magnetic properties and morphology of materials,the hot-deformed PrNd-Fe-B magnets were prepared by the addition of Pr-Ga alloys using a dual-alloys diffusion.The room-temperature coercivity of the hot-deformed PrNd-Fe-B magnets increases substantially from 1.68 to 2.34 T,while the remanence decreases from 1.42 to 1.24 T,by the addition of 5 wt%Pr-Ga alloys.Moreover,the thermal stability of coercivity improves from-0.46%/℃to-0.42%/℃.Two types of grain boundary phases(PrNd-rich and PrNd-Ga-rich)are generated at grain boundaries by microstructural analysis,resulting in the decrease of Fe element concentration from more than 60%to about 10%at grain boundaries.The decrease of ferromagnetic element concentration at grain boundaries and the refinement of grain are considered to be the main reasons for the improvement of coercivity and thermal stability.

Effect of surface Nd-rich phase and oxygen content of melt-spun flakes on formation of coarse grains in hot-pressed Nd-Fe-B magnet

Nd-Fe-B hot-pressed(HP) magnet prepared from melt-spun MQU-F flakes features coarse grains(CG)with the average size of both 200 nm(CGS) and 700 nm(CGL) at flake boundary.The grain growth at the flake boundary of Nd2 Fe14B/α-Fe composite HP magnet before and after diffusion of low-melting-point Pr82Cu18 phase was investigated,revealing the indispensable role of surface RE-rich phase of meltspun flakes in the formation of CG in HP magnet.The dominant role of surface oxygen content of melt-spun flakes in the formation of CGL has been clarified with etching method.The HP magnet prepared from the etched flakes with dramatically decreased oxygen content exhibits the CG regions merely with homoge neous equiaxed CGS at flake boundary.Consequently,the coercivity(μ0 Hc) shows significant increase while remanent magnetization(μ0 Mr) inappreciable change.Further investigation with sieving method reveals the elimination of CGL via removal of the fine Nd-Fe-B flakes smaller than 54 μm due to their much higher oxygen content,confirming the dominant role of oxygen content in the formation of CGL.The quantitative analysis on the magnetic properties of the above HP magnets reveals the monotonic increase of coercivity(μ0 Hc) and negligible change of remanent magnetization(μ0 Mr) with decreased oxygen contents of Nd-Fe-B flakes.The maximum value of coercivity(μ0 Hc) increases from2.26 to 2.47 T as the oxygen content decreases from 0.1692 wt% to 0.079 wt%.

Coercivity enhancement of Ce-containing hot-deformed magnets by grain boundary diffusion of DyF_(3)

The samples with full density were prepared by hot pressing the the melt-spun powders mixed with DyF_(3)powders of different mass fractions followed by hot-deformation process.The magnetic properties and temperature dependence of coercivity were obtained by BH tracer and VSM,respectively.The microstructure were analyzed by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The coercivity of Ce-containing hot-deformed magnets is increased from 1.41 to 1.95 T by grain boundary diffusion of 3 wt%DyF_(3),and is further enhanced to 2.05 T after annealing treatment.The thermal stability of coercivity and remanence is improved.The annealing condition in this work crucially plays a role in thickening the grain boundary phase.Microstructure analysis reveals that the continuous and thick grain boundary phase formed after DyF_(3)diffusion can weaken the magnetic coupling between grains,and suppress the platelet shaped grain size and the aspect ratio.The Dycontaining shell structure formed by the partial diffusion of Dy into the main phase can increase the magnetic anisotropy field,which is the main reason for the coercivity improvement.After optimizing the structure by DyF_(3)diffusion,the"dendritic-like"reverse domain is transformed into the"dot scatteredlike"reverse domain.

Sandwiched structure of hot-deformed Nd-Fe-B permanent magnets processed by Nd-Cu eutectic alloys diffusion

A series of sandwiched structures with different near-surface mass fractions x(x=3 wt%,4 wt%,5 wt%)was employed to develop high-coercivity hot-deformed Nd-Fe-B magnets by the addition of 2 wt%Nd-Cu eutectic alloys via adjusting the middle thickness and near-surface thickness.The designed magnet with a pronounced composite structure shows a 23% increase in coercivity with a 6% loss of remanence by adjusting the sandwiched structure at 4 wt% Nd-Cu eutectic alloys in the near-surface regions.The results indicate that the near-surface Nd-Cu-rich "shell" structure can effectively suppress the magnetization reversal of overall magnets,enhancing the coercivity.With the help of loading stress,Nd-Cu liquid enriched at the near-surface regions of the sample is infiltrated into the Nd-Cu-lean middle region,resulting in a concentration gradient.Microstructure characterizations further demonstrate that the infiltrated Nd-Cu eutectic plays a critical role in inhibiting grain growth and intergranular magnetic interaction.The optimized microstructure features suppress the reversed magnetization process,which makes a positive contribution to coercivity.