Oxide evolution in NdDy-Fe-B magnet during aging process

Sintered NdDy-Fe-B magnets were made from strip cast flakes by powder metallurgy method. For the magnet with 6wt.% Dy substituted Nd, a remarkable coercivity increase of 70% was obtained after aging at 900°C for 2h and a high coercivity of 23.5 kOe was achieved after aging at 520°C for 2h. The average diameter of the oxide particles decreased during the following 900°C aging treatment. Dy and O diffusion from oxide particles to Nd-rich phase during the aging process was observed. The oxide evolution and Dy element diffusion were helpful to the coercivity enhancement.     

Influence of dysprosium substitution on magnetic and mechanical properties of high intrinsic coercivity Nd-Fe-B magnets prepared by double-alloy powder mixed method

The double-alloy powder mixed method is very proper for developing new small-mass products by changing the composition of sintered Nd-Fe-B magnets, and there is little research on this aspect. The variation on magnetic and mechanical properties of high intrinsic coercivity Nd-Fe-B magnets prepared by double-alloy powder mixed method was discussed, which is a method blending two-type main phase alloy powders with different components. The results showed that the intrinsic coercivity and density of sintered Nd-Fe-B magnets increased gradually with the increase in Dy content, and the double-alloy powder mixed method could obtain high intrinsic coercivity Nd-Fe-B magnets with good crystallographic alignment and microstructure. The bending strength of sintered Nd-Fe-B magnets declined, and the Rockwell hardness of sintered Nd-Fe-B magnets first declined, and then increased with the increase in Dy content. The microstructure showed that there existed the phenomenon that the Dy element diffused into main phase during sintering process, and the distribution of Dy content in main phase had some variation in homogeneity as a result of incomplete reaction between the double-alloy powder types.     

Improvement of coercivity and corrosion resistance of Nd-Fe-B sintered magnets by doping aluminium nano-particles

Nd-Fe-B permanent magnets with a small amount of Al nano-particles doping were prepared by conventional sintered method. Effect of Al content on magnetic property, corrosion resistance and oxidation properties of the magnets were studied. Investigation showed that the coercivity rose gradually, while the remanence decreased simultaneously with increase of Al doping amount. Further investigation revealed that most Al element diffused into the main phase and some Al element diffused into the Nd-rich phase. The autoclave test results showed that the corrosion rate of the magnets decreased with Al content increasing. After oxidation, the maximum energy product losses of the magnets with 0.0 wt.% and 0.2 wt.% Al nano-particles doping were 6.13% and 3.99%, respectively. Therefore, Al nano-particles doping was a promising way to enhance the coercivity and corrosion resistance of sintered Nd-Fe-B magnet.     

Influence of Al/Cu content on grain boundary diffusion in Nd-Fe-B magnet via in-situ observation

The influence of aluminum and copper content in the starting Nd-Fe-B magnet on grain boundary diffusion process(GBDP) was studied by observing the phase transformation behaviors of the magnets in-situ at high temperature. A higher coercivity increment is discovered in the sample with higher AI/Cu despite the fact that its Dy diffusion amount is the same as the other. DSC analysis shows an evident melting behavior in the higher Al/Cu sample. Laser scanning confocal microscopy(LSCM) in-situ characterization shows a large amount of melted intergranular phase spills out to the surface simultaneously at around 600 ℃ in the high Al/Cu sample, while the phase spills out gradually one after another in the range between 623 and680 ℃ in the other sample, which indicates that the intergranular phase can be more easily melted in the sample containing more AI/Cu. The area fraction of matrix phase remarkably shrinks while that of intergranular phase enlarges after LSCM heating, which demonstrates the outer region of the Nd_2 Fe_(14)B grains melt at the temperature of 900 ℃. Electron probe microanalyzer result(EPMA) shows that the Nd and Dy concentrate in edge regions and subsequently mix into the intergranular phase with the melting of the grain edge, while a large amount of AI and Cu in the intergranular phase spill out. Nevertheless, the sample with higher starting AI/Cu still remains higher residual contents after LSCM experiments, and that could probably be the main reason why the high AI/Cu magnet shows smaller coercivity decrement after LSCM experiment. Overall, the increase of AI/Cu in the starting magnet optimizes the Dy distribution and the wettability of intergranular phase, enhancing coercivity increment effect further.     

晶界扩散中Tb对烧结NdFeB磁体微观组织和磁性能的影响

通过晶界扩散技术提升烧结钕铁硼(NdFeB)磁体矫顽力的方法已获得广泛应用,为了研究重稀土磁粉对磁体综合磁性能的影响,本文采用喷涂扩散的方法将重稀土Tb含量为6.0%(质量分数)的磁粉作为复合扩散源的一部分进行晶界扩散并制备了高性能烧结NdFeB磁体。结果表明,当主扩散源占比为60%(质量分数)时,Nd₄₀Tb₆₀对应扩散磁体的矫顽力最高达到21.52 kOe,矫顽力增幅明显。经过微观组织结构和XRD表征分析,重稀土元素Tb沿晶界相扩散进入磁体内部的同时发生了晶格取代反应,可在晶粒表层生成磁晶各向异性场更强的(Nd,Dy/Tb)₂Fe₁₄B硬磁相,显著增强了磁体矫顽力。当主扩散源占比为20%、40%和80%(质量分数)时,Nd₈₀Tb₂₀,Nd₆₀Tb₄₀和Nd₂₀Tb₈₀对应扩散磁体的矫顽力增幅较小,其中Nd₈₀Tb₂₀扩散...     

Coercivity enhancement of Ce-containing hot-deformed magnets by grain boundary diffusion of DyF3

The samples with full density were prepared by hot pressing the the melt-spun powders mixed with DyF₃powders 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%DyF₃,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 DyF₃diffusion 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 DyF₃diffusion,the"dendritic-like"reverse domain is transformed into the"dot scatteredlike"reverse domain.     

晶界扩散Dy60Co35Ga5合金对烧结钕铁硼磁体 磁性能及热稳定性的影响

研究了晶界扩散Dy₆₀Co₃₅Ga₅合金对烧结钕铁硼磁体磁性能及其热稳定性的影响.随着扩散温度的升高,磁体的矫顽力（H_cj）呈现出先增加后减少的趋势,并在890 ℃扩散3 h,480 ℃回火5 h的工艺条件下,矫顽力达到较优,从1 209 kA/m提高到1 624 kA/m,磁体的剩磁只有轻微的下降,从1.38 T降低到1.32 T.高温下测试磁体的磁性能,原始磁体和890 ℃晶界扩散Dy₆₀Co₃₅Ga₅合金磁体的矫顽力都呈下降趋势,但晶界扩散Dy₆₀Co₃₅Ga₅合金磁体的矫顽力在高温下要明显优于原始磁体.原始磁体及890 ℃晶界扩散Dy₆₀Co₃₅Ga₅合金磁体在不同温度下保温2 h的不可逆磁通损失分别为63 %和45 %.且DSC结果显示,890 ℃晶界扩散Dy₆₀Co₃₅Ga₅合金磁体的居里温度（T_c）要明显高于原始磁体的居里温度,这表明晶界扩散磁体的热稳定性得到了很大的改善. XRD图谱显示,890 ℃晶界扩散磁体RE₂Fe₁₄B相的衍射峰较原始磁体向右偏移,说明Dy原子及Co原子少部分已进入主相晶粒.      

晶界扩散Dy–Al–Ga对钕铁硼磁体的磁性能和微观组织的影响

通过晶界扩散Dy₇₀Al₁₀Ga₂₀合金研究了烧结Nd-Fe-B磁体的磁性能和热稳定性能.用NIM-500C高温永磁测量仪和MLA650扫描电镜测出了磁体在扩散前后的磁性能和微观组织的变化.结果表明，在Dy₇₀Al₁₀Ga₂₀合金扩散热处理后，磁体矫顽力从原始的1 080.968 kA/m显著提升到1 671.600 kA/m，提升幅度约为55 %，而剩磁下降很少. Dy、Al、Ga元素在晶界处扩散，很好地隔绝了磁交换作用，提升矫顽力. SEM图显示在扩散Dy₇₀Al₁₀Ga₂₀合金后，可以很明显地看到晶粒外延层有一层灰色的壳层包覆着主相晶粒，很好地起到了隔离晶粒的磁交换作用. XRD显示主相的峰普遍往右偏移，这归因于重稀土元素Dy进入晶粒外延层形成（Nd, Dy）₂Fe₁₄B核壳结构. Dy的原子半径比Nd小，导致峰往右移.      

晶界扩散Tb对烧结Nd-Fe-B组织与磁性能的影响

用磁控溅射法在烧结Nd-Fe-B磁体表面沉积Tb金属薄膜并进行晶界扩散处理,对比经不同热扩散温度及时间处理后的磁体组织和磁性能变化.结果 表明,925℃×10 h+500℃×2 h为最佳晶界扩散工艺,可将磁体矫顽力提高到1630.9 kA· m-1,较原始磁体提升50％,同时剩磁和磁能积无明显下降,磁体仍具有较高的退磁...     

镝扩渗对烧结钕铁硼磁体组织结构与磁性能的影响

对比研究了38UH、42SH和N50薄片状钕铁硼磁体晶界镝扩渗前后的组织结构与磁性能,发现经过镝扩渗处理后磁体的矫顽力提高了400 kA·m-1以上,而剩磁几乎不变,最大磁能积因为矫顽力和方形度的提高而提高。经组织结构分析认为,钕铁硼磁体晶界镝扩渗提高矫顽力主要是通过提高Nd₂Fe₁₄B晶粒外延层的各向异性和形核场实现的。根据Fick第一扩散定律,对磁体晶界镝扩渗进行了模拟计算,可近似得到定温热处理不同时间后渗镝深度及对应的镝的质量浓度及质量分数。      

Technology of melting REM–Fe–Co–B alloys for thermally stable magnets

A technology of making REM–Fe–Co–B (REM = rare-earth metal) alloys for thermally stable magnets is developed. This technology ensures a stable chemical composition (REM content deviation of ±1.0 wt % from the calculated value, Co and B content deviation of ±0.5 wt %) and a low impurity content (Al or Ni ≤ 0.2 wt %, [O] ≤ 0.1 wt %). This technology makes it possible to make Pr–Dy–Fe–Co–B alloys and more complex compositions with additional REM, e.g., gadolinium. Fe–Pr and Fe–Dy master alloys are chosen and melted, and the possibility of using them to make Pr–Dy–Fe–Co–B alloys is studied.     

Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys

Different from the grain boundary diffusion process(GBDP),which is suitable for modifying thin magnet,a green-pressing agents permeation process(GAPP) that uses low melting point alloys was applied to the Nd-Fe-B green compact with a thickness over 15 mm to reconstruct the boundary microstructure of a sintered Nd-Fe-B magnet.The coercivity increases from 12.3 kOe for the sample free of Pr_(80)Al_(20) to16.8 kOe for the sample with 2 wt% Pr_(80)Al_(20).By further increasing the Pr_(80)Al_(20) content to 3 wt%,the coercivity increases slightly,but the remanence and H_k/H_(cj) deteriorate obviously.The optimal comprehensive properties of H_(cj)=16.8 kOe,B_r=13.4 kG and H_k/H_(cj)=0.975 are obtained at 2 wt% Pr_(80)Al_(20),since matrix phase grains are separated by relatively continuous thin grain boundary layers,which weaken the magnetic coupling between adjacent grains.The coercivities of the samples from the GAPP that use2 wt% Pr_(80)Al_(20),Pr_(70)Al_(30) and Pr_(60)Tb_(20)Al_(20) alloys,respectively,can be enhanced to a large extent.However,the coercivity of the magnet reconstructed with Pr_(80)Al_(20) is lower than that of the sample with Pr_(60)Tb_(20)Al_(20) but is higher than that of the sample reconstructed with Pr_(70)Cu_(30) alloy.Moreover,the coercivity of the sample from the GAPP using 2 wt% Pr_(80)Al_(20) is much higher than that of the sample from the GBDP,which is due to a nearly uniform boundary microstructure from the surface to the interior of the thick magnet from the GAPP,thus providing new insights into the fabrication of thick and bulky permanent magnets with high coercivity.     

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 (CG_S) and 700 nm (CG_L) at flake boundary. The grain growth at the flake boundary of Nd₂Fe₁₄B/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 melt-spun flakes in the formation of CG in HP magnet. The dominant role of surface oxygen content of melt-spun flakes in the formation of CG_L 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 homogeneous equiaxed CG_S at flake boundary. Consequently, the coercivity (μ₀H_c) shows significant increase while remanent magnetization (μ₀M_r) inappreciable change. Further investigation with sieving method reveals the elimination of CG_L 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 CG_L. The quantitative analysis on the magnetic properties of the above HP magnets reveals the monotonic increase of coercivity (μ₀H_c) and negligible change of remanent magnetization (μ₀M_r) with decreased oxygen contents of Nd-Fe-B flakes. The maximum value of coercivity (μ₀H_c) increases from 2.26 to 2.47 T as the oxygen content decreases from 0.1692 wt% to 0.079 wt%.     

Relationship between controllable preparation and microstructure of NdFeB sintered magnets

The double hard magnetic phase magnets with nominal compositions of Nd_30–xDy_xFe₆₉B₁(x=2, and 4) (wt.%) were prepared. The magnetic properties of the magnets were measured with a NIM-2000H hysteresigraph. The crystalline structures of the magnets were identified by X-ray diffraction (XRD). The Rietveld refinement was carried out using the FULLPROF software. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses were carried out in order to investigate the microstructure of the magnets. It showed that the magnets consisted mainly of Nd₂Fe₁₄B phase, and some Nd-rich phase. Two types of matrix-phase grains in dark grey and light grey were found in the magnets with x=2 and 4. The Dy content was obviously different in the two types of grains, which proved that the double hard magnetic phases (Dy-rich and Dy-lean phases) coexisted in the magnet. It revealed that the Nd-rich phases in junction regions had fcc structure, with the unit cell parameter of about 0.52–0.56 nm. The weak superlattice spots were found in the SAD patterns of the junction Nd-rich phases with large scale. The double hard magnetic phase structure seemed to improve the magnetic properties of NdFeB magnets with high coercivity, while decrease the consumption of Dy element, compared with the single alloy magnet.     

Enhanced magnetic properties in chemically inhomogeneous Nd-Dy-Fe-B sintered magnets by multi-main-phase process

Homogeneous substitution of Dy for Nd in the hard magnetic 2:14:1 phase can effectively enhance coercivity to ensure the high temperature operation,however,inevitably deteriorate remanence at expense.In this work,we performed a comparative investigation of the two magnets prepared by multimain-phase(co-sintering Nd_2 Fe_(14)B and(Nd,Dy)_2 Fe_(14)B powders) and single-main-phase(sintering(Nd,Dy)_2 Fe_(14)B powders) approaches.The comparative investigation reveals that at the same Dy substitution level(2.16 wt%),such chemically inhomogeneous multi-main-phase magnet possesses better roomtemperature magnetic properties as well as thermal stability than those of the single-main-phase one with homogenous Dy distribution in the matrix grains.Room-temperature magnetic properties H_(Cj)=1664 kA/m,B_r=1.33 T and(BH)_(max)=350.4 kJ/m~3 for the multi-main-phase magnet are all better than those for the single-main-phase magnet with H_(Cj)=1536 kA/m,B_r=1.29 T and(BH)_(max)=318.4 kj/m3.In addition,over the temperature range from 295 to 423 K,both the temperature coefficients of coercivity and remanence for the multi-main-phase magnet are also lower than that for the single-main-phase magnet.Such superior magnetic performance is attributed to the short-range magnetic interactions inside individual 2:14:1 phase grains and the long-range magnetostatic interactions between adjacent grains with inhomogeneous Dy distribution.Our work provides a feasible approach of enhancing coercivity and retaining energy product simultaneously in the Nd-Dy-Fe-B permanent magnets.     

Effect of rare earth doping on thermo-physical properties of lanthanum zirconate ceramic for thermal barrier coatings

The effect of rare earth doping on thermo-physical properties of lanthanum zirconate was investigated. Oxide powders of various compositions La2Zr2O7 were synthesized by coprecipitation-calcination method. High-temperature dilatometer, DSC, and laser thermal diffusivity methods were used to analyze thermal expansion coefficient （TEC）, specific heat, and thermal diffusivity. The results showed that CeO2 doped pyrochlores La2（Zr1.8Ce0.2）2O7 and La1.7（DyNd）0.15（Zr0.8Ce0.2）2O7 had higher TEC than La2Zr2O7 and La1.7Dy0.3Zr2O7. La2（Zr1.8Ce0.2）2O7, La1.7Dy0.3Zr2O7, and La1.7（DyNd）0.15（Zr0.8Ce0.2）2O7 had lower thermal conductivity than undoped La2Zr2O7. The Dy2O3, Nd2O3, and CeO2 codoped composition showed the lowest thermal conductivity and the highest TEC. Thermo-physical results also indicated that TEC of rare earth oxide doped La2Zr2O7 ceramic was slightly higher than that of conventional ZrO2-8Wt.% Y2O3 （8YSZ）, and its thermal conductivity was lower than that of 8YSZ.     

Effect of aluminum on microstructure and magnetic properties of sintered Nd-Fe-B magnets processed by grain boundary diffusion of Tb-Al

The grain boundary diffusion process(GBDP) of Tb can improve the coercivity of sintered Nd-Fe-B magnets.In this study,the effect of AI on the diffusion of Tb in the GBDP was investigated.The content of diffused Tb-Al was precisely controlled by adjusting the magnetron sputtering process.The Tb equivalent of Al was also studied.Results show that AI promotes the diffusion of Tb deeper into the magnet,reducing the thickness of the shell in the core-shell structure.This study is helpful for further ...     

Characterization of Microstructure and Magnetic Properties of NdFeB Magnet with Dy, Al and Cu Additions

A new alloy of Nd33.5Dy0.99Febal.Al0.52Cu0.1B1.15 (%, mass fraction) was fabricated by powder metallurgy. The effects of Dy, Al and Cu additions on the microstructure and magnetic properties of sintered NdFeB magnet were investigated. The additions of Dy, Al and Cu are effective to refine grains and improve coercivity. Moreover, suitable amounts of Dy, Al and Cu lead to a demagnetization curve with good rectangularity. It is found that the sintered NdFeB magnet has relatively high magnetic performance of Br=12.17kGs, jHc=13.52kOe and (BH)max=34.71MGOe. The sintered NdFeB sample was examined by magnetic force microscope which revealed the domain structures at the surface. It is revealed that the mean Nd2Fe14B grain size is significantly larger than the average scale of the magnetic contrast. An explanation about this is that most Nd2Fe14B grains in sintered NdFeB alloy are dominated with the muhidomain structures when the magnet is in thermally demagnetization state.     

Modification effect of lanthanum on primary phase Mg2Si in Mg-Si alloys

The modifying effect of La addition on primary phase Mg2Si in Mg-5Si alloys was investigated. The results showed that a proper amount of La could effectively modify the primary phase Mg2Si. Based on the present experiment, the optimal modification effect was obtained with an addition of about 0.5 wt.% La. The size of the primary phase Mg2Si was considerably reduced to 25 μm or less and the morphology was modified from a coarse dendritic shape to a polyhedral shape. However, when the addition of La increased to 0.8 wt.% or higher, the primary Mg2Si grew into a coarse dendritic morphology again. Moreover, it was found that some LaSi2 compounds were formed during solidification and the amount of the compounds appeared to increase gradually with increasing La content.     

Effects of Ni on austenite stability and fracture toughness in high Co-Ni secondary hardening steel

Three kinds of high Co-Ni secondary hardening steels with different Ni contents were studied.The nanoscale austenite layers formed at the interface of matensite laths were observed.Both observation and diffusion kinetic simulation results showed that both Ni and Co did not obtain enough time to get the equilibrium content in this system.The Ni content in austenite layers decreased significantly,and Co content increased slightly with the decrease of Ni content in overall composition.The austenite stability was estimated by Olson-Cohen model,in which both chemical and mechanical driving force could be calculated by equilibrium thermodynamic and Mohr′s circle methods,respectively.Simulation and mechanical test results showed that the decrease of Ni content in austenite layers would cause the change of austenite stability and decrease the fracture toughness of the steels.When the Ni content in the overall composition was lower than 7wt.%,the Ni content inγphase would be lower than 20 wt.%.And the simulation value of Mσs(stress-induced critical martensite transformation temperature)would be up to 80°C,which was about 60°C higher than room temperature.Based on the analysis,the Ni content in the overall composition of high Co-Ni secondary hardening steels should be higher than 8wt.% in order to obtain a good fracture toughness.