RE-rich Phase Research Articles

Nd-Fe-B Magnets: The Gradient Change of Microstructures and the Diffusion Principle after Grain Boundary Diffusion Process.

The diffusion of Tb in sintered Nd-Fe-B magnets by the grain boundary diffusion process can significantly enhance coercivity. However, due to the influence of microstructures at different depths, the coercivity increment and temperature stability gradually decreases with the increase of diffusion depth, and exhibit good corrosion resistance at a sub-surface layer (300–1000 μm). According to the Electron Probe Micro-analyzer (EPMA) test results and the diffusion mechanism, the grain boundary and intragranular diffusion behavior under different Tb concentration gradients were analyzed, and the diffusion was divided into three stages. The first stage is located on the surface of the magnet, which formed a thick core-shell structure and a large number of RE-rich phases. The second stage is located in the sub-surface layer, forming a uniform and continuous RE-rich phase and thin core-shell structure. The third stage is located deeper in the magnet, and the Tb enrichment only existed at the triangular grain boundary.

Coercivity enhancement of nanocrystalline hot-deformed Nd-Fe-B magnets by low-melting eutectic MM-Cu (MM=La, Ce, Pr, Nd) alloys addition

MM85Cu15 (MM = La, Ce, Pr, Nd) eutectic alloys were added into the hot-deformed Nd-Fe-B magnets to enhance the coercivity. It is found that three endothermic peaks occur on the differential scanning calorimetry curve of the MM-Cu melt-spun ribbons at 432.2, 451.1 and 516.5 °C. The peaks substantially correspond to three types of MM-Cu low-melting eutectic phase. The coercivity of magnets increases when the MM-Cu content is lower than 4 wt%, and then keeps almost no change with the content further increasing to 5 wt%. The coercivity of the hot-deformed magnets with 4 wt% and without MM-Cu addition is 948 and 683 kA/m, respectively. Nearly all the platelet-shaped grains are isolated by the thickened intergranular phase after MM-Cu addition. Moreover, the average grain size of the magnets with MM-Cu addition decreases compared with that of the magnet without MM-Cu addition. Scanning electron microscopy images show that the areal fraction of the RE-rich grain boundary phase increases from 8.6% to 15.1% after MM-Cu addition. The La, Ce together with Cu and Ga aggregate at the grain boundary regions separating neighboring grains and smoothing the grain boundaries. Therefore, both the thickened grain boundary and decreased mean grain size result in the enhancement of coercivity after MM-Cu eutectic alloy addition.

Effect of Aging on the Structure and Transformation Behavior of Cu–12Al–3.5Ni–0.7Ti–0.05RE High Temperature Shape Memory Alloy

In this study, the effect of isothermal aging on the thermoelastic martensitic transformation and microstructure of the as-quenched Cu–12Al–3.5Ni–0.7Ti–0.05RE (RE = Ce, La) high temperature shape memory alloy was studied. The results showed that the alloy microstructure and martensitic transformation are strongly influenced by the aging temperature rather than aging time. During aging at 350 °C the alloy was prone to both the precipitation of the γ2 phase and the bainitic transformation, resulting in a loss of martensitic transformation and damping capacity. The prolonged aging at 350 °C caused a decomposition of parent phase into the equilibrium γ2 phase alongside the α phase which produced a significant hardness increment. On the other hand, aging at 250 °C affected the microstructure only slightly producing insignificant shift in the transformation temperatures. It was found that, the secondary phases including Ti-rich X-phase and the RE-rich phase were not influenced by the aging process. The results prescribe a high temperature order of the stability of martensitic transformation for this new alloy which is important for its high temperature shape memory applications.

Corrosion behavior of as-cast ZK60 alloy modified with rare earth addition in sodium sulfate medium

The corrosion resistance of the as-cast ZK60 alloy with and without 1.5 %wt rare earth (RE) addition and submitted to a dip sol-gel coating was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 0.1 mol/L Na2SO4 solution. Analysis using scanning Kelvin probe force microscopy (SKPFM) was carried out to investigate the corrosion attack mechanism on these alloys. The RE addition has accelerated the ZK60 alloy dissolution by formation of a less noble RE-rich second phase. The dip sol-gel coating was not confirmed as a powerful tool to enhance the protection against corrosion in the as-cast ZK60 alloys.

Enhanced magnetic properties and improving thermal stability for sintered Nd-Fe-B magnets prepared by two-step grain boundary diffusion processes

Abstract Two-step grain boundary diffusion processes (GBDP) were carried out to obtain the diffused magnets with excellent magnetic properties and high thermal stability, using Dy and Al coated films as diffused sources in turn, prepared by magnetron sputtering. It is found that through the two-step diffusion with Dy/Al films, the highest coercivity, up to 1382 kA/m is achieved, increasing by 42.3% compared to as-prepared magnet, moreover, the maximum energy product is improved significantly as well. Besides, whether for as-prepared magnets or one-step diffused magnets with Al film and Dy film, respectively, the thermal stability and magnetic properties of two-step diffused magnet with Dy/Al film, are the highest among magnets. Microstructural investigation reveals that Dy is preferred to diffuse along the RE-rich phase and forming the “shell” of (Nd, Pr, Dy)2Fe14B phase around the outer region of main phase, restraining the nucleation of anti-magnetization domain, while the Al tends to locate in the RE-rich region, playing the role of modifying the interface between RE-rich phase and 2:14:1 main phase. Eventually, the temperature coefficient of coercivity (|β|) and remanence (|α|) with 0.65%/K and 0.12%/K, can be obtained, respectively.

Effect of RE addition on solidification process and high-temperature strength of Al−12%Si−4%Cu−1.6%Mn heat-resistant alloy

Abstract The effect of RE addition on solidification process and high-temperature strength of Al−12%Si−4%Cu−1.6%Mn (in wt.%) heat-resistant alloy was investigated by microstructure observation and tensile test. A great number of fine needle-like RE-rich phases are observed in the alloys with RE addition. Solutionizing treatment does not change their morphologies and sizes, indicating that they have good thermal stability. The addition of RE totally alters the solidification process of eutectic CuAl2 phase, from network-like phase in the form of segregation at the final eutectic grain boundaries to discretely blocky phase growing on the hair-filamentous RE-rich needles. In the alloys with Ce addition, blocky CuAl2, particulate Al15Mn3Si2 and needle-like RE-rich needle phases grow together, but they did not occur in the alloy with only La addition. The addition of RE does not considerably improve the strength of the alloy at high temperatures. The formation of RE-rich phases also does not significantly alter the originating and propagating of micro-cracks in the alloy during tensile test.

Effects of hot pressing temperature on the alignment and phase composition of hot-deformed nanocrystalline Nd-Fe-B magnets

Abstract The magnetic properties, alignment and phase composition of nanocrystalline Nd-Fe-B magnets hot deformed (HD) at 830 °C employing different hot-pressed precursors fabricated at various hot pressing (HP) temperatures were investigated. Both the remanence and coercivity simultaneously increased first and then decreased with the HP temperature from 450 °C to 650 °C. The optimum magnetic properties of HD magnets were obtained at HP temperature of 550 °C with remanence and coercivity of 1.40 T and 924 kA/m, respectively. The usage of the HP temperatures lower than 500 °C was resulted in high porosity and low crystallization degree for hot pressed magnets, causing aggregation of the RE-rich phase and small amount of residual amorphous after hot deformation. This procedure increased the volume fraction of nonmagnetic phase and reduced magnetic properties. Furthermore, the platelet-shaped grains were separated by thick triple junctions, which deteriorated the alignment of grains. When HP temperature was 550 °C, the hot-deformed magnet gained the optimum alignment, and the stacked platelet-shaped grains were separated by smooth thin grain boundary. However, at HP temperature of 650 °C equiaxed coarse grains were dominant instead of platelet-shaped grains in the hot-deformed magnet. The high HP temperature led to apparent grain growth, and then the nucleated grains further grew and were hard to be deformed, developing a poor alignment. A schematic model of the microstructure evolution for hot-deformed magnet is proposed.

Attaining high magnetic performance in as-sintered multi-main-phase Nd-La-Ce-Fe-B magnets: Toward skipping the post-sinter annealing treatment

Two-step post-sinter annealing (PSA) treatment, due to its universal applicability, has been one of the most effective strategies for enhancing the coercivity of Nd-Fe-B sintered magnets. Here we report a peculiar phenomenon that the as-sintered multi-main-phase (MMP) Nd-La-Ce-Fe-B magnet exhibits the highest reported to date coercivity of 13.0 kOe upon 25 wt% La-Ce substituting for Nd, combined with Br = 13.12 kG, and (BH)max = 41.67 MGOe. Further annealing cannot enhance the coercivity whereas the remanence and energy product are drastically lowered, i.e. to 11.0 kOe, 12.57 kG, and 38.71 MGOe after two-step PSA, being remarkably different from the conventional Nd-Fe-B. Microstructural analysis reveals that the as-sintered Nd-La-Ce-Fe-B magnet with coexisting REFe2 (Fd3¯m) and RE-rich (Fm3¯m) intergranular phases possesses continuous GB layer isolating adjacent ferromagnetic grains. Hence the beneficial role of PSA on modifying the microstructure and strengthening the coercivity is tiny. Additionally, high-temperature PSA destroys the initial chemical heterogeneity of MMP magnets, leading to the formation of REs homogeneously distributed grains and resultant reduction of the intrinsic magnetic properties. As evidenced by in-situ Lorentz TEM characterization, PSA sample with reduced chemical gradient exhibits quick domain wall motion, compared to the as-sintered one with basically unchanged magnetic domain structure upon the same applied field. The proof-of-principle micromagnetic simulation further confirms that retaining the inhomogeneous La/Ce/Nd distribution is essential to suppress the magnetic dilution effect. These findings demonstrate the realistic prospect of skipping PSA treatment in MMP magnets, which can not only reduce the production process and cost, but also delight the scenario for designing high-performance 2:14:1-type sintered magnets based on abundant La/Ce.

Study on the phase separation behavior of (U,Nd)3O8 powder by high temperature oxidation

Several samples of (U1−xNdx)3O8 with different Nd content were prepared by different process. After high temperature oxidation, the final oxidized products were found to be a mixture of small particles Nd-rich (U1−yNdy)O2+v phases and large particles Nd-poor (U1−zNdz)3O8−w phases by XRD and SEM analysis. This study indicated that (U1−xNdx)3O8 powder formed a solid solution firstly at 1100 °C or high. The recrystallization of this solid solution happened and the RE-rich fluorite phase formed concurrently when the temperature began to decrease. The present work will provide more information to understand the oxidation behavior of rare-earth doped U3O8 phase at high temperature.

A novel method of selectively enriching and separating rare earth elements from rare-earth concentrate under super gravity

The Bayan Obo rare-earth concentrate was obtained through mineral processing and massively disposed at the tailing dams. For efficient extraction of the rare earth elements (REEs) from the rare-earth concentrate, a novel approach was proposed to selectively enrich and separate REEs as rare earth oxide fluoride ([Ce,La,Pr,Nd]OF) phase under super gravity. Firstly, the mineral reconstruction behavior of the rare-earth concentrate revealed that REEs were selectively enriched into the rare earth oxide fluoride phase with the temperature’s rising to 1473 K–1773 K under a reductive atmosphere. Subsequently, selective separation of the rare earth oxide fluoride phase was conducted under super gravity at the enriching temperature range, where the REEs were enriched as the single Re-rich phase while other minerals formed the molten slag. Accordingly, high-purity rare earth oxide fluoride phase with a high ∑ReO content of 90.35 wt.% was efficiently separated from the rare-earth concentrate under super gravity at 1773 K with G = 1000 for 10 min, which was accurately characterized for the first time to supplement some lacking data for the Re-rich phase. In addition, this manuscript provided a novel physical separation method for mineral processing.

A green method of respectively recovering rare earths (Ce, La, Pr, Nd) from rare-earth tailings under super-gravity.

A green method was proposed for respectively recovering rare earths (REEs) under super-gravity from rare-earth tailings which are massively stockpiled in the Bayan Obo tailings dams. Firstly, the REEs (Ce, La, Pr, Nd) were discovered to be precipitated as the rare earth oxyfluoride, rare earth ferrate and britholite phases respectively at various temperature ranges of 1773-1673 K, 1673-1473 K and 1473-1373 K. However, the Re-rich phases were intimately intertwined with each other in the normal-gravity. Consequently, respective recovery of REEs (Ce, La, Pr, Nd) at their corresponding precipitation temperatures was conducted under the super-gravity. 98.38% of (Ce) were firstly enriched into the rare earth oxyfluoride and separated from the tailings as driven by the super-gravity, 97.70% of (La) were enriched into the rare earth ferrate and separated subsequently, and the residual REEs were precipitated further into britholite. Accordingly, high-purity of rare earth oxyfluoride, rare earth ferrate and britholite phases were attained respectively, achieving the green and efficient recovery of REEs (Ce, La, Pr, Nd) from the tailings with no additives, no hazardous wastes and no secondary pollution.

Permanent Rare-Earth Magnets—The Need to Protect Them against Corrosion

The RE-Fe-B permanent magnets have a complex microstructure and they are susceptible to corrosion process. In this paper, the commercial nickel coatings adhesion was investigated. The microstructure of the RE-Fe-B without coating was analyzed by scanning electronic microscopy and electrochemical techniques. The interface magnet/coating was studied by scanning electron microscopy and the nickel-plated Nd-Fe-B commercial magnets were tested in a salt spray chamber. The ferromagnetic and RE-rich phases were observed. After the anodic polarization curve, a strong intergranular corrosion was observed and the RE-rich phase was preferentially attacked. The interface magnet/Ni coating presented inter-granular corrosion that can affect the nickel coating adherence. This attack had probably occurred during the electrodeposition process. Not all the samples suffered localized corrosion during the salt spray tests and the Ni triple-layer coating presented a few corrosion points. RE-Fe-B alloy magnets need to be protected with appropriate coatings to each environment to which they will be exposed and the protective coating must not be damaged.

Influence of Rare Earth Additions on the Microstructure and Mechanical Properties of Al7Si0.3Mg Alloys Processed by Semi-Solid Die Casting and Gravity Die Casting

Microstructures with fine globular grains and refined eutectic structures are important to enhance the mechanical properties of A356 alloys processed by semi-solid and gravity die casting. Rare earth (RE) additions have been shown to be capable of refining both the α-Al particles as well as modify the eutectic phase of alloys. In semi-solid die casting, Al7Si0.3Mg alloys with RE concentrations (0, 0.1 and 0.4 wt.%) were used to prepare semi-solid slurries using the SEED (Swirling Enthalpy Equilibrium Device) method, and subsequently semi-solid die cast. The same compositions of alloys were also applied to gravity die casting. The microstructure and mechanical properties of castings in two processes have been characterized. Compared to the grains produced in gravity die casting, globular grains with small size (260 μm) in the semi-solid die casting significantly enhance the UTS and elongation of alloys. Although the size of grains had no change with increasing RE concentrations in alloys. The Al-Si eutectics were changed to refined morphology with the 0.1 wt.% RE addition, which enhanced the ductility of alloys in two processes. When increasing the RE addition to 0.4 wt.%, the RE-rich phases precipitated at grain boundaries, which decreased the UTS and elongation of alloys.

Microstructure and improved properties of sintered Nd-Fe-B magnets by grain boundary diffusion of non-rare earth

Abstract Al film, which was coated in sintered Nd-Fe-B magnets prepared by magnetron sputtering, was employed for grain boundary diffusion source. Effects of the grain boundary diffusion processes (GBDP) on the microstructure evolution and properties were investigated in detail. Through grain boundary diffusion processes, the highest coercivity of 1184 kA/m and maximum energy product of 238 kJ/m3 could be obtained, increasing by 21.8% and 3.9%, respectively, compared with the initial magnet. Meanwhile, our results showed that fine, uniform and continuous intergranular phase induced by Al diffusion, was the main reason for properties improvement, while a weak ferromagnetic phase and the vague interface between main phase and RE-rich phase should be responsible to the deterioration of coercivity when the diffused temperature exceed 600 °C. Besides, the corrosion resistance of Al-diffused magnets was also greatly improved, owing to the fact that the Al element could promote the electrochemical potential of RE-rich phase and the more thin, continuous intergranular phase would also narrow the corrosion channel.

Anisotropic corrosion behavior of sintered (Ce0.15Nd0.85)30FebalB permanent magnets

In this work, the corrosion behavior of the surface parallel to the oriented c-axis (c∥) and perpendicular to c-axis (c⊥) in the (Ce0.15Nd0.85)30FebalB dual main phase magnet was studied. With the addition of Ce, the volume fraction of RE-rich phase shown in the backscattered electron (BSE) images is basically approximate on the two surfaces. The free corrosion potential (Ecorr) of the c∥ surface is more negative, which shows the worse corrosion resistance from the perspective of thermodynamics. While the reaction kinetics parameters with the smaller free corrosion current (icorr) and the larger transfer resistance (Rct) react the opposite conclusion in 3.5 wt% NaCl solution. Moreover, the c∥ surface performs smaller ions concentration of corrosion product and less damaged corrosion morphology compared to c⊥ surface after the free corrosion. The inconformity is not affected by the RE-rich phase, but by the anisotropy of the grains that the c∥ surface has larger density of the atoms and the lower ratio of (Nd, Ce)/Fe.

Effects of (Nd, Pr)-Hx addition on the coercivity of Nd-Ce-Y-Fe-B sintered magnet

Y-Ce co-substituted RE-Fe-B (RE, rare earth) sintered magnets with good thermal stability have attracted growing attention as low-cost hard magnetic materials. The inferior intrinsic properties of Y2Fe14B and Ce2Fe14B to Nd2Fe14B, however, limit the substitution of Y-Ce for Nd at a low level. In this work, (Nd, Pr)-Hx powders are introduced into (Nd, Pr)22.8(Y, Ce)7.7FebalM1.3B1.0 sintered magnets to further enhance the coercivity. By incorporating 1 wt% (Nd, Pr)-Hx, the coercivity is increased to 9.7 kOe, which is 16.9% higher than that for the starting magnet (8.3 kOe). Continually increasing (Nd, Pr)-Hx addition to 2 and 3 wt% lead to a very limited coercivity increment, i.e. to 10.0 kOe and 10.3 kOe, respectively. The formation of continuous RE-rich intergranular phase by the extra Nd/Pr after (Nd, Pr)-Hx dehydrogenation can enhance the magnetic isolation effect between adjacent matrix phase grains, resulting in coercivity enhancement. However, more Nd atoms exist in the RE-rich phase rather than form the Nd-rich hardening shell, which is distinct from the available Nd-Ce-Fe-B system. The REs distribution within the matrix phase grain is similar before and after (Nd, Pr)-Hx incorporation for the Nd-Ce-Y-Fe-B system. It is deemed that the preferential occupancy of Y in the 2:14:1 matrix phase hinders the diffusion of Nd into the outer layer of Y-Ce enriched regions, being responsible for the limited coercivity increment with higher (Nd, Pr)-Hx addition. The present work suggests that tuning the distribution of Y is of crucial importance to further improve the coercivity of Y-containing permanent magnets.

Grain boundary diffusion of Dy films prepared by magnetron sputtering for sintered Nd–Fe–B magnets

Dy films, deposited on the surface of sintered Nd–Fe–B magnets by magnetron sputtering, were employed for grain boundary diffusion source. High coercivity sintered Nd–Fe–B magnets were successfully prepared. Effects of sputtering power and grain boundary diffusion processes (GBDP) on the microstructure and magnetic properties were investigated in detail. The dense and uniform Dy films were beneficial to prepare high coercivity magnets by GBDP. The maximum coercivity value of 1189 kA m−1 could be shown, which was an amplification of 22.3%, compared with that of as-prepared Nd–Fe–B magnet. Furthermore, the improved remanence and maximum energy product were also achieved through tuning grain boundary diffusion processes. Our results demonstrated that the formation of (Nd, Dy)2Fe14B shell surrounding Nd2Fe14B grains and fine, uniform and continuous intergranular RE-rich phases jointly contribute to the improved coercivity.

Microstructure and magnetic properties of core-shell Nd-La-Fe-B sintered magnets

To balance the utilization of rare earth (RE) resource, abundant La has been incorporated into the Nd-Fe-B sintered magnets via the binary main phase (BMP) approach. Microstructure and magnetic properties of the Nd-La-Fe-B BMP magnets with increasing La substitution from 0 to 45 wt% were investigated. The results show that RE-rich intergranular phase with high oxygen content forms easily in the triple junction region for the La-substituted magnets. Those hcp-RE2O3 precipitates exhibit poor wettability with the matrix phase, leading to the direct exchange coupling of adjacent grains. As a result, the coercivity of Nd-La-Fe-B BMP magnets is dramatically reduced. Fortunately, the remanence can be maintained at a high level, ascribing to the inhomogeneous RE distributions among the grains (core-shell morphology), and the stable tetragonal phase after high-temperature sintering. With 27 wt% La substitution for Nd in the BMP magnet, the remanence decreases from 13.15 to 13.01 kGs by merely 1.06%. This work suggests that if one can optimize the microstructure by constructing magnetically isolated grains, low-cost Nd-La-Fe-B BMP magnets can be promising candidates for mass production.

Novel hydrogen decrepitation behaviors of (La, Ce)-Fe-B strips

La and Ce substitution for Nd in the 2:14:1-type sintered magnet is of commercial interest to reduce the material cost and to balance the utilization of rare earth (RE) sources. As hydrogen decrepitation (HD) is widely utilized to prepare the magnetic powders during magnets fabrication, incorporating La and Ce into the Nd-Fe-B permanent magnets, however, may exert complex influences on the decrepitation behavior. In the present work, through a comparative study of the HD behaviors between the (La, Ce)-Fe-B strips and the conventional Nd-Fe-B ones, we find that similar to the Nd-Fe-B system, increasing hydrogen pressures from 2.5 to 5.5 MPa do not break the 2:14:1 tetragonal structure of (La, Ce)-Fe-B strips. The enhanced hydrogen absorption behaviors are observed with increasing pressure, which are still inferior to that of the Nd-Fe-B strips. This should be ascribed to the higher oxygen affinity of La and Ce than that of Nd, leading to the decreased amount of active RE-rich phase and limited hydrogen diffusion channel. As a result, the hydrogen absorption of 2:14:1 matrix phase is significantly suppressed, dramatically weakening the exothermic effect. This finding suggests that La and Ce with stable 2:14:1 tetragonal structure upon HD process are promising alternatives for Nd, despite that more precise oxygen control is necessary for the microstructure modification and magnetic performance enhancement of (La, Ce)-Fe-B sintered magnets.

Magnetic performance change of multi-main-phase Nd–Ce–Fe–B magnets by diffusing (Nd, Pr)Hx

The grain boundary diffusion process (GBDP) is effective to enhance coercivity of the single-main-phase (SMP) RE2Fe14B (rare earth (RE)) magnets through forming magnetic hardening shells surrounding the hard grain cores. Here, the GBDP was applied to the multi-main-phase (MMP) (Nd, Pr)22.3Ce8.24FebalM1.0B1.0 (wt.%) magnets prepared by sintering the mixture of Ce-free and Ce-containing 2:14:1 powders, which have shown superior magnetic properties, especially coercivity, to the SMP ones at the same average composition. The remanence of the (Nd, Pr)Hx diffused magnets increases gradually with the increase of diffusion temperature from 480 to 880 °C, the coercivity, however, slightly decreases. The highest (BH)max of 350.1 kJ m−3 is achieved when diffusing at 680 °C, which is 9.2% higher than 320.7 kJ m−3 for the as-prepared magnet. The remanence increment is due to the diffusion of Nd/Pr into the 2:14:1 phase grains, enlarging the intrinsic saturation magnetic polarization. The slight coercivity reduction is due to the gradual homogenization of RE distribution within the 2:14:1 grains of the undiffused parts, i.e. approaching the ‘close to equilibrium (or SMP)’ state, which offsets the positive contributions from the enrichment of Nd/Pr in the Ce-rich 2:14:1 phase and the formation of continuous RE-rich intergranular phase. These findings suggest that the GBDP effect on coercivity of the MMP Nd–Ce–Fe–B magnets is distinctly different from the SMP ones, and that the chemical heterogeneity should be carefully controlled to improve the magnetic properties of such high cost-performance permanent magnets.