Pr 2Fe14B Research Articles

Design and Qualification of Pr–Fe–Cu–B Alloys for the Additive Manufacturing of Permanent Magnets

AbstractThe direct use of an advanced binder‐free additive manufacturing technique, namely laser powder bed fusion (L‐PBF), does not easily allow obtaining variously shaped, fully dense Nd–Fe–B magnets with high coercivity. The process inherently leads to the re‐melting of the powder and appearance/disappearance of undesired/desired microstructural features responsible for low and large coercivity. In this work, the development of a useful microstructure responsible for high coercivity in Pr21Fe73.5Cu2B3.5 and Nd21Fe73.5Cu2B3.5 alloys and a possible way to produce fully dense permanent magnets via additive manufacturing processes is demonstrated using: (i) suction casting technique, which provides a high cooling rate and thus similar microstructures as in L‐PBF but requires only very small amounts of powder; (ii) conventional L‐PBF processing using kg of powder, and (iii) a subsequent annealing treatment that is similar to a conventional sintering treatment. The subsequent heat treatment is necessary to develop high coercivity by forming a novel microstructure: hard magnetic (Nd,Pr)2Fe14B grains embedded in a matrix of intermetallic (Nd,Pr)6Fe13Cu phase. Furthermore, it is demonstrated that Pr21Fe73.5Cu2B3.5 exhibits a higher coercivity than Nd21Fe73.5Cu2B3.5 because of a finer and more homogeneous grain size distribution of the Pr2Fe14B phase. The final L‐PBF printed Pr21Fe73.5Cu2B3.5 samples provide a coercivity of 0.75 T.

Easy-Magnetization-Axis Arrangements of Sm 2Co 17 and RE 2Fe 14B

In order to prepare the bulk samples with high residual magnetization of magnetic compounds, such as (Sm, La) 2(Co, Cu, Fe, Zr) 17, (Sm, La) 1 (Co, Cu, Fe) 5, Nd 2Fe 14B, and Pr 2Fe 14B, directly prepared by solidification, or hot-deformation, it is the first thing to explore the possibilities of the easy magnetization axis of the whole bulk samples to be arranged in one designed direction. α is defined as the angle between the axis and the direction. In Sm-La-Co-Fe-Cu-Zr system, whether α is equal to 0° or 90° depends upon not only alloy compositions but also the ratio of the temperature gradient at the solid/liquid interface and the crystal growth rate. To some alloys, α can be changed from 90° to 0° if the ratio is increased to be higher than a critical value, so the c-axis texture orientation can be obtained. In Nd-Fe-B system, the easy magnetization axis of Nd 2Fe 14B is always perpendicular to the preferential growth direction [100], and the easy magnetization axes of Nd 2Fe 14B grains are randomly distributed in the plane normal to the growth direction even if the growth rate is decreased from 250 to 12 μm·s −1. But if the magnetization axis of the anisotropic magnet substrate is perpendicular to the heat flux direction of the laser melting solidification layer, c-axis texture of the columnar Nd 2Fe 14B grains in the layer can be obtained, which is the same as that of the substrate, if the laser scanning rate is not less than 25 μm·s −1. Also the c-axis texture [006] can be achieved through hot-deformation of Pr xFe 93.5-xB 5Cu 1.5( x = 15 ∼ 19) under the conditions of hot-pressing temperature 973 ∼ 1273 K, strain rate 10 −3 S −1, and strain 50% ∼ 80%.

Microstructure and Magnetic Properties of Rapidly Quenched Pr 9Fe 91– xB x ( x=4–12) Nanocomposite Magnets

The microstructure and magnetic properties of rapidly quenched nanocomposites Pr 9Fe 91- x B x ( x=4, 5, 6, 7, 8, 8.5, 9, 10.5 and 12) have been studied in this work. The results showed that the phases present in the Pr 9Fe 91– x B x nanocomposites were strongly dependent on the B content. The samples consist of Pr 2Fe 14B and α-Fe phases when the B content was low (4–8 at%), and Pr 2Fe 14B, α-Fe and Fe 3B when the B content was sufficiently high (>11 at%). The metastable Pr 2Fe 23B 3 phase appears in the intermediate B content (8–10.5 at%) and is associated with poor magnetic performance. It was further found that this metastable phase can be transformed into Pr 2Fe 14B, α-Fe and Fe 3B by annealing the sample at temperatures above 750 °C, leading to a more square demagnetization curve. However, the B r of such nanocomposites decreases slightly due to the excessive grain growth during high-temperature annealing. A comparison of microstructure and magnetic properties between Pr 2Fe 14B/α-Fe and Pr 2Fe 14B/Fe 3B was also made in this work. It was found that the Pr 2Fe 14B/α-Fe exhibited a finer and more uniform microstructure with an average grain size of ∼10 nm, whereas a relatively coarse microstructure (grain size up to 100 nm) was observed in the Pr 2Fe 14B/Fe 3B. It was also found that the Pr 2Fe 14B/α-Fe yielded higher B r, ( BH) max and a more square demagnetization loop, although the H ci was slightly lower. The superior magnetic properties observed in the Pr 2Fe 14B/α-Fe can be attributed to the more favorable intrinsic magnetic properties of α-Fe (higher M s and lower K) and the much finer microstructure which produces stronger inter-grain exchange coupling between the Pr 2Fe 14B and α-Fe phases. The results indicate that RE 2Fe 14B/α-Fe nanocomposites are more desirable than RE 2Fe 14B/Fe 3B for future development of RE-Fe-B nanocomposite magnets.

Nanocrystalline magnetic PrFeCrB alloys

Ribbons of Pr 5Fe 77− x Cr x B 18 ( x = 0 , 1, 2, 2.5, 3, 4, 5) were produced by melt spinning and then annealed to develop an enhanced-remanence nanocrystalline magnetic material. These nanocomposites with Cr present a coercive field at least 50% higher than the Cr-free ones, which makes them promising materials for bonded magnets. Four different types of annealing were used in order to develop the nanocrystalline state and to optimize the magnetic properties of these alloys. The first was a conventional annealing, where the ribbons were wrapped in a tantalum foil and annealed in an argon atmosphere, but not encapsulated. The second was a flash annealing, where the ribbons were annealed by passing a current through them. The third was a conventional annealing in an external magnetic field. Finally, the fourth was a conventional annealing, where the ribbons were wrapped in a tantalum foil and encapsulated in quartz tubes with argon gas and then annealed. The annealed samples were studied by magnetic measurements, X-ray diffraction, scanning and transmission electron microscopy and atomic force microscopy. The best magnetic properties are found for Pr 5Fe 74Cr 3B 18, annealed by the fourth method, which resulted in the lowest oxygen content in the annealed nanocrystalline material as confirmed by scanning electron microscopy. The value for the coercive field for this composition is at least 50% higher than for the material without Cr (≈560 vs. ≈320 kA/m) and 40% higher than for the Nd 2Fe 14B/Fe 3B nanocomposite with Cr. Curie temperature measurements and X-ray diffraction data showed that the main phases present in all the samples are Pr 2Fe 14B, Fe 3B and α-Fe, Pr 2Fe 14B being the majoritary phase. From Curie temperature measurements it was also found that Cr atoms preferentially dissolve in the Fe 3B phase.

Magnetic properties and Mössbauer studies of Pr yFe 90− yB 10 ( y=8–11.76) nanocomposites

Magnetic properties and phase evolution of melt spun Pr y Fe 90− y B 10 ( y=8–11.76) nanocomposite ribbons have been studied. Based on TMA measurements and a Mössbauer analysis, four phases, Pr 2Fe 14B, Pr 2Fe 23B 3, Fe 3B and α-Fe, were found for y=8–9.5 and three phases, Pr 2Fe 14B, Pr 2Fe 23B 3 and Fe 3B, for y=10. For y=11.76, only the phase Pr 2Fe 14B was detected. An exchange coupling effect (evidenced by a Henkel plot) was found, to exist in all the ribbons if a suitable crystallization treatment was employed. In this study, optimal values for B r=9.6 kG, i H c=8.3 kOe and ( BH) max=16.2 MGOe were achieved in Pr 9.5Fe 80.5B 10. They result from a proper combination of magnetic phases in the ribbons.

Crystal-field interactions and magnetism in rare-earth transition-metal intermetallic compounds

The description of basic experiments of the rare-earth hard magnetic materials, high-field magnetization curves on single-crystalline samples of Ho 2Co 17 and Nd 2Fe 14B is reviewed with the aim to derive parameters physically relevant on atomic scale. The anisotropic magnetic properties have been found to be of single-ion origin. The successful description of the rare-earth compounds Ho 2Co 17, Nd 2Fe 14B, Pr 2Fe 14B, ErNi 5, PrRu 2Si 2, ErRu 2Si 2 in terms of the crystal-field, spin–orbit and inter-site magnetic interactions proves the existence, in a solid, the discrete atomic-like low-energy electron structure that predominantly governs the magnetic and electronic properties. The macroscopic properties have been correlated with the atomic-scale parameters. These studies reveal the importance of the low-symmetry crystal-field interactions (high-order charge multipolar interactions) in description of the magnetic properties of 4f compounds.

Effects of Metastable Phase Formation on the Magnetic Properties of Pr<SUB>4.5</SUB>Fe<SUB>77</SUB>B<SUB>18.5</SUB> Amorphous Ribbons

The effects of isothermal annealing on the magnetic properties of Pr4.5Fe77B18.5 amorphous ribbons have been investigated. Two separate reactions increasing coercive force ( Hc) were recognized during the progress of annealing. In the first reaction, hard magnetic Pr 2Fe14B particles with nanoscale diameters precipitated in the vicinity of Fe 3B and Pr2Fe23B3 particles. The precipitation of fine Pr 2Fe14B particles increased Hc largely but reduced saturation magnetization ( Ms) significantly. In the second reaction, the Pr 2Fe23B3 particles decomposed into Pr2Fe14B particles containing very fine α-Fe particles within them. This resulted in the second increase in Hc due to both the volume fraction increase of Pr2Fe14B particles and the exchange-coupling interaction between the Pr2Fe14B grains and α-Fe particles within them. This phenomenon was observed originally in the present study. In the final stages of annealing, both Hc and Ms were decreased substantially by the decomposition of Pr2Fe14B into PrFe4B4 particle in addition to the coarsening of α-Fe particles.

Relation between grain alignment and magnetic properties of Pr–Fe–B sintered magnets

This paper presents the results of investigations carried out to correlate magnetic properties and grain alignment in some as-sintered and heat treated Pr–Fe–B magnets. Sintered magnets based on the composition Pr 16Fe 76B 8 were prepared using the hydrogen decrepitation process. Magnetic behaviour ( B r and i H c) was studied as a function of degree of grain alignment ( 〈 cos θ〉 ). The volume fraction ( f) of the matrix phase Φ (Pr 2Fe 14B grains) in the permanent magnets was also estimated and correlated with intrinsic coercivity.

Microstructure refinement and magnetic property enhancement of nanocomposite Pr 2Fe 14B/α-Fe magnets by small substitution of M for Fe (M=Cr, Nb, Ti and Zr)

A comprehensive study on the effect of various substitutions of M on the magnetic and structural properties of melt-spun nanocomposite Pr 8Fe 84M 2B 6 (M=Cr, Nb, Ti and Zr) magnets has been performed with the aim of refining the microstructure and therefore enhancing the hard magnetic properties. It has been found that magnetic properties are improved by all the substitutions. The largest enhancement is obtained in Nb substituted Pr 8Fe 84Nb 2B 6 magnets where a coercivity of 6.5 kOe and a maximum energy product of 17.9 MGOe have been obtained, as compared to the coercivity of 4.3 kOe and the energy product of 9.1 MGOe in the Pr 8Fe 86B 6 magnet. Microstructure studies revealed a finer and more uniform 2:14:1/α-Fe nanoscale microstructure with M substitutions. The most uniform microstructure with the smallest average grain size of 10–20 nm is developed in the Nb substituted magnets. The enhancement of magnetic properties by M substitution is believed to be due to the microstructure refinement which leads to an enhanced exchange coupling between Pr 2Fe 14B and α-Fe.

Optimization of the magnetic properties of nanocrystalline Pr 5Fe 77− xCr xB 18 alloys

Ribbons of Pr 5Fe 77− x Cr x B 18 ( x=0, 1, 2, 2.5, 3, 4, 5) alloys were produced by melt spinning and then annealed to develop enhanced-remanence nanocrystalline magnetic material. Four different types of annealing were used in order to optimize the magnetic properties of these alloys. A conventional annealing, where the ribbons were encapsulated in quartz tubes with argon gas, was the method that led us to the best Pr 2Fe 14B/Fe 3B nanocomposite with Cr. The annealed samples were studied by magnetic measurements, X-ray diffraction and scanning electron microscopy. The best magnetic properties are found for Pr 5Fe 74Cr 3B 18. The value for the coercive field for this composition is at least 50% higher than for the material without Cr and 40% higher than for the Nd 2Fe 14B/Fe 3B nanocomposite with Cr. X-ray diffraction data and Curie temperature measurements showed that the main phases present in these samples are Pr 2Fe 14B, Fe 3B and α-Fe.

Effects of quenching rate on the phase transformation and magnetic properties of melt-spun Pr 8Fe 86B 6 ribbons during annealing

Phase transformations and magnetic properties of overquenched Pr 8Fe 86B 6 ribbons during annealing treatment have been investigated. The as-quenched structure of the ribbons was varied by changing the quenching rate during the melt spinning process. It was found that, depending on their as-quenched structures, the phase transformations of the ribbons during annealing may take place in one of the following sequences: (1) Amorphous phase (Am)+Pr 2Fe 14B+α-Fe→Pr 2Fe 14B+α-Fe; (2) Am+α-Fe→ Am′+α-Fe→α-Fe+Pr 2Fe 23B 3+Pr 2Fe 14B→Pr 2Fe 14B+α-Fe; and (3) Am→Am′+α-Fe→Pr 2Fe 23B 3+α-Fe→ Pr 2Fe 14B+α-Fe. However, for all the ribbons, the microstructure after optimal annealing was found to consist only of two magnetic phases: Pr 2Fe 14B and α-Fe. The optimum magnetic properties (intrinsic coercivity H ci and remanent polarization J r), and the squareness of the demagnetization curves of the annealed ribbons deteriorated drastically with increasing quenching rate of unannealed precursors. This deterioration can be attributed to the formation of a coarser and more irregular microstructure during annealing of the samples initially melt spun with higher wheel speeds, which was confirmed by our TEM observations, analysis of the temperature dependence of coercivity and measurement of irreversible susceptibility.

Refinement of α-Fe and its improvement on magnetic properties of melt-spun nanocomposite Pr 2Fe 14B/α-Fe magnets by low wheel speed spinning

Nanocomposite Pr 2Fe 14B/α-Fe magnets with uniform microstructure and enhanced magnetic properties have been synthesized by melt-spinning a Pr 8Fe 86B 6 alloy at low wheel speed range. Microstructural and magnetic studies showed that there is an optimum wheel speed (about 16.7 m/s) at which a uniform Pr 2Fe 14B/α-Fe structure with fine α-Fe grains (about 10 nm) is developed directly from the melt. Lower speed gives larger grains for both the 2:14:1 and α-Fe, while higher speed leads to the appearance of an amorphous phase which will result in a Pr 2Fe 14B/α-Fe structure with larger α-Fe grains after a subsequent crystallization annealing. Magnetic properties obtained under the optimum-quenching followed by a subsequent annealing are M s = 176.6 emu/g, M r = 118.2 emu/g, M r/M s = 0.67, H c = 5.4 kOe and (BH) m = 12.6 MGOe. The coercivity and energy product are about 20% higher than those obtained by crystallization of over-quenched samples due to the refinement of α-Fe size which leads to an enhanced exchange coupling between Pr 2Fe 14B and α-Fe.

Effect of wheel speed and subsequent annealing on the microstructure and magnetic properties of nanocomposite Pr 2Fe 14B/α-Fe magnets

Nanocomposite Pr 2Fe 14B/α-Fe magnets have been synthesized by melt-spinning a Pr 8Fe 86B 6 alloy at low wheel speed in the range from 10 to 21.7 m/s. Microstructural and magnetic studies showed that there is an optimum wheel speed (about 16.7 m/s) at which a uniform Pr 2Fe 14B/α-Fe microstructure with fine α-Fe grains is developed directly from the melt. Lower speed gives larger grains for both the 2 : 14 : 1 and α-Fe, while higher speed leads to the appearance of an amorphous phase which will result in a Pr 2Fe 14B/α-Fe structure with larger α-Fe grains after a subsequent crystallization annealing. Magnetic properties obtained under the optimum-quenching followed by a subsequent annealing are M s=176.6 emu/g, M r=118.2 emu/g, M r /M s =0.67, H c =5.4 kOe and ( BH) m=12.6 MGOe. The coercivity and energy product are about 20% higher than those obtained by over-quenching and then annealing due to the refinement of α-Fe size which leads to an enhanced exchange coupling between Pr 2Fe 14B and α-Fe. The fine α-Fe development is associated with its solidification behavior characterized by higher nucleation rate and lower growth rate during the rapid solidification.

Microstructural and Magnetic Improvements in Tube-Cast Pr-Fe-B-Cu Alloys Via Heat Treatments and Rapid Upset Forging

ABSTRACTAlloys of Pr-Fe-B-Cu were cast into Cu and stainless steel tubes. It could be seen that a preferred crystallographic orientation was obtained after cooling. Fine platelets of Pr2Fe1 4B matrix phase were formed, surrounded by various grain boundary phases, and there was a significant reduction in the amount of free Fe present, in comparison to conventional slab cast alloys. In order to improve the magnetic properties in the as-cast state, two alternative routes were used. The first involved a series of two step heat treatments to develop an improved microstructure. After an optimum heat treatment of 1000 'C for 24 hours + 500°C for 3 hours, significantly improved magnetic properties were obtained for a 17.5 % Pr alloy; Br = 752 mT, Hci = 613 kAm−1 and BHmax = 96 kJm−3. The second route involved a rapid upset forging [RUF] process, with a strain rate of 11.5 s−1, to hot deform the alloys. Following post-forging heat treatments, properties of Br = 966 mT, Hci = 780 kAm−3 and BHmax = 160 kJm−3 were obtained for a 15% Pr alloy. That nature of the improvements in properties as a result of heat treatments and RUF were investigated and are discussed in the present work.

Metastable and stable magnetic phases in as-cast and annealed Pr 80Fe 15(B 1− xC x) 5 alloys (0.0≤ x≤1.0)

In as-cast Pr 80Fe 15(B 1− x C x ) 5, samples metastable A 1 ( T c=225 °C) was the predominant magnetic phase in the whole composition range, with intrinsic properties that were not affected with increasing C content. Up to x=0.75 this phase coexists with an additional minor magnetic phase having T c=263 °C which has been labelled by us to as A 3. Upon annealing at 600 °C A 1 dissolves and the following stable phases were observed: a) Pr 2Fe 14B and A 3 for 0.0≤ x≤0.75, and; b) an unknown stable phase D 1 with coercivity around 2.1 kOe and Curie temperature of 230 °C for x=1.0. D 1 is the predominant phase for annealing times less than 8 h while for 8 and 16 h annealing an additional phase with T c=17 °C appears. The latter has been tentatively identified as Pr 2Fe 17. SEM and X-ray microanalysis studies were performed on Pr 80Fe 15C 5 samples in the as-cast state and after 16 h of annealing. The as-cast sample shows large Pr-rich grains immersed in a fine eutectic microstructure consisting of Pr and Fe. In annealed samples, both large square or polygonal grains and a needle-like phase are formed. The latter is believed to be D 1.

NMR study of magnetic properties of Pr 2Fe 14B

141Pr NMR measurements have been done on Pr 2Fe 14B and (Pr 0.9Lu 0.1) 2Fe 14B at 1.4K by the spin-echo method. The observed hyperfine fields in Pr 2Fe 14B are 2574 and 2803kOe for the f and g sites, respectively, and in (Pr 0.9Lu 0.1) 2Fe 14B 2560 and 2804kOe for the f and g sites, respectively. The hyperfine fields due to the 4f shell of the Pr 3+ ion were evaluated taking into account the transferred hyperfine field from the Fe sublattice. The magnetic moments of Pr 3+ in Pr 2Fe 14B were estimated to be 2.9 and 3.0μB for the f and g sites, respectively.

High-field transitions in Pr 2Fe 14B and Nd 2Fe 14B

The field-induced transitions present in the magnetisation curves of Pr 2Fe 14B and Nd 2Fe 14B have been investigated in detail. A first-order type of transition is observed in Nd 2Fe 14B, whereas the experimental data suggest that the transitions in Pr 2Fe 14B are not of the first order and originate from a fast but nevertheless continuous rotation of the magnetic moment.

Microstructure and coercivity of cast hot-pressed PrFeB magnets with copper addition and neodymium substitution for praseodymium

Microstructure and coercivity of cast hot-pressed Pr 19Fe 76B 5 magnets with copper addition and neodymium substitution for praseodymium are studied. It is found that copper strongly segregates at the Pr 2Fe 14B grain boundaries and markedly suppresses the grain diffusion growth during the final annealing. The coercivity is thus enhanced with copper addition. In magnets with neodymium substitution for praseodymium, however, a much lower coercivity is obtained because of their coarse-grained cast structure, which is correlated with the solidification behaviour of the magnets.

The Nd-Fe-B materials for permanent magnets

The microstructure-magnetic property relationship in Nd-Fe-B-type permanent magnetic materials has been investigated by means of transmission electron microscopy and analyses of the temperature dependence of the intrinsic coercivity. Effects of postsintering heat treatment, mean grain size in sintered magnets, magnetic separation of the magnetic grains by non-magnetic phases, and the degree of alignment of the c-axis of the magnetic grains have been studied in permanent magnets based on Pr 2Fe 14B. Results indicate that local demagnetization fields are the main sources of a reduction of the intrinsic coercivity in this type of magnets.

Non-linear magnetic response of 3d-4f intermetallics in high magnetic fields

Non-linear magnetic response of rare-earth-3d intermetallics in high magnetic fields are reviewed with special attention for the different mechanisms that can lead to kinks and jogs in the magnetisation curve: metamagnetism of the 3d partner, field-induced crossing of the rare-earth crystal-field levels, breaking of the antiparallel alignment of the rare-earth and 3d magnetic sublattices and anisotropy-related transitions. The latter result from a competition between the rare-earth crystal-field coefficients of different order. By inspecting the high-field magnetisation curves of YCo 2 and LuCo 2, Pr, Dy 2Co 17 and Pr 2Fe 14B, examples can be presented for all four different types of severe non-linearities.