Constitution of melt-spun NdFeB-magnets studied by analytical field ion microscopy

An analytical field ion microscope was applied to melt-spun NdFeB magnets with various cobalt substitutions. The microstructure of melt-spun NdFeB magnets consists of three phases. Whereas the Nd₂Fe₁₄B- and the metastable Nd₇Fe₃-phase determine the magnetic properties of optimally quenched specimens the Nd_1.1Fe₄B₄-phase additionally influences the magnetic behaviour of overquenched and annealed material. With Co substitution the Nd-rich phase transforms via the metastable Nd₂₃(Fe_1−xCo_x)₇₅B₂ intermediate stage of optimally quenched specimens to the stable Nd(Fe_1−xCo_x)₂-phase in over-quenched and annealed ribbons. From our experimental data we conclude that a nucleation model of coercivity is consistent with the magnetic measurements.     

Magnetic Alignment Study of Hybrid Magnet Consisting of R-Rich and R-Lean Nd-Fe-B Alloys

A hybrid magnet was prepared by the hot-pressing and die-upsetting of the mixture of the R-rich Nd_xFe_93.5–xGa_0.5B₆ (x= 13.5 and 11.8) alloy and the R-lean Nd_xFe_93–xNb₁B₆ (x = 6, 9) alloy melt-spun ribbons. The microstructure and magnetic properties of the hybrid magnet were investigated. In the hot-pressed or die-upset hybrid magnet the R-rich and R-lean alloy regions existed independently without alloying between them. The two alloy regions in the die-upset hybrid magnet were coupled effectively via a magnetostatic interaction. A texture was developed only in the R-rich Nd₂Fe₁₄B single phase alloy region in the die-upset hybrid magnet, and this led to an anisotropic nature in die-upset hybrid magnet. The die-upset hybrid magnets containing higher Nd-content (13.5 at%) host alloy shows consistently a better magnetic alignment with respect to the magnets with lower Nd-content (11.8 at%) host alloy.     

Toughness of Sintered NdFeB Magnets

The failure mechanism of sintered NdFeB magnets was studied from the view of fracture model and fracture mechanism, and the effect of alloying on strength and toughness of the magnets was researched too. The results showed that intercrystalline fracture was the main fracture model and cracks elongated along the interface between 2–14–1 phase and Nd-rich phase. Generally, the methods for improving toughness include enhancing bond strength of the interface between the two phases mentioned just, refining grains of 2–14–1 phase and increasing the amount of particles of Nd-rich phase localized at grain boundary. The fracture model of the Nd_15.5Fe_76.5Al₁Ti_0.5B_6.5 magnets was transcrystalline cleavage fracture, whose value of fracture toughness increased about 30% compared to that of the Nd_15.5Fe₇₈B_6.5 magnets. Otherwise, the value of fracture toughness of the magnets with a proper amount of addition of Ag increased about 70% compared to that of the Nd_15.5Fe₇₈B_6.5 magnets. Moreover, the two sorts of modified magnet mentioned just could be drilled successfully. In addition, development of the research about increasing the strength and toughness of sintered NdFeB magnets is reviewed in this article.     

Magnetic properties and microstructure of Fe_(69.5-x)Nd_7B_(21)Nb_(2.5)Ga_x(x=0-1)alloys

The Fe_(69.5-x)Nd_7 B_(21)Nb_(2.5)Ga_x(x = 0-1)permanent magnets in the form of rods were prepared by annealing the bulk amorphous alloys.The magnetic properties,phase evolution and microstructure of the alloys were investigated systematically.It is found that the glass forming ability(GFA), microstructure and magnetic properties are sensitive to Ga content for Fe_(69.5-x)Nd_7 B_(21)Nb_(2.5)Ga_x(x = 0-1)bulk alloys.The annealed alloys are mainly composed of soft α-Fe,hard Nd_2 Fe_(14)B and nonmagnetic Nd_(1.1)Fe_4 B_4 phases.When x = 0.3,the optimally annealed magnets exhibit magnetic properties of the remanence Br = 0.63 T,intrinsic coercivity H_(cj) = 368.68 kA/m and maximum energy product(BH)_(max) = 33.73 kJ/m~3.Furthermore,magnetic field heat treatment at the temperature close to Curie temperature of Nd_2 Fe_(14)B phase was applied to the annealed Fe_(69.2)Nd_7 B_(21)Nb_(2.5)Ga_(0.3) magnet.The results of X-ray diffraction(XRD)and transmission electron microscopy(TEM)indicate that the magnetic field heat treatment can be beneficial for the precipitation of α-Fe.Thus,the B_r,H_(cj) and(BH)_(max) are enhanced by 8.7%,6.3% and 16.3%,respectively.     

Microstructure improvement related coercivity enhancement for sintered NdFeB magnets after optimized additional heat treatment

The microstructure, especially the Nd-rich phase and the grain boundary, in sintered NdFeB magnets plays an important role in magnetic reversal and coercivity mechanism. To better understand the effects of the microstructure on the coercivity, we investigated the microstructure and properties improvements of a commercial sintered NdFeB magnet after optimized additional heat treatment. The coercivity is enhanced from 1399 to 1560 kA/m. This enhancement has been explained in terms of the evolution of the grain boundary structure, and the formation of continuous thin layers of Nd-rich phase is important for high coercivity. The micromagnetic simulation together with the numerical analysis based on the nucleation model suggest that the reversed magnetic domains nucleate mainly at the interface of multi-junctions of Nd₂Fe₁₄B grains with high stray fields during the demagnetization process. Both improved anisotropy fields at grain boundaries and reduced stray fields at multi-junction Nd-rich phases contribute to the coercivity enhancement. This work has importance in understanding the crucial microstructure parameters and enhancing the obtainable properties for sintered NdFeB magnets.     

Effect of Rare Earth Substitution on Machinability of R-Fe-B Sintered Magnets

The machinability of sintered Nd-Fe-B magnets with nominal compositions of (Nd_1–xDy_x)₁₆Fe₇₈B₆ (x = 0, 0.05, 0.10, 0.15) and (Nd_1–yPr_y)₁₆Fe₇₈B₆ (y = 0, 0.33, 0.67, 1), has been investigated. The bending strength, fracture toughness and the Vickers Hardness were measured. It shows that the Dy substituted magnets shows higher Vickers Hardness than the Pr substituted magnets. The brittle index for investigated magnets improves monotonously with increasing Dy content, reduces with increasing Pr content, respectively, which seems to relate closely to the change of the lattice parameters.     

Microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys

In this paper, we systematically investigated the microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys by transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) analyses. The rod-like NdH_2+x phases with diameters of 10–20 nm are embedded into α-Fe matrix, which hereditarily leads to textured grains in HDDR alloy. The migration of NdH_2+x from Nd-rich region to α-Fe matrix during hydrogen absorption process contributes to the uniform redistribution of Nd-rich phases after HDDR treatment. The HDDR alloy with single domain grain sizes of 200–300 nm exhibits relatively low coercivity of 1.01 T that arises from pinning magnetic domain motion. The weak c-axis orientation of HDDR alloy results in a lower reverse magnetic field (coercivity) to reduce remanence to 0. Moreover, the direct contact of Nd₂Fe₁₄B grains and the high concentration of ferromagnetic elements (Fe content ≈ 66.06 at%, Co content ≈ 0.91 at%) in Nd-rich grain boundary layer lead to strong magnetostatic coupling effect among Nd₂Fe₁₄B grains. The nano-sized α-Fe inside Nd₂Fe₁₄B matrix makes the magnetization reversal easily and decreases the coercivity of HDDR alloy.     

Superior corrosion resistance and corrosion mechanism of dual-main-phase (Ce15Nd85)30FebalB1M magnets in different solutions

The electrochemical corrosion behavior of both(Ce₁₅Nd₈₅)₃₀Fe_balB₁ M sintered magnets prepared with dual-main-phase method and N45-type magnets was studied in 3.5 wt% NaCl,1.1 wt% NaH₂ PO₄,and2.5 wt% NaOH solutions,respectively.The(Ce₁₅Nd₈₅)₃₀Fe_balB₁ M sintered magnets perform superior corrosion resistance than N45-type magnets in the tested solutions.In general,two ...     

Effect of yttrium substitution on magnetic properties and service performances of Nd-Fe-B sintered magnets

We successfully fabricated partial Y substituted NdY-Fe-B magnets with nominal compositions of(Nd_1-xY_x)_13.80Fe_ba1Al_0.24Cu_0.1B_6.04(at%,x=0,0.1,0.2,0.3,0.4) by powder metallurgy process and the magnetic properties as well as service performances of the magnets were also systematically investigated.The phase constituents of the magnets have no obvious variation within the whole range of Y content,while the main phase grain...     

Spark Plasma Sintering Nd-Fe-B Permanent Magnets With Good All-Around Property

In present study, sintered Nd-Fe-B permanent magnets with different compositions were fabricated by using both Spark Plasma Sintering (SPS) technique and conventional sintering technique. Microstructure and compositions of both magnets are observed by scanning electron microscope with energy dispersive X-ray detector. Magnetic properties, mechanical properties, and chemical stabilities of both Nd-Fe-B magnets are investigated. Compared with the conventional sintered magnets, SPS Nd-Fe-B magnets possess comparable magnetic properties, better corrosion resistance and mechanical properties. Further investigation shows that the good all-around properties of the SPS magnets result from their unique microstructure. In detail, the grain size of Nd₂Fe₁₄B main phase is fine and uniform, only a few Nd-rich phase forms along the grain boundaries of Nd₂Fe₁₄B, while most of them agglomerates into the triple junctions. As a result, SPS process is expected to be a promising method for the production of new Nd-Fe-B magnets with good all-around properties.     

Effects of Nd–Ga intergranular addition on microstructure and magnetic properties of heavy-rare-earth-free Nd-Fe-B sintered magnets

In this study, we propose an approach of grain boundary modification that can significantly increase the coercivity of the B-lean Nd-Fe-B sintered magnets by intergranular addition of Nd–Ga. The coercivity is substantially enhanced from 1.51 to 2.04 T through optimizing the microstructure and adjusting the phase composition for the grain boundary phase in the annealed magnets. The matrix grains are covered by a continuous thin grain boundary phase accompanying the formation of intermetallic Nd₆Fe₁₃Ga phases. The analysis of magnetic behaviors above Curie temperature confirms that the grain boundary phase of annealed Nd–Ga doped magnets appears to be non-ferromagnetic, facilitating the intergrain exchange decoupling. Microstructure observation in grain boundary area indicates that some surface of the matrix grain is dissolved in the formation process of the Nd₆Fe₁₃Ga phase. It gives rise to a decrease in the proportion of matrix grains and saturation magnetization of the magnet. The detailed relationship between magnetic properties and microstructure is discussed based on these results.     

Multilayer Magnets

After experimental evidence of intergrain exchange coupling was reported, nanocomposite magnets with high remanence and large energy products were predicted. However, the experimental values of the maximum magnetic energy product of nanocomposite bulk magnets have been much less than the theoretically predicted ones. We gave a brief review on advances in multilayer magnets. The exchange coupling and remanence enhancement were realized in nanocomposite (Nd,Dy)(Fe,Co,Nb,B)_5,5/α-Fe thin films prepared by sputtering and heat treatments. Well-designed multilayer films consist of magnetically hard Nd₂Fe₁₄B-type phase with the grain size of 40 nm and magnetically soft α-Fe phase existing in the form of the continuous layers. Furthermore, we reported the structural and magnetic properties of anisotropic Nd-Fe-B thin films. The effects of thickness, deposition rates, substrate temperature, annealing temperature were studied. A high maximum energy product of (BH)_max = 270 kJ/m³ was obtained for anisotropic Nd-Fe-B thin films.     

Preparation and magnetic properties of anisotropic Nd_2Fe_(14)B/Sm_2Co_(17) hybrid-bonded magnets

In the present work, anisotropic Nd_2 Fe_(14) B/Sm_2 Co_(17) hybrid-bonded magnets were prepared with different Nd-Fe-B contents. It is found that the particle distributions and ratios between the two magnetic phases have important roles in the magnetic properties, microstructures and thermal stability of the magnets. With increase of Nd-Fe-B content, the saturation magnetization of the anisotropic hybrid magnet increases significantly, however, coercivity decreases, and the demagnetization curves show magnetically single-phase behavior. The anisotropic Nd_2 Fe_(14) B/Sm_2 Co_(17) hybrid-bonded magnets exhibit a maximum energy product and remanence of 14.15 MGOe and 99.53 A·m~2/kg, respectively, when the NdFe-B content is 70 wt% at room temperature. Furthermore, the hybrid magnets also have better thermal stability at elevated temperatures due to the interaction between the two magnetic particles.     

Effect of La–Y co-substitution on magnetic properties and microstructure of NdFeB alloy ribbons

Committed to obtaining cost-effective NdFeB based permanent magnets, Nd_27–xLa₃Y_xFe_balAl_0.1Cu_0.1B₁ (x = 0–3) alloys were fabricated to detect the magnetic properties and microstructure. When x = 1.8, coercivity of 1004 kA/m and the magnetic remanence of 0.75 T are obtained, which are close to those of the original Nd₃₀Fe_balAl_0.1Cu_0.1B₁ alloy ribbons. The temperature coefficient of coercivity (β) and the temperature coefficient of remanence (α) of the LaY-substituted alloys are better than those of the original alloys. The research results manifest that La is mainly distributed in the grain boundary phases and plays the role of refining the main grains, optimizing the grain boundary phases and improving the wettability between the main phases and the grain boundary phases, while Y tends to enrich in the main phases and enhances the short-range exchange coupling.     

A comparative study of NdY-Fe-B magnet and NdCe-Fe-B magnet

Low cost and high abundance rare earth elements Y and Ce have attracted tremendous interests of the industrial and scientific societies for fabricating the highly cost-performance efficient rare earth permanent magnets. However, the effect of separate replacement of Nd by Y or Ce on the performances of NdFeB-type magnet under the same atomic ratio and preparation conditions is still unclear. In this work, we systematically investigated the magnetic properties, thermal stabilities and service performances of (Nd_0.8Y_0.2)_13.80Fe_balAl_0.24Cu_0.1B_6.04 (atomic fraction, denoted as 20Y) and (Nd_0.8Ce_0.2)_13.80Fe_balAl_0.24Cu_0.1B_6.04 (atomic fraction, denoted as 20Ce) magnets. The results demonstrate that the μ₀M_r, μ₀H_c and (BH)_max of 20Y magnet are respectively 1.325 T, 1.173 T and 343.467 kJ/m³, which are comprehensively higher than those of 20Ce magnet (μ₀M_r = 1.310 T, μ₀H_c = 0.948 T, (BH)_max = 321.105 kJ/m³). Moreover, the 20Y magnet has higher thermal stability compared with 20Ce magnet which is favorable for the magnetic performances at elevated temperatures. The investigation of microstructure and elemental distribution indicates that the excellent magnetic performances of NdY-Fe-B magnet can be attributed not only to the preferable intrinsic properties 4πM_s, H_a and T_c of Y₂Fe₁₄B, but also to the in-situ core–shell structure of the 2:14:1 matrix phase grain with Y-rich core and Nd-rich shell, along with the thicker grain boundary layer between the adjacent matrix phase grains in NdY-Fe-B magnet. Furthermore, the 20Y magnet exhibits better mechanical property and higher corrosion resistance than 20Ce magnet, which are helpful for prolonging the service life of the magnet in practical application.     

Effect of Terbium addition on the coercivity of the sintered NdFeB magnets

The effects of Tb addition on the microstructure and magnetic properties of the NdFeB magnets prepared by HD method were investigated by X-ray diffraction (XRD) and BH magnetometers. The results of the microstructure showed that both the Tb-doped and undoped permanent magnets were composed mostly of Tetragonal phase Nd₂Fe₁₄B (space group P42/mnm) and a trace amount of Nd-rich phase. Accordingly, addition of Tb led to a decrease of the pole density factor of (004), (006) and (008) crystal plane of the Nd₂Fe₁₄B phase calculated by Horta formula, but the coercivity of the magnets increased from 2038 kA/m up to 2302 kA/m as a consequence of Tb addition. The study of the H_c(T)/M_s(T) versus H^min_N/(M_s(T) behavior showed that the nucleation was the dominating mechanism for the magnetization reversal in both sintered magnets, and the microstructural parameters of α_k and N_eff were obtained also. The Kronmüller-plot showed an increase of the α_k responsible for an increase of the coercivity.     

Phase Composition and Magnetic Properties of Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B/α-Fe Nanocomposites Prepared by Mechanical Alloying

Combined studies of hard magnetic Nd₂Fe₁₄B/α-Fe nanocomposites are performed. They were prepared by mechanical alloying of melt-quenched Nd_7.4Pr_2.0Fe_76.6Co_4.2Zr_3.4B_6.4 and Nd_5.8Fe₈₀Co_4.9Ti_1.5Si_2.5B_5.3 alloys taken in mass proportions of 90/10 and 70/30. It is found that, after mechanical alloying, an amorphous–crystalline structure is formed; it consists of the hard magnetic Nd₂Fe₁₄B and soft magnetic (amorphous and α-Fe) phases. Subsequent annealing at ~500°C initiates the decomposition of the amorphous phase and the formation of the nanocrystalline Nd₂Fe₁₄B and α-Fe phases. This leads to an increase in the coercivity and the residual magnetization-to-saturation magnetization ratio (σ_r/σ_s ≥ 0.5). It is assumed that the magnetic hardening of powders is due to the formation of an exchange-coupled state, which results from the exchange interaction between α-Fe nanocrystals and the Nd₂Fe₁₄B phase.     

Electrical and Magnetic Properties of Nd2Fe14B Magnet in Demagnetized and Magnetized States at Low Temperatures

Below room temperature the spin reorientation phase transition was observed for Nd₂Fe₁₄B magnet with T_sr ∼ 135 K. Spin reorientation refers to a change in the preferred direction of the M magnetization. The Nd₂Fe₁₄B magnet changes from a uniaxial or easy-axis material at TT_sr to an easy-cone material at T <T_sr. The transition from easy-axis to easy-cone anisotropy is an interesting physical phenomenon, which is important in a practical sense. The purpose of the present work is to study the electrical and magnetic properties of Nd₂Fe₁₄B magnet in the interval of temperatures ∼ 15–273 K in demagnetized and magnetized states. For the first time measurements of electrical resistance (ρ) and a complex magnetic susceptibility (χ = χ′+iχ″) of Nd₂Fe₁₄B alloy in demagnetized state and at two directions of magnetization in a range of temperatures ∼ 15–273 K were carried out. The following new effects were found out: (i) It was established that occurrences of the positive contribution to the value of ρ at the perpendicular magnetization and the negative contribution at the parallel magnetization. (ii) It was shown that, the value of the magnetic contribution in resistivity at the parallel magnetization, Δρ|, has a minimum in a vicinity of temperature of the spin reorientation transition, T_sr. (iii) It was established the occurrence of maximums of χ′ and χ″ values in a vicinity of temperature of the spin reorientation transition. (iv) It was established that T_sr value at perpendicular magnetization goes down and at parallel magnetization increases in comparison with the demagnetized sample. The received data have the practical importance for the application of Nd₂Fe₁₄B based permanent magnets at low temperatures.     

Effect of cobalt on the oxidation resistance of Pr(Nd)–Dy–Fe–Co–B materials

The effect of cobalt on the oxidation resistance of (Nd_0.85Dy_0.15)_16.4(Fe_0.89Co_0.11)_74.4Ti_1.3B_7.9 and (Pr_0.56Dy_0.39Sm_0.05)_14.5(Fe_0.75Co_0.25)_78.8B_6.7 alloys has been studied. The storage of magnet blanks made from these alloy in air for 200 h does not affect the magnetic properties of the sintered magnets owing to the presence of the phases (Pr, Dy)(Fe, Co)₂, (Pr, Dy)(Fe, Co)₂B₂, (Pr, Dy)(Fe, Co)₄B, (Pr, Dy)(Fe, Co)₃B₂, and (Pr, Dy)(Fe, Co)₃, which are resistant to oxidation and ensure liquid-phase sintering of magnets. After 200-h exposure to air, oxidation of the blanks takes place, the rate of which decreases by more than two times at the expense of an increase in the cobalt content in the alloy.     

Role of boron in the formation of the magnetic properties of sintered Nd-Dy-Fe-Co-B materials with a high cobalt content

Sintered materials having the compositions (Nd_{1 ? x} Dy _x ) _S (Fe_{1 ? y} Co _y )_balB_6–8 (atomic fractions x = 0.09–0.71, y = 0.19–0.34, S = 13.3–15 at %) have been studied. It is shown that, as the content of the boron-containing (Nd, Dy)(Fe, Co)₄B phase increases, the coercive force decreases abruptly. This effect is explained by the transition of boron atoms from the principal magnetic boron-containing (Nd, Dy)₂(Fe, Co)₁₄B phase to the (Nd, Dy)(Fe, Co)₄B phase during sintering.