Effect of Thermal Treatment on Formation of High Magnetic Parameters of Sintered Magnets Dd-Fe-B

The present paper is dedicated to study effect of thermal treatment parameters on the properties (residual magnetic induction fir, coercive force on magnetization _iH_c, temperature coefficient of magnetic induction α_b within the temperature range 300–373 K) of sintered permanent magnets [Dd_1–x(Dy, Tb, Ho)_x]₁₅(Fe_1–yCo_y)_bal(Al,Cu,Ga,Ti)_0.1–1.4B_5.0–8.5 (at.%), where x=0.2–0.4; y = 0.2–0.3; Dd - didymium. For the magnets (Dd_1–xDy_x)₁₅(Fe_1–yCo_y)_baiB_5.0–5.6 discontinuities were observed at demagnetization curves caused by appearance of magnetic phases with reduced magnetic anisotropy. _iH_c value nearly does not depend on temperature of thermal treatment within the range 750–1275 K at increasing of boron content (7.0–7.5at.%). Magnets doping by such elements as aluminium, copper, gallium shifts optimal temperature of the last step of thermal treatment from 900 K to 725–825 K, at the same time magnetic properties (B_r, _iH_c α_b) improve, especially with increasing of Co and Dy content. It was possible to attain the following properties for the magnets (Dd_0.6Dy_0.4)₁₅(Fe_0.74Co_0.26)_bai(Al,Cu,Ga)_1.1B_8.5 after optimal thermal treatment {1175 K (7, 2 ks) with further cooling at the rate 1–2 K/min, then 900 K (3.6 ks) - hardening + 775 K (3.6 ks) - hardening}: B_r= 1,0 T, _iH_c = 1600 kA/m, BH_max = 192 kJ/m³, α_b= −0,024 %/K in the temperature interval 293–373 K.     

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.     

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...     

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.     

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.     

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.     

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.     

Effect of samarium on the temperature induction coefficient of Nd-Dy-Fe-Co-B materials

Sintered (Nd_{1 ? x ? z} Dy _x Sm₂)-(Fe_{1 ? y} Co _y )-B (atomic fractions x = 0.27, 0.32, 0.39; y = 0.17, 0.23, 0.28; and z = 0.04–0.18) materials have been studied. It is shown that the magnetic moments of samarium ions and the iron-cobalt sublattice are ordered antiferromagnetically and, as the samarium content increases, the content of dysprosium and samarium in the basic A magnetic phase decreases and increases, respectively. As the samarium content (z) in sintered magnets (Nd_{0.61 ? z} Dy_0.39Sm _z )_17.5(Fe_0.72Co_0.28)_76.3B_6.2 increases, the thermal induction coefficient is found to decrease monotonically from 0.021 to 0.006%/°C, i.e., by 3.5 times. In this case, residual inductance B _r is unchanged (0.8 T).     

Influence of gadolinium and dysprosium substitution on magnetic properties and magnetocaloric effect of Fe78−xRExSi4Nb5B12Cu1 amorphous alloys

Amorphous Fe_78−xRE_xSi₄Nb₅B₁₂Cu₁ (RE = Gd, Dy) ribbons with different RE contents were prepared by melt spinning to investigate the effect of heavy rare earth (Gd, Dy) substitution on the hyperfine structure, magnetic properties and magnetocaloric effect. The Curie temperature of RE substituted alloys, hyperfine field and magnetic moments of Fe atoms initially increase up to 1 at% RE content and then decrease monotonously for increasing RE content up to 10 at%. The dependence of magnetic entropy change (–ΔS_M) and refrigeration capacity (RC) of the alloys on RE contents displays the same tendency. The RC_AREA values of the alloys substituted with 1 at% Gd and Dy are similar to those of recently reported Fe-based metallic glasses with enhanced RC values compared with those of Gd₅Ge_1.9Si₂Fe_0.1. Enhanced –ΔS_M and RC values, negligible coercive force and hysteresis commonly make these Fe_78−xRE_xSi₄Nb₅B₁₂Cu₁ amorphous alloys as low-cost candidates for high-temperature magnetic refrigeration.     

Sintering of Pr-Dy-Fe-Co-B materials

The properties of sintered magnets (Pr_{1 ? x} Dy _x )-(Fe_{1 ? y} Co _y )-B (x ? 0.6 at %, y ? 0.2 at %) as functions of the milling time and the sintering conditions are studied. Minima in the coercive force and the density as functions of the sintering temperature are detected. The dependence of the minimum depth on the milling time is detected. In the case of long-term sintering or additional sintering, the values of these parameters are restored. The minimum in the (Pr_{1 ? x} Dy _x )-(Fe_{1 ? y} Co _y )-B (x ? 0.6 at %, y ? 0.2 at %) materials is assumed to be related to the deceleration of diffusion processes in them as compared to the (Nd_{1 ? x} Dy _x )-(Fe_{1 ? y} Co _y )-B (x = 0.71 at %, y = 0.34 at %) system, in which this minimum is insignificant.     

Role of interphase boron diffusion in the formation of the magnetic properties of sintered (Pr,Dy)-(Fe,Co)-B materials

Sintered (Pr, Dy)-(Fe_{1 ? y} Co _y )-B materials (y = 0.15–0.5 at %) have been studied. It is found that, with increasing cobalt content, the coercive force of the sintered materials decreases down to zero at y ～ 0.5. The volume contents of all phases present in the compositions with y = 0.2–0.9 are determined. The interphase boron diffusion is shown to play a key role in the formation of the magnetic properties during sintering and annealing.     

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.     

Effect of Annealing of a Pr–Dy–Fe–Co–B Alloy on Its Phase Composition and the Properties of Related Sintered Magnets

The effect of annealing of a (Pr_0.41Ce_0.12Dy_0.47)_13.46(Fe_0.64Co_0.36)_80.3B_6.24 alloy at 1000°C on its microstructure and the properties of sintered magnets made of it has been studied. In the annealed state, the composition of the main magnetic phase (Pr,Dy)₂(Fe,Co)₁₄B changes sharply (dysprosium content changes), the residual induction of the sintered magnets made of the annealed alloy increases by 6%, and the coercive force determined from magnetization increases by 8.5%. The volume content of the main magnetic phase R₂(Fe,Co)₁₄B in sintered magnets (Nb,Dy)–(Fe,Co)–B and (Pr,Dy)–(Fe,Co)–B is found. The content of this phase in neodymium-based magnets is approximately half as much.     

Magnetic properties of Pr-Dy-Fe-Co-B sintered materials with a high cobalt content

Materials (Pr_1?x Dy _x )_12–17(Fe_1?y Co _y )_balB_5–15 (x = 0.44–0.51 at %, y = 0.15–0.87 at %) have been studied. It is shown that the abrupt decrease in the coercive force of the (Pr_1?x Dy _x )_12–17(Fe_1?y Co _y )_balB_5-15 sintered materials with x = 0.44–0.51 and y > 0.25 is determined by the amount of boron-containing phases. In this case, an increase in the boron content in the (Pr_0.72Dy_0.28)_14.5(Fe_0.50Co_0.50)_balB _z material with an unchanged dysprosium content leads to an increase in the coercive force by 4.5 times. It is assumed that the escape of boron from the near-surface areas of R ₂ F ₁₄B-phase grains leads to the formation of regions characterized by a low anisotropy constant, which serve as nuclei for the formation of reverse-magnetization domains.     

Effect of copper on the properties of Pr-Dy-Fe-Co-B sintered magnets

The effect of copper on the properties of magnets (Pr_0.52Dy_0.48)₁₃(Fe₆₅Co_0.35)_{80.3 ? x} Cu _x B_6.7 (x = 0–10) has been studied. Alloying with copper is shown to decrease the sintering temperature and to increase the content of the principal (Pr,Dy)₂(Fe,Co)₁₄B magnetic phase. For compositions with x = 1.3–3.3, copper is found to affect the value and sign of the temperature induction coefficient (TIC). It is shown that the effect of copper on the TIC is determined by the substitution of copper ions for iron ions in lattice sites, which are coupled via an antiferromagnetic exchange interaction.     

Chemical State, Site, Solid Solubility, and Magnetism of Fe in the Ferropericlase (Mg1–x Fe x )O Produced by Ball Milling of MgO and Fe

Ferropericlase (Mg_1–x Fe _x )O solid solution was prepared by ball milling of the mixture of MgO with a rock-salt structure and metal Fe powders in atmosphere and at room temperature. Differing from (Mg_1–x Fe _x )O prepared at high temperature by using MgO and FeO as starting materials, the solution of Fe in MgO is not continuous but limited in the ball milling process, and the solubility limit is less than 30 at. pct. About 92 pct of the Fe ions occupy the site of tetrahedral oxygen coordination in trivalent Fe (Fe³⁺) with high spin, whereas about 8 pct of the Fe ions occupy the site of octahedral oxygen coordination in bivalent Fe (Fe²⁺) with high spin. The Fe³⁺ and Fe²⁺ ions do not show a ferromagnetic but show a paramagnetic state. The as-milled (Mg_1–x Fe _x )O is metastable and decomposes to ferropericlase (Mg_1–y Fe _y )O (where y < x) and MgFe₂O₄ with spinel structure as annealed above 773 K (500  °C), and the content of Fe in the (Mg_1–y Fe _y )O increases with increasing annealing temperature. A bulk (Mg_1–x Fe _x )O was fabricated by annealing the as-milled (Mg_1–x Fe _x )O powders at 973 K (700  °C). It shows n-type conductivity, which is attributed to an electronic small polaron with an activation energy of 0.135 eV.     

Influence of Heat Treatment Parameters on Properties of NdRFeMCoB Sintered Magnets

Influence of multi-step heat treatments on magnetic properties and structure of three magnet types NdRFeMCoB: high-energy (B_t < 1, 3 T) (A); high-coercive (_iH_c < 2000 kA/m) (B) and thermally stable (α_b> |0, 02| %/K in the temperature interval 293–373K) (C) was investigated. Significant affect of condition of Nd₂Fe₁₄B phase on high-coercive condition development is shown. High values of _iH_c in magnets of NdRFeMCoB type can be provided by creating of elasto-stressed condition of Nd₂Fe₁₄B phase and minimal stresses values near boundary phases during heat treatments. The highest results on _iH_c are achieved after multi-step heat treatments. The temperature of the last step of heat treatment for optimal chemical compositions of the alloys (in dependence on the ratio of such elements as Al, Cu, Ga, Pr, Co) is in the interval of 725–905K (preferably, 775–825K). The value of _iH_c of magnet (B) Nd_13,2Tb_1,8Fe_baiCo₅Mo_3,8Al_0,9B₉(at. %) after heat treatment {1175–1075K (4–12 ks) - hardening + 625–725K (3,6–36 ks) + 810K (3, 6 ks) - hardening} was 2600 kA/m. It was possible to attain the following properties for the magnets (C) (Nd_0.22Pr_0.44Tb_0.20Ho_0.10Gd_0.04)₁₅(Fe_0.73Co_0.27)_baiAl_0.99Cu_0.1Ga_0.1B_8.5(at. %) after optimal thermal treatment (1175K (3,6–7,2 ks) with slow (12–16 ks) cooling to 675K and subsequent endurance at T=775K for 3,6 ks - hardening}: B_r= 1,0 T, _iH_c = 1560 kA/m, BH_max= 192 kJ/m³, α_b> |0, 015| %/K in the temperature interval 293–373K.     

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.     

Effect of C on Phase Evolution,Microstructure, and Magnetic Properties of Pr2Fe14B-Type Nanocomposites

The phase evolution, microstructure and magnetic properties of melt-spun Pr-rich Pr₁₁Fe_bal.B_8-yC_y (y = 0–8) and Pr-lean Pr₉Fe_bal.Ti_xB_11–yC_y (x = 0, 2.5 and 4; y = 0–11) ribbons have been investigated intensively. A slight substitution of C for B (y=2) was proven to be effective in improving the magnetic properties of Pr-rich Pr₁₁Fe_bal.B_8–yC_y nanocomposites. C atoms prefers to enter 2:14:1 phase in forming Pr₂Fe₁₄(B, C). But the volume fraction of Pr₂Fe₁₇C_z, α-Fe and 1:2 carbide increases, due to the dissociation of 2:14:1 phase and the suppression of Fe₃B phase, with further increase of carbon content. The optimal magnetic properties of B_r = 9.4 kG, _iH_c = 9.3 kOe, and (BH)_max = 17.3 MGOe were obtained for Pr₁₁Fe_bal.B₆C₂. In contrast, the increase of C substitution in Pr₉Fe_bal.Ti_2.5B_11–yC_y (y=0–11) ribbons degrades the B_r, _iH_c, and (BH)_max monotonically, which is arisen from the increase of vol % of Pr₂Fe₁₇C_z and α-Fe phases, and the rapid decrease of 2:14:1 phase. In comparison with those of Pr₉Fe_bal.B_11–yC_y (y=0–5.5) ribbons, improved magnetic properties of (BH)_max=15.3–17.8 MGOe with higher coercivity of _iH_c=9.7–10.8 kOe have been obtained in Ti-containing Pr₉Fe_bal.Ti_2.5B_11–yC_y (y=0–5.5) ribbons. However, in higher Ti concentration Pr₉Fe_bal.Ti₄B_11–yC_y, ribbons, a slight substitution of C for B (y = 0.5–1) is beneficial in improving the coercivity and magnetic energy product, simultaneously. The optimal properties of B_r = 9.4 kG, _iH_c = 11.1 kOe, (BH)_max = 18.0 MGOe and α = −0.146 %/°C, β = −0.576 %/°C were achieved in Pr₉Fe_bal.Ti₄B_10.5C_0.5 nanocomposites.     

Pr-Dy-Fe-Co-B materials for navigation instruments

The temperature dependence of the magnetization of (Pr_{1 ? x} Dy _x )_13–14(Fe_{1 ? y} Co _y )_balB₇ (x = 0.48–0.73, y = 0.24–0.35) sintered materials has been studied in a temperature range of 20–100°C. A theoretical model is proposed to describe the temperature dependence of the magnetization of materials. The results of testing the materials with a pendulous accelerometer are reported and compared with the results of testing the existing device materials.