Coercivity and microstructure of sintered Nd-Fe-B magnets diffused with Pr-Co,Pr-Al,and Pr-Co-Al alloys

The commercial 42 M Nd-Fe-B magnet was treated by grain boundary diffusion(GBD)with Pr₇₀Co₃₀(PC),Pr₇₀Al₃₀(PA)and Pr₇₀Co₁₅Al₁₅(PCA)alloys,respectively.The mechanism of coercivity enhancement in the GBD magnets was investigated.The coercivity was enhanced from 1.63 T to 2.15 T in the PCA GBD magnet,higher than the 1.81 T of the PC GBD magnet and the 2.01 T of the PA GBD magnet.This indicates that the joint addition of Co and Al in the diffusion source can further improve the coercivity.Microstructural investigations show that the coercivity enhancement is mainly attributed to the exchange-decoupling of the GB phases.In the PCA GBD magnet,the wider thin GB phases can be formed and the thin GB phases can still be observed at the diffusion depth of 1500μm due to the combined action of Co and Al.At the same time,the formation of the Pr-rich shell can also be observed,which is helpful for the coercivity enhancement.     

Corrosion behaviour of hot-pressed and die-upset nanocrystalline NdFeB-based magnets

Isotropic and anisotropic nanocrystalline Nd₁₄Fe₈₀B₆ and Nd₁₂Dy₂Fe_73.2Co_6.6Ga_0.6B_5.6 magnets have been produced from melt-spun materials by hot pressing and subsequent die-upsetting. The microstructure has been characterized using XRD, scanning electron microscope and energy dispersive X-ray analysis. The corrosion behaviour of die-upset NdFeB-based magnets has been studied in 0.1 M H₂SO₄ by inductively coupled plasma solution analysis and electrochemical polarization techniques and compared with their hot-pressed counterparts. Texturing of hot-pressed (isotropic) NdFeB-based magnets via die-upsetting significantly modifies their corrosion performance. Textured Nd₁₂Dy₂Fe_73.2Co_6.6Ga_0.6B_5.6 magnets exhibit the highest corrosion resistance in this study. The low effective diffusivity of corrosion hydrogen inside the bulk magnet and the reduction in the strength of galvanic coupling between magnet phases are the main reasons for the observed improvement in the corrosion resistance. The corrosion behaviour of the magnets in relation to their phase composition and phase distribution is discussed in terms of dissolution, hydrogenation and pulverization. Pulverization trends are correlated with hydrides formation and hydrogen-trapping sites using thermal desorption analysis.     

Effect of hot deformation on texture and magnetic properties of Sm-Co and Pr-Co alloys

Nanocrystalline PrCo₅, SmCo₅ and Sm₂(Co,Fe,Mn)₁₇ alloys were subjected to a high-degree plastic deformation at 950 °C with the height reduction ranging from 70% to 95%. With increasing degree of deformation, the PrCo₅ and SmCo₅ magnets showed improvement of the deformation-induced [0 0 1] texture. The PrCo₅ alloys, known to develop a superior texture at the lower degrees of deformation, showed only modest improvement and their magnetic performance was undermined by a low coercivity. The SmCo₅ alloys had their texture markedly enhanced and, after height reduction by 94.5%, they exhibited a remanence of 8.6 kG, maximum energy product of 18 MGOe and an intrinsic coercivity of 22.8 kOe. No induced texture was found in the alloys based on the Sm₂Co₁₇ structure. The microstructures of the hot-deformed alloys were studied with a transmission electron microscopy, and possible mechanisms of the texture development in the RCo₅ alloys (R=Pr, Sm) are briefly discussed.     

Effect of die-upset level on the mechanical properties and microstructure of Nd-Fe-B magnets prepared by spark plasma sintering

The mechanical properties and microstructure of Nd-Fe-B magnets produced at various die-upset levels have been investigated. The results showed that the Vickers hardness and the fracture toughness of Nd-Fe-B magnets first increased, and then rapidly decreased with the increase in die-upset level. The optimum Vickers hardness and the fracture toughness were obtained at the die-upset level of 40%. The peak intensity of the (1 0 5) peak is the maximum value, the relative intensities of the (2 1 2), (2 1 4) and (3 1 4) peaks decline and the relative intensity of the (0 0 6) peak increases with the increase in die-upset level. The microstructures show that the variation in the size of small spherical grains is not obvious, and the volume fraction of small spherical grains declines gradually with the increase in die-upset level.     

Enhanced alignment and magnetic properties of die-upset nano-crystal Nd2Fe14B magnets with Nb addition

It is difficult to obtain the crystallographic alignment for stoichiometric Nd₂Fe₁₄B alloys by applying the melt-spun and subsequent hot-pressing and hot-deformation techniques. However, the enhanced alignment and magnetic properties of die-upset nano-crystal Nd₂Fe₁₄B magnets have been obtained by Nb addition in the present paper. The magnetic properties studies show that Nb addition leads to the remarkable increase of remanence B_r and intrinsic coercivity H_ci, which is due to the improvement of c-axis texture and refinement of microstructure. Microstructure studies using transmission electron microscopy (TEM) and X-ray diffraction (XRD) reveal that Nb atoms are enriched at grain boundary and the NbFeB phase is observed with increasing Nb content. Since some Fe atoms in the Nd₂Fe₁₄B phase participate in the formation of NbFeB phase, the excessive Nd atoms may be enriched at grain boundary, which may improve the physical property of grain boundary and provide a mass transport pass for preferential growth of oriented Nd₂Fe₁₄B grains, thus leading to the enhanced alignment and magnetic properties.     

Electromagnetic properties and structure of polydisperse mechanically activated Fe-Si materials

Complex investigation of the physical properties and structure of sintered powder materials of the Fe-Si system after grinding of a mixture of powders in two different mechanical aggregates has been performed. It is shown that fragmentation of powders in both a BW-micro vibration grinder and in a Fritsch-type centrifugal planetary mill leads to a nonmonotonic change in the properties of sintered materials. This result is explained by mechanical activation of powders during grinding. Original Russian Text ? G.A. Dorogina, I.A. Kuznetsov, E.S. Gorkunov, Yu.V. Subachev, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 1, pp. 141–144.     

Strain aging and thermally activated mechanisms in alloys

The dynamic strain phenomena have been reported in many metals containing solute atoms. In this paper the motion of dislocations in the field of solute atoms which also move in the lattice is investigated. Both processes are assumed to be thermally activated. The dislocation velocity and the friction stress are calculated. The appearance of jerky flow, the occurrence of the plateau stress and anomalies in the stress dependence of the strain rate sensitivity are discussed.     

Kinetic effects in the mechanically activated solid-state reduction of haematite

Nanometer-sized -Al₂O₃-Fe composites were obtained by solid-state reduction in ball mills of Fe₂O₃ and Al in nitrogen and air. Together with -Al₂O₃ and Fe, the formation of Hercynite and clusters of Fe in alumina are observed in both cases.     

Frequency dependence of ferromagnetic resonance in amorphous magnets: Ferromagnetic and reentrant alloys

We report measurements on magnetic resonance in a number of transition metal-glass former complexes. Data were taken over a wide range of frequencies (2–35 GHz) and temperatures (2–300 K). In the ferromagnetic regime all the systems have a term in their linewidth which points strongly to noncolinearity of spin in glassy magnets even in a nominally saturated state. At low temperatures, in the reentrant alloys, there is a characteristic exponential term describing the increase in linewidth, with reducing T. Such a contribution appears in most random spin systems. There is no adequate microscopic picture to account for many of the results.     

Hard magnetic properties of anisotropic Sm-Fe-N magnets produced by compression shearing method

Anisotropic Sm-Fe-N bulk magnets were produced by the compression shearing method using a hardened steel plate and a tungsten-carbide (WC) plate. It was found that the magnets retained the original Sm₂Fe₁₇N₃ phase structure without any appreciable decomposition of the Sm₂Fe₁₇N₃ phase. The anisotropic Sm-Fe-N bulk magnet produced using a WC plate had a higher density and higher crystallographic alignment of the Sm₂Fe₁₇N₃ phase than that produced using a hardened steel plate, and exhibited high maximum energy products of 228 kJ/m³ with a high coercivity of 0.88 MA/m.     

Random fields in Ge-Si alloys produced by doping

A calculation is performed and estimates made of the binary correlation functions of the random field produced by the difference in the atomic pseudopotentials of solvent and dissolved materials. Also evaluated are elastic deformation fields in unordered Ge-Si solid replacement solutions with low Si content (up to 10 at. %).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 41–46, February, 1987.The author heartily thanks V. L. Bonch-Bruevich for his interest in the current study and many valuable discussions.     

Preparation and characterization of activated carbon produced from pomegranate seeds by ZnCl2 activation

In this study, pomegranate seeds, a by-product of fruit juice industry, were used as precursor for the preparation of activated carbon by chemical activation with ZnCl₂. The influence of process variables such as the carbonization temperature and the impregnation ratio on textural and chemical-surface properties of the activated carbons was studied. When using the 2.0 impregnation ratio at the carbonization temperature of 600 °C, the specific surface area of the resultant carbon is as high as 978.8 m² g⁻¹. The results showed that the surface area and total pore volume of the activated carbons at the lowest impregnation ratio and the carbonization temperature were achieved as high as 709.4 m² g⁻¹ and 0.329 cm³ g⁻¹. The surface area was strongly influenced by the impregnation ratio of activation reagent and the subsequent carbonization temperature.     

Study of the photodegradation of brilliant green on mechanically activated powders of zinc oxide

A study of the photocatalytic activity of commercial and mechanically activated zinc oxide powders has been carried out based on the example of the decomposition of Brilliant Green. The goal of this work was to study the effect of the grinding time (0, 1, 3, 5, and 7 min) on the structure of zinc oxide and its photocatalytic activity under visible and ultraviolet (UV) radiation. It has been found that, when UV radiation is used, the constant of the dye oxidation rate for samples activated for 1 min increases compared with unactivated powders, whereas further mechanical activation leads to a decrease in the photocatalytic activity. When using visible radiation, samples activated for 1 min showed the minimum photocatalytic activity and further mechanical activation led to an increase in the efficiency of photocatalysis.     

Etching of slip produced antiphase domain boundaries in iron-silicon alloys

A method for revealing the slip produced antiphase domain boundaries in nearly stoichiometric ordered Fe₃Si alloys by chemical etching in a solution of HF + H₂O₂ + H₂O is reported. The method was applied to determine the character of superlattice dislocations in strained crystals. Perfect superlattice dislocations were found at the beginning of deformation. Imperfect superlattice dislocations producing antiphase domain boundaries were found at higher deformations.     

Thermally activated deformation in some neutron ibradiated binary nickel alloys

Low temperature deformation behaviour of some neutron irradiated, binary nickel alloye, Ni-1%Ti, Ni-5%Ti and Ni-8%Fe has been investigated in 77–300 X temperature range. The studies have confirmed the irradiation induced defects responsible for both the changes in thermally activated deformation parameters and radiation hardening     

Effect of Cu and Ga additions on the anisotropy of R2Fe14B/α-Fe nanocomposite die-upset magnets (R=Pr,Nd)

Fully dense nanocomposite magnets containing hard R₂Fe₁₄B and soft α-Fe phases were produced from both melt-spun and mechanically milled alloys by hot pressing and subsequent die upsetting. Although R-lean R–Fe–B alloys that do not contain the grain-boundary R-rich phase are known not to be susceptible to texture development by means of die upsetting, we found that small additions of Cu make the texturing possible. The resulting microstructure of oriented platelet grains is similar to that of the R-rich die-upset magnets. Properties of the Cu-containing R₂Fe₁₄B/α-Fe die-upset magnets can be further improved by adding Ga. The anisotropic Pr₁₂Fe₈₀Cu₁Ga₁B₆ magnet made from mechanically milled alloy and containing 17.2 wt% α-Fe had a remanence of 13 kG and a maximum energy product of 23.4 MG Oe. The Pr_11.25Fe_80.75Cu₁Ga₁B₆ magnet made from melt-spun alloy and containing 16.2 wt% α-Fe had a maximum energy product of 19.9 MG Oe. The low coercivity of 3–4 kOe typical for the Cu-containing R₂Fe₁₄B/α-Fe die-upset magnets is due to the relatively coarse α-Fe grains. The latter grains are too large for intergranular exchange interaction, but, nevertheless, they are well coupled with the R₂Fe₁₄B grains by a long-range magnetostatic interaction.     

Metastable surface alloys produced by ion implantation,laser and electron beam treatment

Ion Implantation, Laser and Electron-beam Treatment (LET) of metals have been employed extensively to produce metastable surface alloys. Recent published work on implanted alloys is reviewed first. The dilute implanted alloys (solute concentration <10 at. %) are shown to lead to crystalline metastable solid solutions. At higher solute concentrations, an amorphous phase has been observed for several binary systems and recently for a ternary system. The physical mechanisms at play, are discussed in detail. A review of the surface alloys produced by LET of metals is then presented—with an emphasis on the mechanisms involved. In particular, general criteria governing formation of metastable solid solutions under LET are proposed and shown to have excellent agreement with available data on metals and Si.     

Thermal stability of high concentration Fe-Ag and Fe-Cu alloys produced by vapor quenching

Mössbauer effect measurements were carried out for sputtered fcc Fe-Ag and Fe-Cu alloys annealed at various temperatures. At temperatures higher than 300 °C, the metastable fcc phases decompose by removing saturated Fe atoms. During the phase separation processes, the ejected Fe atoms form clusters, which initially have a fcc structure and transform to bcc particles as their sizes grow beyond a critical value.     

Organic- and molecule-based magnets

The discovery of organic- and molecule-based magnets has led to design and synthesis of several families with magnetic ordering temperatures as high as ∼125°C. Examples of soft and hard magnets with coercivities as high as 27 kOe have also been reported. Examples from our laboratory of organic-based magnets using the tetracyanoethylene radical anion, [TCNE]^•−, are discussed. In addition, several molecule-based magnets based on Prussian Blue structured materials as well as dicyanamide are discussed.     

Pure multipoles from strong permanent magnets: analytic and experimental results

We present the construction of arbitrary multipole field configurations from strong permanent magnets for trapping charged or neutral particles. A general analytic method for the design of three-dimensional magnetic multipoles is discussed for an idealized continuously varying magnetisation taking advantage of the superposition principle. Simple recipes for constructing magnetic dipole and quadrupole fields are given with two types of elements, axially and radially magnetised rings. Cylindrical magnet components not only give free access to the experimental region of interest, but also allow for some tunability to reduce undesirable higher multipole orders. Measurements confirm theoretical predictions achieving useful magnetic fields of 1 T and steep gradients of 3 T/cm with high purity over several ccm.