Neodymium-rich Phase Research Articles

Microstructures of Al coatings prepared through magnetron co-sputtering with microcrystalline Ti doping

Sintered NdFeB magnets are being utilised in numerous applications because of the excellent magnetic properties. However, due to its main components are Fe, B and rare earth metals. There is a potential difference between the neodymium rich phase, boron rich phase and the main phase, which was very easy to form a primary battery and cause electrochemical corrosion on the surface of the material. After the materials are easy to be corroded or the components and structures change, the performance of the materials will become weak or even lost, which will affect the service performance and life of the devices or equipment. According to the requirements of surface protection technology for NdFeB magnets, the preparation of protective coating by magnetron sputtering was an efficient means to enhance the corrosion resistance of magnets. Pure Al coating was deficient in microstructure, hardness and corrosion resistance, and the preparation of composite coating by doping co-sputtering had become a development trend. In the current work, AlTi coatings with various Ti contents were prepared from an Al target and a Ti target using direct and radio frequency hybrid magnetron co-sputtering. The surface morphology, microstructure, and elemental compositions of the AlTi coatings were explored through XRD (X-ray diffraction), SEM (scanning electron microscopy), and EDS (energy dispersive spectroscopy). The anticorrosion performances of the AlTi coatings were evaluated by performing electrochemical tests. According to SEM and EDS, the AlTi composite coating was denser and more homogeneous than that of the Al single layer. Furthermore, the results demonstrated that microcrystalline Ti doping reduced the grain size of Al coatings. Therefore, the corrosion current density of the sintered NdFeB magnets plated with Al(120w)/Ti(10w) was approximately more than a hundred times inferior to that of the pure Al layer, and the hardness of AlTi composite coatings also improved.

Magnetic structure and nanomechanical properties of USC-20L Nd–Dy–Fe–B sintered permanent magnets

Scanning electron microscopy (EDS analysis), magnetic force microscopy (MFM) and nanoindentation were used in the metallographic study of the magnetic structure and nanomechanical properties of sintered USC-20L Nd–Dy–Fe–B rare-earth magnets (Ural Strip Casting Technology). The microstructure of the sintered USC-20L Nd–Dy–Fe–B magnet includes the Nd 2 Fe 14 B phase grains separated by lamellas of neodymium-rich phases. Nd–29.1%Fe–6.2%C–2.2%O–1.4%Dy inclusions are located at the triple junctions of Nd 2 Fe 14 B grains. Nd–4.5%Fe–9.1%O–6.7%C–4.5%Fe–2%Dy inclusions are located along grain boundaries and contain neodymium and dysprosium oxides. The chemical composition of grains: Fe–25%Nd–6.9%C–1.6%Dy–1.4%B. It was found that due to irregular grain growth, the neodymium-rich phase interlayers are connected to each other in the region of grain junctions causing internal stress concentration and cracking. A crack propagates along the grain boundary from one wetted grain joint to another due to the occurring mechanical stresses. A phenomenon of intergranular wetting by the neodymium-rich phase is observed at Nd 2 Fe 14 B/Nd 2 Fe 14 B grain boundaries. It was found that Nd-riched phases can pseudo-incompletely (or pseudo-partially) wet such grain boundaries, i.e. form a non-zero contact angle along the grain boundaries and in triple joints. Based on the magnetic force microscopy results, a conclusion was made about the presence of a one-dimensional domain structure; domains cross grain boundaries. The presence of pores and inclusions of Nd and Dy oxides localized along grain boundaries is noteworthy. The average transverse size of the banded-structure domain is ~1 μm, domain wall energy is γ ~ 14 kJ/m 2 , domain wall width is δ ~0.6·10 –9 m. The nanoindentation method was used to measure the values of nanohardness ( H , GPa), elastic modulus ( E , GPa), contact stiffness ( S , N/m), elastic strain energy ( W e , nJ) and plastic strain energy ( W p , nJ) in submicrovolumes of Nd 2 Fe 14 B grains. According to the measurement results, the minimum value of Nd 2 Fe 14 B grain adhesion was estimated: K int = 0.539 MPa·m 0.5 .

Perpendicular magnetic anisotropy in amorphous NdxCo1−x thin films studied by x-ray magnetic circular dichroism

The origin of perpendicular magnetic anisotropy (PMA) in amorphous ${\mathrm{Nd}}_{x}{\mathrm{Co}}_{1\ensuremath{-}x}$ thin films is investigated using x-ray magnetic circular dichroism (XMCD) spectroscopy at the Co ${L}_{2,3}$ and Nd ${M}_{4,5}$ edges. The magnetic orbital and spin moments of the $3d$ cobalt and $4f$ neodymium electrons were measured as a function of the magnetic field orientation, neodymium concentration, and temperature. In all the studied samples, the magnetic anisotropy of the neodymium subnetwork is always oriented perpendicular to the plane, whereas the anisotropy of the orbital moment of cobalt is in the basal plane. The ratio ${L}_{z}/{S}_{z}$ of the neodymium $4f$ orbitals changes with the sample orientation angle, being higher and closer to the atomic expected value at normal orientation and smaller at grazing angles. This result is well explained by assuming that the $4f$ orbital is distorted by the effect of an anisotropic crystal field when it is magnetized along its hard axis, clearly indicating that the $4f$ states are not rotationally invariant. The magnetic anisotropy energy associated to the neodymium subnetwork should be proportional to this distortion, which we demonstrate is accessible by applying the XMCD sum rules for the spin and intensity at the Nd ${M}_{4,5}$ edges. The analysis unveils a significant portion of neodymium atoms magnetically uncoupled to cobalt, i.e., paramagnetic, confirming the inhomogeneity of the films and the presence of a highly disordered neodymium rich phase already detected by extended x-ray-absorption fine structure (EXAFS) spectroscopy. The presence of these inhomogeneities is inherent to the evaporation preparation method when the chosen concentration in the alloy is far from its eutectic concentrations. An interesting consequence of the particular way in which cobalt and neodymium segregates in this system is the enhancement of the cobalt spin moment which reaches 1.95 ${\ensuremath{\mu}}_{B}$ in the sample with the largest segregation.

The effect of nano-TiC addition on sintered Nd-Fe-B permanent magnets

This paper addresses the effect of nano-TiC addition on sintered Nd-Fe-B permanent magnets. TiC nanoparticles were added to sintered Nd-Fe-B magnets with a specific aim to improve the Curie temperature and thermal stability. A standard powder metallurgy route was adopted to prepare the magnets. It was found that introducing nano-TiC prior to jet milling was effective as the nanoparticles dispersed in the final alloy, concentcalcrating in the neodymium-rich phase of the magnets. Magnets with optimal properties were obtained with the addition of 1wt% TiC nanoparticles. The hysteresis loop for such magnets showed an improved shape and VSM analysis a coercivity value of 1188kA/m, a remanence value of 0.96T and a maximum energy product of 132kJ/m3. The maximum working point and the Curie temperature of the developed magnets were 373K and 623K respectively.

Microstructure and Properties of Die-Upset Nd-Fe-B/Dy$_{2}$O$_{3}$ Composite Magnets

Fabrication of rare-earth-based permanent magnets with both high electrical resistivity and excellent magnetic properties for the traction motor applications is highly challenging. In this paper, Nd-Fe-B/Dy2O3 composite magnets have been fabricated by hot-pressing and die-upset. The influence of Dy2O3 filler on the microstructures, electrical resistivity and magnetic properties of the Nd-Fe-B magnets have been investigated by scanning electron microscope, four-probe resistivity meter and hysteresis graph. The results showed that the electrical resistivities of the Nd-Fe-B/Dy2O3 composite magnets increased with the increase of Dy2O3 amount and reached 1270 cm when the Dy2O3 amount increased to 20 wt.%. Whereas the electrical resistivity of the nondoped Nd-Fe-B magnets is 230 cm. The cross-section SEM images of the composite magnets showed that the Dy O phase formed laminated structure and some agglomerate phenomenon. The agglomerate areas became larger with the increase of Dy2O3 amount. The residual magnetic flux density and maximum energy product of the Nd-Fe-B/Dy2O3 composite magnets decreased due to the nonmagnetic Dy2O3 filler. It is interesting to find that coercivity of the Nd-Fe-B/Dy2O3 composite magnets increased from 7.8 kOe to 9.64 kOe when the Dy O amount increased from 10 wt.% to 20 wt.%. The reason may be that some of the dysprosium diffused into the Nd-Fe-B matrix and formed (Nd,Dy)2Fe14B compounds. In the mean time, some of neodymium-rich phase diffused to Dy2O3 phase, replaced dysprosium and formed Nd O . This probably happened during the die-upset process at 850°C.

Detection of Neodymium-Rich Phase for Development of Coercivity in Neodymium-Iron-Boron-Based Alloys with Submicron-Sized Grains Using Positron Lifetime Spectroscopy

In order to evaluate the relationship between positron lifetime and microstructure, which contributes to the development of coercivity in hydrogenation-disproportionation-desorption-recombination (HDDR)-processed Nd-Fe-B-based alloys, detailed studies of positron lifetime spectroscopy were performed on HDDR-processed Nd-Fe-B-based alloys during desorption-recombination (DR) treatment. After the onset of coercivity, the change in positron lifetime closely corresponded to the change in intrinsic coercivity (HcJ) with the progress of DR treatment. This result can be explained in terms of the grain size of the recombined Nd2Fe14B phases and the diffusion length of positrons, which annihilate in the matrix before reaching the grain boundary. Furthermore, positron lifetime spectroscopy was able to detect small changes in the grain boundary region very sensitively compared with thermal desorption spectroscopy (TDS) and X-ray diffraction (XRD). These changes in the grain boundary region caused the onset of coercivity attributed to the formation of Nd-rich intergranular phases. These results indicate that formation of a small amount of the Nd-rich intergranular phase during the DR process, which could be detected by positron lifetime spectroscopy, contributes to the onset of coercivity, even if NdHx phases remain. [doi:10.2320/matertrans.M2009342]

Microstructure and properties of Nd-Fe-B magnets prepared by spark plasma sintering

Anisotropic Nd15.5Dy1.0FeBalCo3.0B6.8Al1.0 magnets were produced by the spark plasma sintering (SPS) technique. The effects of processing conditions on the microstructure, magnetic properties, dimensional precision and density of the magnets were studied. The magnetic properties, microstructure and constituents were investigated by means of a magnetic flux density - magnetic field strength (B-H) loopline instrument, scanning electron microscopy and energy dispersive X-ray analysis. The density of the magnets was determined by the Archimedes method, and the dimensional precision of the magnets was measured by micrometer. It was found that the microstructure of SPS processed Nd-Fe-B magnets is unique; the grain size is fine and uniform while distribution of the neodymium rich phase is heterogeneous. The optimal magnetic properties of SPS processed Nd-Fe-B magnets obtained so far are maximum energy product of 240 kJ m-3 and coercive force of 1260 kA m-1. The dimensional precision of the magnets is ~ 20 μm, and the density of the magnets reaches 7.58 g cm-3.

Electrochemical corrosion characterisation of intermetallic phases occurring in Nd–Fe–B type magnets

AbstractThe corrosion behaviour of Nd–Fe–B type magnets was characterised by conducting gravimetric and electrochemical studies with synthetic preparations of the three maiN intermetallic phases of these materials. Tests were conducted in deaerated sulphate solutIOns at pH 0·3–9; some accelerated atmospheric corrosion tests were also carried out. In acidified solutions the neodymium rich (Nd4Fe) synthetic phase was the most active with a high corrosion rate, which could be further accelerated by cathodic hydrogenation; the boron rich (NdFe4B4) phase was the most noble and had the lowest corrosion rates. In neutral solutions corrosion of the magnets is inhibited by the protective properties of neodymium oxide-hydroxide layers formed on the surface of the neodymium rich phase. The protective properties of these layers disappeared at pH < 4.

Microstructural development and sintering of NdFeB alloys

The reactions and phase changes occurring during sintering of NdFeB permanent magnet alloys were studied by differential thermal analysis and scanning electron microscopy. The powders were produced by hydrogen decrepitation and on heating, hydrogen evolution occurred in two stages: firstly from the matrix phase (∼170 °C) and then from the neodymium-rich phase (350–750 °C). The neodymium-rich phase melted at ∼630 °C and no significant microstructural changes occurred below this temperature. Above 630 °C the neodymium phase is mobile, wetting the matrix grains and leading to their magnetic isolation. Densification occurs by a solution/precipitation mechanism.

NdFeB-based magnets: Magnetic properties and microstructural appearance with copper addition

The microstructure of sintered permanent magnets with a composition 17Nd-76.5Fe-5B-1.5Cu was investigated by optical microscopy, electron probe microanalysis and transmission electron microscopy. Using the hydrogen decrepitation processing route, maximum magnetic properties of H cJ=950 kA m −1 and ( BH) max=230 kJ m −3 were obtained after high temperature homogenization and low temperature annealing. The main effect of the heat treatment procedure was to change the proportion of phases in the four phase microstructure. In addition to Φ and a neodymium-rich phase, two new phases were found: an Nd 1Cu 1-type phase and a phase Nd 30Fe 66Cu 5. The latter seems to play an important role in obtaining high coercivity.

Investigations of the thermal stability of (Nd, Dy)-(Fe, Co)-B based magnets with small additions of niobium and gallium

The magnetic properties and microstructure of (Nd, Dy)-(Fe, Co)-B based sintered permanent magnets containing small amounts of niobium and (Nb, Ga) were studied. The magnetic measurements showed a large increase in coercivity, a distinct improvement in the thermal stability and a practical temperature range up to 120°C when niobium and gallium were added to the (Nd, Dy)-(Fe, Co)-B based magnets. The mechanism of coercivity of both magnets was found to be different from observations of the microstructure. A new phase (Fe, Co) NbB with the CoNbB-type orthorhombic structure was found in both magnets. Furthermore, there were also a boron-rich phase Nd 1+εFe 4B 4, a neodymium-rich phase and a principal phase (Nd, Dy) 2(Fe, Co) 14B all containing finely dispersed niobium-rich precipitates. Dislocations were found around the Fe 2Nb precipitates in the magnet containing (Nb, Ga), but not around Fe 2Nb in the magnet containing niobium alone after the best post-heat treatment. The pinning of the domain walls through Cottrell atmospheres around the dislocations and the gallium-improved wettability of the neodymium-rich phase with respect to the matrix may be the reason that the coercivity of the magnet with (Nb, Ga) is higher than that of the magnet with niobium alone.

A study of Nd2Fe14B and a neodymium-rich phase in sintered NdFeB magnets

The microstructure and properties of NdFeB sintered permanent magnets were analysed by different methods. Samples analysed were sintered and thermally treated. The hard magnetic Nd2Fe14B phase and amorphous neodymium-rich phase were observed by TEM. The neodymium-rich phase contained iron and boron, in elemental and in B2O3 form, which is known as a glass former. At the sintering temperature, Nd2Fe14B and the neodymium-rich phase are supersaturated with iron, which should be dissolved at the annealing temperature to react with neodymium and boron and form additional Nd2Fe14B phase. Iron precipitates of size up to 2 nm were detected in the Nd2Fe14B phase. These superparamagnetic precipitates of α-Fe could affect the hard magnetic properties of NdFeB magnets.

The magnetic and mechanical properties of NdFeB type permanent magnets and the effect of quenching

The fracture strength of Nd/sub 15/Fe/sub 77/B/sub 8/ and Nd/sub 14.5/Dy/sub 1.5/Nb/sub 1/Fe/sub 76/B/sub 7/ sintered magnets has been measured to be 285(40) MPa and 240(48) MPa, respectively. The fracture mechanism appears to be controlled by a critical level of tensile stress. In all the cases considered, Nd-Fe-B-type permanent magnets have been shown to fracture in a brittle manner along the grain boundaries. This is consistent with poor adhesion between Nd/sub 2/Fe/sub 14/B grains and could be due to the lack of the neodymium-rich phase along certain interfaces even in the fully sintered condition. The addition of dysprosium and niobium to the Nd-Fe-B alloy did not alter the fracture behavior of the material, whereas these additions have a marked effect on the magnetic properties. Quenching from 1370 K was found to greatly affect the fracture stress. This is possibly due to rapid cooling through the melting point of the neodymium-rich phase, which prevents sufficient wetting of the grains. Indeed, when the quench rate was increased the effect was so severe that some samples were prone to fail on pulse magnetization. However, where this did not occur, the effect was reversible on annealing at 903 K for 1 h. The intrinsic coercivity decreased progressively with increasing quench temperature above 900 K. This could be due to progressively less wetting with increasing quench temperature.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Hydrogen decrepitation of a permanent magnet Nd-Fe-B alloy

The nature of hydrogen decrepitation when applied to a cast Nd-Fe-B permanent magnet alloy has been studied by following the microstructural changes on polished surfaces of the material exposed to hydrogen at a pressure of 4 bar. The milling of the material decrepitated at a pressure of 10 bar has also been studied by determining the particle size as a function of milling time. The milled particles have also been examined by scanning electron microscopy. It is concluded that preferential swelling of the neodymium-rich phase occurs and that this gives rise to 1. (i) shear stresses leading to fracture at the interface between the particles and the matrix, and 2. (ii) cracks in the iron-rich matrix phase which show 90° intersections characteristic of tensile stress relief.