Coercivity kinetics upon step annealing of sintered Sm(Co_(0.88-x)Fe_xCu_(0.09)Zr_(0.03))_7 magnets

Behavior of the coercivity of the high-temperature Sm(Co_(0.88-x)Fe_xCu_(0.09)Zr_(0.03))_7 magnets depending on the temperature and time of annealing with the temperature decreasing stepwise from 700 to 400℃ was investigated.It is shown that the growth rate of coercivity abruptly increases at the initial stage of annealing in the vicinity of the Curie temperature of the SmCo_5 phase.The origin of the effect is the counter diffusion of Cu and Co atoms through dislocation tubes,which form because of enhanced stresses and a partial breakage of coherent coupling at the interface of the Sm_2 Co_(17) and SmCo_5 phases.Diffusive enrichment of the SmCo_5 phase in Cu close to the interface with Sm_2 Co_(17) leads to relaxation of stresses and increases in the gradient of the magnetic domain-wall energy and coercivity.     

Structure Analysis of Sm2(Fe,Mn)17NX Magnet

In order to understand the coercivity mechanism of Sm₂(Fe, Mn)₁₇N_x magnet, the structure of amorphous phase found in the crystal phase was clarified using a TEM-tomography. It was found that amorphous phases precipitated as a fine wire in the crystal phase that differs with the phase separation in Sm-Co magnets. It may say that different coercivity mechanism should be proposed in each Sm₂(Fe, Mn)₁₇N_x and Sm-Co magnets.     

Structure and permanent magnetic properties of SmFex （x=3-8） melt spun ribbons during heat treatment

Compounds with the composition SmFex（x=3–8） were prepared by melt spun method at a velocity of 40 m/s and subsequent annealing at temperature between 600–1000 ℃. The crystal structures of the as-quenched and as annealed powders were investigated by XRD methods with following Rietveld analysis. The glass forming ability could be enhanced by the increase of Sm content to x≤5.Metastable Sm5Fe17-type structure existed when 3≤x≤5 and temperature was lower than 800 ℃. SmFe2-type structure could be stable up to 1000 ℃ when x〉3 and temperature was under 800 ℃. The content of SmFe2-type decreased with the increase of x value and increased with temperature lower than 800 ℃, from which SmFe2-type started to bring the transition to SmFe3-type. As for Sm5Fe17-type compounds with x=3.4, the highest coercivity of 33.6 kOe could be obtained under a velocity of 30 m/s and heat treated under 700 ℃×1h.     

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.     

Influence of Fe Content on Microstructure and Magnetic Properties of Melt-Spun SmCo-Based Alloy Ribbons

The microstructure and magnetic properties of melt-spun Sm(Co_0.88-xFe_xCu_0.10Zr_0.02)_7.5 (x = 0.1 and 0.2) alloy ribbons have been studied. The results showed that the as-spun ribbons were in a single phase, SmCo₇, with the Cu₇Tb structure. When aged in the temperature range from 720 to 900 °C, the SmCo₇ phase transformed into Sm₂Co₁₇, SmCo₅, and CoFe(Zr) phases with a minor Sm₂Co₃ phase. For the x = 0.1 alloy, a large coercivity, H_c = 8.7 kOe, was observed although the soft magnetic CoFe(Zr) phase was present in the alloy. The volume fraction of the CoFe(Zr) phase increased when the ageing temperature increased from 720 to 760 °C. At higher ageing temperature, the CoFe(Zr) phase was partially re-dissolved. With an increase in the Fe content in the alloy, the CoFe(Zr) phase increased significantly, causing the coercivity to decrease.     

Structure and Magnetic Properties of Sm2Fe17Nx Sintering Magnets Prepared by Spark Plasma Sintering

Bulk Sm2Fe17Nx sintering magnet was fabricated by spark plasma sintering(SPS) technique. The effects of sintering pressure and sintering temperature on the magnetic properties of the Sm2Fe17Nx magnet were investigated. As a result, the density of the magnet is obviously improved with the increase of sintering pressure, but the coercivity drops since Sm2Fe17Nx has decomposed into SmN, α-Fe and N2. When sintering temperature was only above 200 ℃ under 1 GPa sintering pressure, the coercivity even begins to decrease, which indicates that high pressure promotes the decomposition of the Sm2Fe17Nx at lower temperature. The decomposition is also proved by the decrease of nitrogen and increase of α-Fe in the magnets.     

Sintering of Fe–C–BN steel compacts in different atmospheres studied by dilatometry and DTA/TG

Abstract

In the present work, 2%hBN was admixed with Fe–0·8C, and both dilatometric and differential thermal analysis/thermogravimetry investigations were conducted in Ar and N₂ atmospheres, followed by microstructural studies and mechanical testing. The α–γ phase transformation in both atmospheres was found to occur in the temperature range of plain iron and not, as expected, in that common for Fe–C. In the Ar atmosphere, liquid phase formation is recognised by endothermic differential thermal analysis signal and shrinkage in the dilatometer during the heating stage at ～1275°C. In contrast to the activating effect of the inert Ar for the decomposition of hBN, the deactivating effect of the N₂ atmosphere is visible from the dilatometry results: sintering in N₂ even resulted in slight expansion during the isothermal stage. Therefore, the, at first surprising, conclusion can be drawn that the chemically inert Ar is activating the sintering process while the more reactive N₂ passivates it.     

Crystal structure and hard magnetic properties of TbCu_7-type Sm_(0.98)Fe_(9.02–x)Ga_x nitrides

The compound Sm0.98Fe9.02–xGaxNδ(x=0, 0.25, 0.5, 0.75, 1) were prepared by melt-spun method and subsequent annealing and nitriding. The Rietveld analysis showed that the lattice expansion played an important role in improving the Curie temperature. An obvious development of the Curie temperature was obtained with the increased Ga content from x=0–1(ΔTc=90 oC). The optimum coercivity of nitrides was obtained at x=0.25 with the value Hcj=652 kA/m(8.15 kOe) after annealing, which corresponded to a reasonable distribution of grain sizes of both TbCu7-type SmFe9Nδ and α-Fe. However, an excess of Ga doping might lead to an abnormal growth of α-Fe, which in turn deteriorated the magnetic properties. It was concluded that a moderate Ga content was very effective in raising the coercivity and Curie temperament in the TbCu7-type Sm-Fe-N.     

Magnetic properties of melt-spun ribbons(Sm_(1-x)Zr_x)(Fe_(0.92)Ti_(0.08))_(10) with ThMn_(12) structure and their hydrides

The structure and magnetic hysteresis properties of the cast Sm_(1-x)Zr_x(Fe_(0.92)Ti_(0.08))_(10)(x = 0-0.3)alloys and melt-spun ribbons prepared from them were studied.In the cast alloy with x0.2, a considerable amount of the eutectic phase is found in the SEM micrographs.Analysis of the temperature dependences of the magnetic susceptibility and XRD patterns allows amorphous state in the as-spun ribbons with x0.2 to be determined.The specific magnetization measured in a field of 17 kOe and remanence decrease with increasing annealing temperature from 800 to 900 ℃ and weakly depend on Zr concentration.The maximal value of coercivity Hc = 4.7 kOe is obtained on the ribbons with x = 0.2 after annealing at 850℃ for 10 min.After additional hydrogenation of the ribbons,both the coercivity and remanence increase by 54% and 7%,respectively.     

Effects of pre-aging on defects evolution and magnetic properties of Sm-Co-Fe-Cu-Zr magnets

The heterogeneous precipitation in the 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets has been found to contain complex formation and dissociation of defects. Though low-temperature pre-aging has been utilized to promote the precipitate nucleation by the enlarged chemical driving force, how the defects evolve after pre-aging and how the possibly changed defects state affects the subsequent precipitation behavior remain unclear. In this work, a model magnet Sm₂₅Co_47.9Fe_18.5Cu_5.6Zr_3.0 (wt%) was selected to study. Through comparison with the as-solution-treated state, it is found that pre-aging for 2 h at 550 °C reduces the defects density, which was characterized by less cell boundaries (i.e., larger cell size) and less basal stacking faults inside the cells (i.e., higher 2:17R ordering degree). Further studies reveal that after aging for the same time (10 h) at the same temperature (830 °C), the reduced density of defects by pre-aging also leads to slower precipitation/phase transformation kinetics when compared with the non-pre-aged one, which was characterized by the lower 2:17R ordering degree and smaller coercivity for the former. These findings suggest that pre-aging has a strong influence on the density of defects and their evolution during subsequent isothermal aging process, which should be carefully considered to tailor the microstructure and magnetic properties of Sm-Co-Fe-Cu-Zr magnets.     

Magnetic Domains and Coercivity in SmCo 2:17 Type Magnets

The coercivity of SmCo 2:17-type magnets is known to be very sensitive to details of the processing procedure. In this study we have found that the same is true with respect to the domain structure of these magnets. Thus in optimally processed high temperature Sm(Co_0.784Fe_0.1Cu_0.088Zr_0.028)_7.19 magnets, the typical domain width is less than 1 μm, but significantly larger than the typical size of 2:17R cells of about 100 nm. On the other hand, quenching straight from the aging temperature of 850 °C, i.e. omitting slow cooling, leads to a much coarser domain structure (domain width ∼ 10 μm) and a small coercivity (< 0.1 T). The fine domain structure of the optimally processed bulk magnet can be considered as interaction domains - an alternative phenomenon compared to the zigzag shaped domain walls known for thinned specimen.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High density Fe₃Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min^-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe₂Al₅/FeAl₂ (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe₃Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m^-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min^{- 1} heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.

SEM microscopy suggests that agglomerated Fe₂Al₅/ FeAl₂ particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe₂Al₅/FeAl₂ components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity.     

Interstitial atom configurations in stable and metastable Fe-N and Fe-C solid solutions

Mössbauer Fe⁵⁷ spectroscopy allows comparison of Fe?N and Fe?C interstitial solid solutions. The spectra of Fe?N retained austenite indicate that nitrogen atoms are randomly distributed on octahedral sites in the austenite and in the virgin martensite. On heating, austenite decomposes directly to the equilibrium phases α iron and Fe₄N at temperatures above 160°C. Virgin martensite ages at room temperature by local ordering of nitrogen atoms. In that process, three new iron atom environments develop, characteristic of the Fe₁₆N₂ (α″) structure. However, the excessive width of the peaks indicate the perfect order of the Fe₁₆N₂ precipitate is not achieved, except after very long times. Further aging at 100°C leads to the complete decomposition of the virgin martensite to the discrete phases α iron and Fe₁₆N₂. This two phase structure is stable up to 160°C, above which the precipitation of Fe₄N occurs. These results are in contrast to Fe?C data. Carbon atoms in retained austenite tend to be far apart in their octahedral sites, and this nonrandom distribution is inherited by the virgin martensite. Fe?C austenite decomposes by the formation of ∈ carbide below 160°C and precipitation of Fe₃C above 180°C. The carbon atoms in virgin martensite agglomerate at room temperature and regions of ordered Fe₄C are believed to result. Subsequently ∈ carbon is formed at 80°C and Fe₃C precipitates above 160°C.¹     

Synthesis mechanism of Sm2Fe17 alloy produced in reduction-diffusion process

Sm₂Fe₁₇ alloy was the precursor of Sm₂Fe₁₇N_x magnetic materials. Reduction-diffusion (R/D) method was a new preparation process for the Sm₂Fe₁₇ alloy, and had been widely employed as a new preparation method for rare earth-transition metal intermetallic compounds. In this text, thermodynamics and kinetics for the synthesis of the Sm₂Fe₁₇ alloy by reduction-diffusion (R/D) method in the Ca-Sm₂O₃-Fe system were analyzed. The related synthesis mechanism of this reaction was investigated in detail by means of scanning electron microscope (SEM). The results showed that the thickness of the Sm₂Fe₁₇ alloy layer versus the reaction time could be fit by the parabola law, and its growth model was determined to be: (L₀-L)²=43.848 t, the diffusion of Sm into Fe proceeded with the formation of the Sm₂Fe₁₇ phase from the very beginning of the reaction, and rich samarium phases, such as SmFe₂ and SmFe₃, were not formed, and the rate-determining step of the R/D reaction was found to be the peritectic reaction between liquid samarium and solid iron.     

Correlation between microstructure and coercivity of amorphous (fe0.82b0.18)0.90tb0.05la0.05 alloy ribbons

Amorphous alloys of (Fe_0.82B_0.18)_0.90Tb_0.05La_0.05 develop intrinsic coercive forces of about 9 kOe when crystallized at 650 °C. In this work we have investigated the relationship between coercive force and microstructure for these alloys using X-ray and electron diffraction, electron microscopy, and by differential thermal analysis (DTA). The DTA reveals that upon heating, an exothermal process (probably an atomic rearrangement) occurs before crystallization begins. Annealing between the temperatures of 577 °C and 611 °C produces microcrystals of α-Fe, Fe₃B, and R₆Fe₂₃ (R = Rare Earth). Mainly Fe₃B and R Fe₂₃ and a small amount of a-Fe appear in crystalline form between 627 °C and 752 °C, and the crystalline size increases with the temperature. Small amounts of La and an unidentified phase are also present in this temperature interval. Above 752 °C, the La and the unidentified phase disappear, the Fe₃B transforms to a-Fe and Fe₂B, and the grains of α-Fe and R₆Fe₂₃ grow appreciably. The high coercivity of ribbons annealed between 650 °C and 700 °C seems related to the single magnetic domain nature of the small crystallized grains, 100 to 300A. The decrease in coercivity observed by annealing above 700 °C is apparently related to the increase in grain size.     

Effect of niobium substitution on microstructures and thermal stability of TbCu7-type Sm–Fe–N magnets

This paper reports crystal structures, magnetic properties and thermal stability of TbCu7-type Sm_(8.5)Fe_((85.8-x)Co_(4.5)Zr_(1.2)Nb_x(x = 0-1.8) melt-spun compounds and their nitrides, investigated by means of X-ray diffraction, vibrating sample magnetometer, flux meter and transmission electron microscope. It is found that the lattice parameter ratio c/a of TbCu_7-type crystal structure increases with Nb substitution, which indicates that the Nb can increase the stability of the metastable phase in the Sm-Fe ribbons. Nb substitution impedes the formation of magnetic soft phase a-Fe in which reversed domains initially form during the magnetization reversal process. Meanwhile, Nb substitution refines grains and leads to homogeneous micro structure with augmented grain boundaries. Thus the exchange coupling pining field is enhanced and irreversible domain wall propagation gets suppressed. As a result, the magnetic properties are improved and the irreversible flux loss of magnets is notably decreased. A maximum value 771.7 kA/m of the intrinsic coercivity H_(cj) is achieved in the 1.2 at% substituted samples.The irreversible flux loss for 2 h exposure at 120 ℃ declines from 8.26% for Nb-free magnets to 6.32% for magnets with 1.2 at% Nb substitution.     

Magnetic properties and microstructure of nanocrystalline bulk Nd-Fe-B with Ce-Co addition

The influence of Ce-Co alloy addition and sintering holding time on permanent magnetic properties and micro structure of nanocrystalline Nd-Fe-B bulk alloy were investigated.The coercivity of Nd-Fe-B bulk alloy can be enhanced greatly by more than 100% after adding Ce-Co powders.However,when the concentration of Ce-Co is up to 30 wt%,the density of the magnet can reach the maximum value of 7.58 g/cm~3,but the coercivity does not increase significantly.On the other hand,with the increase of holding time to 10 min,the density and coercivity of magnets increase gradually,reaching up to 7.55 g/cm~3 and 1134.3 kA/m,respectively.After the addition of Ce-Co alloy,Ce-Co may easily diffuse into the Nd-Fe-B matrix during hot-pressing and under the high pressure and temperature,thus increasing the content of grain boundary phase and the pinning effect of grain boundary,which leads to the increase of coercivity.The extension of the hot-pressing holding time may be more conducive to the diffusion of CeCo into the Nd-Fe-B matrix.In addition,the effect of Ce-Co addition on the magnetic properties of Nd-FeB with different content of rare earth was also studied.The addition of Ce-Co can effectively increase the coercivity of nanocomposite Nd_2 Fe_(14)B/α-Fe magnets.The addition of Nb to the parent alloy can further improve the coercivity.For Nd_(11)Fe_(81.5)Nb_1 Ga_(0.5)B_6 alloy with 10 wt% Ce-Co addition,the coercivity can increase from 740.28 to 1098.48 kA/m.     

Revisiting the pinning sites in 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

It is still an open debate whether the 1:5H cell boundaries (CBs) or the intersections of 1:3R platelets and 1:5H CBs are the strong pining sites for the cellular nanostructured 2:17-type Sm-Co-Fe-Cu-Zr high temperature permanent magnets despite that they have been widely applied in advanced industries since 1970s. Herein, through tuning the volume fraction of Zr-enriched 1:3R platelets by varying the second-step aging time, the pinning behavior in a model magnet Sm₂₅Co_44.9Fe_21.5Cu_5.6Zr_3.0 (wt%) was investigated. The results show that the volume fraction of 1:3R platelets can be effectively enlarged without changing the cell size (i.e. the volume fraction of CBs) by extending the aging time at 400 °C. Microscopic TEM characterizations combined with macroscopic magnetic measurements reveals that the locally thickened 1:3R platelets after long-term second-step aging reduce the effective pinning area by interrupting the magnetic domain walls at CBs, weakening the average pinning strength and the coercivity of the magnet. Consequently, our work supports that the 1:5H CBs act as the dominating pinning sites instead of the intersections of 1:3R platelets and 1:5H CBs, which may provide an important insight towards understanding the hard magnetism of pinning-controlled permanent magnets.     

Sintering of modified M35MHV HSS under diferent nitrogen pressures

Abstract

The sintering behaviour of high carbon–high vanadium water atomised M35MHV HSS (1·8 wt-%C, 4·2 wt-%V) is analysed as a function of the nitrogen pressure in the sintering atmosphere. Uniaxially pressed compacts were sintered to full density (≥ 98%TD) under different N₂ atmospheres in a range of pressures from vacuum to 8 bar. It is observed that the optimum sintering temperature (OST) depends on the absorbed nitrogen and is as low as 1050°C when the nitrogen content in the steel is 1·2 wt-%. The absorbed nitrogen affects not only the OST but also the matrix and carbides composition and the phases present after sintering. Compared with other powders processed under the same conditions, it is shown that the amount of absorbed nitrogen depends not only on the nitrogen partial pressure in the sintering atmosphere but also on the amount of vanadium and carbon and even on the heating rate. Hardness, fracture toughness, and fracture strength values are reported after heat treatment.     

Hydrogen Decrepitation and Recycling of NdFeB-type Sintered Magnets

With the rapid growth in the use of NdFeB-type magnets and with the growing environmental need to conserve both energy and raw materials, the recycling of these magnets is becoming an ever important issue. In this paper it is demonstrated that hydrogen could play a vital role in this process. Fully dense, sintered NdFeB-type magnets have been subjected to the hydrogen decrepitation (HD) process. The resultant powder has been subsequently processed in one of two ways in order to produce permanent magnets. Firstly, the powder was subjected to a vacuum degassing treatment over a range of temperatures up to 1000°C in order to produce powder that would be suitable for the production of anisotropic bonded or hot pressed magnets. Secondly, the HD-powder has been used to produce fully dense sintered magnets; in which case optimisation of the milling time, sintering temperature and time was carried out. The optimum degassing temperature for coercive powder was found to be 700°C, giving powder with a remanence (B_r) of ∼1350mT (±10 mT) and an intrinsic coercivity (H_cj) of ∼750kAm⁻¹ (±10 kAm⁻¹). The best sintered magnet was produced by very lightly milling the powder (30 min, roller ball mill), aligning, pressing and vacuum sintering at 1080°C for 1 hour. The magnetic properties of this magnet were: (BH)_max = 290 kJm⁻³ (±5 kJm⁻³), Sr = 1240mT (±5 mT) and H_cj = 830 kAm⁻¹ (±5 kAm⁻¹); representing decreases of 15%, 10%, and 20% respectively, from the properties of the initial magnet.