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.     

Optimization of core-shell structure distribution in sintered Nd-Fe-B magnets by titanium addition

In view of the uneven distribution of the core-shell structure of sintered Nd-Fe-B magnets after grain boundary diffusion,this study proposes to use high-melting-point and reactive element titanium(Ti) as an additive to increase the diffusion channels and to enhance the diffusion of heavy rare earth elements along the grain boundary phase.By adding Ti element,the diffusion depth and hence the intrinsic coercivity of magnets are increased significantly.The addition of Ti increases the coercivity ...     

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.     

Coercivity enhancement of nanocrystalline hot-deformed Nd-Fe-B magnets by low-melting eutectic MM-Cu(MM=La,Ce,Pr,Nd) alloys addition

MM_(85)Cu_(15)(MM = La,Ce,Pr,Nd) eutectic alloys were added into the hot-deformed Nd-Fe-B magnets to enhance the coercivity.It is found that three endothermic peaks occur on the differential scanning calorimetry curve of the MM-Cu melt-spun ribbons at 432.2,451.1 and 516.5℃.The peaks substantially correspond to three types of MM-Cu low-melting eutectic phase.The coercivity of magnets increases when the MM-Cu content is lower than 4 wt%,and then keeps almost no change with the content further increasing to 5 wt%.The coercivity of the hot-deformed magnets with 4 wt% and without MM-Cu addition is 948 and 683 kA/m,respectively.Nearly all the platelet-shaped grains are isolated by the thickened intergranular phase after MM-Cu addition.Moreover,the average grain size of the magnets with MM-Cu addition decreases compared with that of the magnet without MM-Cu addition.Scanning electron microscopy images show that the areal fraction of the RE-rich grain boundary phase increases from 8.6% to 15.1% after MM-Cu addition.The La,Ce together with Cu and Ga aggregate at the grain boundary regions separating neighboring grains and smoothing the grain boundaries.Therefore,both the thickened grain boundary and decreased mean grain size result in the enhancement of coercivity after MM-Cu eutectic alloy addition.     

Improvement in magnetic properties,corrosion resistance and microstructure of Nd–Fe–B sintered magnets through intergranular addition of Tb68Ni32

New energy vehicles and offshore wind power industries have a high demand for sintered Nd–Fe–B magnets with high intrinsic coercivity and high corrosion resistance. In this study, the magnetic properties, anticorrosion properties, and microstructure of Nd–Fe–B sintered magnets with the intergranular addition of low-melting-point eutectic Tb₆₈Ni₃₂ alloy powders were investigated. The aim is to determine if the addition of Tb₆₈Ni₃₂ can improve these properties. A low melting-point eutectic alloy Tb₆₈Ni₃₂ powders was prepared as a grain boundary additive and blended with the master alloy powders prior to sintering. The coercivity of the resultant magnets gradually increases from 1468 to 2151 kA/m by adding increasing amounts of Tb₆₈Ni₃₂. At the same time, the remanence first increases and then slightly decreases. After studying the microstructure and elemental composition of the Tb₆₈Ni₃₂ added magnets, it is found that the significant increase in coercivity and the negligible reduction in remanence is due to densification, improved grain orientation, a uniform and continuous boundary phase distribution, as well as the generation of a (Nd,Pr,Tb)₂Fe₁₄B “core–shell” structure surrounding the main-phase grain. Moreover, the corrosion resistance of the magnet is greatly improved owing to the enhancement of electrochemical stability, as well as the optimization of the distribution and morphology of the intergranular phase.     

Influence of Al/Cu content on grain boundary diffusion in Nd-Fe-B magnet via in-situ observation

The influence of aluminum and copper content in the starting Nd-Fe-B magnet on grain boundary diffusion process(GBDP) was studied by observing the phase transformation behaviors of the magnets in-situ at high temperature. A higher coercivity increment is discovered in the sample with higher AI/Cu despite the fact that its Dy diffusion amount is the same as the other. DSC analysis shows an evident melting behavior in the higher Al/Cu sample. Laser scanning confocal microscopy(LSCM) in-situ characterization shows a large amount of melted intergranular phase spills out to the surface simultaneously at around 600 ℃ in the high Al/Cu sample, while the phase spills out gradually one after another in the range between 623 and680 ℃ in the other sample, which indicates that the intergranular phase can be more easily melted in the sample containing more AI/Cu. The area fraction of matrix phase remarkably shrinks while that of intergranular phase enlarges after LSCM heating, which demonstrates the outer region of the Nd_2 Fe_(14)B grains melt at the temperature of 900 ℃. Electron probe microanalyzer result(EPMA) shows that the Nd and Dy concentrate in edge regions and subsequently mix into the intergranular phase with the melting of the grain edge, while a large amount of AI and Cu in the intergranular phase spill out. Nevertheless, the sample with higher starting AI/Cu still remains higher residual contents after LSCM experiments, and that could probably be the main reason why the high AI/Cu magnet shows smaller coercivity decrement after LSCM experiment. Overall, the increase of AI/Cu in the starting magnet optimizes the Dy distribution and the wettability of intergranular phase, enhancing coercivity increment effect further.     

Al与Mo复合添加对NdFeB磁体矫顽力的影响

本文采用晶间合金化工艺将合金元素Mo和Al直接引入烧结Nd-Fe-B磁体晶间区域,改变晶间区域的合金体系和显微组织,以达到提高磁体矫顽力的目的.实验结果表明Mo在低温时效过程中可抑制晶间富Nd相与主相之间的平衡转变,使晶界区域析出细小二次主相晶粒,使矫顽力提高.     

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.     

Effects of grain boundary diffusion process on magnetic properties enhancement and microstructure evolution of hot-deformed Nd-Fe-B magnets

Grain boundary diffusion(GBD) process is an important approach for producing Nd-Fe-B magnets with high coercivity and high thermal stability.The GBD for hot-deformed Nd-Fe-B magnets with nanocrystalline micro structure is more complicated compared to sintered magnets.Here,we investigated the effects of different GBD methods,i.e.,intergranular addition(in-situ GBD 1#),in-situ GBD from magnet surface during hot pressing and hot deformation(in-situ GBD 2#),and conventional GBD,on the magnetic prope...     

Improvement of coercivity and thermal stability of Nd-Fe-B sintered magnets by intergranular addition of Tb80Fe20 alloy

To improve the coercivity and temperature stability of Nd-Fe-B sintered magnets for high-temperature applications, the eutectic Tb₈₀Fe₂₀ (wt%) alloy powders were added into the Nd-Fe-B sintered magnets by intergranular method to enhance the coercivity (H_cj) and thermal stability. The microstructure, magnetic properties and thermal stability of the Nd-Fe-B magnets with different Tb₈₀Fe₂₀ contents were studied. The experimental results demonstrate that the coercivity (H_cj) of the sintered Nd-Fe-B magnet is significantly enhanced from 14.12 to 27.78 kOe, and the remanence (B_r) decreases not obviously by introducing 4 wt% Tb₈₀Fe₂₀ alloy. Meanwhile, the reversible temperature coefficients of coercivity (β) and remanence (α) of the Nd-Fe-B magnets are increased from ?0.5634%/℃ to ?0.4506%/℃ and ?0.1276%/℃ to ?0.1199%/℃ at 20–170 ℃, respectively. The Curie temperature (T_C) of the Nd-Fe-B magnet is slightly enhanced with the increase of Tb₈₀Fe₂₀ content. Moreover, the irreversible flux magnetic loss (h_irr) is obviously reduced as Tb₈₀Fe₂₀ addition increases. Further analysis of the microstructure reveals that a modified microstructure, i.e. clear and continuous RE-rich grain boundary layer, is acquired in the sintered magnets by introducing Tb₈₀Fe₂₀ alloy. The associated mechanisms on improved coercivity and thermal stability were comprehensively researched.     

Microstructural Evolution in Nd2Fe14B/α-Fe Bearing Zr-Nb Nanocomposite Magnet

Microstructural evolution in Nd₂Fe₁₄B/α-Fe bearing Zr-Nb nanocomposite magnet has been investigated. The magnetic properties, especially the coercivity were enhanced with the addition of Zr and Nb elements for the fine and uniform microstructure. The results of three-dimensional atom probe (3DAP) revealed that (Zr,Nb)Fe₂ intergranular phase existed at the boundary of grains, suppressing the grain growth during crystallization process. The interaction coupling between grains was enhanced and the magnetic properties were improved for the refinement of grains.     

The incorporation of self-propagating,high-temperature synthesis-formed Fe-TiB2 into ferrous melts

Steel-matrix particulate composites were processed by direct addition of an Fe-TiB₂ master alloy powder to a BS970:080M30 medium-carbon steel. This powder was produced using a self-propagating, high-temperature synthesis (SHS) reaction and consisted of a dispersion of fine TiB₂ particles (2 to 5 μm), respectively, in an iron binder. The addition of the Fe-TiB₂ powder resulted in the formation a parasitic Fe₂B phase and TiC within the steel microstructure. In response to this, an SHS master alloy composed of Fe-(50 pct TiB₂+50 pct Ti) was manufactured, which, when added to the steel, prevented the formation of Fe₂B and resulted in a composite containing a mixture of TiB₂ and TiC particles. The effect of master alloy composition and addition level on the microstructural phases generated is discussed in detail. The response to heat treatment of composite materials manufactured in this way was also investigated. It was found that an isothermal hold at 840 °C leads to a substantial softening of the material processed using the Fe-TiB₂ additive, while at 1000 °C, a hardness level equivalent to that of the as-cast material was maintained. The same heat treatment of samples in which the formation of Fe₂B was suppressed resulted in no appreciable difference in hardness level or microstructure.     

Magnetic properties enhancement of hot-deformed NdFeB magnets by two different methods of CeNdCu diffusion

Two different ways were used to control the distribution of cerium for the enhancement of coercivity. One was by coating CeNdCu in the prepared magnet and annealing to make CeNdCu diffuse into the grain boundary to increase the coercivity, the other was by mixing CeNdCu with initial magnetic powders and then preparing the hot-deformed magnet. The SEM-EDS result indicates that cerium diffuses more easily into the main phase by the mixing way, while cerium is mainly distributed in the grain boundary via the coating way. The heat-treatment process may be one of the dominant influencing factors for the distribution of Ce. Multi-steps heat treatment in the mixing way, consisting of hot-pressing, hot-deforming and post heat process, easily introduces Ce into the main phase Nd₂Fe₁₄B, which forms the Ce₂Fe₁₄B shell resulting in the decrease of H_A. But the coating way can ensure uniform dispersion of Ce in the grain boundary, which leads to the high coercivity.     

Effects of dispersal and mechanical boron alloying on the thermal stability of hydride phases in the Ti-B-H system

Methods have been applied from scanning electron microscopy, hydrogen thermal desorption, XRD, and differential thermal analysis on the effects of grain size and alloying with boron as regards the thermal stability and decomposition temperatures of hydride phases in mechanical alloys in the Ti-B-H system. The alloys were prepared by high-energy processing for 50 h in a planetary ball mill with mixtures of TiH_1.9 + 9 mass% B + 13 mass% Ti and also with TiH_1.9 + 50 mass% TiB₂ at speeds of 1000 rpm, in addition to mixtures of TiH_1.9 + 40 mass% B and TiH_1.9 + 50 mass% TiB₂, which were treated for 20 min at speeds of 1680 rpm. The dispersal on mechanical treatment and the addition of boron to the titanium hydride powder have substantial effects on the thermal stability. The processing of the mixture TiH_1.9 + 9 mass% B + 13 mass% Ti lasting 50 h in argon gave temperatures for the dissociation of the Ti(B, H)_x hydride phase in the mechanical alloy lower by 300 deg than the decomposition temperature for the initial titanium hydride TiH_1.9. The mechanisms have been identified for the effects of the dispersal and boron alloying on the thermal stability of the titanium hydride.     

The Interface of TiB<Subscript>2</Subscript> and Al<Subscript>3</Subscript>Ti in Molten Aluminum

In the grain refinement of aluminum, Al₃Ti and TiB₂ particles are introduced to reduce the casting grain size down to 200 micrometer level, which makes cold working possible. The particles are brought in by the addition of Al-Ti-B-type master alloys. It is generally believed that TiB₂ particles are stable and nucleate α-Al grains in solidification in the presence of titanium in solution from the dissolution of Al₃Ti particles in the master alloys. The titanium in solution either forms Al₃Ti layers on the surface of TiB₂ particles to promote the nucleation of α-Al grains or remains as solute to restrict the growth of α-Al grains in solidification. However, a consensus on a grain refinement mechanism is still to be reached due to the lack of direct observation of the three phases in castings. This paper presents finding of the TiB₂/Al₃Ti interfaces in an Al-Ti-B master alloy. It demonstrates a strong epitaxial growth of Al₃Ti on the surface of TiB₂ particles, a sign of the formation of an Al₃Ti layer on the surface of TiB₂ particles in grain refinement practice. The Al₃Ti layer has a crystal coherency with α-Al and hence offers a substrate for heterogeneous nucleation of α-Al grains. However, the layer must be dynamic to avoid the formation of compounded Al₃Ti and TiB₂ particles leading to the loss of efficiency in grain refinement.     

Silicide Formation in Ti-3Al-8V-6Cr-4Zr-4Mo

TEM, including convergent beam electron diffraction (CBED), SEM, and EDX techniques, were used to] characterize the silicide (Ti, Zr)₅ Si₃ which forms in Beta-C (Ti-3Al-8V-6Cr-4Zr-4Mo) and other similar titanium alloys. Space group of this silicide is determined as P6/mmm. In addition, solvus of this silicide was determined and a complete phase field diagram was constructed for Beta-C alloy. It was found that morphology and distribution of the silicide strongly depends on thermal and thermomechanical treatments. Continuous grain boundary silicide was found to be detrimental for tensil ductility which results in intergranular fracture. Thermal and thermomechanical treatments were identified which can reduce or eliminate continuous grain boundary silicide precipitation.     

Boron removal by titanium addition in solidification refining of silicon with Si-Al melt

In order to effectively remove B from Si for its use in solar cells, a process involving B removal by solidification refining of Si using a Si-Al melt with Ti addition was investigated. For clarifying the effect of Ti addition on B removal from the Si-Al melt, TiB₂ solubilities in Si-64.6 at. pct Al melt at 1173 K and Si-60.0 at. pct Al melt at 1273 K were determined by measuring the equilibrium concentrations of B and Ti in the presence of TiB₂ precipitates. The small solubilities of TiB₂ in the Si-Al melt indicate the effective removal of B from the Si-Al melt by Ti addition. Further, solidification experiments of Si-Al alloys containing B by Ti addition were performed, and the effect of Ti addition on the solidification refining of Si with the Si-Al melt was successfully confirmed.     

Coating of 6028 Aluminum Alloy Using Aluminum Piston Alloy and Al-Si Alloy-Based Nanocomposites Produced by the Addition of Al-Ti5-B1 to the Matrix Melt

The Al-12 pctSi alloy and aluminum-based composites reinforced with TiB₂ and Al₃Ti intermetallics exhibit good wear resistance, strength-to-weight ratio, and strength-to-cost ratio when compared to equivalent other commercial Al alloys, which make them good candidates as coating materials. In this study, structural AA 6028 alloy is used as the base material. Four different coating materials were used. The first one is Al-Si alloy that has Si content near eutectic composition. The second, third, and fourth ones are Al-6 pctSi-based reinforced with TiB₂ and Al₃Ti nano-particles produced by addition of Al-Ti5-B1 master alloy with different weight percentages (1, 2, and 3 pct). The coating treatment was carried out with the aid of GTAW process. The microstructures of the base and coated materials were investigated using optical microscope and scanning electron microscope equipped with EDX analyzer. Microhardness of the base material and the coated layer were evaluated using a microhardness tester. GTAW process results in almost sound coated layer on 6028 aluminum alloy with the used four coating materials. The coating materials of Al-12 pct Si alloy resulted in very fine dendritic Al-Si eutectic structure. The interface between the coated layer and the base metal was very clean. The coated layer was almost free from porosities or other defects. The coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages (1, 2, and 3 pct), results in coated layer consisted of matrix of fine dendrite eutectic morphology structure inside α-Al grains. Many fine in situ TiAl₃ and TiB₂ intermetallics were precipitated almost at the grain boundary of α-Al grains. The amounts of these precipitates are increased by increasing the addition of Al-Ti5-B1 master alloy. The surface hardness of the 6028 aluminum alloy base metal was improved with the entire four used surface coating materials. The improvement reached to about 85 pct by the first type of coating material (Al-12 pctSi alloy), while it reached to 77, 83, and 89 pct by the coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages 1, 2, and 3 pct, respectively.     

The nucleation of aluminum grains in alloys of aluminum with titanium and boron

The microstructure of a ternary alloy, Al-5 wt pct Ti, 1 wt pct B, has been examined by optical and electron transmission microscopy, by selected area diffraction, and electron probe microscopy, by selected area diffraction, and electron probe microanalysis. Particles of Al₃Ti are found at the center of grains and there exist preferred epitaxial orientations between this compound and the surrounding aluminum. Particles containing titanium and boron occur at aluminum grain boundaries and have no preferred configurations with respect to the aluminum or to one another. It is concluded that the active heterogeneous nuclei are therefore Al₃Ti and that particles of TiB₂, AlB₂, or a ternary compound are not active in this alloy. Grain size measurements in binary Al-Ti alloys suggest that particles of a nucleating phase must be present at concentrations as low as 0.01 wt pct Ti, and it is suggested that these could be Al₃Ti if the existing binary phase diagram Al-Ti is in error.