Nd-Fe-B三元系相图的研究 Ⅱ.过Nd_2Fe_(14)B点的等硼和等Nd垂直剖面相图

根据差热分析测量的相变点和铸态样品的金相观察及微区成分分析,绘制了通过Nd_2Fe_(14)B正分点的等硼(B=5.88at.-%,Nd≤45.41at.-%)与等钕(Nd_2Fe_(14)B-Nd_(1.1)Fe_4B_4)两个三元垂直剖面相图。证明合金液相冷却时,四方结构的Nd_2Fe_(14)B和Nd_(1.1)Fe_4B_4是经包晶反应析出的,而不是固液同成分析出。     

TRANSFORMATION OF Nd_(14)Fe_(73)Co_6B_7 ALLOY UNDER HYDROGEN TREATMENT

The phase constitution and variation of Nd_(14)Fe_(73)Co_6B_7 alloy after hydriding and subsequentdehydriding treatment at 770℃ have been investigated respectively by means of X-raydiffraction,SEM,TEM and magnetic measurement techniques.The original coarse,as castgrain structure,is dissolved into α-Fe,Nd hydride and Fe_2B mixture with average dimensionof 0.1 μm,during hydriding process,and subsequently the triphase mixture recombined intofinely unique Nd_2Fe_(14)B phase with dimension of 0.3 μm by desorbing the hydrogen.It wasfound that the Nd-rich phase distributed at the boundary of as-cast grain structure diffusedinto matrix Nd_2Fe_(14)B grain during hydriding.It remains during dehydriding and distributehomogeneously around some colonies composed of fine grains of Nd_2Fe_(14)B.     

Effect of Heat Treatment Time on Dy–Cu Alloy Diffusion Process in Dy-Containing Commercial Nd–Fe–B Sintered Magnets

In this paper, the grain boundary diffusion process(GBDP) using a Dy_(70)Cu_(30)(at.%) alloy as the diffusion source was performed in a commercial sintered Nd–Fe–B magnet, and the effect of heat treatment time on the microstructure and magnetic properties of the magnet was investigated in detail. For the processed magnets heat-treated at 860℃, as heat treatment time increased, the coercivity and the depth of(Nd,Dy)_2Fe_(14)B core–shell structure increased first and then decreased. However, when the heat treatment time was more than 2 h, the diffusion path of Dy from the Dy-rich shell phase into the Nd_2Fe_(14)B grains was revealed, and a nearly homogeneous(Nd,Dy)_2Fe_(14)B phase was formed, which brought on the decrease in both the depth of visible core–shell structure and the coercivity of Nd–Fe–B magnet.     

Structure and magnetic properties of TbCu_7-type melt-spun Sm–Fe–B alloys

The melt-spun SmFe_(12)B_x(x = 0, 0.50, 0.75,1.00, 1.25 and 1.50) ribbons were prepared at 40 m·s~(-1),and their structure and magnetic properties were studied by powder X-ray diffraction(XRD), vibrating sample magnetometer(VSM) and transmission electron microscopy(TEM). XRD results indicate that SmFe_(12)B_x alloys with 0.50 ≤ x ≤ 1.00 are composed of single-phase TbCu_7-type structure. Moreover, it is found that the boron addition can inhibit the emergence of soft magnetic phase a-Fe and result in the increase in the axial ratio c/a. After annealing at 650 ℃ for 0.5 h, the metastable phase TbCu_7 initially decomposes into the stable phase Sm_2Fe_(14)B(Nd_2Fe_(14)B-type) and a-Fe. The value of magnetic moment per Fe atom increases slightly from 1.75 uB for boron-free sample to 1.80 uB for the x = 0.75 sample and then decreases again.In addition, the best magnetic properties of maximum energy product [(BH)_(max)] of 14.56 kJ·m~(-3), coercivity(H_(cj))of 172.6 kA·m~(-1) and remanence(B_r) of 0.45 T are obtained for the SmFe_(12)B_(1.00) alloy. Based on transmission electron microscopy(TEM) results, the average size of grains is around 197 nm for B-free sample and decreases to 95 nm for x = 1.00 sample, indicating that the addition of boron can refine grains.     

Phase Relationship in Nd_2(Fe_(1-x)Ni_x)_(14) B and Pr_2(Co_(1-y)Ni_y)_(14)B Pseudo-ternary Compounds

The melting behavior,solid state phase transformation and structure of pseudo-ternary compounds Nd_2(Fe_(1-x)Ni_x)_(14)B andPr_2(Co_(1-y)Ni_y)_(14)B were studied using differential thermal analysis,optical microscopy X-ray diffraction,and electron probemicro-analysis techniques.At high temperature,eutectoid decomposition R_2(Ni,M)_(17)→R(Ni,M)_5 x-Ni(M) takes place in thesetwo pseudo-ternary compounds,in the composition range x=0.6～1.0 and y=0.3～1.0,respectively.When x(or y)≤0.2,bothNd_2(Fe_(1-x)Ni_x)_(14)B and Pr_2(Co_(1-y)Ni_y)_(14)B are single phase tetragonal.The phase constitutents of these two systems at room tempera-ture are similar in the composition range 0.6≤x(or y)≤1.0.     

MICROSTRUCTURAL CHARACTERIZATION OF Nd_(34)Fe_(60)O_(6) ALLOY

The microstructure of Nd_(34)Fe_(60)O_6 alloy was characterized by means of scanning electronmicroscope,Auger electron multiprobe,X-ray diffraction,differential thermal analysesand thermomagnetic analyses.It is found that the equilibrium microstructure of the alloy iscomposed of a Nd_2Fe_(17) matrix,an oxygen-containing Nd-rich phase anda Nd-Fe-O ternary ferromagnetic phase (referred to T_1).This phase forms via aperitectic reaction between Nd_2Fe_(17) and Nd-rich phase at 840 ℃.Its composition isaround Nd_(36)Fe_(54)O_(10),and Curie temperature 180℃.The X-ray diffraction peaks can beindexed on a tetragonal cell with lattice parameter of a=0.771nm and c=2.228nm.Thisstable phase is believed to be transformed from the metastable phase (A_1) in Nd-Febinary system as a result of oxygen introduced.     

MICROSTRUCTURE AND MAGNETIC HARDENING OF SINTERED NdFeB MAGNETS DOPPED WIHT Dy OR Dy_2O_3

The magnetic properties,microstructure and magnetic hardening of sintered(Nd_(1-x)Dy_x)_(16)Fe_(77.2)B_(6.8) and Nd_(16)Fe_(77.2)B_(6.8)+ywt-%Dy_2O_3 magnets have been stud-ied.As an addition of Dy to the magnet during smelting,Dy atoms may enter thematrix phase Nd_2Fe_(14)B,so as to enhance H_A,refine grains and improve boundary struc-ture,as well as to increase H_(ci)of the magnet.If Dy_2O_3 is added to powder materialprior to sintering,Dy atoms diffuse into the epitaxial layer of grains of matrix phasecausing enhancement of K′_1,also refinement of grains and improvement of boundarystructure.The NdFeB based permanent magnets with higher H_(ci) and greater(BH)_mmay be produced by adding about 2—3 wt-% Dy_2O_3 which make a favourable conditionfor lower cost.     

MAGNETIC PROPERTIES OF Nd_2(Fe_(1-x)M_x)_(14)B(M=C,Si)ALLOYS

The magnetic properties of Nd_2(Fe_(1-x)M_x)_(14)B(M=C and Si) compounds have been investi-gated.Substituting Fe by small amount of metalloid C and Si atoms,the compounds are intetragonal structure and have uniaxial anisotropy.The substitution of C for part of Fe de-creases the Curie temperature of the compounds and the intrinsic coercivity of bond samples atlow temperature.The replacement of Fe by Si makes the Curie temperature increase with amaximum at the vicinity of x=0.15.When the range of the content of Si is 0≤x≤0.10,thecoercivity distinctly increases at room temperature.from 62.7 kA/m at x=0 to138.7kA/m at x=0.06,where the residual magnetization has a maximum:in the mean timethe saturation magnetization decreases only by a small amount.     

EFFECT OF Nb ADDITION ON MICROSTRUCTURE OF(Nd,Dy)-(Fe,Co)-B MAGNETS

An addition of Nb up to 2 at.-% into(Nd,Dy)-(Fe,Co)-B alloy causes the formation ofdendrite Fe_2Nb and the precipitates in hard magnetic grains.Both increases withincreasing Nb content.During sintering,part of Fe_2Nb dissolves within the 2:14:1 grains,and the Fe_2Nb along grain boundaries hinders the growth of 2:14:1 grains.During an-nealing,the dissolved Nb in(?)-phase precipitates out,and this nearby the grain boundarydiffuses into Nd-rich phase to form new granular Fe_2Nb and to increase the Nb contentin Nd-rich phase,this results in a precipitate-free zone in the(?)-grain near Nd-rich phase.     

CRYSTAL STRUCTURE OF Y_2(Fe_(0.95)Al_(0.05))_(14)B

The site occupancies in permanent magnetic alloy Y_2(Fe_(0.95)Al_(0.05))_(14)B has been studied bymeans of neutron diffraction.The results from profile refinement show that Al enters thetetragonal structure of Nd_2Fe_(14)B and occupies the 4c and 8j_2 sites of the space groupP4_2/mnm preferably.The relation between the site occupancies and the magnetic proper-ties is discussed preliminarily.     

BOUNDARY MICROSTRUCTURE AND MAGNETIC HARDENING OF SINTERED NdFeB MAGNET

The boundary microstructure of sintered alloy Nd_(15.5)Fe_(77)B_-(7.5) has been studied by TEM,AESand SAED.The boundary structure may be distinguished into 4 types.The first three typesremain the same during annealing,and the fourth changes its microstructure remarkably.The4th type is composed of two different regions,i.e.,the central Nd-rich phase and the epitaxiallaver of the Nd_2Fe_(14)B grains.Owing to the atomic diffusion and other types of mass trans-port,magnetic hardening occurred in the epitaxial layer,thus the coercivity of the alloy hasbeen improved.     

CHANGE OF BORIDE IN Fe_(70)Cr_(18)Mo_2SiB_9 WITH THERMAL TREATMENT

The formation of borides M_3B_2 M_2B_2 and M_(23)B_6 may be carried out from the melt-quenchedFe_(70)Cr_(18)Mo_2SiB_9 during 700—1150℃ annealing.As the temperature raising,the M_2B,themajority being Fe_2B.may be gradually replaced by Cr_2B via the co-existence between Fe_2Band Cr_2B.The Cr_2B may be formed by trans formation of Fe_2B through the atomic substitu-tion and structural adjustment.The thin slice of remaining Fe_2B is sandwiched between(100)faces of(Cr,Fe)_2B as stacking fault.The M_2B_2 is virtually composed of theMo_(1+x)(Fe,Cr)_(2-x)B_2 where x(0≤x≤1)increases with the increase of temperature.     

MAGNETIC PROPERTIES AND CRYSTALLIZATION OF RAPIDLY QUENCHED(Fe_(1-x)Ga_x)_(77.5)Nd_4B_(18.5) ALLOYS

The Curie temperature increases but crystallization temperature decreases with the in-crease of Ga content,x,of amorphous(Fe_(1-x)Ga_x)_(77.5)Nd_4B_(18.5) alloys.The averagemagnetic moment of Fe atoms is almost a constant.By X-ray diffraction and ther-momagnetic measurements,the crystalline phases of the alloys,an unknown phase andα-(Fe,Ga)besides Fe_3 B as major one,were identified.The relationship between roomtemperature coercive field and Ga.content was also studied.     

HOT DEFORMATION OF CAST Pr-Fe-B MAGNETS

The hot deformation process of cast Pr_(19)Fe_(74.5)B_5Cu_(1.5) magnets were studied.It isfound that the easy-axes are aligned parallel to the compressive strain direction in vari-ous hot deformation process.During hot deformation,the crushing of Pr_2Fe_(14)B matrixgrains and the relative slip and rotation between the grains take place and the Pr-richmelt is squeezed out of the magnet,leading to a fine,dense and well-aligned microstruc-ture in the fully-deformed magnets.Magnetic properties of B_r=990 mT,_iH_c=880kA/m and(BH)_m=191 kJ/m~3are obtained.The increase of coercivity is attributed tothe fine Pr_2Fe_(14)B grains and the thin Pr-rich phase layer continuously distributed atthe grain boundaries.The increasing remanence is resulted primarily from the develop-ment of the easy-axis alignment as well as from the densification of the Pr_2Fe_(14)B matrix.The easy-axis alignment is developed by the relative slip and rotation of the crushedPr_2Fe_(14)B grains.A full and slow hot deformation is necessary for the good easy-axisalighnment and high magnetic properties.     

Microstructure and magnetic properties of hot-deformed anisotropic Nd–Fe–B magnets prepared from amorphous precursors with different crystallization proportions

Anisotropic magnets were obtained by hot deformation with the partial crystallized precursor prepared via spark-plasma sintering(SPS). Amorphous powders with the nominal composition of Nd_(28.72)Fe_(bal)Co_(5.66)Ga_(0.59)B_(0.92)(wt%) were used as the starting material. The results show that the amorphous powders would suffer varying degrees of crystallization even below the crystallization point during the SPS process under high pressure.And the pre-crystallized grains in precursors have great impacts on the microstructure and magnetic properties of the hot-deformed magnets. The final obtained anisotropic magnets exhibit homogeneous microstructure consisting of well-aligned and platelet-shaped Nd_2Fe_(14)B grains without abnormal growth. It can be found that a reasonable proportion of pre-crystallized gains could promote the preferential orientation in the magnet, leading to the achievement of optimal magnetic properties among the magnets with identical composition and best magnetic performance is achieved in the magnet hot deformed from the 490 °C high-pressure hot-pressed precursor.     

CRYSTALLIZATION AND MAGNETIC PROPERTIES OF AMORPHOUS Nd_6Fe_(81)B_(13)ALLOY

The magnetic properties and the crystallization and its influence on the magnetic and struc-tural properties for rapidly quenched amorphous Nd_6Fe_(81)B-(13) alloy have been studied.The lowtemperature magnetic properties,the Curie temperature and the crystallization behavior arediscussed.A small addition(6at.-%)of Nd in amorphous Fe-B alloys increases largely thecrystallization temperature T_x but causes a decrease of the Curie temperature Tc·A newphase was observed in the sample annealed at 873 K.     

THREE-DIMENSIONAL TOPOLOGICAL ANALYSIS OF Nd-Fe-B AND Pr-Fe-B PHASE DIAGRAMS

With the aid of 3-dimensional topological analysis methodology,relationships amongphage regions on the isothermal section of the Nd-Fe-B phase diagram at 1000℃ andthose on vertical sections of the Pr-Fe-B phase diagram passing through Pr_2 Fe_(14)B pointhave been re-discussed and modified.     

Magnetic properties of Nd_2Fe_(14)B/α-Fe nanocomposite magnets with yttrium addition

The phase evolution and magnetic properties of Nd9?xYxFe72Ti2Zr2B15 (x = 0,0.5,1,and 2) melt-spun nanocomposite ribbons were studied.It is found that Y addition not only enhances the formability of amorphous phase in the alloy,but also stabilizes the amorphous phase during the annealing treatment.The appropriate content of Y addition effectively enhances the remanence (Jr) of the annealed sample.The residual amorphous intergranular phase in the annealed sample optimizes the squareness of the loop,resulting in an larger maximum energy product (BH)max.The best magnetic properties,Jr = 0.78 T,Hci (coercivity) = 923.4 kA/m,and (BH)max = 98.5 kJ/m3,were obtained from the Nd8YFe72Ti2Zr2B15 ribbon spun at Vs = 4 m/s and annealed at 700°C for 10 min,which is composed of Nd2Fe14B,α-Fe,and amorphous phase.     

Preparation and Characterization of Amorphous W–B–C Alloy and Solid Solutions of C in Tungsten Borides

Mixtures of W and B_(13) C_2 powders were mechanically milled and subsequently annealed at 900–1200 °C. It is found that amorphous W–B–C alloy formed as the mixtures were milled for 20–80 h. After annealing the 80 h-milled mixtures at 900–950 °C, solid solutions of C and/or B in tungsten [W(B, C)], C in tungsten boride [W_2 B(C) or WB(C)]formed by the crystallization of amorphous W–B–C. The formation temperature of W_2 B(C) and WB(C) is lower than that of W_2 B and WB reported previously. As the 80 h-milled mixtures were annealed at 1200 °C, W reacted with amorphous W–B–C completely to form WB and W_2B_5 or W_2B_5 instead of the solid solutions of C in tungsten borides, which is determined by the mole ratio of W to B_(13) C_2. The formation mechanisms of the W_2 B(C) and WB(C) solid solutions as well as phase transition rules of the mixtures at annealing temperature and mole ratio were also investigated using first-principle calculation.     

Coercivity,microstructure,and thermal stability of sintered Nd-Fe-B magnets by grain boundary diffusion with TbH_3 nanoparticles

Grain boundary diffusion technique with TbH_3 nanoparticles was applied to fabricate Tb-less sintered NdFe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematically studied. The coercivity and remanence of grain boundary diffusion magnet are improved by 112% and reduced by 26% compared with those of the original magnet, respectively. Meanwhile, both the remanence temperature coefficient(α) and the coercivity temperature coefficient(β) of the magnets are improved after diffusion treatment. Microstructure shows that Tb element enriches in the surface region of Nd_2Fe_(14)B grains and is expected to exist as(Nd,Tb)_2Fe_(14)B phase. Thus, the magneto-crystalline anisotropy field of the magnet improves remarkably. As a result, the sintered Nd-FeB magnets by grain boundary diffusion with TbH_3 nanoparticles exhibit enhanced coercivity.