Rare Metals2020年第1期

Permanent magnetic properties of Nd-Fe-B melt-spun ribbons with Y substitution

Cao-Huan Zhang Yang Luo Dun-Bo Yu Ning-Tao Quan Gui-Yong Wu Ya-Kun Dou Zhou Hu Zi-Long Wang

作者简介：*Yang Luo e-mail:eluoyang@foxmail.com;

收稿日期：10 March 2018

基金：financially supported by the National Key Research and Development Program(No. 2016YFB0700902);

Permanent magnetic properties of Nd-Fe-B melt-spun ribbons with Y substitution

Cao-Huan Zhang Yang Luo Dun-Bo Yu Ning-Tao Quan Gui-Yong Wu Ya-Kun Dou Zhou Hu Zi-Long Wang

National Engineering Research Center for Rare Earth Materials,General Research Institute for Nonferrous Metals

Grirem Advanced Materials Co.Ltd

Abstract：

Phase constituents,microstructures and magnetic properties of melt-spun Nd_12-xY_xFe₈₁B₆ Nb ribbons were investigated systematically.The influence of Y substitution for Nd on the phase stability,grain size and magnetic exchange coupling was analyzed.It is found that all the ribbons crystallize in the tetragonal 2:14:1 structure,i.e.,with single hard magnetic phase at the roll speed of25 m s-1.With the increase in Y doping,Curie temperature(Tc) increases,while the coercivity decreases monoclinic ally.However,remanence magnetization(B_r) and maximum energy product((BH)_max) fluctuate and the maximum value is obtained at certain amount of Y.The optimum magnetic properties of intrinsic coercivity of intrinsic coercivity(H_cj)=908.2 kA·m^-1 and(B_H)_max=118.52 kJ·m^-3 are achieved when x=1.0.It can be attributed to the strengthened exchange coupling between the neighboring nanograins in Nd-Y-Fe-B meltspun powder based on the Henkel curves.Furthermore,Y substitution also significantly improves the temperature stability of magnetic performance.The coercivity temperature coefficient of β=-0.157%·℃^-1 and remanence temperature coefficient of α=-0.32%·℃^-1 are gained,which are greatly reduced compared with those of the undoped Nd-Fe-B compounds.

Keyword：

Rapid quenching; (Nd,Y)-Fe-B alloy; Exchange coupling; Thermal stability;

Received： 10 March 2018

1 Introduction

Since NdFeB permanent magnet materials were discovered,people were committed to finding a variety of ways to improve their thermal s_tability,corrosion r_esistance,mechanical properties and comprehensive magnetic propertie_s,which have made some progress [ 1, 2, 3, 4] ._Wⁱth t_he increasing demand for NdFeB permanent magnets and the extensive use of ^Nd i_n natural rare-earth resources,th_e rising price of raw ^materials has _brought t_remendous pressure to the NdFeB market.Development of low-cost rare-earth permanent magnetic materials has been on the agenda.For this purpos_e,the issue on substituting Nd with the most abundant and inexpensive light rare-earth metal has drawn continuing interests [ 5, 6, 7, 8] .The substitu^ti^onofY(yttrium) could enhance the exchange coupling effect and improve not only ^the remanence and magnetic energy product but also the thermal stability [ 9, 10, 11] .Moreover,the study of yttrium-neodymium-based rare-earth _permanent magnetic materials broadens the application range ^of Y.It is conducive to ^our rare-earth resour_ce balance and sustainable development,and _it is possible to develop a commercially competitive commercial magnet by ap^propriate methods [ 12] .In most cases,^the sintered or hotpressed methods make Y-containing rare-earth permanent magnet materials exhibit higher magnetic properties.Fo^r example,^Tang et al.[₁₃]studied the ^MRE₂Fe₁₄B(MRE=Y+D^y+Nd) sⁱntered magnets,which co_mbined ^the performance advantages of Dy2^Fe14B an_d Y2Fe1₄B,and the microstructure showed that the sintered magnet formed the main phase segregation,thus increasingthe magnetic properties of the temperature coefficient.By contrast,partly replaced magnets prepared by melt-spun methods could also achieve promising magnetic propertie_s.For example,C_hen et al.[₁4]reported that melt-spun Nd_9-xY_xFe_72Ti₂Zr₂B_15alloy exhibited a co_ercivity of923.4 kA·m^-1,which was t_he highest performance components in current rapid quenching Y-containing permanent magnet material_s.L_iu et al.^{[ [15]} recorded th^at the Ysubstitution f^or Dy not only improved the remanence and magnetic energy product,but also improved _the thermal stability.Further effort ^on Y-containing rare-earth pe_rmanent magnet materials,such as optimizing alloy compos_ition or studying microstructure,can become an importan_t direction for future research.

Up to now,most investigations only reported the roomtemperature magnetic properties of (Nd,Y)-Fe-B magnets,but few studies have been performed on the effects of Y on the thermal demagnetization,which is a research on the depth.In this work,the Nd_12-xY_xFe₈₁B₆Nb alloys were prepared via the melt-spun method.The effect of Y substitution for Nd in Nd1_2-xYxFe8₁B6_Nb alloys on phase constituent_s,microstructure_s,magnetic properties and thermal demagnetization was studied.

2 Experimental

The alloy ingots with nominal composition of Nd_12-xYxFe8₁B6Nb were fabricated by arc melting under an argon atmosphere of 99.9%Fe,99.9%Nd,99.9%Y,99.₉%Nb and 97%ferroboron (FeB).^The contents of Y ⁱn the precursors (x) vary from 0 to 6.0 with compositional intervals of 0.₅.Since it is difficult to volatilize by default during the smelting process,no additional compensation is require_d.Each ingot was m_elted at least five times to achieve composition homogeneity._T^he melt-spun ribbons were produced by induction melting ^the ingot in quartz tubes with a 0.7 mm in _diameter bottom orifice under 50,650 Pa of high-purit^y Ar gas.All ribbons were prepared by mel_t spinning onto a rotating Mo disk at a w^heel speed of velocities 25 ^m·s^-1,which has been proven t_o be the best wheel speed for this ingredien^t.Typical ribbons possess a thickness and width of approximately 30-50 and 2-3 m_m,respectively.Subsequently,the ribbons wrappe_d iⁿ tantalum _foil _were annealed at 750℃for 15 min in vacuum(<1.0×10^-3 Pa) and rapidly quenched with room-temperature water.All ribbons were then crushed to powder_s,and crystal structures an_d phase of these powders were examined ^by ^X-ray diffraction (XRD,R_igaku Sm_artLab)using Co Kαradiation.The thermomagnetic curves of al_l samples ^were tested using a vibrating sample ma_gnetometer⁽VSM,Qua_ntum Design VersaLa^b) with an applied field of 1 ^T.Magnetic powder pr^operties forcompacted cylinder (Φ3 mm×5 mm) in resin capsules were measured by ^using a vibrating sample magnetometer(VSM) in fields up to 3 _T.Microstructures of ribbons were observed by transmission electro_n microscopy (TEM,FEITecnai F20).

3 Results and discussion

3.1 Structure and phase analysis

Figure 1 displays some XRD patterns of melt-spun ribbons with optimal magnetic properties in Nd_12-xYxFe81_B6 Nb alloys quenched at the wh^eel velocities of ²5 m·s^-1.It is found that single tetragonal 2:14:1 structure is obtain_ed in al_l cases and there are no additional ph_ases.These findings prove that the substitution of Y ^forms a unique hard magnetic phase and it is independent of the content of the added e_lement.It _also has been shown _that the diffractio_n peak is slightly shifted to the sm_all angle compar_ed wi_th the XRD patterns of melt-spun ri_bbons with different compositions,but the peak intensit^y does not change significantl_y.The reason can be attributed to that light rareearth Y enters the tetragonal phase,resulting in the change in lattice constant to a certain extent.Si_milar work has been done on nanocomposite Nd-Y-Fe-B-Mo bulk magnets prepared by injection casting techn_ique,and Tao et al. [ 16] had c_onfirmed that Y was coⁿcentrated into 2:14:1 hard phase and promoted the content of magnetic hard phase.The lattice parameters of the ribbons have been refined by Rietveld method based on the 2:14:₁ structu_re,as gi_ven in Table 1,which interpret the lattice constant and ax_ial ratio c/^a value versus Y content wel^l.The lattice parameters a and c of the annealed samples are slightly expan_ded compared _with ^the pure Nd2Fe14_B phase (a=0.8792 nm,c=1.2177 nm) [ 17] ,i_ndicating that Y dopⁱng atoms entered the lattice of the Nd₂Fe14B phase.Since the atomicradius of Y and neodymium is nearly equivalent,the substitution of the element does not cause a large change in the covalent bond length,but results in a slight chang_e in lattice constant[18_].The increase in Curie temperature is ascribed to the higher _TC of Y2Fe₁4B (565 K) than that of Pr₂Fe₁₄B or Nd₂Fe_14B [ 19] .^This also indirectl^y suggests that Y atoms enter the 2:14:1 tetragonal phase as an alternative element.

Fig.1 XRD patterns of melt-spun Nd12-xYxFe81B6Nb(x=0,1.0,2.0,3.0,4.0,5.0) alloys prepared at wheel velocities of 25 m·s-1

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Table 1 Lattice parameters a and c and TC from the Rietveld refinement for Nd12-xYxFe81B6Nb ribbons

3.2 Magnetic properties

The melt-spun Nd12-xYxFe81B6Nb ribbons were obtained by quenching at the wheel velocities of 25 m·s-1,crystallization annealed at 750℃for 15 min in vacuum(<1.0×10-3 Pa),which has proven to be an optimum process to obtain best magnetic performance,and the ribbons were subsequent crushed to below 107μm.The variation of intrinsic coercivity (Hcj),remanence (Br) and maximum energy product ((BH)max) with different Ycontents at room temperature of all samples is summarized in an applied field,as shown in Fig.2.The greatest magnetic properties of Br=0.88 T,Hcj=908.2 kA·m-1 and(BH)max=118.52 kJ·m-3 are obtained where the Y atomic weight is 1.0 in annealed ribbons.

Conforming to the situation,the coercivity shows a linear decline trend with Y concentration increasing,which is mainly attributed to the decreased magnetocrystalline anisotropy of the hard magnetic phase (Nd,Y)2Fe14Bcompared to that of pure Nd2Fe14B [ 20] .The remanence and magnetic energy product of the rapidly quenched samples exhibit irregular changes,which,in general,appear an uneven trend of increasing first and then decreasing with the addition of Y.According to previous work,magnetic energy product improves by 12.05 kJ·m-3and the remanence increases by 0.04 T compared to those of Y-free samples.

And in general,when the atomic weight of substitution Y is relatively low,the remanence and magnetic energy product increase linearly,and the best magnetic properties occur when x is 1.0.With the addition of Y increasingcontinuously,the magnetic properties decrease dramatically.But at the point of x=4.5,the magnetic property reaches a larger value of 107.72 kJ·m-3,which is almost close to the theoretical value of saturation.Reasonable explanation as Kneller et al. [ 21] reported that the decrease in the magnetocrystalline anisotropy of the hard magnetic phase caused to larger length of exchange coupling correlation.Zhang et al. [ 22] concluded that the extension of the exchange coupling length led to the improvement in exchange coupling in nanocomposite magnets due to more magnetic grains participated in the exchange coupling,which similarity strengthens the exchange coupling between the neighboring nanograins in Nd-Y-Fe-B meltspun powders in our work.

Fig.2 Room-temperature magnetic properties of Nd12-xYxFe81B6Nb ribbons at 25 m·s-1 followed by optimal annealing treatment

Fig.3 Demagnetization curves of Nd12-xYxFe81B6Nb1(x=0,1.0,2.0,3.0,4.5,6.0)powders

Figure 3 shows the demagnetization curves of Nd12-xYxFe81B6Nb powders.Compared to the original power,it is found intuitively that by adding 1.0 wt%Y,the magnets exhibit a little improvement in its remanence,and the coercivity almost keeps invariable.All the demagnetization curves“step”of samples tested are quite smooth,which indicate that a large number of Y atoms enter the magnetic phase selectivity,and this does not significantly deteriorate its squareness.

The microstructure of the rapidly quenched ribbons was investigated by TEM.Figure 4a shows the initial Nd12Fe81B6Nb without Y.The grain size ranges from60 nm up to some few tens nanometers.In contrast,typical TEM images of Y-containing ribbons are shown in Fig.4b-d.It can be found that the addition of Y improves the morphology of the grains and makes grains even more uniform.In addition,a small amount of Y substitution can make grains refinement.However,the presence of large amounts of Y has further increased grain size,even itsmicrostructures are also extremely fine and the grains are spherical.

3.3 Exchange coupling by Henkel plots

The most common method of quantitatively analyzing the intergrain interactions is by constructing Henkel plots,according to Chen et al. [ 23] .The interaction between two phases is normalized as the following relationship:

where mr(H) and md(H) are defined as the remanent magnetizations after applying a magnetic field H on a thermally demagnetized sample and after applying a reverse field on a previously saturated sample,respectively.For an assembly of noninteracting single-domain particles,the value of magnetization (M) is 0.An increase in the positive deviationδM(H) sugges2s that the exchange coupling is enhanced.A negative deviationδM(H) of the plots isinterpreted as magnetostatic interactions dominating when magnetization reversal occurs [ 24] .

Fig.4 TEM images and inserted size distributions of melt-spun Nd12-xYxFe81B6Nb ribbons with different Y contents:a x=0,b x=1.0,c x=3.0 and d x=6.0

Figure 5 shows the Henkel plots of a various nanocrystalline Nd12-xYxFe81B6Nb ribbons with x=0,1.0,3.0,4.5.In practice,based on its unique characteristics of NdFeB alloy,the soft magneticα-Fe grains are completely exchange coupled with the hard magnetic grains if their grain size does not exceed a certain limit which is approximately equal to the Bloch wall width (δB) of the hard magnetic phase [ 25, 26, 27] ,and can lead to exhibiting a weak exchange coupling effect without any added elements.For Y-containing samples,a larger positive derivation ofδM(H) indicates that intergranular interactions are dominated by exchange and much stronger than that of the initial alloy.Corresponding to its performance,when the Y concentration is 1.0 at%and 4.5 at%,δм(H) is more positive,as the change in hard phase promotes the demagnetization of the two-phase structures,leading to the abrupt enhancement of remanence and magnetic energy product.

3.4 Thermal stability

It is noted that Y2Fe14B has a positive temperature coefficient of anisotropy field over a wide temperature range [ 28] ,which suggests that Y is a potential candidate for enhancing the thermal stability of the Nd-Y-Fe-B magnets.In this work,Y is infiltrated into NdFeB magnets through the process of bonding magnets,cylindrical mold with 10 mm in diameter and 7 mm in height was used and pressure of 0.6 MPa was applied to the powder.Ratio of height to diameter of the bonded magnets was 0.7,which was in the range of controllable error.And its effect on the thermal stability of bonded NdFeB magnets was studied systematically.

Fig.5 Henkel plots of a various nanocrystalline Nd12-xYxFe81B6Nb(x=0,1.0,3.0 and 4.5) ribbons

In order to probe into the thermal stability of magnets,the study began with the following two aspects:One is that the magnet was kept isolated at different time at 150℃and then the loss of magnetic flux was measured at different stages.The other is that the magnetic properties and temperature coefficients of magnets with different Y contents were compared at variable temperatures.

A simple experiment was performed to compare the aging loss of bonded magnets with different Y contents at150℃,as shown in Fig.6.The results show that all bonded magnets have significantly high flux losses after aging during different timeframes.It is likely that low-Y-bonded magnets have better heat resistance compared to Y-free samples.Hence,it should be noted that the flux loss of bonded magnets with a small amount of 0.5 wt%Yremarkably decreases in all test samples.And after keeping in the incubator for 350 h,the magnetic flux losses of x=0.5 and 1.0 ribbons are-2.124%and-2.466%,respectively,which exhibit a certain degree of reduction compared to-2.739%when x=0.This is may be mainly attributed to the special properties of the Y iron base.It is therefore concluded that the thermal stability could be significantly improved by the appropriate addition of Y.However,when the Y content is more than 1.0 at%,the heat resistance of the magnet is deteriorated,and the total flux loss gradually increases with Y content increasing.This may be caused by a significant decay of coercivity,which is due to that the irreversible loss of magnetic flow was strongly affected by Hcj [ 29] .

Figure 7a,b shows the temperature dependence of the remanence and intrinsic coercivity for the samples with different Y contents (x=0,0.5,1.0,3.0,5.5) from 298 to423 K,respectively.It is easy to observe that the remanence and coercivity of all samples exhibit a linear dcrease with temperature changing.The reversible temperature coefficients of remanence (α) and coercivity (β) of the magnets with different Y contents in the temperaturerange of 298-423 K based on the data in Fig.7a,b are shown in Fig.7c.

Fig.6 Flux loss of magnetic flow for samples with different amounts of Y addition after exposure up to 150℃for variable time

Fig.7 Temperature dependence of a remanence and b intrinsic coercivity for samples with different amounts of Y content,c temperature coefficients of remanence (α) and coercivity (β) of magnets from 298 to 423 K

It could be seen that the temperature dependence of the remanence and intrinsic coercivity is both linearly reduced.The values ofαare improved from-0.40 to-0.32%·℃-1 in the temperature range of 298-423 K by adding 0 wt%-5.5 wt%Y,and the correspondingβvalues are improved from-0.216 to-0.157%.℃-1.This is attributed to the weakened temperature dependence of the magnetocrystalline anisotropy and the magnetization as Ypreferably substituted into the 2:14:1 phase [ 30] .

4 Conclusion

The magnetic properties,microstructure and thermal stability of the bonded magnets with Y addition were systematically investigated.By adding a small amount of Y,the coercivity shows a linear decline trend with the Ysubstitution increasing,while the remanence and magnetic energy product of the rapidly quenched samples exhibit irregular changes,and the greatest magnetic properties of Br=0.88 T and (BH)max=118.52 kJ·m-3 are obtained where the Y atomic weight is 1.0 at%in annealed ribbons,which enhances by 12.05 kJ·m-3 compared to that of Y-free samples.The improvement in magnetic performance could be attributed to the strengthened exchange coupling between the neighboring nanograins in Nd-Y-Fe-B melt-spun powder based on the Henkel curves.Moreover,the thermal stability of bonded magnets is effectively improved simultaneously.Bothαandβof the magnets with Y addition are smaller than those of the initial magnets in the temperature range of 298-423 K.The values ofαare improved from-0.40 to-0.32%.℃-1 by adding 0 wt%-5.5 wt%Y,and the correspondingβvalues are improved from-0.216 to-0.157%·℃-1.In addition,the flux losses of magnets with x=0.5 and 1.0 are-2.124%and-2.466%,respectively,which exhibit a certain degree of reduction compared to-2.739%with x=0 after being exposed at 150℃for 350 h.

This work proves that Y enters lattice of the Nd2Fe14Bphase as an additive,and Y might be a potential candidate for preparing low-cost commercial-grade permanent magnets with improved thermal stability in the future.

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