A first principles calculation of site occupancy of the B2 phase in Ti2AlX (X=V,Cr, Fe,Mo, Ta,Nb, Zr,Hf and Re) intermetallics

The site occupancy of the B2 phase in Ti₂AlX (X=V, Cr, Fe, Mo, Ta, Nb, Zr, Hf and Re) intermetallics have been studied using first principles pseudo potential plane wave method.The Ti, Al and X atoms are arranged in five different ways, in the lattice sites corresponding to B2 structure of Ti₃Al. In Ti₃AlX, the X atoms are substituted at the Ti and / or Al sites. Further, the equilibrium lattice constants and the formation energy (E_for) of these intermetallics with different site occupancies in the B2 phase have been predicted. The formation energy values suggest that the B2 phase is stable in all alloys. Amongst the five cases in a particular alloy, stable configuration is identified with the minimum E_for and is further considered for the calculations of mechanical properties. All the alloys are mechanically stable in terms of Born stability criteria and show anisotropic behaviour. All the alloys display ductile behaviour in terms of G/B ratio.     

Magnetocrystalline anisotropy of Ni and Mn substituted Nd2Fe14B compounds

Measurements of the effects of Ni and Mn substitutions for Fe on the magnetic properties of Nd₂Fe₁₄B compounds are reported. The Curie temperature is slightly increased with Ni substitution whilst in the case of replacement of Fe by Mn it is reduced drastically. A monotonic decrease of both the lattice parameters a and c is observed. The saturation magnetization is decreased by both Ni and especially Mn substitutions. The composition dependence of both the reorientation spin transition temperature and the cone angle has been measured. The influence of the 3d metal substitution on the Nd anisotropy has been measured and discussed. The composition dependence of the room temperature anisotropy field values, which is an important figure of merit for permanent magnet applications, decreases slightly in the case of Ni and drastically for Mn substitution. A comparison with the case of Co substitution has been made.     

Positive muon spectroscopy of Nd2Fe14B and Pr2Fe14B

We have performed positive muon spin rotation measurements on polycrystalline samples of Nd₂Fe₁₄B and Pr₂Fe₁₄B in zero applied field. In both samples a single sharp μSR line was observed which was unexpected in this complicated structure. The temperature dependence of the muon frequency for Nd₂Fe₁₄B clearly reflects the spin reorientation below 150 K and can be explained qualitatively by assuming that only the c-axis component of a magnetization is sampled by the muon. A smooth decrease of the muon frequency with increasing temperature is observed for Pr₂Fe₁₄B.     

Atomistic simulation for the phase stability, site preference and thermal expansion of YFe12−xTx (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W)

The effect of the ternary element on the structural properties of YFe_12−xT_x (T=Ti, V, Cr, Mn, Zr, Nb, Mo, W) has been studied by using interatomic pair potentials based on ab initio method and lattice inversion equations. Calculated results show that adding ternary element T makes the crystal cohesive energy of YFe_12−xT_x decrease markedly, which indicates that T helps stabilize the 1:12 phases. The ternary elements T prefer to occupy the 8i sites in these compounds. The calculated results are found in good agreement with the values deduced from experiments. Furthermore, we have calculated the thermal expansion of YFe₁₁V compounds with the ThMn₁₂ structure. The method utilized in the present investigation offers a rather easy and direct way to study the structural properties of YFe_12−xT_x.     

Mössbauer study of Nd2Fe14B and Nd2(Fe1−xCox)14B compounds

The Mössbauer spectra of Nd₂Fe₁₄B and Nd₂(Fe_1?xCo_x)₁₄B compounds (x=0.0.05, 0.10 and 0.16) have been investigated at room temperature and at 77 K. Taking advantage of combined ME and NMR investigations of the Nd₂Fe₁₄B compund, the hyperfine field values and their assignment to the six Fe sites have been determined to be the following sequence: 378(j₂), 346(k₂), 334(j₁), 325(k₁), 322(c), 306(e) kOe. The substitution of Co for Fe decreases the hyperfine fields at all Fe sites. The intensity variations of the subspectra with Co content show that Co atoms have a strong preference to occupy the k₂ site, but have a rather less tendency to enter the j₂ site, which is preferred by Fe atoms.     

钕铁硼的冲击压缩特性

 利用二级轻气炮对恒磁体钕铁硼进行冲击压缩,采用阻抗匹配法进行测量,获得了平均初始密度为7.346 g/cm³的钕铁硼Hugoniot关系数据。实验结果表明,该种钕铁硼在19~78 GPa范围内,其D-u_p满足线性变化关系,即：C₀、λ分别为3.686 km/s、1.059,是一种稳定的压缩过程,其间没有相变产生。而较小的λ值表明该种钕铁硼材料偏向于疏松体结构,且容易被压缩。同时实验结果也为其状态方程和脉冲功率源等方面的研究工作提供了可资参考的实验参数。     

Magnetic and Structural Properties of the Mechanically Alloyed Nd2(Fe100 − x Nb x )14B System

In this work we report the magnetic and structural properties obtained by Mössbauer spectrometry, Vibrating Sample Magnetometer and X-ray diffraction of milled powders with initial composition Nd₂(Fe_{100 ? x} Nb _x )₁₄B with x = 0 and x = 4. The mixtures were ball milled for different times up to 240 h. Structural and microstructural parameters were derived from high statistics X-ray patterns and discussed as a function of milling time. The Mössbauer spectra of the samples were fitted by means of a sextet and an hyperfine field distribution, associated to a pure iron phase (α-Fe) and a disordered iron-based phase, respectively. The α-Fe grain size decreases from 50 nm for 6 h up to 5 nm for 240 h milling time. The Vibrating Sample Magnetometer results allow to conclude that these samples behave as soft ferromagnets.     

Magnetization process related to the singular point detection signal observed at high fields on Pr2Fe14B and Nd2Fe14B

The singular point detection signal, d²M/dt², observed on Pr₂Fe₁₄B and Nd₂Fe₁₄B at high fields and low temperatures is explained on the basis of one sublattice crystal field model. The signal is attributed to a steeper reversible magnetization process and a first order magnetization process for Pr₂Fe₁₄B and Nd₂Fe₁₄B respectively.     

Magnetic Characteristics of Ball-Milled Nd2Fe14B - Gd

Mössbauer measurements using ⁵⁷Fe and ¹⁵⁵Gd have been made on samples of ball-milled Nd₂Fe₁₄B in a gadolinium matrix in order to determine the magnetisation of each phase separately. Although magnetometer measurements did not show any coercivity, both measurements showed the phases were in intimate magnetic contact. The spectra of each phase differed considerably from those of the respective original materials and provide information on the conduction electron polarisation through the transferred hyperfine fields.     

Phase stability and site preference of Sm(Fe,T)12

The structure of intermetallics Sm(Fe,T)₁₂ is analyzed via a quasi-ab initio pair potentials Φ_Fe–Fe(r), Φ_Sm–Fe(r), Φ_Sm–Sm(r), Φ_Sm–T(r), Φ_Fe–T(r) and Φ_T–T(r). The calculation results show that each of Cr, V, Mo and Ti significantly decreases the cohesive energy of Sm(Fe,T)₁₂, and thus stabilizes its structure of ThMn₁₂. The calculated lattice constants coincide quite well with experimental values. The sequence of site preference occupation is 8i, 8j and 8f, with the 8i occupation corresponding to the greatest energy decrease. The calculated results also show that each of Co, Cu, Ni and Sc does not stabilize the system with the structure of ThMn₁₂. The calculated crystal structure can recover after either an overall wide-range macro-deformation or atomic random motion, demonstrating that an Sm–Fe–T system has the stable structure of ThMn₁₂. The crystal space group remaining consistent at different temperatures is also shown in this paper. All of the results verify that the first principle potentials based on the lattice inversion technique are effective.     

Mössbauer spectra of Nd2Fe14B and related compounds

Mössbauer spectra and ac susceptibility data on Nd₂Fe_{1 4} B with substitution of Pr or Sm for Nd indicate that the spinreorientation temperature is depressed by Pr substitution but increased in alloys containing Sm so that it occurs at roomtemperature for Nd_{1 2}Sm₅Fe_{7 6}B₇. In the Nd end-member, the onset of the transition appears about 210 K although the main reorientation is at 134 K. Nickel shows no marked site preference: its effects are to increase slightly the Curie temperature and reduce the iron moment.     

Cr、Mn、Co、Ni掺杂对Al13Fe4相稳定性和力学性能影响的第一性原理研

采用基于密度泛函理论的第一性原理计算方法,系统地研究了不同掺杂浓度下过渡族元素Cr、Mn、Co、Ni在Al13Fe4相中的占位情况、结构稳定性和机械性能. 计算得到所有的Al13(Fe24-xMx) (M=Cr、Mn、Co、Ni;X=1,2,4)相都具有良好的热力学稳定性和机械稳定性. 相同掺杂浓度化合物的形成焓按如下顺序减小：Al78(Fe24-xCrx) > Al78(Fe24-xMnx) > Al13Fe4 > Al78(Fe24-xNix) > Al78(Fe24-xCox).形成焓的降低增加了Al13Fe4相成核驱动力,Co和Ni有利于促进Al-Fe合金中Al13Fe4相形核,细化Al13Fe4相. 过渡族元素可以改善金属间化合物的脆性,增强塑性变形能力. 并且随着掺杂浓度的增加,过渡族元素的加入对脆性的改善呈先增大后减小的趋势.     

辐照诱发中间相Zr(Cr,Fe)2非晶化的原位电子显微研究

 在美国Argonne国家实验室连接有IVEM-Tandem National Facility加速器的Hitatch3000电子显微镜上，通过能量600keV及注量率为2.5×10¹²cm^-2·s^-1的Ne离子原位辐照,研究了锆-4合金中沉淀相hcp Zr(Cr,Fe)₂的重离子辐照效应，结果表明：600keV的Ne离子在350℃原位辐照至0.2dpa时，hcp-Zr(Cr,Fe)₂沉淀相的层错条纹开始消失；0.8dpa时沉淀相的电子衍射斑点大部分消失，发生明显的化学无序；2.7dpa时明场像衬度完全消失，非晶环已非常明显，表明沉淀相已经非晶化。600keV的Ne离子在350℃辐照hcp-Zr(Cr,Fe)₂相非晶化的临界损伤离位率约为2.7dpa。     

Magnetic entropy change in perovskite manganites La0.65Nd0.05Ca0.3Mn0.9B0.1O3 (B=Mn,Cr, Fe)

We have investigated the effect of B-site dopant on magnetic entropy change in perovskite manganite samples of La_0.65Nd_0.05Ca_0.3MnO₃, La_0.65Nd_0.05Ca_0.3Mn_0.9Cr_0.1O₃, and La_0.65Nd_0.05Ca_0.3Mn_0.9Fe_0.1O₃ prepared by sol–gel technique. The maximum ΔS_H is in the order of −1.68 J/kg K and peaks at Curie temperature for La_0.65Nd_0.05Ca_0.3MnO₃ upon 10 kOe applied field change. For the sample with B-site dopant, a decrement of the maximum magnetic entropy change has been observed.     

Mössbauer study of the nanocrystallization of the amorphous system Fe73.5Si13.5B9Cu1Nb1X2 with X=Nb, Mo, V and Zr

Amorphous to crystalline transformation after isochronal annealing of as-quenched ribbons with the composition Fe_73.5Cu₁Nb₁Si_13.5B₉X₂ (X = Zr, Nb, Mo, V) was investigated by the combined use of differential scanning calorimetry and Mössbauer spectroscopy. Alloying raises the thermal stability in the order V < Mo < Nb < Zr. The fraction of the Mössbauer subspectra corresponding to the nanophase for the same annealing conditions decreases in the order V > Mo > Nb > Zr.     

Magnetic and structural properties of the Nd2(Fe100−x Nb x )14B system prepared by arc melting

In this work the magnetic and structural properties are investigated by Mössbauer spectrometry, Vibrating Sample Magnetometry and X-ray diffraction of Nd₂(Fe_100?x Nb _x )₁₄B powdered alloys with x?=?0, 2 and 4 prepared by arc melting. The Mössbauer spectra of the samples were fitted with several contributions from: Nd₂Fe₁₄B, α-Fe and a paramagnetic phase associated with Nd_1.1Fe₄B₄ for x?=?0 and additionally from NbFeB and Nd₂Fe₁₇ for x?=?2 and x?=?4. The relative fractions of α-Fe and Nd₂Fe₁₄B are smaller for x?=?4 than for x?=?0, indicating that the amount of these two phases is reduced with increasing Nb content, while the relative fraction of Nd₂Fe₁₇ increases. The α-Fe grain size slightly decreases while that of the Nd₂Fe₁₄B phase is increasing, when the Nb content increases. The hysteresis loops indicate that these samples behave as hard ferromagnets, with a coercive field which decreases when the Nb content increases, but with rather low remanent magnetization.     

Satellites,hypersatellites and RAE from Ti,V, Cr,Mn and Fe in photoionisation

K x‐ray satellites, hypersatellites and lines due to radiative Auger effect (RAE) of Ti, V, Cr, Mn and Fe were measured after exciting the samples with Ag bremsstrahlung at 35 kV. All the lines were measured with the ‘Spectroscan VY’ spectrometer of Spectron—OPTEL, Russia, in which a curved LiF(200) crystal was used. The spectra were deconvoluted using Voigt functions, and the peak positions of the satellites and hypersatellites were determined with errors of ± (1–5) eV. The energy shifts of the satellite lines with respect to their parent lines were also obtained. Multi‐configuration Dirac–Fock (MCDF) calculations with the inclusion of higher‐order corrections were carried out to predict the peak positions of the satellite and hypersatellite lines. Our data were then compared with our own calculated values and also with the data of others. Our measured energy shifts for K_αL¹, K_αL², K_α^h and K_βL¹, when compared with our MCDF calculations, show a maximum deviation of 8, 12, 2 and 20% respectively. Copyright © 2008 John Wiley & Sons, Ltd.