Local structure of amorphous and microcrystalline Fe-B alloys

Amorphous and microcrystalline Fe-B alloys in the composition range (4–25) at % B, fabricated by melt spinning, were investigated by pulse nuclear magnetic resonance (NMR) on ¹¹B nuclei at 4.2 K. Alloy samples were prepared both from a natural mixture of isotopes and an isotope mixture ⁵⁶Fe-¹¹B. The NMR spectra were measured at different boron contents. The local atomic structure of amorphous Fe-B alloys has been determined. The amorphous alloys consist of microregions (clusters) with short-range order of the tetragonal and orthorhombic Fe₃B-phase types, as well as of the α-Fe type.     

<Superscript>57</Superscript>Fe NMR study of amorphous and rapidly quenched crystalline Fe-B alloys

Amorphous and crystalline Fe-B alloys (5–25 at % B) were studied using pulsed ⁵⁷Fe nuclear magneticr esonance at 4.2 K. The alloy samples were prepared from a mixture of the ⁵⁷Fe and ¹⁰B isotopes by rapid quenching from the melt. In the microcrystalline Fe-(5–12 at %) B alloys, the resonance frequencies were measured for local states of ⁵⁷Fe nuclei in the tetragonal and orthorhombic Fe₃B phases and also in α-Fe. The resonance frequencies characteristic of ⁵⁷Fe nuclei in α-Fe crystallites with substitutional impurity boron atoms in the nearest neighborhood were also revealed. In the resonance frequency distribution P(f) in the amorphous Fe-(18–25) at % B alloys, there are frequencies corresponding to local Fe atom states with short-range order of the tetragonal and orthorhombic Fe₃B phases. As the boron content decreases below 18 at %, the P(f) distributions are shifted to higher frequencies corresponding to ⁵⁷Fe NMR for atoms exhibiting a short-range order of the α-Fe type. The local magnetic structure of the amorphous Fe-B alloys is also considered.     

Short-range order in amorphous ferromagnetic Fe-B alloys

Amorphous and quenched crystalline Fe-B alloys in the composition range of 4–25 at % B were prepared by melt spinning and investigated by ⁵⁷Fe Mössbauer spectroscopy at T = 87 K. The states of iron atoms in the α-Fe phases, including iron atoms having boron atoms in the nearest coordination sphere, and in the orthorhombic (o) and tetragonal (T) Fe₂B phases are detected in the microcrystalline alloys. The short-range order and the local atomic structure of the amorphous Fe-B alloys are determined. The amorphous alloys consist of microregions (clusters) with short-range order of the t- and o-Fe₂B and α-Fe types. The dependence of the content of various types of clusters on the alloy composition is quantitatively estimated.     

Atomic and magnetic structures of Fe<Subscript>70</Subscript>Al<Subscript>5</Subscript>B<Subscript>25</Subscript> amorphous alloys

The short-range order around boron, aluminum, and iron atoms in Fe₇₅B₂₅ and Fe₇₀Al₅B₂₅ amorphous alloys has been studied by ¹¹B and ²⁷Al nuclear magnetic resonance at 4.2 K and ⁵⁷Fe Mössbauer spectroscopy at 87 and 295 K. The average magnetic moment of iron atoms μ(Fe) in these alloys has been measured by a vibrating sample magnetometer. It has been revealed that the substitution of aluminum atoms for iron atoms does not disturb μ(Fe) in the Fe₇₀Al₅B₂₅ alloy, gives rise to an additional contribution to the ¹¹B NMR spectrum in the low-frequency range, and shifts maxima of the distribution of hyperfine fields at the ⁵⁷Fe nuclei. In the Fe₇₀Al₅B₂₅ amorphous alloy, the aluminum atoms substitute for iron atoms in the nearest coordination shells of boron and iron atoms. This alloy consists of nanoclusters in which boron and iron atoms have a short-range order of the tetragonal Fe₃B phase type.     

Mössbauer study of Fe-B alloys containing Nd additives

The influence of a few percent neodynium additives to Fe-B alloy on the hyperfine parameters was investigated. It was found that Nd decreases the hyperfine field of amorphous Fe-B alloys. The samples annealed at temperatures between 600°C and 700°C contain α-Fe and Fe₃B phases. The average hyperfine field of Fe₃B phase increases with increasing Nd content and decreases with increasing annealing temperature. The average isomer shift of Fe₃B phase decreases with increasing Nd content. The experimental data show that the change of hyperfine parameters of the Fe₃B phase in the studied alloys is due to Nd atom.     

Local structure of Fe70Cr15B15 X-ray amorphous alloy

The local atomic and magnetic structure of Fe₇₀Cr₁₅B₁₅ X-ray amorphous alloy is studied by means of ¹¹B nuclear magnetic resonance (NMR) and ⁵⁷Fe Mössbauer spectroscopy. It is determined that Fe₈₅B₁₅ and Fe₇₀Cr₁₅B₁₅ X-ray amorphous alloys consist of microregions (nanocrystals) with short-range orders of t-Fe₃B and α-Fe phases. It was found out that chromium atoms in the Fe₇₀Cr₁₅B₁₅ X-ray amorphous alloy are evenly distributed in these two nanocrystals, forming t-(Fe,Cr)₃B and α-Fe(Cr) phases.     

Microstructure and magnetic properties of a two-phase alloy of α-Fe and metastable Fe3B

Applying the hypercooling technique, the metastable-phase Fe₃B, instead of the stable-phase Fe₂B, is formed directly in the bulk Fe-B eutectic alloy melt and can be further preserved at room temperature. Measurement of magnetic properties shows that, for the bulk Fe-B eutectic alloy with Fe₃B phase, the intrinsic coercivity and retentivity become smaller, and the saturation magnetization is larger, than the stable eutectic alloy (α-Fe/Fe₂B) and some Fe-B amorphous alloys.     

Crystallization of Fe-B amorphous alloy powders produced by chemical reduction

The crystallization behaviour of the Fe?B amorphous alloy powders prepared by the chemical reduction method has been investigated by Mössbauer spectroscopy. In comparison to amorphous ribbons prepared by melt-spinning, a different crystallization behaviour has been observed. After annealing the amorphous samples entirely crystallized into three crystalline phases: α-Fe, Fe₃B, and Fe₂B. In the case of Fe₈₀B₂₀ amorphous alloy ribbons produced by melt-spinning technique eutectic crystallization is commonly observed and results in the crystalline phases: α-Fe and Fe₃B. This kind of crystallization was not observed in the chemically prepared samples. The metastable tetragonal Fe₃B phase transformed completely into α-Fe and Fe₂B after annealing at 973 K for one hour.     

纳米超微晶Fe73.5Cu1Mo3Si13.5B9合金显微结构和磁性的研究

对纳米晶Fe_73.5Cu₁Mo₃Si_13.5B₉合金的原始制备态和各退火态样品进行了室温Mossbauer谱研究,结果表明晶化态的合金存在α-Fe(Si)微晶相和晶界的非晶相。晶相和非晶相内场和面积随退火温度的变化是退火时Cu,Mo,B等成分的扩散和在各相中的再分配引起的。最佳磁性能对应非晶相中的铁量占合金铁总量的30％左右,超微晶合金的双相无规各向异性模型表明,一定量的非晶相对保持纳米晶优异的软 关键词：     

Short range order of the amorphous Fe−B powders prepared by borohydride reduction

Ultrafine Fe?B amorphous alloy powders were prepared by reducing Fe²⁺ ions using KBH₄ and NaBH₄ in aqueous solution. Adjusting technological factors, the amorphous powders around the composition of Fe₆₅B₃₅ can be easily obtained, but in the vicinity of eutectic point (Fe₈₀B₂₀) a certain amount of α-Fe often appears in the samples. From the Mössbauer spectrum, the crystallization products of the Fe₆₃B₃₇ amorphous powder are α-Fe and Fe₂B phases. The measurement of¹¹B spin echo nuclear magnetic resonance (NMR) at 8K showed that Fe₂B-like and Fe₃B-like short range orders (SRO) exist in the amorphous powder of Fe₇₆B₂₄.     

Experimental evidences of clusters with different short range order in amorphous alloys

⁵⁶Fe,⁵⁷Fe,¹⁰B and¹¹B isotopes were used for binary alloys. The signals of B (40,5 MHz) and Fe (43 MHz) from α-Fe in binary Fe−B crystalline and amorphous alloys were found besides the signals of these nuclei in t-and o-phase or clusters like these phases. The NMR on (51)V impurity nuclei in Fe−B alloys was used as well. Only amorphous Fe-(^<15 at.%B) alloys had the clusters with o-, t-Fe₃B and α-Fe short range order.     

Effect of boron in Fe 70 Al 30 nanostructured alloys produced by mechanical alloying

The substitution of aluminum by boron in the Fe₇₀Al₃₀ system prepared by high energy ball milling is studied when the B content ranged from 0 up to 20 at. %, and the milling times were 24, 48 and 72 h. X-ray diffraction (XRD) patterns of Fe₇₀Al₃₀ showed a predominant bcc structural phase with a lattice parameter larger than that of α-Fe. A second (tetragonal) phase arose with the addition of boron. It is associated to the existence of (Fe, Al)₂B, although the values of the lattice parameters are slightly different from those found in the literature. This phase shows high stability; its lattice parameters and the Mössbauer parameters do not show notable variations, either with milling time or composition. It was also evidenced that an increase of boron content and of milling time produced a decrease of the lattice parameter of the Fe-Al bcc structure. This is in agreement with the small atomic radius of boron in comparison with that of aluminum. This also allows boron to occupy interstitial sites in the lattice, increasing the grain size and giving rise to the ductile character of the alloy. On the other hand, 300 K transmission Mössbauer spectra (TMS) were fitted, for low boron concentrations (<8 at.%), with a hyperfine field distribution (HFD) associated with the bcc phase. For high boron content (≥8 at.%), a magnetic component related to the tetragonal phase is added and its broadened lines are attributed to the disordered character of Fe₂B, probably induced by the milling process.     

Local atomic and magnetic structures of nanocrystalline Fe75Cr10B15 alloy

The crystal, local atomic and magnetic structures of Fe₇₅Cr₁₀B₁₅ alloys annealed at 440?C473°C for 5 min have been studied using X-ray diffraction and ⁵⁷Fe M?ssbauer spectroscopy. At the annealing temperature T _a = 440°C, nanocrystals of the ??-Fe phase (??1%) precipitate in the amorphous matrix of the alloy. The complete crystallization of the amorphous alloy occurs at T _a = 473°C with the formation of ??-Fe nanocrystals 26 ± 2 nm in size and nanocrystals of tetragonal boride t-Fe₃B 47 ± 2 nm in size. It has been found that chromium atoms are located in nanocrystals of the ??-Fe and y-Fe₃B types. The distribution functions of hyperfine fields in the nanocrystalline Fe₇₅Cr₁₀B₁₅ alloy reconstructed from the M?ssbauer spectra (at T _a = 473°C) show that there are three allowed states of iron atoms in the ??-Fe phase and three equally probable crystallographic nonequivalent states of iron in the t-(Fe,Cr)₃B phase. The chromium concentration x in the ??-Fe(Cr) phase is found to be ??10 at %. The substitution of chromium atoms for iron atoms in t-Fe₃B substantially decreases local magnetic moments of the iron atoms.     

非晶合金Fe78Si9B13在脉冲电流作用下的单相晶化

对非晶合金Fe₇₈Si₉B₁₃进行了超短脉冲电流处理，实现了晶化时α-Fe（Si）单相结构析出．可以认为，脉冲电流作用时，电子运动与非晶中空位型结构缺陷间的周期性排斥效应促进了类金属原子从非晶结构单元中析出，使Fe（Si）原子局部富集，导致基体金属相在较低温度下优先成核．而在空位的定向迁移的同时，将伴随B原子的扩散，则B原子局域富集，Fe-B化合物的形核析出就要受到这两个因素的抑制 关键词：     

Some applications of NMR to the study of magnetically-ordered materials with emphasis on the short-range order in (Fe-B)-based crystalline and amorphous alloys

In this review, we summatize recent developments in nuclear magnetic resonance (NMR) studies on (Fe-B)-based crystalline and amorphous alloys, focusing on the application of NMR in identifying the existence of short-range order (SRO), determining the types of SRO, characterizing the behavior of the SRO and exploring the effect of the SRO on the magnetic properties for the Fe-B system. NMR experiments reveal that certain local environments surrounding the B atoms exist in both crystalline and amorphous Fe-B alloys. The type of SRO existing in this rapidly quenched system can be either o-Fe₃B or bct-Fe₃B, or a mixture, depending on the composition and processing factors, especially the carbon content and quenching speed. The SRO originates from a strong covalent bonding between the B and Fe atoms. As this interaction plays the same role in both crystalline and amorphous Fe-B alloys, the SRO which occurs in the amorphous Fe-B alloys is similar to the SRO which exists in their crystalline counterparts. NMR, in combination with magnetization measurements, provides evidence indicating that the SRO existing in the amorphous Fe-B alloys has a significant effect on their soft magnetic properties and that different types of SRO may act differently, thus providing an opportunity to improve the magnetic properties by changing the SRO. In connection with reviewing the achievements of NMR studies in recent years, brief comments concerning the advantages and potential of NMR experiments in the investigation of other magnetically-ordered materials will also be presented.     

A combined NMR and Mössbauer study of SRO in Fe80B20−x C x,amorphous alloys

The short range order (SRO) of amorphous Fe₈₀B_20?xC_x (X=0, 2, 4, 7, 9) alloys has been investigated by means of Mössbauer effect (ME) and nuclear magnetic resonance (NMR). Both the amorphous samples and the samples annealed at lower temperatures (693–713K) for a short time (10 minutes) were used. The Mössbauer measurement shows that the experimental spectra of annealed samples consist of the subspectra of α-Fe and bct-Fe₃B for X=0, While of α-Fe, bct-Fe₃B and o-Fe₃ (B, C) for X=2, 4, 7, 9. Moreover, the relative intensities of the subspectra originating from o-Fe₃ (B, C) increases with carbon concentration. These results are consistent with those of NMR very well.     

CEMS investigation of near surface structure

Conversion Electron Mössbauer Spectroscopy (CEMS) studies are reported for as-cut and laser melted surfaces of single phase crystalline Fe₂Y, Fe₂₃Y₆, Fe₂Zr, Fe₂B and FeB ingots. Disorder and the appearance of a new phase with a low value of the room temperature hyperfine field was observed for the Fe?Y and Fe₂Zr ingots even on the as-cut surfaces due to the mechanical processing. In case of these ingots surface melting by ns laser pulses resulted in the formation of amorphous alloys. In case of the Fe?B ingots the formation of amorphous phase by laser melting was observed for Fe₂B only, while in case of FeB the low temperature α-FeB modification appeared both, for mechanical processing and laser melting.     

Hyperfine and X-ray investigations of amorphous Fe2Er and Fe2Ce alloys and the effect of hydrogenation on short-range order

The effect of hydrogenation on the short-range order of amorphous Fe₂Er and Fe₂Ce alloys has been investigated by Mössbauer, X-ray and magnetization measurements. The hydrogenation leads to drastic changes in the short-range order. The results of Mössbauer measurements show two different distributions of magnetic hyperfine fields for amorphous Fe₂CeH₄ alloys (a-Fe₂CeH₄). For a-Fe₂ErH₃ alloys we found drastic changes in magnetic structure, which is different from the well-known magnetic structures.     

热处理对非晶态合金Nd4Fe96-xBx磁性的影响

本文研究了用单辊急冷方法制备的非晶态合金Nd₄Fe_96-xB_x的晶化,以及热处理对其硬磁性和相组成的影响,发现非晶态合金Nd₄Fe_96-xB_x的晶化温度比相同B含量的非晶态合金Fe_100-xB_x高120—190K,X射线衍射和热磁测量表明,15≤x≤25的样品晶化相是由Nd₂Fe₁₄B(T 关键词：     

Research on preparing compact bulk nanocomposite Nd2Fe14B/α-Fe magnetic materials by hot extrusion

In this paper, compact bulk nanocomposite Nd₂Fe₁₄B/α-Fe magnetic materials were prepared by hot extrusion of amorphous and nanocrystalline powders, which were prepared by high-energy ball-milling (HEBM) of the Nd₂Fe₁₄ B-type hard magnetic phase with 20 vol% of α-Fe as soft magnetic phase. The extrusion temperature has important influence on magnetic properties and microstructure of magnetic materials. The results show that the grain size of Nd₂Fe₁₄B and α-Fe phase increases steadily with increasing extrusion temperature. Furthermore, optimal extrusion temperature of 1223 K occurs, at which the highest magnetic properties and relative density can be obtained.