Study of nanocrystalline and amorphous powders prepared by mechanical alloying

The process of mechanical alloying consists of intimate mixing and mechanical working of elemental powders in a high-energy ball mill. It has been well established that this process is able to produce nanocrystalline and amorphous material. In this study, the structural effects of mechanical alloying of pure Fe, Fe₅₀W₅₀ and Fe₅₀Mo₅₀ powders were investigated by X-ray diffraction and Mössbauer spectroscopy. For all cases, nanocrystalline and/or amorphous fractions were found after milling. The resulting particle size was determined by X-ray diffraction. Pure Fe does not amorphize even after prolonged milling times. For the nanocrystalline powder, significant changes in the linewidth and the hyperfine field are found. Powder mixtures of Fe₅₀Mo₅₀ and Fe₅₀W₅₀ are completely amorphous after milling times of 10 h, as seen by Mössbauer spectroscopy, but nanocrystalline fractions of the non-iron part are still found in X-ray diffraction. Also in the amorphous state, further changes in the hyperfine parameters are found with increasing milling time.     

Synthesis and characterization of in situ nanocrystalline intermetallic phase reinforced AlTiSi amorphous matrix composite

Composites with partially amorphous matrix were synthesized by mechanical alloying of an Al₅₀Ti₄₀Si₁₀ elemental powder blend in a high energy planetary ball-mill, followed by high pressure (8 GPa) low temperature (350–450°C) sintering. Microstructural studies and compositional micro-analysis were carried out using scanning and transmission electron microscopy, and energy dispersive spectroscopy, respectively. Phase evolution as a function of milling time and isothermal temperature and their thermal stability was determined by X-ray diffraction at room or elevated temperature and differential scanning calorimetry, respectively. The microstructure of composites sintered between room temperature and 450°C showed nano-size (≈50 nm) crystalline precipitates of Al₃Ti dispersed in an amorphous matrix. The composites sintered at 400°C with 8 GPa pressure exhibited the highest density (3.58 Mg/m³), nanoindentation hardness (8.8 GPa), Young's modulus (158 GPa) and compressive strength (1940 MPa). A lower hardness and modulus on sintering at 450°C is attributed to additional amorphous to nanocrystalline phase transformation and partial coarsening of Al₃Ti.     

Structure evolution of a zirconium-based amorphous alloy upon megaplastic strain in the bridgeman chamber

The structure evolution of the Zr₅₀Ni₁₈Ti₁₇Cu₁₅ massive metal glass upon megaplastic strain (MPS) in the Bridgeman chamber is investigated in a wide interval of strain degrees at room temperature. It is demonstrated that upon MPS with n = 1/2 rotation of the Bridgeman anvil, nanocrystalline phases are precipitated in the alloy. With increasing strain, these phases are dissolved. It is suggested that nucleation centers of nanocrystalline phases (nanoclusters) are formed in the amorphous matrix upon MPS under compression without shear in the Bridgeman chamber. These phases grow in the shear bands transforming into the nanocrystalline phase upon MPS.     

Formation of boride phases from individual powder components during the mechanical synthesis of Ni<Subscript>80</Subscript>Mo<Subscript>7</Subscript>B<Subscript>13</Subscript> alloy

Ni₈₀Mo₇B₁₃ nanocrystalline alloy containing a Ni(Mo,B) face-centered cubic (FCC) solid solution of the substitutional-interstitial type was obtained by high-energy ball milling of the component mixtures. In the temperature range 400–700°C, the metastable solid solution Ni(Mo,B) decomposes, leading to the formation of metastable FCC Ni(Mo) and HCP MoB₄ phases. Upon isothermal annealing at 1000°C for1 h, the alloy transforms into the stable state and contains the equilibrium phases FCC Ni(Mo), cubic Ni₂₁Mo₂B₆, and orthorhombic Ni₃B.     

非晶FeMoSiB合金机械晶化的动力学机制

 研究了非晶(Fe_0.99Mo_0.01)₇₈Si₉B₁₃（FMSB）合金的机械晶化过程和机制,并讨论了局域高压的作用。结果表明：非晶FMSB合金的晶化过程及其产物与球磨强度和球磨时间有密切关系,在低能球磨FMSB非晶过程中,晶化相只有α-Fe(Mo,Si)固溶体,而在高能球磨过程中,除了α-Fe(Mo,Si)固溶体结晶相之外,还分别有(Fe,Mo)₃B和Fe₂B相析出。其晶化机制可归因于由碰撞引起的局域高压和局域高温共同作用的结果。实验结果还表明,机械球磨不仅对非晶FMSB的常压热晶化温度有重要影响,而且对其热晶化结果亦有重要影响。     

Formation and structure of nanocrystals in bulk Zr<Subscript>50</Subscript>Ti<Subscript>16</Subscript>Cu<Subscript>15</Subscript>Ni<Subscript>19</Subscript> metallic glass

The possible formation of a nanocrystalline structure in controlled crystallization of a bulk Zr₅₀Ti₁₆Cu₁₅Ni₁₉ amorphous alloy has been studied using differential scanning calorimetry, transmission and high-resolution electron microscopy, and x-ray diffraction. It was established that crystallization of the alloy at temperatures above the glass formation point occurs in two stages and brings about the formation of a nanocrystalline structure consisting of three phases. Local spectral x-ray analysis identified the composition and structure of the phases formed.     

High-frequency magnetic properties of soft magnetic cores based on nanocrystalline alloy powder prepared by thermal oxidation

FeSiBNbCu nanocrystalline alloy powder was thermally oxidized in an air atmosphere to enhance an oxide layer formation on the surface of the powder and subsequently toroidal shape FeSiBNbCu nanocrystalline alloy powder cores were prepared by compaction at room temperature. The phase change on the surface of FeSiBNbCu nanocrystalline alloy powder by thermal oxidation was analyzed and its effect on the high frequency magnetic properties of the compacted cores was investigated. By thermal oxidation, the formation of the oxide layer consisting of Fe₂O₃, CuO, and SiO₂ on the surface of FeSiBNbCu nanocrystalline alloy powder was enhanced and the thickness of oxide layer could be controlled by changing the thermal oxidation time. FeSiBNbCu nanocrystalline alloy powder core prepared from the powder treated by thermal oxidation exhibits a stable permeability up to high frequency range over 10 MHz. The core loss could be reduced remarkably and the dc-bias property could be improved significantly, which were due to the formation of oxide layer consisting of Fe₂O₃, CuO, and SiO₂ on the FeSiBNbCu nanocrystalline alloy powder. The improvement in high-frequency magnetic properties of the FeSiBNbCu nanocrystalline alloy powder cores could be attributed to the effective electrical insulation by oxide layer between the FeSiBNbCu nanocrystalline alloy powders.     

Room-temperature deposition of nanocrystalline PbWO 4 thin films by pulsed laser ablation

Pulsed laser ablation (PLA) was applied to synthesize nanocrystalline PbWO₄ thin films onto glass substrates. The effects of Ar background gas pressure on phase evolution, microstructures and optical characteristics of PbWO₄ thin films were investigated in detail. The PLA processes were carried out at room temperature without substrate heating or post-annealing treatment. XRD and HR-TEM results revealed that the PbWO₄ thin films are composed of nanocrystalline and amorphous phases. Moreover, the films contained a high density of lattice defects such as twin boundaries and edge dislocations. The crystallite size and crystallinity increased, which were associated with a change in surface morphology as the Ar pressure increased. Reduced tungsten states W⁵⁺ or W⁴⁺ induced by oxygen vacancies were observed at 10 Pa and the atomic concentration of all constituent element was almost stoichiometric, especially the [Pb]/[W] ratio, which was nearly unity above 50 Pa. The optical energy band-gap was 3.03 eV at 50 Pa and increased to 3.35 eV at 100 Pa, which are narrower than the reported value (4.20 eV). This optical band-gap narrowing could be attributed to localized band-tail states and new energy levels induced by the amorphous structure and inherent lattice defects. PACS 81.15.Fg; 78.20.-e; 68.55.-a; 73.22.-f     

Nanogranular Phase Formation During Devitrification of Fe(NiCo)ZrB Amorphous Alloys

A comprehensive review of our recent experimental and theoretical developments in the processing of nanocrystalline soft magnetic materials made by crystallization of amorphous precursors and containing new nanocrystalline phases is given. The relationship between the structures of the metastable and equilibrium phases and their transformations are discussed. Nickel-rich amorphous precursors with stoichiometry Ni₆₄Fe₁₆Zr₇B₁₂Au₁ were produced by melt-spinning technique and then heat-treated at temperatures ranged from 420 °C to 600 °C for one hour to form nanostructured alloy. The transformation from the amorphous state into the nanocrystalline state was investigated by the differential scanning calorimetry (DSC), the x-ray diffraction (XRD), the vibrating sample magnetometry (VSM) and Mössbauer techniques. The annealing favours the emergence of cubic Fe_xNi_23-xB₆ crystalline grains (10-25 nm in diameter). Magnetic measurements made at 4.2-1100 K reveal rather high value of saturation magnetization (nearly 60 and 40 Am²/kg at 4.2 K and room temperature, respectively) in amorphous as well as in nanocrystalline states. These facts are consistent with 300 K ⁵⁷Fe Mössbauer results which are well supported by the calculations of Ni and Fe magnetic moments in Ni₂₃B₆ and Fe₂₃B₆ phases, using the spin polarized tight binding linear muffin-tin orbital (TB-LMTO) method. However, anomalous high magnetic moments of Fe and Co atoms were found in some inequivalent positions in the crystal structure.     

Effect of thermal treatment on the microstructure and magnetic properties of a bulk amorphous Fe72Al5P10Ga2C6B4Si1 alloy

The structure and chemical composition of the phases that form in the controlled crystallization of a bulk amorphous Fe₇₂Al₅P₁₀Ga₂C₆B₄Si₁ alloy are studied by differential scanning calorimetry, transmission electron microscopy, and x-ray diffraction. It was established that, when the alloy is annealed at a temperature above the glass transition point, a nanocrystalline structure consisting of three phases arises. The magnetic properties of amorphous and nanocrystalline samples were studied with a vibrating-sample magnetometer. The coercive force and saturation magnetization of amorphous samples were found to be 1 Oe and 130 emu/g, respectively. The structure and chemical composition of the forming phases and their correlation with the magnetic properties of the samples were determined. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 12, 2004, pp. 2158–2163. Original Russian Text Copyright ? 2004 by Abrosimova, Aronin, Kabanov, Matveev, Molokanov, Rybchenko.     

Structural and magnetic properties of nanocrystalline bismuth manganite obtained by mechanochemical synthesis

We have studied the formation of BiMnO3 (BMO) nanocrystalline perovskite powder produced by high-energy milling of the constituent oxides. The crystal structure and the amount of crystalline and amorphous phases in the powder as a function of the milling time were determined with XRPD using Rietveld refinement. BMO perovskite formed directly from highly activated nano-sized constituent oxides after 240 min of milling and subsequently grew during prolonged milling. The morphology, structure, and chemical composition of the powder were investigated by SEM and TEM. A clear ferromagnetic transition was observed at T _C ~66 K for a sample milled for 240 min and increased with milling time. The magnetic hysteresis behavior is similar to that of a soft ferromagnet. The magnetic properties of the obtained BMO powders were found to change as a function of milling time in a manner consistent with variations in the nanocomposite microstructure.     

退火诱导机械合金化Fe42.5Al42.5Ti5B10合金的结构演变与晶粒生长动力学

利用XRD和TEM方法研究Fe_42.5Al_42.5Ti₅B₁₀合金在机械合金化及等温热处理过程中的结构演变及晶粒生长动力学,讨论了机械合金化合成机理和热处理过程中的晶粒生长机理.结果表明,球磨过程中Al,Ti,B原子向Fe晶格中扩散,形成Fe(Al,Ti,B)固溶体.机械合金化合成Fe(Al,Ti,B)遵循连续扩散混合机理.球磨50h后,金属Fe,Al,Ti,B已完全合金化,球磨终产物为纳米晶Fe(Al,Ti,B).球 关键词： XRD TEM 42.5Al_42.5Ti₅B₁₀合金')" href="#">Fe_42.5Al_42.5Ti₅B₁₀合金 机械合金化     

Nanostructured Y-Fe Alloys Produced by Ball Milling

Ball milling was used to produce nanostructured Y-Fe alloys. Depending on preparation conditions, nanocrystalline and amorphous components are formed to coexist. The transmission Mössbauer spectra exhibit YFe₂ and amorphous components. The influence of superparamagnetic YFe₂ particles was separated from the amorphous part by measuring at 77 K. The thermal stability of the samples and the growth of equilibrium phases was studied by annealing.     

Temperature dependence of the Mössbauer spectra of amorphous and nanocrystallized Fe86Zr7Cu1B6

Mössbauer measurements have been performed on amorphous and nanocrystalline alloy ribbons of nominal composition Fe₈₆Zr₇Cu₁B₆. The nanocrystalline samples were obtained by annealing the as-quenched alloy at different temperatures in the range between 650 and 870 K. Mössbauer spectra of the as-quenched amorphous sample have been recorded at 77 K, room temperature and above the Curie temperature (330 K) at 360 K. We have also performed Mössbauer measurements at room temperature in the nanocrystalline alloys to characterize the phases that appear after the annealing and their relative concentration. The as-quenched sample spectra reveal the existence of two inequivalent sites for Fe. Such a feature is also observed in the remaining amorphous phase of the annealed samples. In the first steps of crystallization, -Fe precipitates and its concentration increases with the annealing temperature. The experimental results suggest that the composition of the whole amorphous phase does not suffer large changes during crystallization.     

Preparation and dielectric properties of the lead-free BaFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics obtained from mechanically triggered powder

In the paper the influence of mechanical activation of the powder on the final dielectric properties lead-free Ba(Fe_1/2Nb_1/2)O₃ (BFN) ceramic was examined. The BFN ceramics were obtained by 3-steps route. Firstly, the substrates were pre-homogenized in a planetary ball mill. Than, the powder was activated in vibratory mill (the shaker type SPEX 8000 Mixer Mill) for different duration between 25 h and 100 h. The influence of the milling time on the BFN powder was monitored by X-ray diffraction. The diffraction data confirmed that the milling process of the starting components is accompanied by partial synthesis of the BFN materials. The longer of the high-energy milling duration the powders results in increasing the amount of amorphous/nanocrystalline content. The mechanically activated materials were sintered in order to obtain the ceramic samples. During this temperature treatment the final crystallisation of the powder appeared what was confirmed by XRD studies. The performed dielectric measurements have revealed the reduction of the dielectric loss of the BFN ceramics compared to materials obtained by classic methods.     

静高压下有表面化学反应的非晶合金晶化研究

在静高压3—5GPa,510—660℃温度下,研究了在晶化过程中其表面与Al发生反应的非晶(Fe_0.99,Mo_0.01)₇₈Si₉B₁₃合金的晶化过程。发现在4GPa左右,510—660℃的温度范围内,非晶FMSB晶化为纳米α-Fe(Al)相,在其他压力下,晶化为α-Fe(Mo,Si),(Fe,Mo)₃B或Fe₂B相。利用简单固体模型对其晶化的热力学机制 关键词：     

Room-temperature synthesis of apatite-type lanthanum silicates by mechanically milling constituent oxides

Apatite-type lanthanum silicates have been successfully prepared at room temperature by dry milling hexagonal A−La₂O₃ and either amorphous or low cristobalite SiO₂. Milling a stochiometric mixture of these chemicals in a planetary ball mill with a moderate rotating speed (350 rpm), allows preparing the target phase in only 6 h although longer milling times are needed to eliminate all SiO₂ traces. The mechanically activated chemical reaction proceeds faster when using amorphous silica than low cristobalite as silicon source and pure phases are obtained after only 9 and 18 h respectively. As obtained powder phases are not amorphous and show an XRD pattern as well as IR and Raman bands characteristic of the lanthanum silicate.     

Microstructure characterization of mechanosynthesized nanocrystalline NiFe2O4 by Rietveld's analysis

Nanocrystalline nickel ferrite (NiFe₂O₄) is synthesized at room temperature by high-energy ball milling the stoichiometric mixture of (1:1 mol%) of NiO and α-Fe₂O₃ powders. The structural and microstructural evolution of NiFe₂O₄ caused by milling is investigated by X-ray powder diffraction. The relative phase abundance, particle size, r.m.s. strain, lattice parameter changes of different phases have been estimated employing Rietveld structure refinement analysis of X-ray powder diffraction data. Particle size and content (wt%) of both NiO and α-Fe₂O₃ phases reduce rapidly with increasing milling time and a significant amount of nanocrystalline NiFe₂O₄ is formed within 1 h of ball milling. Particle sizes of all the phases reduce to ∼10 nm within 5 h of milling and remain almost unchanged with increasing milling time up to 20 h. Lattice parameter of cubic NiO decreases linearly with increasing milling time, following the Vegard's law of solid-solution alloy. A continuous decrease in lattice parameter of cubic NiFe₂O₄ phase clearly suggests that smaller Ni atoms have occupied some of the vacant oxygen sites of ferrite lattice. Cation distribution both in octahedral and tetrahedral sites changes continuously with milling time and the normal spinel lattice formed at early stage of milling, transforms to inverse spinel lattice in the course of milling. High-resolution transmission electron microscope (HRTEM) micrographs of 11 h milled sample corroborates the findings of X-ray profile analysis.     

Annealing temperature effect on microstructure,magnetic and microwave properties of Fe-based amorphous alloy powders

Fe₇₄Ni₃Si₁₃Cr₆W₄ amorphous alloy powders were annealed at different temperature (T) for 1.5 h to fabricate the corresponding amorphous and nanocrystalline powders. The influences of T on the crystalline structure, morphology, magnetic and microwave electromagnetic properties of the resultant samples were investigated via X-ray diffraction, scanning electron microscopy, vibrating sample magnetometer and vector network analyzer. The results show that the powder samples obtained at T of 650 °C or more are composed of lots of ultra-fine α-Fe(Si) grains embedded in an amorphous matrix. When T increases from 350 to 750 °C, the saturated magnetization and coercivity of the as-annealed powder samples both increase monotonously whereas the relative real permittivity shows a minimal value and the relative real permeability shows a maximal value at T of 650 °C. Thus the powder samples annealed at 650 °C show optimal reflection loss under −10 dB in the whole C-band. These results here suggest that the annealing heat treatment of Fe-based amorphous alloy is an effective approach to fabricate high performance microwave absorber with reasonable permittivity and large permeability simultaneously via adjusting T.     

FeCo基纳米晶合金高温交换耦合作用机理

研究了500和600℃真空退火后的纳米晶Fe_38.4Co₄₀Si₉B₉Nb_2.6Cu合金初始磁导率随温度的变化规律,发现较高温度(600℃)退火的FeCo基纳米晶合金,在非晶相居里温度以上较宽温度范围内磁导率没有明显的衰减,这是在双相纳米晶合金中观察到的一种新现象,其磁特性不同于Fe基纳米晶合金.为了探明这种现象的起源,估算了与剩余非晶相同成分的非晶合金的居里温度及纳米晶粒间发生交换耦合作用的参数 关键词： 交换耦合作用 非晶相居里温度 交换耦合穿透深度