Effect of micro-alloying chromium on the corrosion resistance of nanocrystalline nickel aluminide intermetallic produced by mechanical alloying process

In this study, Ni₅₀Al_50 − xCr_x nanocrystalline intermetallic compound was synthesized by using the high energy mechanical milling of pure Ni, Al and Cr elemental powders for 16 h. The morphological investigation was done by using the optical and scanning electron microscope. The corrosion behavior of the samples was studied by using the electrochemical impedance spectroscopy in 3.5% NaCl solution. The results showed that when the micro-alloying Cr content is increased, the particles distribution is modified and the size of particles is decreased. Therefore the passive film which is formed on the surface of samples is less porous, so the corrosion resistance is increased.     

Interface microstructure of diffusion bonded Ni3Al intermetallic alloy and austenitic stainless steel

The diffusion bonding of a Ni₃Al intermetallic alloy to an austenitic stainless steel has been carried out at temperatures 950, 1000 and 1050 °C. The influence of bonding temperature on the microstructural development and hardness across the joint region has been determined. The microvoids in the interface have been found to decrease with increasing bonding temperature. The intermetallic phase Al₃Ni has been detected at the Ni₃Al side of the diffusion couple. Diffusion of Cr and Fe from the stainless steel to the Ni₃Al alloy has been observed.     

Effects of Al and Zn additions on mechanical properties and precipitation behaviors of Mg-Sn alloy system

For the Mg-Sn-Al alloys, the ultimate strength (UTS) of an extruded Mg-9wt.%Al-2wt.%Sn alloy reached 390 MPa. TEM observation indicated that plate-like Mg₁₇Al₁₂ precipitates having Burgers orientation relationship with the matrix are responsible for the strength. This alloy also exhibits an age hardening behavior: the peak hardness appears after 15-20 h of heat treatment at 473 K. On the other hand, the UTS of the Mg-Mg-Sn-Zn alloys are on the order of 300 MPa. The precipitates in these alloys are composed of the Mg₂Sn and MgZn₂ particles. It was found that these phases often precipitate together, suggesting that the MgZn₂ phase can act as a nucleation sites for Mg₂Sn.     

Improvement of magnetic characteristics and electrical properties of FeCoHfO/AlOx multilayered films

High permeability magnetic films can enhance the inductance of thin-film inductors in DC-DC converters. In order to obtain high permeability, effective uniaxial anisotropic field should be as low as possible. A multilayered technique (laminating the magnetic layers with oxide spacers) was exploited to improve the magnetic properties of thick films. The FeCoHfO/AlO_x multilayered films were fabricated by dc reactive magnetron sputtering. Inserting an insulator (AlO_x) layer can decrease the magneto-elastic anisotropy by reducing the residual stress of the FeCoHfO magnetic films. The anisotropic field and resistivity of the FeCoHfO/AlO_x multilayered films were evidently improved by multilayered coating. With this optimum configuration of 9 layers structure [FeCoHfO (133 nm)/AlO_x (10 nm)]₉, low anisotropic field (H_K = 65 Oe) and high resistivity (ρ ∼ 1350) μΩ cm were achieved.     

Fe@Ag core-shell nanoparticles with both sensitive plasmonic properties and tunable magnetism

Monodisperse Fe@Ag core-shell nanoparticles with relatively uniform Fe cores and Ag shells have been successfully fabricated by a seed mediated method in a two-step reducing process, and then characterized by electron microscopy techniques (HRTEM, EDX), X-ray diffraction (XRD), UV-vis spectroscopy,and magnetometry. The results demonstrate unique optical and magnetic properties for Fe@Ag core-shell nanoparticles. The surface plasmon resonance of Fe@Ag core-shell nanoparticles is red shifted as compared with that of pure colloidal nano-silver, while the plasmon band of Fe@Ag core-shell nanoparticles with thinner Ag shells is shifted to a longer wavelength. Fe@Ag core-shell nanoparticles have a narrow plasmon band and therefore sensitive plasmonic properties. The magnetism of Fe@Ag nanoparticles can be tuned from superparamagnetic to ferromagnetic by modifying the proportion between Fe and Ag contents. The multifunctional Fe@Ag core-shell nanoparticles have potential in optoelectronic, spintronic, and biomedicine applications.     

Enhanced magnetic properties of cobalt nanoparticles on FeMn films

Monodisperse cobalt nanoparticles are synthesized by the thermal decomposition of Co₂(CO)₈ in phenyl ether and subsequently deposited on antiferromagnetic (AFM) FeMn films and glass substrates, respectively. Magnetic measurement shows that the as-prepared Co nanoparticles are superparamagnetic and can be transformed into ferromagnetic (FM) through thermal treatment. While keeping monodisperse, the annealed FM Co nanoparticles on AFM FeMn films show a much larger coercivity than the ones on glass substrates due to FM/AFM exchange coupling. Accordingly, we propose a convenient method to enhance magnetic properties of nanoparticles.     

The microstructure and magnetic properties of NdFeB magnets directly solidified at a low cooling rate

Nd_12.5Fe₈₂B_5.5 ribbons have been prepared by a melt spinning technique at a wheel speed ranging from 5 to 22 m/s. It was found that the wheel speed v_R dramatically influences the microstructure and magnetic properties of the ribbons. At an optimized v_R, a structure with single-phase Nd₂Fe₁₄B and an enhanced coupling between grains give rise to excellent magnetic properties. For example, the coercivity, remanence ratio, and maximum energy product of the best sample quenched at a wheel speed of 13 m/s are 1227.4 kA/m, 0.76, and 182.6 kJ/m³, respectively. In comparison with the samples from under- and over-quenched conditions and all the samples with annealing, the as-quenched sample fabricated at the optimized wheel speed has the best magnetic properties.     

Carbon microspheres with embedded magnetic iron oxide nanoparticles

Narrow-size disperse porous carbon microspheres with embedded magnetite nanoparticles were prepared by annealing Fe(III)-containing microspheres composed of a copolymer of acrylic acid and divinylbenzene at 800 °C under inert atmosphere. The Fe(III)-containing microspheres were prepared by uptake of Fe²⁺ ions through ion exchange process by poly(acrylic acid-divinylbenzene) microspheres that were prepared by distillation-precipitation polymerization, followed by annealing at 250 °C at ambient atmosphere. The carbonization of the microspheres created micropores with a maximum pore diameter of about 0.38 nm and a BET surface area of ~ 200 m²/g. The saturation magnetization of the magnetic carbon microspheres was 31.5 emu/g with a low remnant magnetization and coercivity.     

Synthesis and characteristic of FeNi3 intermetallic compound obtained by electrical explosion of wire

In this work, the FeNi₃ permalloy nanopowder was synthesized by wire explosion in deionized water. X-ray diffraction was used to identify and characterize the prepared nanopowder exhibiting a FeNi₃ phase. The morphology and size of the prepared powders were observed by field emission-scanning electron microscopy and transmission electron microscopy. The compositions of the wire before exploding and the nanopowder were determined with electron probe microanalysis. The results indicate that the FeNi₃ permalloy nanopowder was synthesized with an average particle size ∼50 nm. The magnetic properties of the nanopowder indicate a symmetric hysteresis loop of ferromagnetic behavior with coercivity of 199.3 Oe and saturation magnetization of 56.7 emu/g.     

Synthesis of a nanostructured MgH2-Ti alloy composite for hydrogen storage via combined vacuum arc remelting and mechanical alloying

To improve the hydrogen kinetics of magnesium hydride, TiCr_1.2Fe_0.6 alloy was prepared by vacuum arc remelting (VAR) and the alloy was co-milled with MgH₂ to process nanostructured MgH₂-5 at.% TiCr_1.2Fe_0.6 powder. The hydrogen desorption properties of the composite powder were studied and compared with pure magnesium hydride. X-ray diffraction (XRD) analysis showed that the composite powder prepared by VAR/mechanical alloying (MA) procedure consisted of β-MgH₂, γ-MgH₂, bcc Ti-Cr-Fe alloy, and small amount of MgO. The average size of particles and their grain structure after 4 h MA were determined by a laser particle size analyzer and XRD method and found to be 194 nm and 11 nm, respectively. The hydrogen desorption temperature of magnesium hydride decreased from 426 °C to 235 °C by the bcc Ti alloy and the utilized processing method, i.e. combined VAR/MA.     

Exchange-coupled nanocomposites with novel microstructure and enhanced remanence by a new approach

Exchange-coupled nanocomposites with hard/soft magnetic phases are promising for the next generation of permanent magnets.Chemical methods have an advantage in controlling the nanoscale size of both phases.The nanocomposites obtained by the chemical method generally consist of a hard phase core and a soft phase shell.However,the soft-phase shell is easily oxidized leading to small enhancement of remanence.Here,a novel microstructure of Fe@FePt nanocomposites with Fe soft phase core and FePt hard phase shell has been synthesized by replacement reaction,in which the size of core and shell can be controlled below 10 nm by adjusting the ratio of Fe nanoparticles to PtCl4.Excellent exchange-coupling(single-phase-like demagnetization curves) between soft-hard phases was observed due to the precise size control of both phases,and substantial enhancements both in remanence (32 ％) and saturation magnetization (81％) were obtained in optimal nanocompistes.This work provides an alternative routine to prepare heterostructure materials with various applications.     

Oxidation of Au4Al in gold ballbonds

Two different 4N (99.99% purity) gold wires were ballbonded on 1 μm thick Al-1 wt.% Si-0.5 wt.% Cu bondpad metallisation and subjected to high temperature storage (HTS) at 175 °C in air. Each wire type showed ball lift failures, Type A after 500 h and Type B after 1500 h, which in both cases was a result of Au₄Al oxidation. With wire Type A the dominant compound underneath the ball was Au₈Al₃. A thin layer of Au₄Al (≈ 1 μm thick) was observed between the Au₈Al₃ and the gold ball. Ball lift failures occurred in the Au₄Al layer, which appeared to disintegrate due to oxidation and the resulting by products of oxidation were deposited on the underlying and unoxidised Au₈Al₃. With wire Type B, a double layer Au₄Al was dominant after long term ageing and Au₈Al₃ was confined to the ball periphery. Consequently, because of the much greater volume of Au₄Al, compound oxidation resulted in the formation of a large amount of a completely new microstructure consisting of gold precipitates embedded in a dark oxide matrix. The Au₈Al₃ compound remained unoxidised. It is speculated that internal stress and contamination may accelerate the oxidation reaction.     

Ion beam mixing to study the metallic glass formation of the Cu-Zr system

An ion beam mixing experiment for Cu-Zr system was conducted and two supersaturated solid solutions were observed with compositions of 16 atom% Zr in Cu and 17 atom% Cu in Zr, respectively, which are much greater than almost nil found from the equilibrium phase diagram of Cu-Zr system. The observation indicates that Cu-Zr metallic glasses could possibly be obtained in composition range bounded by the two observed solid solubilities, i.e. 16-83 atom% Zr. Besides, a unique Cu₆₅Zr₃₅ metallic glass was obtained by ion beam mixing and its composition is very close to the so-called best composition referred in the literature.     

Magnetic properties of SBA-15 mesoporous nanocomposites with CoFe2O4 nanoparticles

A novel mesoporous nanocomposite based on SBA-15 with CoFe₂O₄ and Fe₂O₃ magnetic nanoparticles was prepared via the hydrothermal treatment and impregnation method. We showed that Fe₂O₃ nanoparticles were anchored in the frame and CoFe₂O₄ nanoparticles were confined in the mesopores of SBA-15 in the prepared nanocomposite. Our results indicated that the magnetic properties could be adjusted by the addition of CoFe₂O₄ and Fe₂O₃ magnetic nanoparticles. The presence of couple exchange interaction was confirmed with Kelly-Hankel (δM) curves, which enhanced the magnetic properties of the CoFe₂O₄/Fe₂O₃-SBA-15 mesoporous nanocomposite.     

Ambient temperature synthesis and magnetic properties of aluminum borate—Fe nanocomposites

Nanocomposites in which fine Fe particles are dispersed in a ceramic aluminum borate Al_xB_yO_z matrix were synthesized at ambient temperatures by simultaneous hydrolysis of aluminum butoxide and reduction of Fe²⁺ with sodium borohydride. The nanocomposites were characterized by X-ray powder diffraction, transmission electron microscopy, SQUID magnetometry, and Mössbauer spectroscopy. The as-prepared samples consist of crystalline Fe particles dispersed in an amorphous aluminum borate matrix, which on annealing in H₂ atmosphere at T ≥ 800 °C transforms to crystalline aluminum borate; the crystalline aluminum borate exhibits a needle-like morphology. The as-prepared samples all seem to consist of a mixture of ferromagnetic and superparamagnetic Fe particles. The superparamagnetic fraction vanishes completely on annealing in H₂ atmosphere at 900 °C. The magnetic results are supported by Mössbauer spectra recorded at room temperature.     

Mechanical properties of sputter-deposited nanocrystalline Cr3Si film

The sputter-deposited Cr-Si film prepared by glow discharge was comprised of equiaxed nanocrystalline Cr₃Si with a grain size of less than 5 nm. The hardness of the nanocrystalline Cr₃Si film was 2.37 times higher than that of coarse-grained Cr₃Si, whereas the elastic modulus of nanocrystalline Cr₃Si was equal to that of coarse-grained Cr₃Si. Compared with the coarse grain of Cr₃Si, the sputter-deposited nanocrystalline Cr₃Si film exhibited higher ductility or toughness due to the GB-mediated deformation.     

Fe3O4纳米粒子修饰碳纳米管的制备及其磁学性能（英文）

通过FeCl2.4H2O和FeCl3.6H2O混合共沉淀,合成平均粒径为6 nm和10 nm的Fe3O4纳米粒子。然后将两种Fe3O4纳米粒子分别与经HNO3氧化处理的多壁碳纳米管(MWCNTs)置于乙醇水溶液(水和乙醇的体积比为1∶1)中,在超声波作用下制备Fe3O4/MWCNT复合材料。用高分辨透射电子显微镜、X-射线光电子能谱、振动样品磁强计、X射线衍射仪、热重分析仪对所制备的Fe3O4/MWCNT复合材料进行表征。结果表明:由6 nm和10 nm Fe3O4纳米粒子所制备的Fe3O4/MWCNT复合材料中,Fe3O4的质量分数分别为26.65%和29.3%,相应的磁饱和强度分别为16.5 emug-1和7.5 emug-1。     

On the microsegregation of Ti-45Al-(8-9)Nb-(W, B, Y) alloy

The morphology, distribution and origin of microsegregation of an as-cast high Nb containing TiAl alloy and the effect of the microsegregation on as-forged and heat-treated microstructures had been investigated. The results showed that there were three kinds of microsegregations, designated as L-segregation, β-segregation and α-segregation, existing on boundaries of primary β grains, on triple junctions of lamellar colonies, within lamellar colonies, respectively. The segregations were formed during solidification and phase transformation of the alloy due to high cooling rate and low atom diffusion rate. After forging in (α + γ) region β-segregation and α-segregation could be eliminated after, but L-segregation could not be eliminated. L-segregation was detrimental to heat treatment of the as-forged material.     

Formation of rod‐shaped BaFe12O19 nanoparticles with well magnetic properties

A facile approach to preparing rod‐shaped nanoparticles of barium ferrite was developed by combination of hydrothermal and annealing process in air. The magnetic nanorods with diameters ∼ 40 nm and lengths ∼ 150 nm were clearly visible in TEM and SEM images. Magnetic measurements showed that the rod‐like BaFe₁₂O₁₉ nanoparticles exhibited a great coercive field and high saturation magnetization of up to 4511 Oe and 67.3 emu/g, respectively. It is found that heat treatment in air can create less oxygen vacancies, which dramatically affects the Fe-O-Fe superexchange coupling. The results of this paper suggest that oxygen vacancies should be responsible for the decrease of saturation magnetization in nanosized magnetic materials.