Study of mechanically synthesized carbide Fe<Subscript>7</Subscript>C<Subscript>3</Subscript>

Magnetic properties and hyperfine interactions in the carbide Fe₇C₃ prepared by mechanical synthesis (MS) of α-Fe in toluene have been studied. It has been found that the boundary of temperature stability of the MS carbide does not exceed 775 K. The Curie temperature of the synthesized phase varies between 509 and 525 K. The Mössbauer experiments (⁵⁷Fe) have demonstrated that the unit cell of Fe₇C₃ contains 32 iron atoms distributed over five types of crystallographic positions with a multiplicity of 4: 8: 8: 8: 4. The hyperfine interaction parameters have been calculated for each nonequivalent position. It may be conjectured from the specific features of the decomposition at T ≥ 775 K that the most probable sequence of the phase transitions in the MS carbides Fe-C is → θ, where -Adcock carbide Fe₇C₃.     

Effect of silicon on the phase formation in mechanically activated systems based on Fe(75)C(25): Temperature-induced transformations in mechanosynthesized composites

Transformations realized in mechanosynthesized amorphous-nanocrystalline Fe(75)C(25 − x)Si(x) (0 ≤ x ≤ 10 at %) alloys during heating have been studied using dynamic magnetic susceptibility measurements, X-ray diffraction, and metallography. In contrast to mechanosynthesized alloys consisting of α-Fe, Fe₃C, and amorphous phases, the annealed alloys with x > 5 at % were found to exhibit the formation of an additional phase such as Fe₅SiC. After heating to 700 and 800°C, the powder particles of alloys contain a large amount of uniformly distributed graphite particles of ∼0.5 μm in size. The formation of particles results from the cementite decomposition, which is accelerated at the expense of partial silicon dissolution in cementite and in the presence of α-Fe nanograins as well.     

Formation of carbide phases upon the mechanosynthesis of the (Fe<Subscript>0.93</Subscript>Cr<Subscript>0.07</Subscript>)<Subscript>75</Subscript>C<Subscript>25</Subscript> alloy compared with other carbide-forming processes

Methods of X-ray diffraction, differential thermal analysis, and measurements of the dynamic magnetic susceptibility have been used to investigate the sequence of phase transformations upon the mechanical alloying of a mixture of powders of the initial components of the composition (Fe_0.93Cr_.07)₇₅C₂₅. It has been shown that, at later stages of mechanical alloying, the phase composition is determined by the conditions of the dynamic equilibrium between the crystalline and amorphous phases. A change in the conditions of mechanical alloying leads to a shift in this equilibrium and to a change in the phase composition of the alloy. A comparison of carbide formation in the Fe–C system upon the mechanosynthesis, tempering of martensite, the saturation of iron with carbon from the gaseous medium, the quenching of the melt, and the sputtering deposition of films has been performed. Some general regularities have been established, from which it follows that an important role in phase formation upon the mechanosynthesis, just as in other abovementioned processes, is played by the thermally activated phenomena.     

Structure and magnetic properties of mechanically alloyed Tb0.2Pr0.8（Fe0.4Co0.6）1.93 and magnetostriction of its epoxy composite

The C15 Laves phase with composition Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93 was synthesized by mechanical alloying (MA) and subsequent annealing process. The structure and magnetic properties of Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93 were investigated by means of X-ray diffraction (XRD), a vibrating sample magnetometer, and a standard strain technique. The effect of annealing on the structure and magnetic properties was studied. The analysis of XRD shows that the high Pr-content Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93 alloy with the single phase of MgCu₂-type structure can be successfully synthesized by MA method. The sample annealed at 450°C is found to have a coercivity of 196 kA/m at room temperature. An epoxy/Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93 composite was produced by a cold isostatic pressing technique. A large magnetostriction of 400 ppm, at an applied magnetic field of 800 kA/m, was found for the composite. The epoxy-bonded Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93 composite combines a high magnetostriction with a significant coercivity, which is a promising magnetostrictive material.     

Instability of the Fe<Subscript>23</Subscript>B<Subscript>6</Subscript> phase during thermomechanical treatment

Experimental results that indicate an unusual state detected in the fcc structure of the metastable Fe₂₃B₆ phase upon compacting and additional annealing are presented. This state manifests itself macroscopically in the form of the processes of α-Fe dissolution and precipitation without changing the magnetic-transition temperature T _C of the mechanically synthesized alloy. The Mössbauer data of the mechanically synthesized Fe₂₃B₆ alloy suggest that the nature of this phenomenon is determined by changes in the parameters of the local surrounding of iron atoms in crystallographic position Fe(4a) in the fcc unit cell.     

The formation of structure and phase composition and magnetic properties of Fe(Fe<Subscript>3</Subscript>C,Fe<Subscript>5</Subscript>SiC)-SiO<Subscript>2</Subscript> nanocomposites upon mechanical alloying

Structural and phase transformations that occur in Fe(Fe₃C, Fe₃SiC)-SiO₂ (amorphous quartz) systems during mechanical alloying in an Ar atmosphere and in air have been studied by X-ray diffraction, Mössbauer spectroscopy, IR spectroscopy, electron microscopy, and magnetometry. It has been shown that the mechanoactivation treatment leads to the formation of isolated particles 2–20 nm in size with a complex phase composition (Fe, FeSi alloy, oxides, silicates, and carbides), which depends on the milling atmosphere. It has been found out that the magnetic properties of such systems strongly depend on the oxygen and carbon compounds existing in the system, which cannot be detected by X-ray diffraction, but their presence is testified by the data of Mössbauer spectroscopy.     

Preparation and properties of a nano TiO<Subscript>2</Subscript>/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> composite superparamagnetic photocatalyst

Nano TiO₂/Fe₃O₄ composite particles with different molar ratios of TiO₂ to Fe₃O₄ were prepared via sol-gel method. X-ray diffraction, transmission electron microscopy, and vibration sample magnetometry were used to characterize the TiO₂/Fe₃O₄ particles. The photocatalytic activity of the particles was tested by degrading methyl blue solution under UV illumination (254 nm). The results indicate that with the content of TiO₂ increasing, the photocatalytic activity of the composite particles enhances, while the magnetism of the particles decreases. When the molar ratio of TiO₂ to Fe₃O₄ is about 8, both the photocatalytic activity and magnetism of the TiO₂/Fe₃O₄ particles are relatively high, and their photocatalytic activity remains well after repeated use.     

Magnetic anisotropy of the Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B compound and its hydride Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>BH<Subscript>4</Subscript>

Magnetization curves of single crystals of Nd₂Fe₁₄B and its hydride Nd₂Fe₁₄BH₄ have been measured along their principal crystallographic directions in a temperature range of 4.2–280 K. The magnetic anisotropy constants, which allow one to describe experimental magnetization curves as well as the low-temperature “easy-cone”-type magnetic structure and field-induced first-order magnetic phase transitions, have been determined in terms of a collinear-ferrimagnetic-ordering model. The anisotropy constants were shown to decrease in magnitude upon hydrogenation. In this case, the ratios of the effective fourth-and sixth-order constants to the second-order constant increase, whereas the constants responsible for the basal-plane anisotropy exhibit a more than threefold decrease. At the same time, the spin-reorientation temperature and opening of the magnetization cone at 4.2 K remain virtually unchanged. The results obtained are discussed in terms of the single-ion-anisotropy theory.     

Free energy of formation of Mn<Subscript>7</Subscript>C<Subscript>3</Subscript> from vapor pressure measurements

The Knudsen cell has been employed to determine the free energy of formation of Mn₇C₃ in the temperature range 800° to 950°C. A value of 66,440 cal was found for ΔH°₀ for α-manganese. Measurements of the pressure of manganese over a mixed carbide, (Fe,Mn)₇C₃, points to a power relationship between \({a_{M{n_7}{c_3}}}\) and \({N_{M{n_7}{c_3}}}\).     

Effect of the nanocrystalline state and electrical resistance of Fe and Fe<Subscript>75</Subscript>Si<Subscript>25</Subscript> powders produced by the method of high-energy ball milling on the frequency dispersion of microwave material parameters

The influence of the nanocrystalline state of Fe and Fe₇₅Si₂₅ particles and their electrical resistance on the microwave properties of composite materials that contain these particles has been investigated experimentally. The main factors that determine changes in the frequency dispersion of the permeability are the skin effect and the decrease in the internal field of anisotropy of the particles. In the case of Fe particles, the role of skin effect of prevails.     

Effect of crystallization annealing on the dynamic magnetoelastic properties of the Fe<Subscript>73.7</Subscript>Cu<Subscript>1.0</Subscript>Nb<Subscript>3.2</Subscript>Si<Subscript>12.7</Subscript>B<Subscript>9.4</Subscript> amorphous alloy

Double electromagnetic acoustic transformation method was used to study dynamic magnetoelastic properties of the Fe_73.7Cu_1.0Nb_3.2Si_12.7B_9.4 amorphous alloy subjected to crystallization annealing at different temperatures. Anomalous variations in the differential magnetostriction and ultrasound damping and velocity were found. According to Mössbauer spectroscopy data, they are related to the formation of a component of magnetic moments that is perpendicular to the ribbon surface.     

Switching of domains and domain walls in Fe<Subscript>50</Subscript>Mn<Subscript>50</Subscript>/Ni<Subscript>81</Subscript>Fe<Subscript>19</Subscript> bilayers with non-180° ferromagnetic domains

We have studied FeMn/Ni₈₁Fe₁₉ bilayers using vectorial vibrating-sample magnetometry (VSM) and the magnetooptic-indicator-film (MOIF) technique and observed stationary antiferromagnetic (AF) domain walls throughout the hysteresis process. The FeMn/Ni₈₁Fe₁₉ bilayer was demagnetized at 450 K and then cooled to 300 K in a zero field. The resultant hysteresis loop at 300 K clearly shows two loops, one shifted to the left of the origin and the other, to the right. Moreover, one loop is slanted and the other is square, indicating that the two sets of ferromagnetic (FM) domains have noncollinear magnetizations. At the zero applied field, the MOIF images clearly reveal the FM domains and domain walls. These domain walls, however, are hybrid domain walls connecting both FM and AF domain walls. With the application of a magnetic field, the FM domain walls can be swept. But, regardless of the applied field, including the case where the FM layer has been made into a single domain, domain walls at the original locations are still visible. Of the two sets of the FM domains with different magnetic-anisotropy axes, one set becomes reversed by rotation during magnetization reversal, while the other set becomes reversed by domain-wall nucleations and propagations.     

The effect of natural ageing of quickly quenched amorphous Fe<Subscript>75</Subscript>Si<Subscript>12</Subscript>B<Subscript>10</Subscript>Nb<Subscript>1</Subscript>Cu<Subscript>2</Subscript> alloys on their corrosion behavior sin acidic sulfate environments

The effect of natural ageing on the electrochemical and corrosion behavior of an amorphous Fe₇₅Si₁₂B₁₀Nb₁Cu alloy in acidic sulfate environments is studied. In its shape, the polarization curve is similar to that of the alloy, which was heat treated at 400°C. The ageing made the alloy brittle and unable to be soldered with acidic fluxes.     

Electrodeposition and properties of composite coatings modified by fullerene C<Subscript>60</Subscript>

Composite electrochemical coatings (CECs) based on nickel and fullerene C₆₀ as the dispersed phase are obtained. The process of electrodeposition of these coatings is studied. The method of secondary ion-mass spectrometry was used to determine the composition of nickel–fullerene C₆₀ CECs. It is found that coefficients of sliding friction of the obtained CECs decrease by from 1.8 to 2.5 times as compared to similar nickel coatings with no dispersed phase. Corrosion behavior of nickel–fullerene C₆₀ CECs is studied in 0.5M H₂SO₄ solution.     

Effect of Ga addition on the magnetic properties and microstructure of nanocrystalline Nd_（12.3）Fe_（81.7）B_（6.0） ribbons

Nd 12.3 Fe 81.7 x Ga x B 6.0 (x = 0-1.8) ribbons were prepared by melt spinning at 22 m/s and subsequent annealing treatment. The influences of Ga addition and annealing conditions on the magnetic properties and microstructure of the nanocrystalline alloys were systematically investigated. After being annealed at 620℃ for 20 min, the J r and H ci increased from 0.85 T and 582.6 kA/m for Ga-free sample to 0.97 T and 734.6 kA/m for the x = 0.9 sample, respectively. The (BH) max for the x = 0.9 sample increased by about 40% from 96.3 to 135.5 kJ/m 3 compared with that of the Ga-free one. The significant improvement of magnetic properties originated from the refinement of grains in the samples by introducing Ga, which led to a stronger exchange coupling between the neighboring grains in comparison with that in Ga-free samples. The microstructure and magnetic properties of the samples depended strongly on annealing parameters, while the sensitivity of micro-structure to annealing conditions could be significantly suppressed by the addition of Ga element.     

High-Temperature Exposure Studies of HVOF-Sprayed Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-25(NiCr)/(WC-Co) Coating

In this research, development of Cr₃C₂-25(NiCr) + 25%(WC-Co) composite coating was done and investigated. Cr₃C₂-25(NiCr) + 25%(WC-Co) composite powder [designated as HP2 powder] was prepared by mechanical mixing of [75Cr₃C₂-25(NiCr)] and [88WC-12Co] powders in the ratio of 75:25 by weight. The blended powders were used as feedstock to deposit composite coating on ASTM SA213-T22 substrate using High Velocity Oxy-Fuel (HVOF) spray process. High-temperature oxidation/corrosion behavior of the bare and coated boiler steels was investigated at 700 °C for 50 cycles in air, as well as, in Na₂SO₄-82%Fe₂(SO₄)₃ molten salt environment in the laboratory. Erosion-corrosion behavior was investigated in the actual boiler environment at 700 ± 10 °C under cyclic conditions for 1500 h. The weight-change technique was used to establish the kinetics of oxidation/corrosion/erosion-corrosion. X-ray diffraction, field emission-scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS), and EDS elemental mapping techniques were used to analyze the exposed samples. The uncoated boiler steel suffered from a catastrophic degradation in the form of intense spalling of the scale in all the environments. The oxidation/corrosion/erosion-corrosion resistance of the HVOF-sprayed HP2 coating was found to be better in comparison with standalone Cr₃C₂-25(NiCr) coating. A simultaneous formation of protective phases might have contributed the best properties to the coating.     

Mechanical synthesis and rapid consolidation of nanocrystalline 3Fe<Subscript>0.67</Subscript>Cr<Subscript>0.33</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite by high frequency induction heating

Nanopowders of 3Fe_0.67Cr_0.33 and Al₂O₃ were synthesized from CrO₃ and 2FeAl powders by high-energy ball milling. A highly dense nanocrystalline 3Fe_0.67Cr_0.33-Al₂O₃ composite was consolidated by a high frequency induction heated sintering (HFIHS) method within three minutes from mechanically synthesized powders of Al₂O₃ and 3Fe_0.67Cr_0.33. The average grain size and mechanical properties of the composite were investigated.     

Effect of Nozzle Geometry on the Microstructure and Properties of HVAF-Sprayed WC-10Co4Cr and Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-25NiCr Coatings

Thermally sprayed hard metal coatings are the industrial standard solution for numerous demanding applications to improve wear resistance. In the aim of improving coating quality by utilising finer particle size distributions, several approaches have been studied to control the spray temperature. The most viable solution is to use the modern high velocity air-fuel (HVAF) spray process, which has already proven to produce high-quality coatings with dense structures. In HVAF spray process, the particle heating and acceleration can be efficiently controlled by changing the nozzle geometry. In this study, fine WC-10Co4Cr and Cr₃C₂-25NiCr powders were sprayed with three nozzle geometries to investigate their effect on the particle temperature, velocity and coating microstructure. The study demonstrates that the particle melting and resulting carbide dissolution can be efficiently controlled by changing the nozzle geometry from cylindrical to convergent–divergent. Moreover, the average particle velocity was increased from 780 to over 900 m/s. The increase in particle velocity significantly improved the coating structure and density. Further evaluation was carried out to resolve the effect of particle in-flight parameters on coating structure and cavitation erosion resistance, which was significantly improved in the case of WC-10Co4Cr coatings with the increasing average particle velocity.