Electric-field-enhanced crystallization of amorphous Fe86Zr7B6Cu1 alloy

The effect of external electric field on the crystallization of amorphous Fe₈₆Zr₇B₆Cu₁ alloy was studied. It is shown that its crystallization process is markedly enhanced by external electric field. The crystallization time at 600 °C decreases from 1 to 0.5 h on application of a modest (10⁴ V/m) external electric field. The volume fraction of crystalline -Fe increases from 65.2 to 68.6%, while the grain size decreases from 13.2 to 11.9 nm. The electric-field-enhanced crystallization may result from the fact that the external electric field results in the increases of the crystal nucleation rate and the crystal growth rate. The complex permeability shows an apparent improvement in soft magnetic properties induced by electric field, thereby demonstrating the practical utility of electric-field-enhanced crystallization technique.     

Magnetic, electrical and plastic properties of Fe76Nb2Si13B9, Fe75Ag1Nb2Si13B9 and Fe75Cu1Nb2Si13B9 amorphous alloys

Influence of 1 h annealing in vacuum on magnetic, electrical and plastic properties of Fe₇₆Nb₂Si₁₃B₉, Fe₇₅Ag₁Nb₂Si₁₃B₉ and Fe₇₅Cu₁Nb₂Si₁₃B₉ melt spun ribbons were carefully investigated. It was shown that in all cases soft magnetic properties can be significantly enhanced by applying 1-h annealing at characteristic temperatures T_op. This optimization annealing causes that permeability increases more than 15-times and magnetic losses (tangent of loss angle) achieves a minimum in relation to the as quenched state. Using structural examinations (X-ray and HRTEM) it was shown that for the Fe₇₅Cu₁Nb₂Si₁₃B₉ alloy the optimized microstructure corresponds to a nanocrystalline αFe(Si) phase whereas in other alloys to a relaxed amorphous phase free of iron nanograins. As a consequence of this fact the Fe₇₆Nb₂Si₁₃B₉ and Fe₇₅Ag₁Nb₂Si₁₃B₉ alloys show higher plasticity in comparison to the nanocrystalline Fe₇₅Cu₁Nb₂Si₁₃B₉ alloy. Temperatures of the first stage of crystallization, and related diffusion parameters were determined using measurements of resistivity versus temperature with different heating rates.     

The corrosion behavior of amorphous and nanocrystalline Fe73.5Cu1Nb3Si15.5B7 alloy

The potentiodynamic polarization curves in 0.5 M NaCl solution before and after crystallization of Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy have been studied in relation to the microstructure and alloy composition. It was shown that the corrosion resistance of the alloy strongly depending on these two factors. The observed decrease in corrosion resistance of the alloy after the heat treatment up to 480 °C in comparison to the corrosion resistance of the alloy in the as prepared state is attributed to the increased inhomogeneity of the alloy that coincides with the first appearance of Fe₃Si phase. Further heating (up to 600 °C) resulted in an increase in the number of Fe₃Si nanocrystallites and the appearance of a FeCu₄ phase. After annealing at 600 °C the lowest corrosion rate, 0.004 mm a⁻¹, was observed. Annealing of the samples at higher temperatures (>600 °C) induced formation of six crystalline phases which proved detrimental to the corrosion resistance of the Fe_73.5Cu₁Nb₃Si_15.5B₇ alloy. Solid corrosion products were identified on the surface of the samples after anodic polarization.     

Magnetic properties of crystalline and amorphous GdFe2Hx alloys prepared by hydrogenation

Magnetic properties of crystalline c-GdFe₂H_4.3 and hydrogen-induced amorphous a-GdFe₂H_3.3 which were prepared by hydrogenation of crystalline CdFe₂ with the C15 structure were investigated by magnetization measurements and ⁵⁷Fe Mössbauer spectroscopy. The hydrogenation decreases the magnetic ordering temperature. The Curie temperatures are 360 K in c-GdFe₂H_4.3 and 469 K in GdFe₂H_3.3, which are noncollinear ferrimagnets with the compensation temperatures of 202 and 45 K, respectively. The Mössbauer spectra of c-GdFe₂H_4.3 at 297, 77.3 and 4.2 K consist of a sharp quadrupole doublet and a broadened Zeeman sextet, which indicates that a quarter of the Fe atoms do not carry an ordered magnetic moment even at 4.2 K. The spectra of a-GdFe₂H_3.3 show a broadened sextet with a distribution of hyperfine fields. The maximum of the distribution probabilities is located at 365 kOe at 4.2 K, a value which is larger than that of α-Fe.     

Kinetics and formation mechanism of amorphous Fe52Nb48 alloy powder fabricated by mechanical alloying

A single phase amorphous Fe₅₂Nb₄₈ alloy has been synthesized through a solid state interdiffusion of pure polycrystalline Fe and Nb powders at room temperature, using a high-energy ball-milling technique. The mechanisms of metallic glass formation and competing crystallization processes in the mechanically deformed composite powders have been investigated by means of X-ray diffraction, Mössbauer spectroscopy, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. The numerous intimate layered composite particles of the diffusion couples that formed during the first and intermediate stages of milling time (0–56 ks), are intermixed to form amorphous phase(s) upon heating to about 625 K by so-called thermally assisted solid state amorphization, TASSA. The amorphization heat of formation for binary system via the TASSA, ΔH_a, was measured directly as a function of the milling time. Comparable with the TASSA, homogeneous amorphous alloys were fabricated directly without heating the composite multilayered particles upon milling these particles for longer milling time (86 ks–144 ks). The amorphization reaction here is attributed to the mechanical driven solid state amorphization. This single amorphous phase transforms into an order phase (μ phase) upon heating at 1088 K (crystallization temperature, T_x) with enthalpy change of crystallization, ΔH_x, of −8.3 kJ mol⁻¹.     

Structural, magnetic and Mössbauer spectral studies of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite powders

Nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ powders, synthesized by a combustion method are investigated by X-ray diffraction, vibrating sample magnetometry and Mössbauer spectroscopic techniques. We adopt a strategy to systematically control the particle sizes between 4 and 45 nm simply by changing the elemental stoichiometric coefficient, Φ_e, of the combustion mixture. Curie temperature of the superparamagnetic particles of size 4 nm is higher than that of the bulk particles. Interestingly, bigger particles (45 nm) show a comparable room temperature saturation magnetization and exceptionally very high Curie temperature of 833 K, when compared to that of the bulk Ni_0.5Zn_0.5Fe₂O₄ material (563 K).     

Development and characterization of composite Ni–Cr–C–CaF2 laser cladding on γ-TiAl intermetallic alloy

Mössbauer spectroscopy has been used to study the crystallization behaviour in paramagnetic bulk amorphous steels Fe₅₀Cr₁₄Mo₁₄C₁₄B₆X₂ (X = Y and Dy), prepared by arc-melting and Cu mould casting techniques. Room temperature Mössbauer spectra of as-cast samples exhibit a broadened and asymmetric doublet-like structure that indicates the paramagnetic and amorphous nature of the as-cast alloys. During heat-treatment alloys crystallized into γ-Fe and η-Fe₃Mo₃C iron phases and crystallization completed at 1123 K. Alloy containing Dy shows better performance against crystallization at low temperature as compared to the Y-containing alloy.     

The influence of chemical disorder enhancement on the martensitic transformation of the Ni50Mn36Sn14 Heusler-type alloy

The effect of chemical disorder over the martensitic phase transformation of the Ni₅₀Mn₃₆Sn₁₄ Heusler-type alloy was systematically investigated by performing X-ray diffractometry (DRX), DC magnetization and ⁵⁷Fe-doping and ¹¹⁹Sn-Mössbauer spectroscopy measurements. DRX patterns are characteristics of a L2₁-type chemically disordered structure, where the presence of this disorder was first evaluated by analyzing the relative intensity of the (1 1 1) DRX reflection, which varies in the case of Fe-doped and practically disappears for the milled samples. In consequence, the magnetic properties of Fe-doped well-milled samples related to the martensitic phase transformation change substantially. 300 K ⁵⁷Fe-Mössbauer spectroscopy data suggest that the changes in the magnetic properties related to the martensitic transformation are intrinsically correlated to the ferromagnetic and paramagnetic fractions, which are respectively associated with Fe atoms replacing Mn- and Sn-sites. In the case of milled samples, the drastic reduction of alloy magnetization was explained by the increase of the number of Mn atoms in the shell regions, which have a reduced magnetic moment comparatively to those in the grain cores. The magnetization change and the temperature transition in the martensitic transformation are governed by the grain core. The initial magnetic properties and martensitic transformation can be recovered by a subsequent annealing on the milled sample.     

Structural changes of Co70Fe5Si10B15 amorphous alloy induced during heating

In this study we present the results on complex structural changes of the Co₇₀Fe₅Si₁₀B₁₅ amorphous alloy induced during heating in the temperature range between 20 and 1000 °C. The structural and phase transformation changes were correlated with DTA, XRD and SEM properties. It is shown that initial Co₇₀Fe₅Si₁₀B₁₅ alloy during heating undergoes complex crystallochemical changes. In the range between ambient temperature and near 400 °C, investigated alloy retains the solid-state amorphous properties. Prolonged heating induces complete transformation to crystalline solid state. The solid–solid amorphous to crystalline state transformation process is completed at 500 °C, when two nanocrystalline phase alloy systems are formed. Prolonged thermal treatment between 600 and 1000 °C, influenced further elemental segregation and phase transition. At 1000 °C, the composite material consisting of two FCC cobalt-rich alloys and a hexagonal unidentified alloy are formed.     

Nanocrystalline Fe1−xCoxSn2 solid solutions prepared by reduction of salts in tetraethylene glycol

In an effort to improve the electrochemical performance of tin intermetallic phases as electrode active material for lithium-ion batteries, Fe_1−xCo_xSn₂ solid solutions with x = 0.0, 0.25, 0.3, 0.5, 0.6 and 0.8 were prepared by chemical reduction in tetraethylene glycol. Precise control of the synthesis conditions allowed single-phase nanocrystalline materials to be prepared, with particle diameters of about 20 nm and cubic, nanorods, and U-shaped morphologies. The substitution of iron by cobalt induced a contraction of the unit cell volume. The hyperfine parameters of the ⁵⁷Fe Mössbauer spectra were sensitive to the Co/Fe substitution and revealed a superparamagnetic behaviour. In lithium cells nanocrystalline Fe_1−xCo_xSn₂ active materials delivered reversible capacities above 500 mAh g⁻¹ that depended on the composition and cycling conditions. The intermediate compositions exhibit better electrochemical performance than the end compositions CoSn₂ and FeSn₂.     

Stress corrosion cracking and hydrogen-induced cracking of amorphous Fe74.5Ni10Si3.5B9C2

Stress corrosion cracking (SCC) in NaCl solution and hydrogen-induced cracking (HIC) during dynamic charging of Fe_74.5Ni₁₀Si_3.5B₉C₂ amorphous alloy were investigated through sustained load tests. The normalized threshold stress of SCC was σ_SCC/σ_F=0.04, where σ_F is fracture strength in air. Anodic polarization and addition of As₂O₃ into the solution did not change σ_SCC/σ_F, but cathodic polarization increased σ_SCC/σ_F from 0.04 to 0.31. Cathodic polarization increased but anodic polarization decreased the time to failure during SCC at the constant load of σ=0.27σ_F.The normalized threshold stress of HIC, σ_HIC/σ_F, was linearly decreased with the increase in logarithm of hydrogen concentration (C₀) and kept a minimum constant when C₀ was larger than a critical value, i.e., σ_HIC/σ_F=1.58−0.36lnC₀ (C₀?74.4 wppm) and 0.1 (C₀?74.4 wppm). The threshold stress of HIC during dynamic charging with the maximum current was larger than that of SCC at open-circuit potential. Fracture surfaces of HIC were also different with that of SCC. Experiments indicated that SCC of the amorphous alloy in the NaCl solution is controlled by anodic dissolution process instead of hydrogen.     

Magnetic and structural studies of mechanically alloyed (Fe50Co50)62Nb8B30 powder mixtures

Amorphous (Fe₅₀Co₅₀)₆₂Nb₈B₃₀ powder mixture was prepared by mechanical alloying from elemental Fe, Co, B and Nb powders in a planetary ball mill under argon atmosphere. Structural, thermal and magnetic properties were performed on the milled powders by means of X-ray diffraction, differential scanning calorimetry and magnetic measurements. The amorphous state is reached after 125 h of milling. The excess enthalpy due to the high density of defects is released at temperature below 300 °C. Crystallisation and growth of crystal domains are the dominating processes at high temperatures. The saturation magnetisation decreases rapidly during the first 25 h of milling to about 15.24 A m²/kg and remains nearly constant on further milling. Coercivity, H_c, value of about 160 Oe is obtained after 125 h of milling.     

Magnetic and electrical properties of the UCu2T3Al7 alloys

The UCu₂T₃Al₇-type compounds with T = Cr, Mn and Fe crystallize in the tetragonal ThMn₁₂-type structure (I4/mmm space group). Magnetic properties have been examined in magnetic fields up to 50 kOe in the temperature range 1.9–400 K using a SQUID magnetometer. The Cr based pseudoternary alloy is paramagnetic with the linear field dependence of the magnetization at 1.9 K. The temperature dependence of the magnetic susceptibility is described by a modified Curie–Weiss (MCW) law. The Mn containing compound is ferrimagnetic and the field dependence of the magnetization does not reach saturation while the magnetic susceptibility follows the MCW law at high temperature. The Fe alloy is ferromagnetic and at higher temperature the MCW law is fulfilled. The temperature dependence of the electrical resistivity of all materials was examined between 4.2 and 300 K showing metallic character. The Mn and Fe based compounds demonstrate some anomalies in the ρ(T) plot at low temperature. The results are discussed in relation to the anomalies observed in magnetic measurements.     

Influence of thermally induced structural transformations on hardness in Fe89.8Ni1.5Si5.2B3C0.5 amorphous alloy

Effect of heat treatment on mechanical behavior of Fe_89.8Ni_1.5Si_5.2B₃C_0.5 amorphous alloy was investigated by measuring microhardness. It was shown that the as-prepared amorphous alloy has an unexpectedly high microhardness. This can be attributed not only to boron dispersed in the alloy, but also to the structure which exhibits aspects of a nanocomposite of nanoparticles dispersed in an amorphous matrix. As the alloy crystallizes at temperatures above 540 °C, microhardness decreases continuously as a function of heating temperature. This is attributed to separation of boron out of the amorphous matrix into nanocrystals of Fe₂B phase. Further decrease in microhardness is attributed to crystallite growth with the accompanying change in the dominant nature of the interfaces from amorphous/crystal to crystal/crystal, and creation of a porous structure. When the crystallization is complete, the alloy exhibits microhardness close to that of a hypothetical mixture of α-Fe and Fe₂B phases of the same composition.     

Magnetic study of Ti-substituted NiFe2O4 ferrite

The Ni_1+xTi_xFe_2−2xO₄ (0 ≤ x ≤ 0.1) ferrite systems prepared by a semi-chemical route, have been studied by electron paramagnetic resonance (EPR) at X-band, Mössbauer spectroscopy and magnetization measurements at various temperatures. EPR spectra of these samples comprise generally a broad and asymmetric EPR signal. The variation of g_eff and peak-to-peak line width ΔH_pp, with Ti concentration and temperature are attributed to the variation of dipole–dipole interaction and the superexchange interaction. Mössbauer spectra comprise two sets of sextet attributed to Fe³⁺ at two distinct sites-A and -B. Ti⁴⁺ ions are concluded to occupy the octahedral B-sites. Magnetic moment is found to decrease with the increase of Ti⁴⁺ concentration. The effective magnetic field H_eff at the A-sites also follows a similar trend. The reason is attributed to the canted structure of spins in the Ti-doped samples. An anomalous behavior at x = 0.015 is observed in the properties studied here and some sort of phase change is believed to occur at 473 K in these ferrites.     

Magnetic properties of Ce3−xGdxCo11B4 borides

The structure and magnetic properties of Ce_3−xGd_xCo₁₁B₄ borides have been studied by X-ray powder diffraction (XRPD), magnetization and differential scanning calorimetry (DSC) measurements. X-ray analysis reveals that the compounds crystallize in the hexagonal Ce₃Co₁₁B₄-type structure with P6/mmm space group. The substitution of Gd for Ce leads to an increase of the unit-cell parameter a and the unit-cell volume V, while the unit-cell parameter c decreases linearly. Magnetic measurements indicate that all samples are ordered magnetically below the Curie temperature. The Curie temperatures increase as Ce is substituted by Gd. The saturation magnetization at 4 K decreases upon the Gd substitution up to x = 1, and then increases.     

退火温度对Fe64Co16Zr7B12Cu1纳米晶合金软磁性能的影响

在Hitperm合金基础上制备了成分为Fe64Co16Zr7B12Cu1的非晶纳米晶合金带,并对其组织特征及软磁性能进行了研究。结果表明:Fe64Co16Zr7B12Cu1合金具有较好的非晶形成能力和热稳定性;非晶态合金随着温度的升高晶化程度增加,断裂方式逐渐转变为沿晶断裂。合金经过适当的热处理后,饱和磁感应强度Bs和最大磁导率μm均有一定提升,最高分别达到Bs=1.47T,μm=2.67×104。     

Magnetic properties of R2CoGa3 (R = rare earth) compounds

R₂CoGa₃ compounds (R = Gd, Th, Dy, Ho, Er and Y) crystallize in the hexagonal P6,lmcm space group. In these compounds the Co atoms have practically no magnetic moment. The heavy rare earth compounds order ferromagnetically with relatively low Curie temperatures ( 3 K (Er) T_c 50 K (Gd)). The compounds with Tb and Dy, and to a smaller extent the Ho compound, present irreversibilities in their magnetization processes, and there are maxima in the zero-field-cooled susceptibility curves.