Microstructure and magnetic properties of HVOF thermally sprayed Fe75Si15B10 coatings

Partially amorphous Fe₇₅Si₁₅B₁₀ coatings were prepared from nanostructured feedstock powders by using high velocity oxy-fuel spraying. Scanning electron microscopy, X-ray diffraction, Vickers indenter and magnetic measurements were used to investigate microstructural, structural, microhardness and magnetic properties of the coatings. The Rietveld refinement of the X-ray diffraction patterns reveals the presence of an amorphous phase, nanocrystalline α-Fe(Si,B) structure having a lattice parameter close to 0.2841 nm and an average crystallite size of about 78-83 nm in addition to small amounts of Fe₃O₄ oxide (104 nm) and Fe₂B boride (151 nm), which disappear completely with increasing coating thickness. Coercivity and microhardness values are 15.5 Oe and 478 Hv, respectively, for 84 μm thickness.     

In situ structural investigation of amorphous and nanocrystalline Fe40Co38Mo4B18 microwires

Thermal evolution of the structure of glass-coated nanocrystalline FeCoMoB microwire during its devitrification has been studied. It is shown that annealing at the temperature above 411 °C leads to the formation of crystalline α-FeCo grains with diameter ∼12 nm. Annealing at higher temperature increases the crystalline weight fraction up to 40% at 565 °C. However, crystalline grains size increases very weakly to ∼13 nm. The thermal expansion coefficient of nanocrystalline microwire decreases by one half comparing to that of the amorphous precursor.     

Unusual devitrification behaviour in rapidly solidified Ti45Zr38Ni17 alloy

In this study, Ti₄₅Zr₃₈Ni₁₇ ribbons have been elaborated using planar flow casting method (v_quenching = 10⁶ K s⁻¹). The rapidly quenched samples, displaying a dispersion of nanoscaled β phase particles in an amorphous matrix, have been extensively characterised using transmission electron microscopy (TEM) and X-ray diffraction (XRD). Devitrification behaviour, investigated by four-probe resistivity measurements and differential scanning calorimetry as well as high resolution TEM analysis, revealed the formation of nanometric quasicrystals (QC) during the first exothermic phase transformation as well as the precipitation of omega domains inside β particles. On the basis of these data, it has been noticed that nanoscaled β metastable particles had adopted the same temperature dependence as β metastable bulk. Furthermore, particular orientation relationships have been observed for QCs in the vicinity of β particles which have suggested probable influence of crystalline structure on the QCs growth, although the presence of such peculiar materials after annealing treatment could be mainly explained by an icosahedral short-range order prevailing in Ti–Zr–Ni melt and favouring their nucleation.     

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 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.     

The microstructure development in Fe32Cu20Ni28P10Si5B5 immiscible alloy and possibilities of formation of amorphous/crystalline composite

Durch Messungen mittels differentieller Thermal-Analyse und dazu komplementäre Untersuchungen der Mikrostrukturen an Fe-Cu Legierungen ergibt sich erstmals der Nachweis für das Auftreten von kritischer-Punkt Benetzung an einer vollständig metastabilen Mischungslücke. Die damit verbundene vollständige Benetzung tritt in dem Zusammensetzungsbereich von 50–65 at.% Fe nahe der kritischen Konzentration auf. Glaseingelagerte Proben zeigen die vollständige Benetzung des Glases durch die Cu-reiche Schmelze während des Unterkühlens bis zur metastabilen Mischungslücke. Im Zusammensetzungsbereich der vollständigen Benetzung erfolgt die Entmischung bei der zugehörigen Binodaltemperatur ohne zusätzliche Unterkühlung. Bei der Erstarrung der phasenseparierten Schmelze aus tiefer Unterkühlung führt dies zu einem grobskaligen Entmischungsgefüge. Im Gegensatz dazu zeigt die Entmischungsreaktion bei Zusammensetzungen außerhalb des Intervalls vollständiger Benetzung eine merkliche Unterkühlung unterhalb der jeweiligen Binodaltemperatur. Das zugehörige Erstarrungsgefüge zeigt die Phasentrennung auf einer deutlich kleineren Skala. Ohne Glaseinlagerung befinden sich die Proben in Kontakt zu dem Al₂O₃-Tiegel und zu einer Oberflächenschicht aus Eisenoxid. Unter diesen Bedingungen findet kritischer-Punkt Benetzung auf beiden Seiten der kritischen Zusammensetzung statt. Diese Ergebnisse demonstrieren daß kritischer-Punkt Benetzung unabhängig von der Probenumgebung erfolgt, daß jedoch die benetzende Phase oberflächenempfindlich ist und durch die Probenumgebung selektiert wird.

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.     

New fracture morphology of amorphous Fe78Si9B13 alloy

The tensile fracture morphology of a brittle amorphous Fe-based ribbon was investigated by scanning electron microscopy（SEM）. The fracture surface consists of mirror, mist and river-pattern zones with crack propagating. The formation of nanoscale damage cavity structure is a main characteristic morphology on the fracture surface. Approaching the fracture source in the mirror, these damage cavities assemble and form the nanoscale periodic striation patterns, which are neither Wallner lines nor crack front waves. At shear deformation stage, the apparent surface energy yf of amorphous Fe78Si9B13 ribbon is much smaller than that of less-brittle amorphous alloys, which indirectly indicates amorphous Fe7sSi9Bl3 ribbon is perfectly brittle. The crack branching appears at the moment of final fracture due to the high crack propagating velocity.     

Fast consolidation of a nanocrystalline Al2O3 reinforced 3.7Fe0.54Cr0.18Al0.26Si0.016 composite from mechanical synthesized powder through pulsed current activated heating

Nanopowders of Fe_0.54Cr_0.18Al_0.26Si_0.016 and Al₂O₃ were synthesized from Fe₂O₃, Cr, Si, and Al powders using high-energy ball milling. A high-density nanocrystalline 3.7Fe_0.54Cr_0.18Al_0.26Si_0.016-Al₂O₃ composite was consolidated with mechanically synthesized powders of Al₂O₃ and 3.7Fe_0.54Cr_0.18Al_0.26Si_0.016-Al₂O₃ through a pulsed current activated sintering (PCAS) method within 1 min. The hardness of the composite and the average grain sizes of Al₂O₃ and Fe_0.54Cr_0.18Al_0.26Si_0.016 were investigated.     

Structural study on Zr0.39Ni0.61 and (Zr0.39Ni0.61)D0.59 amorphous alloys by neutron and X-ray diffraction

Neutron and X-ray diffraction and reverse Monte Carlo (RMC) simulation were performed on Zr_0.39Ni_0.61 and (Zr_0.39Ni_0.61)D_0.59 amorphous alloys to investigate the rearrangement of metal atoms due to the deuterium absorption. The RMC models showed that the Zr–Zr distances slightly increase but the Zr–Ni and Ni–Ni distances remain virtually unchanged after the deuterium absorption. Additionally, the Voronoi polyhedral analysis of the RMC configurations showed that there is not much difference in the local structure around Zr and Ni between Zr_0.39Ni_0.61 and (Zr_0.39Ni_0.61)D_0.59 amorphous alloys.     

Influence of low-temperature annealing on magnetic properties of (Nd0.625Ni0.375)85Al15 metallic glass

After a review of the selection process of (Nd_0.625Ni_0.375)₈₅Al₁₅ as a metallic glass with a relatively high glass-forming ability, we investigate the influences of its phase transitions by duplicating the heating process of the isochronal thermal analysis with low-temperature annealings. The structure, thermal stability and magnetic properties are characterized. And the influences on magnetic properties are particularly discussed with emphasis. Both the annealing processes, to the glass-transition temperature and to the onset temperature of crystallization, bring about a higher coercivity of the sample and a higher freezing temperature of the spin-glass-state. For the sample annealed to the onset temperature of crystallization, the influence is quite obvious and is ascribed to the formation of ferrimagnetic Nd₇Ni₃ phase, as detected by XRD. For the sample annealed to the glass-transition temperature, the indistinct influence is further identified with the analysis of the frequency dependence of the spin-glass-state, and it is mainly attributed to the change of the short-range order in the amorphous matrix.     

The synthesis of nanocrystalline Ni75Nb12B13 alloys by high energy ball milling of elemental components

Ni₇₅Nb₁₂B₁₃ alloys were synthesized by mechanical alloying (MA) of individual Ni, Nb and B components. X-ray investigation showed the formation of Ni (Nb, B) solid solution and amorphous phase at the intermediate stage of milling. Metastable phases formed by MA turned into Ni (Nb), Ni₂₁Nb₂B₆ and Ni₃Nb stable phases during heating up to 720 °C. The exothermal effects on DSC curves were caused with these processes. The disintegration of Ni (Nb, B) solid solution and crystallization of an amorphous phase resulted in the stable phases formation during the milling prolongation as well as after thermal treatment.     

A mechanical spectroscopy study of the Zr69.5Cu12Ni11Al7.5 alloy

The vibrating reed technique has been used to investigate the Zr_69.5Cu₁₂Ni₁₁Al_7.5 glass forming alloy. The phase transformations of the material, as can be seen through the changes of the resonance frequency of the sample as a function of temperature, have been studied and the results have been compared with TEM and DSC measurements confirming the development, above T_g, of a metastable quasicrystalline state prior to crystallisation. Hydrogen-induced damping peaks were then observed in the different phases of the material with particular attention to the quasicrystalline state, where it seems that the microscopic reorientation mechanism responsible for the internal friction peaks should be similar to the one already known in the amorphous phase (hydrogen jumps between tetrahedral sites). Shortly after crystallisation, three damping peaks were observed, two of which are attributed to relaxation processes (Zener-type or intercrystalline Gorsky effect) taking place in the tetragonal CuZr₂ phase.     

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.     

Studying nearest neighbor correlations by atom probe tomography (APT) in metallic glasses as exemplified for Fe40Ni40B20 glassy ribbons

A next nearest neighbor evaluation procedure of atom probe tomography data provides distributions of the distances between atoms. The width of these distributions for metallic glasses studied so far is a few Angstrom reflecting the spatial resolution of the analytical technique. However, fitting Gaussian distributions to the distribution of atomic distances yields average distances with statistical uncertainties of 2 to 3 hundredth of an Angstrom. Fe₄₀Ni₄₀B₂₀ metallic glass ribbons are characterized this way in the as quenched state and for a state heat treated at 350 °C for 1 h revealing a change in the structure on the sub-nanometer scale. By applying the statistical tool of the χ² test a slight deviation from a random distribution of B-atoms in the as quenched sample is perceived, whereas a pronounced elemental inhomogeneity of boron is detected for the annealed state. In addition, the distance distribution of the first fifteen atomic neighbors is determined by using this algorithm for both annealed and as quenched states. The next neighbor evaluation algorithm evinces a steric periodicity of the atoms when the next neighbor distances are normalized by the first next neighbor distance. A comparison of the nearest neighbor atomic distribution for as quenched and annealed state shows accumulation of Ni and B. Moreover, it also reveals the tendency of Fe and B to move slightly away from each other, an incipient step to Ni rich boride formation.     

Fe73.2Si16.2B6.6Nb3Cu1非晶合金粉体电磁屏蔽性能研究

为防控电磁污染问题,亟待开发更好的电磁屏蔽材料。本文设计了成分为Fe_73.2Si_16.2B_6.6Nb₃Cu₁的合金并制备了非晶合金条带,经过高能球磨处理得到非晶合金粉体。研究了球磨处理时间对合金粉体软磁性能、微观结构、形貌和电磁波吸收性能的影响。结果表明,非晶合金粉体球磨后产生了α-(Fe,Si)相,提高了软磁性能,饱和磁化强度最高可达137.94emu/g。非晶粉体形貌为椭球状,随球磨时间增加,合金粉体粒径减小,提高了电磁波吸收性能,最小平均粒径为8.42μm。合金具有优异的电磁波吸收性能,电磁波损耗机制主要为磁损耗。球磨50h后的非晶合金粉体具有最佳的电磁波吸收性能,在频率为4.57GHz处取得的最小反射损耗达到-42.26dB,吸波剂厚度为2.5mm时的最大有效吸收(<-10dB)带宽高达5.78GHz。     

Effect of accumulated shear on the microstructure and morphology of severely deformed Cu60Zr30Ti10 metallic glass

Cu₆₀Zr₃₀Ti₁₀ metallic glass sample was subjected to different amount of continuous rotational straining by high pressure torsion. Deformation dependent morphology, microstructural and thermal properties were characterized by scanning electron microscopy, synchrotron radiation, X-ray diffraction, and calorimetry, respectively. In order to estimate the temperature rise in the metallic glass during high pressure torsion, quasi three-dimensional heat conduction equation with a source term was considered. Solutions indicate that the final temperature strongly depends on the total amount of plastic deformation while details of the microstructure are mainly determined by the balance between the local shear rate and temperature.     

Synergistic microwave and structural studies of C-type Ho2Si2O7

Ho₂Si₂O₇ material exists in four polymorphs, a triclinic low temperature phase (type-B), a monoclinic modification (type-C), high temperature monoclinic (type-D), and high temperature orthorhombic modification (type-E). The structural properties are measured by XRD and the morphology is noted through scanning electron microscopy (SEM). The dc electrical resistivity (ρ) as a function of temperature and dielectric properties of C-type Ho₂Si₂O₇ in the microwave region is measured. The activation energy is calculated from ln ρ versus 1/k_BT plot. The activation energy is 0.119 ± 0.001 eV. Both the real (?′) and imaginary parts of permittivity (?″) decrease slightly as the frequency increases up to 1.5 GHz, after that ?′ increases while ?″ decreases as the frequency increases. At around 2.45 GHz, resonance is observed.     

Magnetic and electrical transport properties of La0.5A0.5CoO3 nanoparticles (A = Sr, Ca, and Ba)

We have investigated the effect of particle size on the electrical transport and magnetic properties of La_0.5A_0.5CoO₃ (A = Sr, Ca, and Ba) nanoparticles synthesized by sol-gel technique. A size-induced metal to insulator transition is observed in the resistivity behaviour of Sr- and Ba-doped samples as the dimension changes from higher to lower ones. The magnetoresistance exhibits almost linear behaviour throughout the studied field range. The zero-field-cooled (ZFC) and field-cooled (FC) magnetizations display a broad paramagnetic to ferromagnetic transition at T_C with a large magnetic irreversibility. The magnetization results indicate that Co³⁺ ions are in the intermediate spin state but Co⁴⁺ ions stay in a mixture of intermediate and high spin states. The observed frequency dependent shoulder in the in-phase and the peak in the out of phase component of the ac susceptibility indicate the glassy nature of the samples. The analysis of the ac magnetization results suggests that the magnetic behaviour is consistent with the cluster glass model.