Fe-rich Region Research Articles

Development of quaternary Fe-based bulk metallic glasses

Fe-based bulk metallic glasses have been designed using several modeling criteria. Quaternary alloys were successfully developed in the Fe-rich compositional region where the only solutes used were Cr, Nb, and B. Furthermore, the exceedingly high level of Fe, exceeding 75 wt.% in some alloys, makes this series of alloys, compositionally very close to conventional steel alloys. Therefore, their production using conventional facilities and materials is more easily attainable than previously developed bulk metallic glasses. The modeling criteria used, simultaneously analyzes the thermodynamics and kinetics of the vitrification behavior in a potential glass-forming alloy. A liquidus model, which determines and ranks the presence of deep eutectics, is used to determine the optimal compositional region. This criterion is cross-checked with an elastic strain model. Alloys compositionally located near a deep eutectic, while simultaneously containing a topology that induces significant elastic strain in a developing crystalline lattice exhibits experimentally good glass forming ability.

Suzaku Observation of the RCW86 Northeastern Shell

This paper reports on Suzaku results concerning the northeast shell of RCW86. With both spatial and spectral analyses, we separated the X-rays into three distinct components: low ($kT_{\rm e}\sim$0.3keV) and high ($kT_{\rm e}\sim$1.8keV) temperature plasmas and a non-thermal component, and discovered that their spatial distributions are different from each other. The low-temperature plasma is dominated at the east rim, whereas the non-thermal emission is brightest at the northeast rim, which is spatially connected from the east. The high-temperature plasma, found to contain the $\sim$6.42keV line (K$\alpha$ of low-ionized iron), is enhanced at the inward region with respect to the east rim, and has no spatial correlation with the non-thermal X-rays (the northeast). This result suggests that the Fe-K$\alpha$ line originates from Fe-rich ejecta heated by reverse shock. A possible scenario to explain these morphologies and spectra is that a fast-moving blast wave in a thin cavity collided with a dense interstellar medium at the east region very recently. As a result, the reverse shock in this interior decelerated, and arrived at the Fe-rich region of the ejecta and heated it. In the northeast rim, on the other hand, the blast wave is still moving fast, and is accelerating electrons causing them to emit strong synchrotron X-rays.

Fe onto GaN(0001) grown in a full MOVPE process

Abstract To date successful spin injection in spintronics devices has been reported for two different experimental approaches: using diluted magnetic semiconductors or ferromagnetic metals as spin aligners on top of a semiconductor structure. In this work we present our studies on the deposition of Fe on (0 0 0 1)GaN fully accomplished via metalorganic vapour phase epitaxy (MOVPE). Special attention has been devoted to the Fe nucleation onto the GaN surface and the growth has been monitored online by spectroscopic ellipsometry. The structural and magnetic characterization of the layers has been carried out using high-resolution X-ray diffraction, atomic force microscopy, high-resolution transmission electron microscopy and SQUID magnetometry. Depending on the growth parameters, two different kinds of samples can be fabricated: paramagnetic ones, with Fe-alloy nanocrystals a few nanometers below the sample surface, and ferromagnetic ones, with Fe-rich pits and regions close to the surface. Increasing surface roughening due to the Fe deposition in both types of samples, as well as surface reconstruction could be observed from HRTEM measurements. The presence of Fe in proximity of the surface solely indicates the modest diffusion of the magnetic ions achieved by optimizing the growth parameters.

First principles study on the electronic structure and magnetism of Fe1-xCoxSi alloys

Electronic and magnetic properties of Fe1-xCoxSi alloys were investigated by using a full-potential linear augmented-plane-wave method based on density functional theory. Electronic structure calculation demonstrates that half-metallic property appears in the Fe-rich region of 0 0.25. The concentration dependence of the magnetic moment of the alloys can be understood by the fixed Fermi level at minority band in Fe-rich region, as well as at the majority band in Co-rich region. In Fe-rich alloys, the electronic structure and the magnetic properties at Fe site depend mainly on the spin-polarization of nearest neighbouring Co atoms, while in Co-rich alloys, these features at Co site arise mainly from the neighbours of Fe atoms.

On the thermal stability of vitrified industrial wastes using microscale synchrotron radiation based techniques

The effect of annealing on the local coordination of Fe in a series of vitrified industrial wastes is studied by means of x-ray fluorescence mapping, and micro- and conventional x-ray absorption fine structure (XAFS) spectroscopies. It is demonstrated that annealing causes the formation of Fe- and Pb-rich microcrystallites which are embedded in the glass matrix. The local coordination of the Fe ion depends on the local variations of its concentration, i.e., Fe occupies octahedral sites in the Fe-rich crystalline regions and tetrahedral sites into the vitreous network. The percentage of the Fe atoms that belong to the crystalline inclusions depends on the waste content and the annealing temperature, and the stability of the vitrified product is discussed in relation to the nature of the formed microcrystallites. More specifically, when the microcrystallites are mixed Pb and Fe oxides, the material is safe since Pb is trapped both in the crystalline and vitreous regions. Finally, the effect of the different types of crystalline phases and crystalline ratio on the characteristics of the preedge peak in the near edge XAFS spectra is also discussed.

Magnetism of epitaxial Fe xNi 1−x films on Cu 90Au 10(1 0 0)

We have investigated the effect of an expansion of the lattice parameter on the magnetic properties of fcc FeNi films. Epitaxial Fe x Ni 1− x ultrathin films (0.58< x<0.92) were grown on a Cu 90Au 10(1 0 0) single crystal, a substrate with lattice parameter 1.4% larger than the pure copper. The magnetism of the FeNi films was explored by using linear magnetic dichroism in the angular distribution of the photoelectrons (LMDAD). The LMDAD measurements were carried out at 160 K, using synchrotron radiation with an energy of 140 eV. Although we have observed a gradual decrease of the Fe 3p LMDAD asymmetry in the 67–80 at% Fe concentration range, as already reported for Fe x Ni 1− x on Cu(1 0 0), for the Fe-rich region ( x>∼0.80) we have obtained a Fe 3p asymmetry of 6%, suggesting that the lattice expansion of the FeNi due to the epitaxy on Cu 90Au 10(1 0 0) stabilizes the high-spin ferromagnetic state up to higher Fe concentration values.

First-principles calculation of phase equilibria and phase separation of the Fe-Ni alloy system

Theoretical investigation of the phase equilibria of the Fe-Ni alloy has been performed by combining the FLAPW total energy calculations and the Cluster Variation Method through the Cluster Expansion Method. The calculations have proved the stabilization of the L1 2 phase at 1:3 stoichiometry, which is in agreement with the experimental result, and predicted the existence of L1 0 as a stable phase below 550 K; this L1 0 phase has been missing in the conventional phase diagram. The calculations are extended to the Fe-rich region that is characterized by a wide range phase separation and has drawn considerable attention because of the intriguing Invar property associated with a Fe concentration of 65%. To reveal the origin of the phase separation, a P-V curve in an entire concentration range is derived by the second derivative of free energy functional of the disordered phase with respect to the volume. The calculation confirmed that the phase separation is caused by the breakdown of the mechanical-stability criterion. The newly calculated phase separation line combined with the L1 0 and L1 2 order-disordered phase boundaries provides phase equilibria in the wider concentration range of the system. Furthermore, a coefficient of thermal expansion (CTE) is attempted by incorporating the thermal vibration effect through harmonic approximation of the Debye-Gruneisen model. The Invar behavior has been reproduced, and the origin of this anomalous volume change has been discussed.

Application of energy-filtering TEM to the nanocrystallization process in amorphous Fe84Nb7B9 alloy

The nanocrystallization process in amorphous Fe84Nb7B9 alloy was studied by means of energy-filtering transmission electron microscopy. Energy-filtered diffraction intensity provided quantitative structural analysis for the as-quenched state with the help of reverse Monte Carlo simulation. Using the Fe L-edge it was found from chemical mapping that chemical segregation of Fe with Fe-rich and Fe-poor regions starts to occur at the later stage of nanocrystallization.

Effect of Fe content in Fe–Ti–B system on fabricating TiB 2 particulate locally reinforced steel matrix composites

In situ TiB 2 particulate locally reinforced steel matrix composites were successfully fabricated by self-propagating high-temperature synthesis (SHS) reactions in a Fe–Ti–B system containing 10, 30, 50 and 70 wt.% Fe, respectively, in the molten high chromium alloy steel during casting. The locally reinforced regions of the Fe–TiB 2 composites with different Fe contents in the Fe–Ti–B system consist of three different regions, i.e. the Fe-rich region, the fine TiB 2 particulate region and the larger TiB 2 particulate region. The portion of the Fe-rich region decreased and the larger TiB 2 particulate region became unclear with the increase in Fe content in the Fe–Ti–B system. In addition, only a small amount of micropores is found in the fine TiB 2 particulate region of the Fe–TiB 2 composite synthesized in a 70Fe–Ti–B system, and the Fe content has obvious effect on the wear resistance of the locally reinforced region of the Fe–TiB 2 composites.

A microspectroscopic study of the electronic homogeneity of ordered and disordered Sr2FeMoO6

Disordered Sr2FeMoO6 shows a drastic reduction in saturation magnetization compared to highly ordered samples, moreover magnetization as a function of the temperature for different disordered samples shows qualitatively different behaviours. We investigate the origin of such diversity by performing spatially resolved photoemission spectroscopy on various disordered samples. Our results establish that extensive electronic inhomogeneity, arising most probably from an underlying chemical inhomogeneity in disordered samples, is responsible for the observed magnetic inhomogeneity. It is further pointed out that these inhomogeneities are connected with composition fluctuations of the type Sr2Fe1+x Mo1-x O6 with Fe-rich (x > 0) and Mo-rich (x < 0) regions.

Analysis of nanocrystallization process in Fe 73.5Cu 1Nb 3Si 15.5B 7 alloys by cumulant expansion of EXAFS

Amorphous-nanocrystalline transformation in Fe 73.5Cu 1Nb 3Si 15.5B 7 is investigated by extended X-ray absorption fine structure (EXAFS) and analysed by the cumulant method, which allows a refined study of parameters that are able to describe the local structure during the nanocrystallization. The nearest-neighbour interatomic distance and coordination number have been determined for Fe 3Si and Fe; these elucidate the nanocrystallization in two stages. The first stage revealed the formation of DO 3 nanocrystals and the second stage show evidence of a disordered Fe-rich interfacial region formed around the DO 3 nanograins. Thus the cumulant analysis of EXAFS is sensitive to the different crystalline fraction arising from annealing at various temperatures.

Fabrication of steel matrix composites locally reinforced with in situ TiB2–TiC particulates using self-propagating high-temperature synthesis reaction of Al–Ti–B4C system during casting

Abstract Steel matrix composite locally reinforced with in situ TiB2–TiC particulates was successfully fabricated utilizing the self-propagating high-temperature synthesis (SHS) reaction of Al–Ti–B4C system during casting. Microstructure characterization of the composites reveals that the microstructure is made up of two types of TiB2 and TiC particulates concerning particulate size and distribution and white Fe-rich regions. Moreover, the results show that the hardness and wear resistance of the composites are higher than those of the steel matrix.

In situ synthesis of TiB 2–TiC particulates locally reinforced medium carbon steel–matrix composites via the SHS reaction of Ni–Ti–B 4C system during casting

The fabrication of medium carbon steel–matrix composites locally reinforced with in situ TiB 2–TiC particulates using self-propagating high-temperature synthesis (SHS) reaction of Ni–Ti–B 4C system during casting was investigated. X–ray diffraction (XRD) results reveal that the exotherm of 1042 °C initiated by heat release of the solid state reaction in the differential thermal analysis (DTA) curve is an incomplete reaction in Ni–Ti–B 4C system. As-cast microstructures of the in situ processed composites reveal a relatively uniform distribution of TiB 2–TiC particulates in the locally reinforced regions. Furthermore, the particulate size and micro-porosity in the locally reinforced regions are significantly decreased with the increasing of the Ni content in the preforms. For a Ni content of 30 and 40 wt.%, near fully dense composites locally reinforced with in situ TiB 2 and TiC particulates can be fabricated. Although most of fine TiB 2 and TiC particulates which form by the reaction–precipitation mechanism during SHS reaction are present in the locally reinforced region, some large particulates which form by the nucleation–growth mechanism during solidification are entrapped inside the Fe-rich region located in the reinforcing region or inside the matrix region nearby the interface between matrix and reinforcing region. The hardness of the reinforcing region in the composite is significantly higher than that of the unreinforced medium carbon steel. Furthermore, the hardness values of the composites synthesized from 30 to 40 wt.% Ni–Ti–B 4C systems are higher than those of the composites synthesized from 10 to 20 wt.% Ni–Ti–B 4C systems.

Study of annealing induced devitrification of stabilized industrial waste glasses by means of micro-X-ray fluorescence mapping and absorption fine structure spectroscopy

The distribution of Fe in as-casted and annealed vitrified Pb- and Fe-rich solid industrial waste is studied by means of X-ray fluorescence mapping and micro-X-ray absorption fine structure spectroscopy. The studied samples were produced by co-melting of lead and iron contaminated ash with the appropriate quantities of vitrifying (SiO 2) and flux (Na 2O) agents at high temperatures (1400 °C) and were subjected to thermal treatment in the temperature range 400–800 °C for 2 h. It is demonstrated that annealing at temperatures higher than the glass transition temperature (440 °C) induces loss of homogeneity in the sample and promotes the formation of Fe-rich crystalline regions, embedded into the glass matrix. In addition to that, annealing modifies the coordination environment around the Fe 3+ atom, which occupies octahedral sites in the crystalline regions and tetrahedral sites in the amorphous, glassy phase where it acts as a network former. Finally, the effect of the coordination environment of Fe on the chemical stability of vitreous and vitroceramic materials containing industrial wastes is disclosed.

Half-metallic character and electronic properties of inverse magnetoresistantFe1−xCoxSialloys

Based on ab initio calculations we find that ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ alloys in the Fe-rich region behave as a disordered ferromagnetic half metal. Half metallicity can explain the recently found electronic, magnetic, and transport properties. We can also determine a sharp transition from the paramagnetic metallic behavior to the ferromagnetic half-metallic state as a function of Co concentration. At concentrations higher than 0.25 the system starts to segregate Co from Fe atoms, this giving rise to the disappearance of half metallicity and to reflect it in the decreasing value of the magnetic moment.

Spin dynamics and spin-glass state in Fe-doped cobaltites

A thorough study of the magnetic and transport properties of La0.5Sr0.5Co1−xFexO3 (0⩽x⩽0.6) compounds has been made. The Fe substitution destroys the metallic state and the resistivity increases by orders of magnitude even with a very small extent of Fe substitution. The charge localization due to Fe substitution is likely to have its origin in the electronic configuration rather than in its ionic size. The hole-poor regions, corresponding to the Fe-rich regions, would also dilute the magnetic lattice and thereby prevent the occurrence of long-range order. Spin-glass behavior was observed for x⩾0.5 compositions and is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co3+ and Co4+, and the antiferromagnetic interactions like Co-O-Fe and Fe-O-Fe. A dynamic scaling analysis of ac susceptibility data using conventional critical slowing down indicates a finite spin-glass phase-transition temperature Tg≈85 K and a dynamic exponent zv≈12.4, for x=0.5 composition.

In situ Synthesis of TiB2 Particulate Locally Reinforced Steel Matrix Composite by the Self-propagating High-temperature Synthesis Reaction of Al-Fe-Ti-B System during Casting

In-situ TiB2 particulate locally reinforced steel matrix composites were successfully fabricated by the self-propagating high-temperature synthesis (SHS) reaction of 10Al-20Fe-Ti-B and 40Al-20Fe-Ti-B systems, respectively, in the molten high chromium alloy steel during casting. Microstructure analysis of the composite showed that the interface bonding between the matrix and locally reinforced region is very good and the locally reinforced region consists of three different regions, i.e. the Fe-rich region, the fine dispersion of TiB2 particulates region and the larger TiB2 particulates region. The wear test showed that the wear resistance of the locally reinforced regions of the composites is higher than that of the steel matrix.

NMR study of the crystallization kinetics in FINEMET-type materials

We have investigated the evolution of the structural and magnetic properties of Fe(Si)-based (FINEMET-type) nanocrystalline alloys using the NMR technique at zero field. NMR is a technique which provides structural information on different local atomic environments, and also allows the evaluation of the local magnetic anisotropy. Melt-spun ribbons of Fe 73.5Cu 1Nb 3Si (22.5− x) B x ( x=4, 6, and 9), which contain both amorphous and crystalline phases were studied as a function of the annealing temperatures ( T A). The 93Nb and 11B resonances were observed, and the magnetic stiffness associated with different local environments of 93Nb nuclei were determined. We observed that the Nb frequencies decrease with increasing T A, which suggests that Nb atoms migrate from either Fe-rich regions to Fe-poorer regions or to the B-richer phases. The local measurement of the anisotropy as a function of T A shows an equivalence to the behavior of the bulk coercivity as a function of T A.

Phase transformations and reaction kinetics during the temperature-induced oxidation of natural olivine

This work describes the sequence of transformations and their reaction kinetics during the oxidation of olivine in air at high temperature. A natural olivine sample from the layered series of the Ivrea-Verbano igneous complex (Western Alps, Italy) was heated in the temperature range 25-1300 °C in air and investigated by in situ, real time powder X-ray diffraction (PXRD). The evolution of the peaks (measured integrated intensities) was followed in non-isothermal conditions using variable heating rates (b = 20, 22, 25, 27, and 30 °C/min). The total time of the experiments ranged from about 256 (b = 30 °C/min) to 277 (b = 20 °C/min) min including the time for the data collections. An additional isothermal run was performed at 800 °C. The analysis of the kinetic data was attempted with the use of different equations including the classical Avrami theory for solid-state reactions. The kinetic results were confirmed by independent experimental data from electron microscopy (SEM, TEM). In the transformation sequence, hematite appears at about 600 °C producing amorphous segregations. of silica that later recombine with forsterite to form pyroxene Hematite is stable up to 1130 °C where it is transformed into magnetite. The rate limiting step for the formation of hematite is a twodimensional diffusion with constant or decelerating nucleation rate and apparent activation energy of 15 kcal/mol. The concentration of Fe 3+ in Fe-rich regions favors the heterogeneous nucleation of hematite, which may take place on existing defects or at the grain boundaries with impurity phases such as serpentine. At 1130 °C, magnetite is formed at the expenses of hematite, with a contracting volume interface-controlled reaction in two or three dimensions with an apparent activation energy of 30-31 kcal/mol. The hematite to magnetite transformation is direct, without a metastable amorphous intermediate. It is described by the “shrinking core model,” with the formation of a magnetite outer layer at the surface of the hematite particles that proceeds toward the core of the reacting hematite by diffusion of the oxygen throughout the newly formed magnetite layer. Its rate is limited by the advancement of the reaction front. The rate-limiting step for the formation of pyroxene is two-dimensional diffusion with decelerating nucleation rate with an apparent activation energy of 29 kcal/mol.

Fe–Ag magnetic granular systems: a SANS study with polarised neutrons

We present small angle scattering measurements performed on Fe–Ag granular systems using polarised neutrons. Various samples with different iron concentration have been investigated and the analysis of the experimental data points out that samples morphology does not change remarkably with Fe content. Finally, the study of the polarised signal indicates the presence of Fe rich regions in the nanometer range of size having a negligible magnetisation.