Fe-Si Alloys Research Articles

Thermal conductivity of Fe-Si alloys and thermal stratification in Earth’s core

Light elements in Earth's core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron-electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ∼100 to 110 W⋅m-1⋅K-1 for liquid Fe-9Si near the topmost outer core. If Earth's core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core-mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core-mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core.

The alumina enrichment and Fe-Si alloy recovery from coal fly ash with the assistance of Na2CO3

A clean process for the enrichment of alumina and preparation of Fe-Si alloy from coal fly ash (CFA) with the assistance of Na2CO3 was proposed in the present work. This process mainly consists of two stages, e.g., vacuum carbothermal reduction and magnetic separation. The effects of Na2CO3 addition, reduction temperature and reduction duration on the decomposition of mullite (Al6Si2O13) in CFA and the growth of Fe-Si alloy particles were investigated thoroughly. Moreover, the effect of Na2CO3 on the magnetic separation of Fe-Si alloys was also discussed. The results show that the addition of Na2CO3 under vacuum condition could effectively accelerate the decomposition of mullite and enhance the formation of Fe-Si alloys. The mean particle size of the Fe-Si alloy increased from 23.94 μm to 42.74 μm by adding 10 wt% Na2CO3, and the separation efficiency reached 88.95% with the assistance of the Na2CO3. After magnetic separation, the alumina content, silica content, and the mass ratio of alumina/silica in the non-magnetic part of the separation product was 80.36 wt%, 6.49 wt%, and 12.38, respectively, indicating that a highly efficient recovery of alumina was achieved. This process will not pollute the environment and there almost no waste residue or waste acid is generated. Therefore, this provides a good potential for the industry application and delivers favourable conditions for commercial viability.

Enhanced microwave absorption of flaky FeSiAl/ZnO composites fabricated via precipitation

In this study, we investigated the effects of ZnO loading on flaky FeSiAl alloys and assessed their material structure, morphology, magnetism, and microwave absorption characteristics. The structure and morphology indicated a ZnO precipitation limit on the flaky FeSiAl alloy surfaces, and the surface precipitation and mixture were tunable via precipitation technology. The results also indicated that the surface deposition weight ratio of ZnO to FeSiAl should not exceed 2:5, as excessive ZnO deposition resulted in the formation of large ZnO nanoparticles. In addition, nonmagnetic ZnO caused a steady decrease in saturation magnetization from 89.3 to 48.2 emu/g. Following the ideal impedance matching and high dielectric loss, we obtained a minimum reflection loss of −47.85 dB and bandwidth of 3.24 GHz. Compared to conventional oxides coated with flaky FeSiAl, the low density and semiconductor ZnO composites exhibited greatly improved microwave absorption performance for future lightweight and microwave absorption applications.

Enhancing mechanical properties of twin-roll cast Al–Mg–Si–Fe alloys by regulating Fe-bearing phases and macro-segregation

The existence of coarse Fe-bearing phases and macro-segregation under the twin-roll casting (TRC) condition has always been a restrictive factor for the preparation of high-performance Al alloys. In this work, the effect of Mn contents on the formation of Fe-bearing phases, macro-segregation behavior and mechanical properties of TRC Al-0.5Mg-0.7Si-0.1Fe-xMn (x = 0, 0.1, 0.3 and 0.5 wt.%) alloys have been investigated. The macro-segregation region in the as-cast 0Mn alloy is found to contain coarse secondary phases consisting of Al, Fe, Mg and Si. In as-cast 0.1Mn and 0.3Mn alloys, macro-segregation has been alleviated due to the transformation of π-AlFeMgSi phase to α-Al(FeMn)Si phase, as a result of the inhibited peritectic reaction to form the π-AlFeMgSi phase by the addition of Mn. However, with further increasing the amount of Mn, coarse intermetallic compounds at the center region tend to form in the 0.5Mn alloy, which aggravates macro-segregation again. The 0.1Mn alloy exhibits a better combination of yield strength (287 MPa) and elongation (18.9%) than that of the 0Mn alloy (274 MPa, 15.9%) under the artificial aging condition. The higher yield strength of the 0.1Mn alloy is mainly due to the formation of fine α-Al(FeMn)Si particles that make a strong contribution to dispersion strengthening. This work provides a reference for weakening macro-segregation via regulating Fe-bearing phases, which is of great importance in developing AA6xxx series TRC alloys with high performance.

Anomalous Nernst effect in Fe–Si alloy films

We experimentally investigated the anomalous Nernst effect (ANE) in an Fe3Si film, whose ANE was predicted to be large, based on the topological property and the Berry curvature, and systematically compared it with other compositions of iron-silicide, viz., Fe2Si, FeSi, and FeSi2 films. Although both the ferromagnetic Fe3Si and Fe2Si films showed an ANE voltage, the highest ANE coefficient SANE = 1.0 μV K−1 was obtained for Fe3Si, which is larger than that for Fe. We also measured the Seebeck and anomalous Hall effects to estimate the transverse thermoelectric conductivity αyx, suggesting that the contribution of αyx was dominant in the Fe3Si ANE enhancement.

Corrosion and Interfacial Behavior of FeSi Alloy in Organic Electrolyte Solutions

Corrosion and Interfacial Behavior of FeSi Alloy in Organic Electrolyte Solutions

Behavior of phosphorus during solidification of the Si-Fe-P ternary alloy melt used in purification of MG-Si

The behavior of phosphorus during the solidification of the Si-Fe-P ternary alloy melt used in purification of MG-Si was investigated using thermodynamic calculations and design of Si-Fe-P alloy materials with specific composition. It was found that when the concentration of Fe in the liquid phase (LFe) is less than 23 wt%, Fe can promote the decomposition of SiP and the solid solution of P in primary Si, basing on positive activity interaction between Fe-P. After LFe exceeds 23 wt%, Fe will attract P to form clusters and enrich in unsolidified liquid phase or Fe3Si7 phase during the primary crystallization of Si and the eutectic of Si and Fe3Si7(A). Then, the eutectic of Fe3Si7(B) and Si4P4Fe ends the solidification of the alloy melt. These characteristic behaviors lead to the distribution coefficient of P between the silicon matrix and A-Fe3Si7 reaching 0.4868, limiting the effectiveness of P removal from MG-Si by Si-Fe alloy refining. Experiments on in-situ etching and acid leaching in different acid agents showed that HCl+HF and Aqua regia were the optimal acid leaching systems for the removal of P enrichment in Fe3Si7 and Si4P4Fe, respectively.

Broadband electromagnetic wave absorbing metamaterial based on FeSiAl alloy

FeSiAl alloy material is a typical soft magnetic material, which have important application potentials in the areas of electromagnetic wave absorption, and thus it is widely used in civilian and military field. Currently, researchers mostly adopts the changes of microstructure and doped components to improve electromagnetic wave absorbing properties of FeSiAl alloy materials. In this paper, a metamaterial designing method is used for the optimal design of a broadband electromagnetic wave absorbing material based on FeSiAl alloy. We investigate the geometrical dimensions and array period on the absorption performance about the FeSiAl metamaterial. The absorption mechanism of FeSiAl metamaterial is discussed through the propagation of electromagnetic field and Smith diagram. We find that the absorption performance can easily be regulated with the macrostructural size. Moreover, we create a broadband FeSiAl metamaterial. The designed FeSiAl metamaterial exhibit a −10 dB absorption bandwidth from 4 GHz to 8 GHz and 10 GHz to 15 GHz, and it is sensitive at large incidence angle, but do not sensitive within 45°.

In situ observation of the Rayleigh–Taylor instability of liquid Fe and Fe–Si alloys under extreme conditions: Implications for planetary core formation

Rayleigh–Taylor (RT) instability, which occurs when a heavy fluid overlies a light fluid in a gravitational field, is an important scenario for planetary core formation, especially beneath the planetary magma ocean. This process has been discussed based on numerical simulations and experiments using analog materials. However, experiments on the RT instability using the core-forming melt have not been performed at high pressures. In this study, we perform in situ observation of the RT instability of liquid Fe and Fe–Si (Si = 10 and 20 wt. %) alloys under high pressure using a high-power laser-shock technique. The observed perturbation on the Fe–Si surface grows exponentially with time, while there is no obvious growth of perturbations on the Fe in the measured time range. Therefore, the growth rate of the RT instability increases with Si content. The timescale of the initial growth of the RT instability in planetary interiors is likely to be much faster (by more than two orders of magnitude) than the 30–40 × 106 year timescale of planetary core formation.

Non-metallic Inclusions in Different Ferroalloys and Their Effect on the Steel Quality: A Review

Ferroalloys have become increasingly important due to their indispensable role in steelmaking. In addition, the demand for improved steel qualities has increased considerably, which in turn highlights the quality of ferroalloys. This is due to the fact that the impurities in ferroalloys directly and significantly influence the quality of steel products. To gain a better understanding of the main trace elements and inclusions in ferroalloys (such as FeSi, FeMn, SiMn, FeTi, FeCr, FeMo, FeNb, FeV, FeB, some complex ferroalloys) and their behaviours in steel melt after the additions of these ferroalloys, information from a large number of previous results on this topic was extensively reviewed in this work. The applications of different ferroalloys and their production trends were discussed. In addition, the effects of some trace element impurities from ferroalloys on the inclusion characteristics in steel were also discussed. The possible harmful inclusions in different ferroalloys were identified. Overall, the results showed that the inclusions present in ferroalloys had the following influence on the final steel cleanliness: (1) MnO, MnS and MnO–SiO2–MnS inclusions from FeMn and SiMn alloys have a temporary influence on the steel quality; (2) the effect of large size SiO2 inclusions (up to 200 μm) in FeSi and FeMo alloys on the steel cleanliness is not fully understood. The effect of Al, Ca contents should be considered before the addition of FeSi alloys. In addition, Al2O3 inclusions and relatively high Al content are commonly found in FeTi, FeNb and FeV alloys due to their production process. This information should be paid more attention to when these ferroalloys are added to steel; (3) except for the existing inclusions in these alloys, the Ti-rich, Nb-rich, V-rich carbides and nitrides, which have important effects on the steel properties also should be studied further; and (4) specific alloys containing REM oxides, Cr–C–N, Cr–Mn–O, Al2O3, Al–Ti–O, TiS and Ti(C, N) have not been studied enough to enable a judgement on their influence on the steel cleanliness. Finally, some suggestions were given for further studies for the development of ferroalloy productions.

Ce-Bearing FeSi Alloy Inoculation of Electrically Melted, Low Sulphur Grey Cast Irons for Thin Wall Castings

Electrically melted and over-heated (>1500 °C) grey cast iron at less than 0.04%S, as commonly used, solidifies large amounts of carbides and/or undercooled graphite, especially in thin wall castings; this is necessary to achieve a stronger inoculation. The efficiency of Ce-bearing FeSi alloy is tested for lower ladle addition rates (0.15 and 0.25 wt.%), compared to the base and conventional inoculated iron (Ba,Ca-bearing FeSi alloy). The present work explores chill and associated structures in hypoeutectic grey iron (3.6–3.8%CE, 0.02%S, (%Mn) × (%S) = 0.013–0.016, Alres < 0.002%), in wedge castings W1, W2 and W3 (ASTM A 367, furan resin sand mould), at a lower cooling modulus (1.1–3.5 mm) that is typically used to control the quality of thin wall iron castings. Relatively clear and total chill well correlated with the standard thermal (cooling curve) analysis parameters and structural characteristics in wedge castings, at different wall thickness, displayed as the carbides/graphite ratio and presence of undercooled graphite morphologies. The difference in effects of the two inoculants addition is seen as the ability to decrease the amount of carbides and undercooled graphite, with Ce-bearing FeSi alloy outperforming the conventional inoculant, especially as the wall thickness decreased. It appears that Ce-bearing FeSi alloy could be a solution for low sulphur, electric melt, thin wall iron castings production.

Ultra-reduced phases in ophiolites cannot come from Earth’s mantle

Abstract Several recent papers have purported to find ultra-reduced minerals—as natural examples—within ophiolitic mantle sections, including SiC moissanite, Fe-Si alloys, various metal carbides, nitrides, and borides. All those phases were interpreted to be mantle derived. The phases are recovered from mineral concentrates and are assigned to the deep mantle because microdiamonds and other ultrahigh-pressure (UHP) minerals are also found. Based on these findings, it is claimed that the mantle rocks of ophiolite complexes are rooted in the transition zone (TZ) or even in the lower mantle, at redox states so reduced that phases like SiC moissanite are stable. We challenge this view. We report high-temperature experiments carried out to define the conditions under which SiC can be stable in Earth’s mantle. Mineral separates from a fertile lherzolite xenolith of the Eifel and chromite from the LG-1 seam of the Bushveld complex were reacted with SiC at 1600 K and 0.7 GPa. At high temperature, a redox gradient is quickly established between the silicate/oxide assemblage and SiC, of ~12 log-bar units in fO2. Reactions taking place in this redox gradient allow us to derive a model composition of an ultra-reduced mantle by extrapolating phase compositions to 8 log units below the iron-wüstite equilibrium (IW-8) where SiC should be stable. At IW-8 silicate and oxide phases would be pure MgO end-members. Mantle lithologies at IW-8 would be Fe° metal saturated, would be significantly enriched in SiO2, and all transition elements with the slightest siderophile affinities would be dissolved in a metal phase. Except for the redox-insensitive MgAl2O4 end-member, spinel would be unstable. Relative to an oxidized mantle at the fayalite-magnetite-quartz (FMQ) buffer, an ultra-reduced mantle would be enriched in enstatite by factor 1.5 since the reduction of the fayalite and ferrosilite components releases SiO2. That mantle composition is unlike any natural mantle lithology ever reported in the literature. Phases as reduced as SiC or Fe-Si alloys are unstable in an FeO-bearing, hot, convecting mantle. Based on our results, we advise against questioning existing models of ophiolite genesis because of accessory diamonds and ultra-reduced phases of doubtful origin.

An innovative route for valorising iron and aluminium oxide rich industrial wastes: Recovery of multiple metals

Several industrial wastes including biomass, fly ashes, red mud, mill scales, water treatment residues, have significant concentrations of metal oxides: Fe2O3, Al2O3, TiO2, SiO2 etc. Several efforts have been made towards recovering metals within these wastes. Rather than recovering one metal at a time, we report a novel approach for simultaneously extracting multiple metals from mixed oxides in a single process step. Using three distinct furnaces/heating regimes, the carbothermic reduction of Fe2O3/Al2O3/SiO2 system was investigated at 1450–1700 °C for up to 2 h over a wide composition range. Complete reduction was achieved for both Fe2O3 and SiO2 in all cases leading to the formation of Fe and Fe–Si alloys. The reduction of alumina at moderate temperatures was the key challenge. No alumina reduction was observed during reductions at 1450 °C. A partial reduction of alumina and the formation of Fe–Al alloys was detected in the Al2O3/Fe2O3/C system at 1550 °C. The formation of Fe–Si–Al alloys was also observed in the Fe2O3/SiO2/Al2O3/C system at 1550 °C. Complete reduction of alumina was observed at 1600–1700 °C, even for up to 50 wt% alumina in the system. Optimal operating conditions and reference standards were established for the simultaneous recovery of multiple metals from waste oxides. While conserving natural resources, this novel route will lower the burden on waste storage facilities with significant contributions to the economic and environmental sustainability of industrial waste management.

Theory of kink migration in dilute BCC alloys

Plastic deformation in elemental BCC metals and dilute alloys is controlled by the slower of the kink pair nucleation and kink migration processes along screw dislocations. In alloys nucleation is facilitated and migration inhibited, leading to a concentration- and temperature-dependent transition from nucleation dominance to migration dominance. Here, an analytical statistical model for the stress-dependent kink migration barrier in dilute BCC alloys is developed and validated. The barrier depends only on a clearly-defined solute/screw dislocation interaction parameter, the kink width, and dislocation length between jogs. The analytic model is extensively validated via fully atomistic nudged-elastic band calculations and stochastic simulations in a model Fe-Si alloy. Combined with a recent validated double-kink nucleation theory, a fully-analytic model for the temperature- and concentration-dependent flow stress is obtained that includes the transition from nucleation to migration control. The overall model is applied to Fe-Si and W-Re using independently-determined material properties and good agreement is obtained with experiments over a range of concentrations and temperatures. Overall, the two theories represent a unified, fully-statistical, parameter-free understanding of screw dislocation strength in dilute BCC alloys.

Study on the electromagnetic properties and microwave absorbing mechanism of flaky FeSiAl alloy based on annealing and phosphate coating

A low-frequency composite absorbing material with high permeability and low permittivity in low frequency was prepared after the flaky FeSiAl alloy powder was annealed in N2 atmosphere and modified with zinc-based phosphate solution. The effects of annealing temperature and phosphating time on the phase composition, morphology and static magnetic properties of the flaky FeSiAl alloy were investigated with x-ray diffraction (XRD), scanning electron microscope (SEM) and vibrating sample magnetometer (VSM). With the aid of vector network analyzer (VNA), the electromagnetic properties and absorbing performance of the powder treated in two ways were studied. The results showed that after annealing in N2 atmosphere, the crystallization degree of FeSiAl powder increased, the crystal type underwent a phase transition of A2-A2/B2-A2/B2/D03, the permittivity decreased, and the permeability was greatly improved. After phosphating, the phosphating reaction products were deposited on the surface of FeSiAl particles, the permittivity was significantly reduced, the low-frequency impedance matching and low-frequency absorption performance were improved. The causes and frequency changes of electromagnetic wave absorption peaks were analyzed with the λ/4 interference cancellation theory, the interface reflection model and the impedance matching principle.

Persistent slip observed in TiZrNbHfTa: A body-centered high-entropy cubic alloy

Slip properties of TiZrNbHfTa were evaluated to obtain ψ–χ relationships, where χ is defined as the angle between the maximum shear stress plane and (1¯01). ψ is defined as the angle between the apparent slip plane and (1¯01). Bending tests were conducted with micrometer-sized cantilevers fabricated from a single grain with a focused ion beam. Slip bands appear to be straight on a micrometer length-scale, as for Fe–Si alloys, which suggests a persistent slip. However, the ψ–χ relationship indicated that ψ is nearly the same as χ, suggesting that there is no persistent slip. Detailed observations with an atomic force microscope confirm that cross-slipping is so frequent at a sub-micrometer length-scale that the slip bands appear to be straight and the maximum shear stress plane is observed on a micrometer length-scale. Here, TiZrNbHfTa shows abnormally frequent cross-slipping, which is a novel characteristic of high-entropy body-centered cubic alloys.

Thermal expansion of liquid Fe-S alloy at high pressure

Local structure and density of liquid Fe-S alloys at high pressure have been determined in situ by combined angle and energy dispersive X-ray diffraction experiments in a multi-anvil apparatus, covering a large temperature and compositional range. Precise density measurements collected for increasing temperature allowed us to directly derive the thermal expansion coefficients for liquid Fe-S alloys as a function of composition. In turn, thermal expansion has been used to refine thermodynamic models and to address the crystallization regime of telluric planetary cores by comparing the adiabatic temperature gradient and the slope of the liquidus in the Fe-FeS system.For Fe-S cores of asteroids and small planetesimals, top-down solidification is the dominant scenario as the compositional domain for which the slope of the liquidus is greater than the adiabatic gradient is limited to a narrow portion on the Fe-rich side. However, bottom-up growth of the inner core is expected for S-poor cases, with this compositional domain expanding to more S-rich compositions with increasing pressure (size of the planetary body). In particular, bottom-up crystallization cannot be excluded for the Moon and Ganymede.

Zigzag domain boundaries in magnetostrictive Fe-Ga alloys

A recent development in magnetic materials research on a non-Joulian magnetostriction phenomenon [H. D. Chopra and M. Wuttig, Nature 521, 340 (2015)] highlights peculiar cellular magnetic domains with zigzag boundaries in Fe-Ga alloys. The cause of zigzag boundaries of cellular domains is attributed to hypothetical charge density waves beyond classical magnetic domain theory. In this paper, we report observations in Fe-Ga alloys of zigzag boundaries that form conventional stripe domains. The responses of stripe domains to external magnetic fields are observed, and the behaviors of zigzag boundaries are examined, which are further compared with those in cellular domains. It shows that both cellular and stripe domains and the constituent zigzag boundaries in magnetostrictive Fe-Ga alloys can be explained well by classical magnetic domain theory, without resorting to theory based on hypothetical charge density waves. In particular, our findings provide convincing evidence that zigzag boundaries in Fe-Ga alloys are conventional V lines commonly observed in cubic magnetic crystals like Fe-Si alloys. The intricate cellular domain structure is shown to correlate with the simple stripe domain structure in terms of zigzag V lines and flux-closure surface domains.

Tuning the electronic and magnetic properties of CoZrxNb1−xFeSi alloys for spintronic and thermoelectric applications

AbstractThis ab‐initio investigation presents the effect of substitution of Zr by Nb atom on the structural, electronic, magnetic, and transport properties of the quaternary Heusler CoZrFeSi alloy. The ab‐initio calculations were performed using the exchange‐correlation of the modified Becke‐Johnson potential. Our results predict that the crystalline phase for each of CoZrFeSi and CoNbFeSi alloys is classified as Type‐I. The calculated formation energy value of the Nb‐doped alloys indicates the thermodynamic stability of the alloys and their possible creation. The bulk modulus value predicts that the rigidity increases with increasing the concentration of the Nb substitute atoms in the alloy. The electronic band structure and the density of states indicate a half metallicity of the alloys with metallic and semiconducting behaviors in the spin up and down, respectively. The studied alloys have shown ferromagnetic character with a high magnetic moment value when increasing the Nb content up to 75%. The calculated Seebeck coefficient and the charge carrier concentration as a function of temperature predict that the present alloys prefer the n‐type doping with the ability to switch between hole and electron for transporting the charges through the Fermi level of CoZr1−xNbxFeSi (x = 0, 0.25, 0.75, and 1) alloys. However, CoZr0.5Nb0.5FeSi alloy prefers electrons as a charge carrier. Due to their determined properties, beneficial applications are predicted for the present alloys, such as magnetic tunnel junctions and spintronic applications.

Grain growth in soft magnetic ferritic stainless steels under compressive stresses at high temperatures

Precipitation-hardened ferritic stainless steels are used for soft magnetic actuators of engines, as these steels exhibit high strength and soft magnetic properties. These ferritic stainless steels are hardened by nanoscale precipitates formed during an aging process after solution treatment. Furthermore, texture control of these steels is required in order to improve the soft magnetic properties. Therefore, texture control of ferritic stainless steels with <100> fiber texture by deformation at high temperatures is attempted in this study, as it has been demonstrated that the texture of ferritic Fe-Si alloys is significantly changed by deformation conditions, including temperature and strain rate. The results demonstrate that grains with the <100> fiber texture in the present stainless steels are preferentially evolved by compressive deformation under specific strain rates at high temperatures.