Fe-Si Alloys Research Articles

Binary hybrid filler composite formulations of surface modified Fe–Si–Al alloys for multifunctional EMI shielding and thermal conduction

Sendust/expanded graphite and Sendust/boron nitride binary composite fillers adhered through surface silane modification and their non-modified, simply mixed, polymer composites were fabricated for optimization of multifunctional EMI shielding and thermal conduction properties. The effect of filler type, filler content, surface modification, and dispersion properties in thermoplastic polyurethane matrix were examined. Through extensive screening of hybrid composite compositions, tunable properties in EMI shielding (2.8–32.1 dB at 3 GHz and 7.6–48 dB at 10 GHz, sample thickness 0.6 mm), through-plane thermal conductivity (0.3–1.694 W/mK), and in-plane thermal conductivity 1.102–6.084 W/mK) were elucidated through electrical conductivity measurements and dispersion morphology within the polymer matrix. The composite materials database reported in this study allows for the selection of customizable properties according to desired application.

Composition-dependent thermal equation of state of B2 Fe-Si alloys at high pressure

AbstractSolid iron-silicon alloys play an important role in planetary cores, especially for planets that formed under reducing conditions, such as Mercury. The CsCl (B2) structure occupies a considerable portion of the Fe-Si binary phase diagram at pressure and temperature conditions relevant for the core of Mercury, yet its thermodynamic and thermoelastic properties are poorly known. Here, we report in situ X-ray diffraction measurements on iron-silicon alloys with 7–30 wt% Si performed in laser-heated diamond-anvil cells up to ~120 GPa and ~3000 K. Unit-cell volumes of the B2 phase at high pressures and high temperatures have been used to obtain a composition-dependent thermal equation of state of this phase. In turn, the thermal equation of state is exploited to determine the composition of the B2 phase in hcp+B2 mixtures at 30–100 GPa and to place constraints on the hcp+B2/B2 phase boundary, determined to vary between ~13–18 wt% Si in the considered pressure and temperature range. The hcp+B2/B2 boundary of Fe-Si alloys is observed to be dependent on pressure but weakly dependent on temperature. Our results, coupled with literature data on liquid equations of state, yield an estimation of the density contrast between B2 solid and liquid under Mercury’s core conditions, which directly relates to the buoyancy of the crystallizing material. While the density contrast may be large enough to form a solid inner core by the gravitational sinking of B2 alloys in a Si-rich core, the density of the B2 solid is close to that of the liquid at solidus conditions for Si concentration approaching ~10 wt% Si.

Stable hexagonal ternary alloy phase in Fe-Si-H at 28.6–42.2 GPa and 3000 K

Hydrogen (H) and silicon (Si) are considered as important light elements for the planetary cores. A large amount of H is able to alloy with pure Fe metal at high pressures. Si can also alloy well with Fe. However, it remains uncertain how much H can alloy with iron silicides and if it alloys how H can alter the crystal structures of Fe-Si alloys at high pressures-temperatures ($P\text{\ensuremath{-}}T$). We performed experiments on Fe-9Si and Fe-16Si alloys (9 and 16 wt % Si, respectively) in a H medium up to 42.2 GPa and 3000 K in diamond-anvil cells coupled with pulsed laser heating and gated synchrotron x-ray diffraction techniques. We found conversion of the Fe-Si alloys into Fe-rich (fcc and dhcp $\mathrm{Fe}{\mathrm{H}}_{x}$), Si-rich (B20 and B2 FeSi), and intermediate (${\mathrm{Fe}}_{5}{\mathrm{Si}}_{3}{\mathrm{H}}_{x}$) phases. The new ${\mathrm{Fe}}_{5}{\mathrm{Si}}_{3}{\mathrm{H}}_{x}$ phase has a structure similar to the hexagonal ${\mathrm{Fe}}_{5}{\mathrm{Si}}_{3}$ phase but with expanded volumes, and thus, possible H incorporation. Both the observed volume expansion and the H content estimated by density-functional theory calculations support a significant amount of H with H/Fe \ensuremath{\approx} 0.6 in the crystal structure. Because ${\mathrm{Fe}}_{5}{\mathrm{Si}}_{3}$ is known to break down above \ensuremath{\sim}1300 K at \ensuremath{\sim}18 GPa, our results suggest that hydrogen stabilizes the hexagonal structure at high $P\text{\ensuremath{-}}T$. These results have implications for the crystallization of Fe-rich liquid at the solid-to-liquid boundary of planetary cores and possible existence of chemical heterogeneities in the solid cores.

Towards reconciling experimental and computational determinations of Earth's core thermal conductivity

The thermal conductivity (κ) of Earth's core is a critical parameter that controls predictions of core cooling rate, inner core age and the power available to the geodynamo. However, the values of core thermal conductivity inferred from recent studies span a wide range due to the challenges of extrapolating to the pressure-temperature-composition (P-T-C) conditions of the core liquid. In particular, extrapolations of κ from direct experimental determinations are lower than ab initio calculations conducted at core conditions. We have performed density functional theory (DFT) calculations to determine the thermal conductivity and resistivity (ρ) of solid FeSi alloys with two compositions, 4 mol % and 15 mol % Si, at a range of temperatures (850-4350 K) and pressures (60-144 GPa) for ease of comparison with recent directly measured κ values. In agreement with recent experiments, our calculations show that for the larger Si composition the resistivity of the mixture increases substantially, compared to pure Fe, reaching its saturated value already at the lowest temperature investigated. As a result, the thermal conductivity of the mixture is also correspondingly reduced. We also analysed the effect of possible errors in the DFT calculations due to the neglect of electron-electron scattering (EES) processes. Our results show that experimental and EES-corrected DFT calculations of κ are actually consistent within uncertainties when compared directly at overlapping P-T-C conditions. We present new core thermal history models using our EES-corrected estimates of κ=75−81 Wm−1K−1 at core-mantle boundary (CMB) conditions, which support previous determinations of late inner core formation around 400-700 Myrs ago and an early molten lower mantle.

Electromagnetic properties of Sm-doped FeSiAl alloy absorbing materials prepared by melt spinning

The flaky Sm1-xFexSiAl (x = 0,1,2) alloy powders were prepared by melt-spun technique with a wheel speed of 30 m s−1, followed by ball milling method. The flake-shaped FeSiAl alloy particles with the average particle size range from 5µm to 40µm were obtained. The XRD spectra show that the single α-Fe phase without other intermediate phase can be detected for all samples. Complex permittivity decreased remarkable VS complex permeability decreased somewhat with increasing Si content. With the thickness of 1.8 mm, the minimum Reflection Loss (RLmin) of Fe70Si20Al10 alloy powder is −3.4 dB at 2 GHz, while that of Sm2Fe68Si20Al10 alloy powder is −5.8 dB at 3.8 GHz. It is found that the Sm-doped FeSiAl composites can be potential microwave absorbing materials.

Properties of Fe-Si Alloy Anode for Lithium-Ion Battery Synthesized Using Mechanical Milling.

Silicon (Si)-based anode materials can increase the energy density of lithium (Li)-ion batteries owing to the high weight and volume capacity of Si. However, their electrochemical properties rapidly deteriorate due to large volume changes in the electrode resulting from repeated charging and discharging. In this study, we manufactured structurally stable Fe–Si alloy powders by performing high-energy milling for up to 24 h through the reduction of the Si phase size and the formation of the α-FeSi2 phase. The cause behind the deterioration of the electrochemical properties of the Fe–Si alloy powder produced by over-milling (milling for an increased time) was investigated. The 12 h milled Fe–Si alloy powder showed the best electrochemical properties. Through the microstructural analysis of the Fe–Si alloy powders after the evaluation of half/full coin cells, powder resistance tests, and charge/discharge cycles, it was found that this was due to the low electrical conductivity and durability of β-FeSi2. The findings provide insight into the possible improvements in battery performance through the commercialization of Fe–Si alloy powders produced by over-milling in a mechanical alloying process.

Developing Al–Fe–Si alloys with high thermal stability through tuning Fe, Si contents and cooling rates

The use of recycled aluminum alloys can greatly reduce energy consumption and carbon emissions, but the high iron content limits the scope of application of recycled aluminum alloys. The development of alloys with good performance under high Fe content can enable iron-rich recycled aluminum alloys to have a new application direction. The key is to control the type and morphology of the Fe-rich phase in the alloy. In this work, the microstructure evolution of Al–Fe–Si alloys with varying Fe, Si contents and cooling rates was studied. It is found that as the Si content decreases, the iron-rich phases transform from the β-Al9Fe2Si2 to the eutectic α-Al8Fe2Si. Within a certain composition range, increasing the cooling rate will not only refine the microstructure but also promote the transformation of β-Al9Fe2Si2 to α-Al8Fe2Si. In this study, a microstructure containing only one type of second phase, in the form of fine eutectic α-Al8Fe2Si, has been obtained by water cooling of Al–5Fe–3Si melt. The mechanical properties of the alloy are similar to those of the Al–Si alloy under the same conditions, while the thermal stability of the microstructure is significantly better than that of the Al–Si alloy. These findings not only provide new insights for avoiding β-Al9Fe2Si2 phase in aluminum alloys, but also provide guidance for the development heat-resistant alloys with Al - α-Al8Fe2Si eutectic structures.

Revisiting high-temperature phase transition and magnetocaloric effect of LaFe11.6Si1.4 alloy

Sufficient understanding on the formation mechanism of the La(Fe,Si)13 phase (1:13 or τ1) in La–Fe–Si alloys would be helpful in optimizing the parameters for high-temperature annealing. In-situ annealing experiments of a centrifugally as-cast LaFe11.6Si1.4 plate were performed by using a differential scanning calorimeter (DSC) in the present study. We confirmed that the τ1 phase is resulted from a peritectic reaction between La-rich liquid phase and α(Fe) solid phase when isothermal annealing at 1423 K. A new finding of a eutectoid decomposition that may occur within the residual La-rich liquid phase upon cooling was revealed. The isothermal transformation kinetics of the τ1 phase in the as-cast plate followed a simplified model of diffusion-controlled continuous network growth. The isothermal entropy change (|ΔSM|) was calculated using the Maxwell equation on M−H data. The maximum |ΔSM| value reached 26.89 J/(kg·K) under 30 kOe in the LaFe11.6Si1.4 sample annealed at 1423 K for 120 min.

Growth, interface structure, and magnetic properties of Fe/GaAs and Fe3Si/GaAs hybrid systems

Abstract Ferromagnetic Fe and Fe3Si films are grown on GaAs(001) substrates by molecular beam epitaxy. This review article summarizes the important aspects of the heteroepitaxial growth and the associated structural properties of the Fe/GaAs and Fe3Si/GaAs heterostructures. The magnetic anisotropy of the thin Fe epilayers is analyzed and correlated to the interface structure, which is determined by high-resolution transmission electron microscopy. The structural and electrical properties of Fe3Si alloys depend on the exact stoichiometry and the long-range atomic ordering that is controlled by the growth conditions during the molecular beam epitaxy. The interface reactivity and the thermal stability are critically discussed for both heterosystems.

The atomic structure, magnetic properties and bending ductility of a novel Fe-P-C-B-Si amorphous alloy investigated by experiments and ab initio molecular dynamics

The atomic structure, soft magnetic property and bending ductility of Fe-P-C-B-Si amorphous alloys via the change of Si content were investigated. The minor and appropriate Si addition can increase the Curie temperature and saturated magnetization (Ms) effectively. Compared with the 0 at% Si, the Ms of Fe85P10C2B2Si1 and Fe85P9C2B2Si2 can be enhanced up to 1.58 T and 1.61 T, respectively, which is related to the increase of the total magnetic moments analyzed via ab initio molecular dynamics (AIMD) simulation. Furthermore, the Ms of the annealed amorphous alloys can be continued to increase and the highest Ms of the Fe85P9C2B2Si2 amorphous alloys can reach up to 1.76 T and the lowest coercivity (Hc) can be reduced to 5.15 A/m after the ribbons were annealed at 627 K. These phenomena are related to the release of inner stress and the formation of α-Fe clusters in amorphous alloys caused by annealing. All ribbons remain good bending ductility annealed at optimal temperature (627 K) determined by Ms and Hc. The combination of high Ms, low Hc, good bending ductility after annealing as well as low production cost and high manufacturability makes the Fe-P-C-B-Si amorphous alloys with high Fe content promising as complement and even replacement of the Si-Fe alloys in magnetic and electric devices.

Hydrogen solubility in FeSi alloy phases at high pressures and temperatures

AbstractLight elements alloying with metallic Fe can change the properties and therefore play a key role in the structure and dynamics of planetary cores. Hydrogen and silicon are possible light elements in the rocky planets’ cores. However, hydrogen storage in Fe-Si alloy systems remains unclear at high pressures and high temperatures because of experimental difficulties. Taking advantage of pulsed laser heating combined with high-energy synchrotron X-ray diffraction, we studied reactions between FeSi and H in laser-heated diamond-anvil cells (LHDACs) up to 61.9 GPa and 3500 K. We found that under H-saturated conditions the amount of H alloying with FeSi (0.3 and <0.1 wt% for the B20 and B2 structures, respectively) is much smaller than that in pure Fe metal (>1.8 wt%). Our experiments also suggest that H remains in the crystal structure of FeSi alloy when recovered to 1 bar. Further density functional theory (DFT) calculations indicate that the low-H solubility likely results from the highly distorted interstitial sites in the B20 and B2 structures, which are not favorable for H incorporation. The recovery of H in the B20 FeSi crystal structure at ambient conditions could open up possibilities to understand geochemical behaviors of H during core formation in future experiments. The low-H content in FeSi alloys suggests that if a planetary core is Si-rich, Si can limit the ingassing of H into the Fe-rich core.

Inner core composition paradox revealed by sound velocities of Fe and Fe-Si alloy

Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (VS) and high Poisson’s ratio (σ) in the solid inner core. To date, neither VS nor σ of Fe and Fe-Si alloy have been measured under core conditions. Here, we present VS and σ derived from direct measurements of the compressional wave velocity, bulk sound velocity, and density of Fe and Fe-8.6 wt%Si up to ~230 GPa and ~5400 K. The new data show that neither the effect of temperature nor incorporation of Si would be sufficient to explain the observed low VS and high σ of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease VS and increase σ. However, the physical property-based Fe-Si-C core models seemingly conflict with the partitioning behavior of Si and C between liquid and solid Fe.

Role of scan strategies and heat treatment on grain structure evolution in Fe-Si soft magnetic alloys made by laser-powder bed fusion

A major goal in printing soft magnetic Fe-Si steels using additive manufacturing is to take advantage of the potential for complex geometric designs and site-specific grain control. One major step in the processing of these alloys is understanding how processing parameters might impact how the as-built microstructure responds to annealing (i.e. the annealing response). The impact of scan strategy on the annealing response for thin wall geometries is specifically explored. Two scan strategies were explored for a thin wall geometry that produced a strongly columnar grain structure and equiaxed grain structure. Samples from both scan strategies annealed at 1200 °C showed a marked difference in annealing response with the more equiaxed sample seeing full recrystallization and grain growth, while the more columnar grain structure saw little change in microstructure. After analysis through characterization techniques and thermal-mechanical simulations Differences in internal energy within the grains were ruled out because calculated GND density values were similar for both samples. The formation of secondary particles was ruled out as a contributing factor due to the type of oxide formations and their size. It was concluded that the contributing factor to the difference in the annealing response were a difference in the resulting grain size and the density of high angle grain boundaries. These two differences were largely attributed to differences in the thermal gradient conditions due to grains preferentially growing in the direction of the steepest thermal gradient. • Different scan strategies result in different microstructures for thin wall samples printed in Soft magnetic Fe-3Si Steel. • Differences in resulting microstructure between samples can be attributed to differences in the resulting thermal gradient. • Annealing of samples show that the sample with a higher number of equiaxed grains saw faster grain growth. • Similar geometric necessary dislocation (GND) densities were noted for the as-built samples but not the annealed samples. • Differences in annealing response is attributed to differences in grain size and density of high angle grain boundaries.

4D synchrotron X-ray tomographic study of the influence of transverse magnetic field on iron intermetallic compounds precipitation behavior during solidification of Al–Si–Fe alloy

Hard and brittle iron intermetallic compounds (IMCs) will be formed during solidification due to the existence of iron element in Aluminum alloy. These IMCs will cause cracks or fractures in the material, which can deteriorate the mechanical properties. Synchrotron X-ray tomography were used to study the real time formation and growth dynamics of IMCs in Al-10 wt.%Si-1 wt.%Fe alloy during solidification under transverse magnetic field (TMF). The results indicated that the precipitation behavior of IMCs was significantly affected by 0.07 T TMF. The IMCs were observed to form only on the periphery of the sample under TMF, comparing with random distribution without TMF. In addition, the growth dynamics of IMCs were also affected by TMF. When solidified without TMF, an explosive nucleation and growth of IMCs was presented at the intermediate solidification stage, where a large number of fine IMCs precipitated at the entire regions of the sample. In contrast, the IMCs located at the periphery areas were observed to grow steadily under TMF. The growth rate and formation mechanism of IMCs were also quantified and revealed, showing difference with and without TMF. The tip growth rate was relatively stable when solidified with TMF, while an accelerating growth stage was presented when TMF is absent. The discrepancy in precipitation behavior of IMCs could be best explained as the convection of the melt resulted from the applied TMF. The TMF was demonstrated to generate a directional flow with a velocity of ∼3.3 mm/s. The flow was supposed to drive the Fe solute from the center to the edge of the sample, providing enough Fe solute for IMCs to nucleate and grow on the surface of the sample. In addition, due to the influence of TMF induced flow on the distribution of Fe solute and temperature, the morphology and tip growth rate of IMCs were also affected. The result presented in current study provides the possibility to regulate the precipitation behavior of Fe-rich IMCs in Al alloys. The imposed TMF also shows the potential application in Fe removal by driving the Fe solute to the surface of sample in Al alloys.

Screw-controlled strength of BCC non-dilute and high-entropy alloys

The impressive mechanical properties of single phase BCC High Entropy Alloys (HEAs) has renewed interest in understanding the mechanisms controlling strength in BCC alloys. Theory and simulation of strengthening of screw dislocations in HEAs shows that screw dislocations become spontaneously kinked along their length to lower the total energy due to solute/screw interactions. Dislocation motion then mainly occurs via two mechanisms, lateral kink migration and breaking of cross-kinks due to kink formation on different glide planes. Existing kink migration models, in spite of successes in capturing some experiments, are based on several invalid assumptions. Here, a new theory for the kink migration in HEAs is developed based on recent understanding in dilute alloys, leading to a fully derived analytical model for the kink migration energy barrier as a function of applied stress and kink spacing. Results emerge to be quantitatively close to those presented in recent theory by Maresca and Curtin but are on much firmer theoretical basis. A revised version of the Maresca-Curtin screw strengthening theory that incorporates the new kink migration model is compared to experiments on non-dilute Fe-Si, Nb-Mo, and Nb-W alloys and shows broad agreement as a function of temperature and composition, establishing the quantitative accuracy of the new theory.

In-situ reactive synthesis of dense nanostructured β-FeSi2 by Spark Plasma Sintering

β-FeSi2 is a promising material for thermoelectric application, especially if it can be nanostructured to decrease the lattice contribution to the thermal conductivity. It can be even more interesting if the entire chain of processes implemented to obtain it is not too time and energy consuming. In this paper we report a simple route to synthesize nano-β-FeSi2 by combining mechanical milling and reactive spark plasma sintering of Fe-Si alloys. A map of the pressure-temperature conditions using a spark plasma sintering setup was performed and shows that it is possible to obtain nano-β-FeSi2 pellets with a density above 90% using dwell times as short as 5 min. The impact, of both, the density and nanostructuring on the hardness of the β-FeSi2 pellets obtained by this process are presented as well as the impact of the thermoelectric properties of the materials.

Kinetic Monte Carlo Simulation of Abnormal Grain Growth in Textured Systems with Anisotropic Grain Boundary Energy and Mobility

The abnormal grain growth behavior of an individual grain in the textured system was simulated by kinetic Monte Carlo method with focusing on the effect of anisotropy configuration and texture intensity. The mobility anisotropy and the energy anisotropy of grain boundary were derived based on the theoretical and experimental data in Fe-Si alloy. The results indicated that the single configured system with anisotropic grain boundary mobility or energy led to apparently unreasonable abnormal growth kinetic and boundary evolution at the growing front of the candidate grains. The composite configured system with anisotropic grain boundary mobility and energy accurately reproduced the microstructure evolution during abnormal grain growth. The 20°− 45° boundaries at the growing front varied with a certain range in the textured systems. The diameter of the abnormal grain increased linearly with the time, and the growth rate decreased with the weakening of matrix texture intensity. It is found that the grain boundary energy anisotropy plays the key role in the system with relatively strong matrix texture, while the grain boundary mobility anisotropy plays the key role in the system with random matrix texture. By converting the simulation time to the real time, the abnormal grain growth rate is estimated to be ∼1188.5–996.5 µm/min, which is roughly consistent with the actual secondary recrystallization. The present simulation is helpful to understand the abnormal grain growth phenomenon, especially the Goss grain growth in grain-oriented electrical steel. Data availabilityThe data required to reproduce these findings cannot be shared at this time since the data forms part of an ongoing study.

Optimizing Low Frequency Electromagnetic Parameters by Controlling the Distribution of Boron in Fesial Alloy

Optimizing Low Frequency Electromagnetic Parameters by Controlling the Distribution of Boron in Fesial Alloy

A New Strategy for Separating Fe-Si Alloys and Alumina by Controlling Fe/Si Ratios During the Vacuum Thermal Reduction of Coal Fly Ash

A New Strategy for Separating Fe-Si Alloys and Alumina by Controlling Fe/Si Ratios During the Vacuum Thermal Reduction of Coal Fly Ash

Improving Oxidation Resistance of Low Fe-Si Alloys Helped by Preheating Treatment

Improving Oxidation Resistance of Low Fe-Si Alloys Helped by Preheating Treatment