Melting and solidification characteristics of Zr-, Ni-, and Mn-containing 354-type Al-Si-Cu-Mg cast alloys

This study investigated the effects of adding Zr, as a base alloying element, besides Ni and Mn in different amounts and combinations on the melting and solidification characteristics of 354-type Al-Si-Cu-Mg alloys. Differential scanning calorimetry (DSC) was used to characterise the sequence of reactions occurring during the heating and/or cooling cycles; whereas scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques were used to observe and identify existing intermetallic phases. Nickel proved to have a retarding effect on the kinetics of precipitation of the α-Al network and the eutectic Al-Si structure. Also, the presence of Ni consumed a considerable amount of Cu to form Al-Cu-Ni particles instead of Al₂Cu particles. Results revealed that solution treatment at 495°C for 5?h was sufficient to dissolve a large amount of Al₂Cu particles in the α-Al matrix, which is mandatory for a successful aging treatment of the alloys studied. Additions of these transition elements produced new intermetallic phases such as (Al,Si)₃(Ti,Zr), (Al,Si)₃Zr, Al₉FeNi, Al₃Ni, Al₃CuNi, and Al₉FeSi₃Ni₄Zr, in addition to the other phases, namely α-Al, eutectic silicon, Al₂Cu, Mg₂Si, Q-phase (Al₅Cu₂Mg₈Si₆), commonly observed in 354-type alloys, and Fe-based intermetallic phases including β-Al₅FeSi, α-Al₁₅(Fe, Mn)₃Si₂, and π-Al₈FeMg₃Si_6. Superheating the melt at 800°C instead of 750°C had an advantageous effect in that Al₃Zr particles originating from the Al-15%Zr master alloy were dissolved and hence coarse Zr-containing particles were barely spotted in the microstructures examined.     

Influence of impurity elements on the nucleation and growth of Si in high purity melt-spun Al–Si-based alloys

The nucleation and growth of Si has been investigated by TEM in a series of high purity melt spun Al–5Si (wt%)-based alloys with a trace addition of Fe and Sr. In the as-melt-spun condition, some twinned Si particles were found to form directly from the liquid along the grain boundary. The addition of Sr into Al–5Si-based alloys promotes the twinning of Si particles on the grain boundary and the formation of Si precipitates in the α-Al matrix. The majority of plate-shaped and truncated pyramid-shaped Si precipitates were also found to nucleate and grow along {111}_α-Al planes from supersaturated solid solution in the α-Al matrix. In contrast, controlled slow cooling decreased the amount of Si precipitates, while the size of the Si precipitates increased. The orientation relationship between these Si precipitates and the α-Al matrix still remained cube to cube. The β-Al₅FeSi intermetallic was also observed, depending on subsequent controlled cooling.     

Intermetallic phase selection during homogenization for AA6082 alloy

The intermetallic phase selection during homogenization in a AA6082 alloy has been investigated. A short homogenization treatment at 580?{\degr}C for 4?h and a subsequent slow cooling with a rate of about 8?{\degr}C/min, rather than a conventional quick quenching into water, results in the formation of the rounded discrete Al₁₅(FeMnCr)₃Si₂ phase, which greatly suppressed the formation of the plate-shaped β-Al₅FeSi phase. Most of the Al₁₅(FeMnCr)₃Si₂ particles were observed to be very fine (about 200?nm) and homogeneously distributed in the α-Al matrix. Fine Mg₂Si particles were also observed to be located in the vicinity of Al₁₅(FeMnCr)₃Si₂ particles. Additionally, the β series precipitates (most likely β″-Mg2Si), large-scale U2-AlMgSi and B′ (type C) phases were also observed within the α-Al matrix. This investigation demonstrates that the size and distribution of the desired intermetallic phases can be influenced by suitable homogenization treatments.     

Study of microstructure and phase evolution of hot-dipped aluminide mild steel during high-temperature diffusion using electron backscatter diffraction

Mild steel was coated by hot-dipping into a molten aluminum bath. The microstructure and phase evolution in the aluminide layer during diffusion at 750 °C in static air were analyzed by electron backscatter diffraction (EBSD). The results showed that the aluminide layer of the as-coated specimen consisted of an outer aluminum topcoat, minor FeAl₃ and major Fe₂Al₅, respectively. Also, Fe₂Al₅ possessed a tongue-like morphology, which caused corresponding serration-like morphology in the steel substrate. A portion of the peaks of serration-like substrate were isolated, after short exposure at 750 °C, and accompanied by the formation of voids, which continued to appear with further exposure at 750 °C. As the aluminum topcoat was consumed, FeAl₃ phase disappeared and left an aluminide layer of Fe₂Al₅ phase. After 60 min of exposure, FeAl₂ and FeAl phases formed at the interface between Fe₂Al₅ and the steel substrate. With increasing exposure time, the voids condensed and the serration-like morphology disappeared, while FeAl₂ and FeAl phases kept growing. After prolonged exposure, the aluminide layer was composed of FeAl₂ and FeAl and possessed a flat interface between FeAl and steel substrate.     

Formation of Three-Component Phases in Silumins Using a Modifying Mixture Based on Refractory Metals

The results are presented from experimental studies of the microstructure and phase composition of AK7ch Al–Si alloy with an iron content of 0.4 wt % before and after introducing a modifying mixture based on ultradisperse powders of metal oxides and cryolite into the melt. The formation of three-component phases α-Al₂FeSi and β-Al₅FeSi is established experimentally. The effect iron has on the crystallization of eutectic mixtures is considered using the phase diagram of the Al–Fe–Si system.     

Heat treatment induced intermetallic phase transition of arc-sprayed coating prepared by the wires combination of aluminum-cathode and steel-anode

A method to prepare intermetallic composite coatings employing the cost-efficient electric arc spraying twin wires assistant with suitable heat treatment was developed. In this study, a Fe-Al composite coating was produced by spraying twin wires, i.e. a carbon steel wire as the anode and an aluminum wire as the cathode. The inter-deposited Fe-Al coating was transformed in-situ to Fe-Al intermetallic composite coating after a post annealing treatment. The effect of annealing treatment conditions on phase composition, microstructure and mechanical properties of the coating was investigated by using XRD, SEM, EDS and OM as well as microhardness tester. The results show that the desirable intermetallic phases such as Fe₂Al₅, FeAl and Fe₃Al are obtained under the annealing condition. The main oxide in the coating is FeO which can partially transform to Fe₃O₄ up to the annealing condition.     

The structures and properties of FeSin/FeSi\hbox{$_{\mathsf{n}}^{+}$}+n/FeSi\hbox{$_{\mathsf{n}}^{-}$}−n (n = 1 ~ 8) clusters

The geometry, stability, and electronic properties of iron-doped silicon clusters FeSi _n /FeSi\hbox{$_{n}^{+}$}+n/FeSi\hbox{$_{n}^{-}$}?n (n = 1 ~ 8) have been systematically investigated using the density functional theory (DFT) approach at the B3LYP/6-311+G* level. Our results show that the ground state structures of FeSi _n /FeSi\hbox{$_{n}^{+}$}+n/FeSi\hbox{$_{n}^{-}$}?n change from planar to three-dimensional for n > 3. Bipyramidal structures, or their face-capped isomers, are favored for the larger clusters. For neutral FeSi _n clusters, their ground state structures are the trigonal, tetragonal, capped tetragonal, capped pentagonal, and combined tetragonal bipyramids for n = 4 ~ 8, respectively. The lowest-energy structures of the anionic FeSi\hbox{$_{n}^{-}$}?n clusters essentially retain similar frameworks to their neutral counterparts, while those of the cationic FeSi\hbox{$_{n}^{+}$}+n clusters are significantly deformed; this is confirmed by their calculated ionization potential and electronic affinity values. For most of the stable structures, the spin electronic configurations are s = 1 or 2 for neutral FeSi _n , s = 3/2 or 5/2 for ionic FeSi\hbox{$_{n}^{+}$}+n/FeSi\hbox{$_{n}^{-}$}?n. The average binding energy values generally increase with increasing cluster size, indicating the clusters can continue to gain energy during the growth process. Fragmentation and second-order energy peaks (maxima) are found at n = 2, 5, and 7 for FeSi _n /FeSi\hbox{$_{n}^{-}$}?n, n = 4 and 6 for FeSi\hbox{$_{n}^{+}$}+n, suggesting that these clusters possess higher relative stability. Furthermore, the HOMO-LUMO gap values show that anionic FeSi\hbox{$_{n}^{-}$}?n have greater chemical reactivity than cationic FeSi\hbox{$_{n}^{+}$}+n and neutral FeSi _n , except when n = 7.     

Ab initio computer simulation of adsorption of a Fe monolayer on Si(111)

Computer simulation of adsorption of an Fe monolayer on Si(111) is carried out in the generalized gradient approximation in the density functional theory. It is shown that mixing of Fe and Si atoms followed by the replacement of Si atoms and the formation of a silicide-like film containing two types of domains (with the A8 and B8 structure) takes place in the course of deposition. Analysis of the atomic structure of this compound shows that the formation of this interface makes it possible to obtain an FeSi film (with the CsCl-type structure) as well as FeSi₂ film (with a CaF₂-type structure).     

Deposition of aluminide and silicide based protective coatings on niobium

We compare aluminide and alumino-silicide composite coatings on niobium using halide activated pack cementation (HAPC) technique for improving its oxidation resistance. The coated samples are characterized by SEM, EDS, EPMA and hardness measurements. We observe formation of NbAl₃ in aluminide coating of Nb, though the alumino-silicide coating leads to formation primarily of NbSi₂ in the inner layer and a ternary compound of Nb-Si-Al in the outer layer, as reported earlier (Majumdar et al. [11]). Formation of niobium silicide is preferred over niobium aluminide during alumino-silicide coating experiments, indicating Si is more strongly bonded to Nb than Al, although equivalent quantities of aluminium and silicon powders were used in the pack chemistry. We also employ first-principles density functional pseudopotential-based calculations to calculate the relative stability of these intermediate phases and the adhesion strength of the Al/Nb and Si/Nb interfaces. NbSi₂ exhibits much stronger covalent character as compared to NbAl₃. The ideal work of adhesion for the relaxed Al/Nb and Si/Nb interfaces are calculated to be 3226 mJ/m² and 3545 mJ/m², respectively, indicating stronger Nb-Si bonding across the interface.     

Synthesis of ultrathin semiconducting iron silicide epilayers on Si(1 1 1) by high-temperature flash

In this work ultrathin iron silicide epilayers were obtained by the reaction of iron contaminants with the Si(1 1 1) substrate atoms during high-temperature flash. After repeated flashing at about 1125 °C, reflection high-energy electron diffraction indicated silicide formation. Scanning tunneling microscopy revealed highly ordered surface superstructure interrupted, however, by a number of extended defects. Atomic-resolution bias-dependent imaging demonstrated a complex nature of this superstructure with double-hexagonal symmetry and (2√3×2√3)-R30° periodicity. Among the possible candidate phases, including metastable FeSi₂ with a CaF₂ structure and FeSi_1+x with a CsCl structure, the best match of the interatomic distances to the measured 14.4 Å × 14.4 Å unit cell dimensions pointed to the hexagonal Fe₂Si (Fe₂Si prototype) high-temperature phase. The fact that this phase was obtained by an unusually high-temperature flash, and that neither its reconstruction nor its semiconducting band-gap of about 1.0 ± 0.2 eV (as deduced form the I-V curves obtained by scanning tunneling spectroscopy) has ever been reported, supports such identification. Due to its semiconducting properties, this phase may attract interest, perhaps as an alternative to β-FeSi₂.     

Initial stages of iron silicide formation on the Si(1 0 0)2 × 1 surface

The initial stages of iron silicide growth on the Si(1 0 0)2 × 1 surface during solid-phase synthesis were investigated by photoelectron spectroscopy using synchrotron radiation. The experiments were made on iron films of 1-50 monolayer (ML) thickness in the temperature range from room temperature to 750 °С. Our results support the existence of three stages in the Fe deposition on Si(1 0 0) at room temperature, which include formation of the Fe-Si solid solution, Fe₃Si silicide and an iron film. The critical Fe dose necessary for the solid solution to be transformed to the silicide is found to be 5 ML. The solid-phase reaction was found to depend on the deposited metal dose. At 5 ML, the reaction begins at 60 °С, and the solid-phase synthesis leads to the formation of only metastable silicides (FeSi with the CsCl-type structure, γ-FeSi₂ and α-FeSi₂). A specific feature of this process is Si segregation on the silicide films. At a thickness of 15 ML and more, we observed only stable phases, namely, Fe₃Si, ε-FeSi and β-FeSi₂.     

Silicide formation in bilayer ultrathin iron and cobalt films on silicon

The processes that occur in ultrathin (up to 1 nm) Fe and Co layers during deposition onto the Si(100)2 × 1 surface in various sequences and during annealing of the formed structures to a temperature of 400°C are studied. The elemental and chemical compositions of the films are analyzed by in situ high-resolution X-ray photoelectron spectroscopy using synchrotron radiation, and their magnetic properties are determined using the magnetic linear dichroism effect in the angular distribution of Fe 3p and Co 3p electrons. It is shown that, when iron is first deposited, the formed structure consists of the layers of FeSi, Fe₃Si, Co-Si solid solution, and metallic cobalt with segregated silicon. The structure formed in the alternative case consists of the layers of CoSi, Co-Si solid solution, Co, Fe-Si solid solution, and Fe partly covered by silicon. All layers (apart from FeSi, CoSi) form general magnetic systems characterized by ferromagnetic ordering. Annealing of the structures at temperatures above 130dgC (for the Co/Fe/Si system) and ～200°C (for Fe/Co/Si) leads to the formation of nonmagnetic binary and ternary silicides (Fe _x Co_{1 ? x} Si, Fe _x Co_{2 ? x} Si).     

The semiconductor-to-ferromagnetic-metal transition in FeSb2

We propose FeSb₂ to be a nearly ferromagnetic small gap semiconductor, hence a direct analog of FeSi. We find that despite different compositions and crystal structures, in the local density approximation with on-site Coulomb repulsion correction (LDA+U) method magnetic and semiconducting solutions for U=2.6 eV are energetically degenerate similar to the case of FeSi. For both FeSb₂ and FeSi (FeSi_1-xGe_x alloys) the underlying transition mechanism allows one to switch from a small gap semiconductor to a ferromagnetic metal with magnetic moment ≈1 μ_B per Fe ion with external magnetic field.     

Recoilless fraction, structural, magnetic and thermal properties of Fe73.5Cu1Nb3Si13.5B9 alloy

Differential scanning calorimetry, X-ray diffraction and room temperature Mössbauer spectrum measurements of Fe_73.5Cu₁Nb₃Si_13.5B₉ (Finemet) alloy have been carried out in order to study its structural and magnetic properties as a function of annealing temperature. The DSC profile of as-quenched Finemet showed two exothermic peaks at 530 and 702 °C, corresponding to two crystallization processes. The Finemet alloy remains amorphous at 450 °C with one broad peak in XRD pattern and one broad sextet in Mössbauer spectrum. When the Finemet alloy was annealed at 550 °C, only well indexed body-center-cubic phase was detected. After being annealed at 650 and 750 °C, the XRD patterns showed the coexistence of α-Fe(Si) and Fe-B intermetallic phases with the increase in XRD peak intensities, indicating the growth of crystallites and the decomposition of Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy at elevated temperatures. The Mössbauer spectra of annealed Finemet alloy could be fitted with 4 or 5 sextets and one doublet at higher annealing temperatures, revealing the appearance of different crystalline phases corresponding to the different Fe sites above the crystallization temperature. The appearance of the nanocrystalline phases at different annealing temperatures was further confirmed by the recoilless fraction measurements.     

Phase evolution of an aluminized steel by oxidation treatment

Effects of temperature and time on the microstructure and phase evolution for different thermal treatments were investigated with respect to the measurement of intermetallic layer thickness, phase identification and microhardness distribution in the aluminized zone of a steel substrate. The intermetallic phases present in the aluminized region after hot dip aluminizing is mainly Fe₂Al₅. The thickness of the intermetallic layers increases with increasing oxidation temperature and time. In the oxidation treatments of the aluminized steel in air, the initial Fe₂Al₅ phase remains at the temperature below 950 °C in 2-h, and the Fe₂Al₅ phase is completely transformed into low iron content Fe-Al intermetallics due to oxidation at 950 °C for 4 h. However, the Fe₂Al₅ phase remains in the outer layer of the aluminized samples diffusion-treated in vacuum regardless of diffusion time. The microhardness values of the Al₂O₃ and the intermetallic Fe₂Al₅, FeAl₂, FeAl and Fe₃Al phases are HV1150, HV1010, HV810, HV650 and HV320, respectively. The oxide layer formed on the steel substrate has an extremely fast adherence to the steel substrate and excellent properties of thermal shock resistance, high temperature oxidation resistance and anti-liquid aluminum corrosion.     

Surface morphology and luminescence characterization of β-FeSi2 thin films prepared by pulsed laser deposition

β-FeSi₂ thin films were prepared on Si (1 1 1) substrates by pulsed laser deposition (PLD) with a sintering FeSi₂ target and an electrolytic Fe target. The thin films without micron-size droplets were prepared using the electrolytic Fe target; however, the surface without droplets was remarkably rougher using the Fe target than using the FeSi₂ target. After deposition at 600 °C and then annealing at 900 °C for 20 h, XRD indicated that the thin film prepared using the Fe target had a poly-axis-orientation, but that prepared using the FeSi₂ target had a one-axis-orientation. The PL spectra of the thin films prepared using the FeSi₂ and Fe targets at a growth temperature of 600 °C and subsequently annealed at 900 °C for 20 h had A-, B- and C-bands. Moreover, it was found that the main peak at 0.808 eV (A-band) in the PL spectrum of the thin films prepared using the FeSi₂ target was the intrinsic luminescence of β-FeSi₂ from the dependence of PL peak energy on temperature and excitation power density.     

Kinetics of ß-phase transformation in the heat treatment of FeSi2- and Fe2Si5-based thermoelectric alloys

FeSi₂- and Fe₂Si₅-based thermoelectric alloys have been fabricated by melt spinning and levitation melting. It was found that the levitation-melted FeSi₂-based alloy must be annealed at 800°C for 10 h to complete transformation of the β phase, while an anneal for only 6 h was needed for the melt-spun alloy. On the other hand, annealing the levitation-melted Fe₂Si₅-based alloy for 4 h was enough to complete β-phase formation, whereas 14 h was required for the melt-spun alloy. Annealing temperature dependence of the Seebeck coefficient showed that the maximum rate of transformation to β phase occurred at about 800°C for all samples. Application of the Johnson-Mehl-Avrami equation revealed that grain-boundary nucleation was predominant in the levitation-melted FeSi₂-based alloy (time exponent n = 1.1), while the zero nucleation mechanism was operative in the melt-spun alloy (n = 3.1). For the eutectoid reactions in the Fe₂Si₅-based alloys, several kinds of nucleation site were active. However, nuclei formed at grain edges were dominant in the melt-spun alloy since n = 1.6.     

Structural transformations of Fe81B13Si4C2 amorphous alloy induced by heating

Thermal stability and crystallization of the Fe₈₁B₁₂Si₄C₂ alloy were investigated in the temperature range 25-700 °C by the XRD and Mössbauer analysis. It was shown that on heating the as-prepared amorphous Fe₈₁B₁₂Si₄C₂ alloy undergoes thermal stabilization through a series of structural transformations involving the process of stress-relieving (temperature range 200-400 °C), followed by a loss of ferromagnetic properties (Curie temperature at 420 °C) and finally crystallization (temperature range 450-530 °C). The process of crystallization begins by formation of two crystal phases: Fe₃B and subsequently Fe₂B, as well as a solid solution α-Fe(Si). With increase in annealing temperature, the completely crystallized alloy involved only two phases, Fe₂B and solid solution α-Fe(Si).XRD patterns established a difference in phase composition and size of the formed crystallites during crystallization depending on the side (fishy or shiny) of the ribbon. The first nuclei of the phase α-Fe₃Si were found on the shiny side by XRD after heat treatment even at 200 °C but the same phase on the fishy side of ribbon was noticed after heat treatment at 450 °C. The largest difference between the contact and free surface was found for the Fe₂B phase crystallized by heating at 700 °C, showing the largest size of crystallites of about 130 nm at 700 °C on the free (shiny) surface.     

Crystallization of some amorphous metallic alloys studied by Mössbauer spectroscopy

The present work provides an analysis of crystallization processes in amorphous metallic alloys Fe₈₀Si₄Cr_1.0B₁₄ and Fe₆₇Co₁₈B₁₄Si₁. Crystallization of the first sample started at the temperature of 648 K. The fully crystalline state was observed after annealing at 748 K. We identified four sextets. One corresponds to crystalline Fe₂B and the three others to FeSi solid solution with 10 at .% of Si. Crystallization of Fe₆₇Co₁₈B₁₄Si₁ started at the temperature of 623 K. We identified two crystalline phases. The first may have its origin as (Fe_1−xCo_x)₃B, the second one may correspond to a Fe−Co solid solution with a different Co content.