Enhanced corrosion resistance properties of Fe-Al-Cr laser cladding coatings on 1045 carbon steel substrates.

Fe-Al-Cr coatings with different content of Cr additive were prepared on 1045 carbon steel substrates by a laser cladding process. The incorporation of Cr atoms can effectively enhance the corrosion resistance of the coatings. In particular, the Fe-28Al-5Cr laser cladding coating exhibits the best film quality without phase segregation. In addition, the interfacial adhesion between the Fe-28Al-5Cr coating and the 1045 carbon steel substrate is improved. As a result, the Fe-28Al-5Cr laser cladding coating exhibits the best corrosion resistance in a 3.5 wt% NaCl solution under both immersion and electrochemical conditions. However, excessive Cr additive lead to the formation of Al8Cr5 in the grain boundaries, resulting in inferior corrosion resistance. Therefore, the new findings demonstrated in this work may inspire the design of high-quality coatings with excellent corrosion resistance.

Fe-Al-Si-Type Iron Aluminides: On the Strengthening by Refractory Metals Borides.

The effect of boron addition into Fe-28Al-5Si-X (X = -, 2Mo, or 2Ti) on the structure and high-temperature yield stress was investigated. Generally, the alloying of binary Fe3Al-type iron aluminides by silicon significantly improves high-temperature mechanical properties by solid-solution strengthening. On the other hand, the workability and ductile properties at room or slightly elevated temperatures get worse with the increasing silicon content. Boron alloying together with titanium or molybdenum alloying is one of the ways to improve the workability of this type of alloy and, at the same time, ensure the formation of a sufficient amount of secondary phase particles required for effective strengthening. In this paper, the influence of 1 at. % of boron on high-temperature yield stress is evaluated in response to structural changes and compared with results obtained previously on the same type of alloy (Fe-28Al-5Si-2X, X= -, Mo, or Ti) but without boron alloying. It can be concluded that the network structure of borides of refractory metals formed due to boron alloying works more effectively for alloy hardening at higher temperatures than a mixture of silicides and carbides present in the boron-free alloy of the same composition.

In situ construction of Fe3Al@Al2O3 core-shell particles with excellent electromagnetic absorption.

To obtain Fe3Al@Al2O3 core-shell absorbents, DO3-type Fe3Al powder was thermal treated in an argon atmosphere containing a trace amount of oxygen at different temperatures. Since Al atoms have a higher diffusion rate than that of the Fe atoms, Al atoms can migrate to the surface of the Fe3Al particle and in-situ convert to Al2O3 nanoparticles during the thermal treatment process. With the increase of the thermal treatment temperature, the Al2O3 nanoparticles grow larger, exhibiting different microwave absorption properties. In particular, the Fe3Al@Al2O3 obtained by controllable oxidation at 800 ℃ exhibits the best microwave absorption properties, with the minimum reflection loss of -34 dB at 11.5 GHz when the thickness is 2 mm, and the bandwidth below -10 dB is as broad as 6.7 GHz. Since a dielectric Al2O3 shell with a proper thickness can increase the impedance matching ratio of the Fe3Al absorbent, more electromagnetic waves can come into the absorbent. In addition, the magnetic Fe3Al core can efficiently attenuate the absorbed electromagnetic waves by dimensional resonance and natural resonance.

The Effect of Hydrogen on the Stress-Strain Response in Fe3Al: An ab initio Molecular-Dynamics Study.

We performed a quantum-mechanical molecular-dynamics (MD) study of Fe3Al with and without hydrogen atoms under conditions of uniaxial deformation up to the point of fracture. Addressing a long-lasting problem of hydrogen-induced brittleness of iron-aluminides under ambient conditions, we performed our density-functional-theory (DFT) MD simulations for T = 300 K (room temperature). Our MD calculations include a series of H concentrations ranging from 0.23 to 4 at.% of H and show a clear preference of H atoms for tetrahedral-like interstitial positions within the D03 lattice of Fe3Al. In order to shed more light on these findings, we performed a series of static lattice-simulations with the H atoms located in different interstitial sites. The H atoms in two different types of octahedral sites (coordinated by either one Al and five Fe atoms or two Al and four Fe atoms) represent energy maxima. Our structural relaxation of the H atoms in the octahedral sites lead to minimization of the energy when the H atom moved away from this interstitial site into a tetrahedral-like position with four nearest neighbors representing an energy minimum. Our ab initio MD simulations of uniaxial deformation along the ⟨001⟩ crystallographic direction up to the point of fracture reveal that the hydrogen atoms are located at the newly-formed surfaces of fracture planes even for the lowest computed H concentrations. The maximum strain associated with the fracture is then lower than that of H-free Fe3Al. We thus show that the hydrogen-related fracture initiation in Fe3Al in the case of an elastic type of deformation as an intrinsic property which is active even if all other plasticity mechanism are absent. The newly created fracture surfaces are partly non-planar (not atomically flat) due to thermal motion and, in particular, the H atoms creating locally different environments.

Effect of Higher Silicon Content and Heat Treatment on Structure Evolution and High-Temperature Behaviour of Fe-28Al-15Si-2Mo Alloy.

This paper describes the structure and properties of cast Fe3Al-based alloy doped with 15 at. % of silicon and 2 at. % of molybdenum. The higher content of silicon is useful for the enhancement of high-temperature mechanical properties or corrosion resistance of iron aluminides but deteriorates their workability due to increased brittleness. It was found that the presence of both alloying elements leads to an increase of values of the high-temperature yield stress in compression. The heat treatment (annealing at 800 °C for 100 h) used for the achievement of phase stability causes the grain coarsening, so the values of the high-temperature yield stress in compression are lower at 600 °C and 700 °C in comparison to values measured for the as-cast state. This stabilization annealing significantly improves the workability/machinability of alloy. Furthermore, the higher silicon content positively affects the values of the thermal expansion coefficient that was found to be lower in the temperature range up to 600 °C compared to alloys with lower content of silicon.

Direct Synthesis of Fe-Al Alloys from Elemental Powders using Laser Engineered Net Shaping.

The laser engineered net shaping (LENS®) process is shown here as an alternative to melting, casting, and powder metallurgy for manufacturingiron aluminides. This technique was found to allow for the production ofFeAl and Fe3Al phases from mixtures of elemental iron and aluminum powders. Theinsitusynthesis reduces the manufacturing cost and enhances the manufacturing efficiency due to the control of the chemical and phase composition of the deposited layers. The research was carried out on samples with different chemical compositionsthat were deposited on the intermetallic substrates that were produced by powder metallurgy. The obtained samples withthe desired phase composition illustrated that LENS® technology can be successfully applied to alloys synthesis.

A naphthylamide based fluorescent probe for detection of Al3+, Fe3+, and CN- with high sensitivity and selectivity.

A naphthylamide based fluorescent chemosensor, N,N'-(1,2-phenylene)bis(1-hydroxy-2-naphthamide) (H4L), for detection of Fe3+ and Al3+ cations as well as CN- anion is reported. This compound has been synthesized by a novel and facile synthetic method with high yield and characterized by FT-IR, 1H NMR, elemental analysis, and UV-Vis spectroscopy. It could detect Fe3+ and Al3+ ions in different media with different excitation and emission wavelengths. In DMSO solution, H4L showed selective ON-OFF quenching of its 451 nm emission in the presence of Fe3+. On the other hand, in DMF solution, H4L exhibited selective OFF-ON fluorescence upon the addition of Al3+, the intensity at 429 nm increases drastically by 24-fold. Also, among the anions, the probe can selectively distinguish CN- by deprotonation of OH and NH groups, as proved by 1H NMR titration. TD-DFT calculation supports the UV-Vis and fluorescence measurements of the chemosensor.

A Quantum-Mechanical Study of Clean and Cr-Segregated Antiphase Boundaries in Fe3Al.

We present a quantum-mechanical study of thermodynamic, structural, elastic, and magnetic properties of selected antiphase boundaries (APBs) in Fe 3 Al with the D0 3 crystal structure with and without Cr atoms. The computed APBs are sharp (not thermal), and they have {001} crystallographic orientation. They are characterized by a mutual shift of grains by 1/2〈100〉a where a is the lattice parameter of a cube-shaped 16-atom elementary cell of Fe 3 Al, i.e., they affect the next nearest neighbors (APB-NNN type, also called APB-D0 3 ). Regarding clean APBs in Fe 3 Al, the studied ones have only a very minor impact on the structural and magnetic properties, including local magnetic moments, and the APB energy is rather low, about 80 ± 25 mJ/m 2 . Interestingly, they have a rather strong impact on the anisotropic (tensorial) elastic properties with the APB-induced change from a cubic symmetry to a tetragonal one, which is sensitively reflected by the directional dependence of linear compressibility. The Cr atoms have a strong impact on magnetic properties and a complex influence on the energetics of APBs. In particular, the Cr atoms in Fe 3 Al exhibit clustering tendencies even in the presence of APBs and cause a transition from a ferromagnetic (Cr-free Fe 3 Al) into a ferrimagnetic state. The Fe atoms with Cr atoms in their first coordination shell have their local atomic magnetic moments reduced. This reduction is synergically enhanced (to the point when Fe atoms are turned non-magnetic) when the influence of clustering of Cr atoms is combined with APBs, which offer specific atomic environments not existing in the APB-free bulk Fe 3 Al. The impact of Cr atoms on APB energies in Fe 3 Al is found to be ambiguous, including reduction, having a negligible influence or increasing APB energies depending on the local atomic configuration of Cr atoms, as well as their concentration.

An Ab Initio Study of Vacancies in Disordered Magnetic Systems: A Case Study of Fe-Rich Fe-Al Phases.

We have performed quantum-mechanical calculations to examine the impact of disorder on thermodynamic, structural and electronic (magnetic) properties of Fe-Al systems with vacancies. A series of supercells was used and their properties were computed employing density-functional theory (DFT) as implemented in the VASP package. Our case study is primarily aimed at a disordered solid solution Fe 81.25 Al 18.75 but we have compared our results also with those obtained for the ordered Fe 3 Al intermetallic compound for which experimental data exist in literature. Both phases are found in Fe-Al-based superalloys. The Fe-18.75at.%Al solid solution was simulated using special quasirandom structures (SQS) in three different disordered states with a different distribution of Al atoms. In particular, we have considered a general disordered case (an A2-like variant), the case without the first nearest neighbor Al-Al pairs (a B2-like distribution of atoms) and also the case without both the first and second nearest neighbor Al-Al pairs (the D0 3 -like variant, in fact, an Fe-rich Fe 3 Al phase). The vacancy formation energies as well as the volumes of (fully relaxed) supercells with vacancies showed a large scatter for the disordered systems. The vacancy formation energies decrease with increasing concentration of Al atoms in the first coordination shell around the vacancy (an anti-correlation) for all disordered cases studied. The computed volumes of vacancies were found significantly lower (by 25-60%) when compared with the equilibrium volume of the missing atoms in their elemental states. Lastly, we have analyzed interactions between the vacancies and the Fe atoms and evaluated vacancy-induced changes in local magnetic moments of Fe atoms.

Quantum-Mechanical Study of Nanocomposites with Low and Ultra-Low Interface Energies.

We applied first-principles electronic structure calculations to study structural, thermodynamic and elastic properties of nanocomposites exhibiting nearly perfect match of constituting phases. In particular, two combinations of transition-metal disilicides and one pair of magnetic phases containing the Fe and Al atoms with different atomic ordering were considered. Regarding the disilicides, nanocomposites MoSi 2 /WSi 2 with constituents crystallizing in the tetragonal C11 b structure and TaSi 2 /NbSi 2 with individual phases crystallizing in the hexagonal C40 structure were simulated. Constituents within each pair of materials exhibit very similar structural and elastic properties and for their nanocomposites we obtained ultra-low (nearly zero) interface energy (within the error bar of our calculations, i.e., about 0.005 J/m 2 ). The interface energy was found to be nearly independent on the width of individual constituents within the nanocomposites and/or crystallographic orientation of the interfaces. As far as the nanocomposites containing Fe and Al were concerned, we simulated coherent superlattices formed by an ordered Fe 3 Al intermetallic compound and a disordered Fe-Al phase with 18.75 at.% Al, the α -phase. Both phases were structurally and elastically quite similar but the disordered α -phase lacked a long-range periodicity. To determine the interface energy in these nanocomposites, we simulated seven different distributions of atoms in the α -phase interfacing the Fe 3 Al intermetallic compound. The resulting interface energies ranged from ultra low to low values, i.e., from 0.005 to 0.139 J/m 2 . The impact of atomic distribution on the elastic properties was found insignificant but local magnetic moments of the iron atoms depend sensitively on the type and distribution of surrounding atoms.

Impact of Nano-Scale Distribution of Atoms on Electronic and Magnetic Properties of Phases in Fe-Al Nanocomposites: An Ab Initio Study.

Quantum-mechanical calculations are applied to examine magnetic and electronic properties of phases appearing in binary Fe-Al-based nanocomposites. The calculations are carried out using the Vienna Ab-initio Simulation Package which implements density functional theory and generalized gradient approximation. The focus is on a disordered solid solution with 18.75 at. % Al in body-centered-cubic ferromagnetic iron, so-called α -phase, and an ordered intermetallic compound Fe 3 Al with the D0 3 structure. In order to reveal the impact of the actual atomic distribution in the disordered Fe-Al α -phase three different special quasi-random structures with or without the 1st and/or 2nd nearest-neighbor Al-Al pairs are used. According to our calculations, energy decreases when eliminating the 1st and 2nd nearest neighbor Al-Al pairs. On the other hand, the local magnetic moments of the Fe atoms decrease with Al concentration in the 1st coordination sphere and increase if the concentration of Al atoms increases in the 2nd one. Furthermore, when simulating Fe-Al/Fe 3 Al nanocomposites (superlattices), changes of local magnetic moments of the Fe atoms up to 0.5 µ B are predicted. These changes very sensitively depend on both the distribution of atoms and the crystallographic orientation of the interfaces.

High-Temperature Oxidation of Fe3Al Intermetallic Alloy Prepared by Additive Manufacturing LENS.

The isothermal oxidation of Fe-28Al-5Cr (at%) intermetallic alloy microalloyed with Zr and B (<0.08 at%) in air atmosphere, in the temperature range of 1000 to 1200 °C, was studied. The investigation was carried out on the thin-walled (<1 mm) elements prepared by Laser Engineered Net Shaping (LENS) from alloy powder of a given composition. Characterization of the specimens, after the oxidation, was conducted using X-ray diffraction (XRD) and scanning electron microscopy (SEM, with back-scatter detector (BSE) and energy-dispersive X-ray spectroscopy (EDS) attachments). The investigation has shown, that the oxidized samples were covered with a thin, homogeneous α-Al2O3 oxide layers. The intensity of their growth indicates that the material lost its resistance to oxidation at 1200 °C. Structural analysis of the thin-walled components' has not shown intensification of the oxidation process at the joints of additive layers.