Binary Fe-Al Alloys Research Articles

Microstructure and stress analyses in nano-structured alumina layers developed on Fe-Al single crystals

In the present work low index single crystal surfaces of a binary Fe-Al alloy were oxidized in atmospheric air at 700 °C. The oxide layer composition, microstructure and texture were investigated by XPS, TEM and XRD. The oxidation stresses were determined in situ by synchrotron XRD. The results revealed the presence of a dual layered oxide scale, consisting of an inner layer of α-Al2O3 and an external layer of α-Fe2O3 and α-Al2O3. Differences in crystal orientation led to variations in the internal stress distribution in the oxide scales, with compressive stress values α-Al2O3 following (110) > (111) > (100).

Solute cluster-induced precipitation and resultant surface hardening during nitriding of Fe–Al–V alloys

The small amount (0.1 at%) of V addition to Fe–Al alloys promotes AlN precipitation during nitriding and results in substantial surface hardening, in contrast to the normal sluggish precipitation kinetics of AlN during nitriding of Fe–Al binary alloys. The addition of V to Fe–Al alloys can increase the surface hardness to a greater extent than the addition of Ti. First-principles calculations of the interaction energy between an Al atom and an N–s cluster (s = V, Ti, Cr, or Al) show that a V–N cluster has an attractive interaction with an Al atom, and this interaction is stronger than that between a Ti–N cluster and an Al atom, in good agreement with the greater hardening induced by V addition than by Ti addition. These experiments and calculations indicate that solute clusters not only contribute directly to the hardening but also induce alloy nitride precipitation.

The medium-range orders transition in liquid Fe–Al alloys

We observed the transition of medium-range orders (MROs) and physical properties in liquid Fe–Al alloys by using ab initio molecular dynamics simulation. Above 66 at. % Al, only Fe–Fe MRO exists, while in other binary Fe–Al alloys studied Fe–Fe and Al–Al MROs coexist, and below 50 at. % Al, Al–Al MRO turns to be dominant. The curve of short-range order around Fe atoms versus Al content also shows a juncture at 66 at. % Al above which <0,3,6,0> polyhedron type increase significantly, correspondingly a significant decrease of the average magnetic moment per Fe atom is observed. In addition, the self-diffusion coefficient of Al atom increases sharply above 66 at. % Al.

Influence of Al content and pre-oxidation on the aqueous corrosion resistance of binary Fe-Al alloys in sulphuric acid

The influence of Al content and pre-oxidation on the corrosion behavior of binary Fe-Al alloys in 0.0126 M H2SO4 was investigated. The minimum Al content for forming a passive film on binary Fe-Al at 25 °C is 15 at.% and passivation improves up to 25 at.% Al. X-ray photoelectron spectroscopy measurements show that the passive film on Fe-25Al is enriched in Al and consists of an outer layer of mixed Al and Fe hydroxides and an inner layer of Al oxide. The oxide layer generated at 1000 °C effectively protected against aqueous corrosion in H2SO4.

Alloying and strain hardening effects in abrasive contacts on iron based alloys

Strain hardening is a widely used effect to reduce wear, hence it can be expedient in tribological contacts. For this work abrasive contacts were simulated in a scratch test at both, two different forces and velocities, aiming for strain hardening effects in various iron based alloys with different alloying contents. Fe-Cr and Fe-Al binary alloys and Fe-Cr-Ni ternary alloys in a certain range were chosen for investigation - Cr from 10 to 20 wt.- %, Cr and Ni from 10 to 20 wt.- % each and Al from 15 to 25 wt.- %. Strain hardening effects were studied with high resolution scanning electron microscopy, nano-indentation and micro-hardness measurements. Statistical analyses were performed on the influence of 4 chosen parameters: alloying elements and content, and scratch force and velocity.Results show a salient influence of all chosen parameters. Deformed zones increase with higher force; strain hardening can be enhanced with higher velocities at all materials. Fe-Cr-Ni ternary alloys show the most distinct strain hardening effect.

Vacancy effects on the mechanical behavior of B2-FeAl intermetallics

The present work addresses the impact of quenched-in thermal vacancies on the mechanical properties of B2 iron aluminides. For this reason different binary Fe-Al alloys with different histories of heat treatment were indented at room temperature. The contributions of mono- and bi-vacancies in addition to triple defects on the elastic, plastic and elastic-plastic transition were evaluated using nanoindentation technique. In the samples containing bi-vacancies, the increase of the hardness was much more pronounced compared to the mono-vacancies.

The effect of the ternary elements B, Ti, Cr, Cu, and Mo on fully lamellar FeAl + FeAl2 alloys

Binary Fe-Al alloys in the composition range of 53–65 at.% Al contain finely lamellar microstructures formed of the intermetallic phases FeAl and FeAl2 as a result of a rapid eutectoid decomposition reaction on cooling. Here, we study the effect of ternary additions of up to 2 at.% B, Ti, Cr, Cu and Mo on a fully lamellar alloy with respect to the resulting microstructures, crystal structures of the phases, thermal stability, and mechanical properties. The ternary alloys containing B or Cu preserve the fully lamellar microstructure while those with additions of Mo and Ti show formation of third, ternary phases (Al8FeMo3 and Al2FeTi, respectively). Melting temperatures as well as the formation temperatures of the lamellar microstructure are either unaffected or decrease by the ternary additions compared to the binary case. The hardness of the FeAl phase is much more affected by the ternary additions than that of the FeAl2 phase. The brittle-to-ductile transition temperatures and the yield stresses were studied for all ternary alloys and compared to the binary case, for which also the effect of variations in the Al content was investigated. Especially in case of B additions, an unexpected increase in the brittle-to-ductile transition temperature was observed. Atom probe tomographic experiments reveal that B does not segregate at the phase boundaries but instead forms fine boride particles inside the FeAl lamellae.

Effect of Small Amount of Manganese on the Interdiffusivities in Fe-Al Alloys

The ability to weld aluminum and steel sheets depends strongly on the formation of intermetallic phases; a process that is, in turn, controlled by the interdiffusion of iron and aluminum across the welded interface. Understanding the interdiffusion behavior, and how it is influenced by tertiary elements such as manganese, will allow for better prediction of the properties of the spot weld. Hence, interdiffusion coefficients and activation energies for interdiffusion were determined in the α solid solution and B2 intermetallic phases of Fe-Al alloys in the presence of 1.5-2 at.% manganese with pseudo-binary diffusion couples investigated at 900-1095 °C. The interdiffusion coefficients in α were found to increase in the presence of Mn at all temperatures compared with those reported in the binary Fe-Al alloys. The activation energies for interdiffusion in α are correspondingly lower than those in the binary Fe-Al alloys. The increase in the main interdiffusion coefficients in the presence of Mn indicates that diffusional interactions between Fe and Al are increased in the presence of Mn. The expected increase in diffusional interactions of Fe and Al are found to be consistent with the thermodynamic interactions between Fe and Al in the binary Fe-Al and ternary Fe-Al-Mn system as estimated from the literature. The presence of Mn is found to decrease the solubility of Al in the α solid solution, which, in turn, is expected to decrease the growth rate of the intermetallic at the interface between steel and aluminum.

X-ray Photoelectron Spectroscopy (XPS) analysis of low oxidation pressure behavior of FeAl –alloys at 600°C and 700°C

The low oxidation pressure behavior of binary FeAl alloys has been examined at different temperatures of 600-700°C. Polycrystalline and single-crystalline samples of Fe-10Al and Fe-25Al were oxidized for 1 h at 10-6 mbar of oxygen in an ultra-high-vacuum chamber (UHV). Prior oxidation, samples were prepared by sputter-annealing cycles in UHV. After oxidation, the samples were analyzed by X-ray Photoelectron Spectroscopy (XPS). The formed oxide layers on the surfaces consist of FeAl2O4 and Al2O3 showing that Al has a dominating role during the oxidation process. From XPS peaks, the composition of the Fe peaks of the oxide layer consists of Fe metal, FeAl-alloys, and FeO. Meanwhile, the composition of the Al peak is changed from Al metal, FeAl-alloys and Al2O3 at 600°C to Al metal, FeAl-alloys and FeAl2O4 on 700°C. From XPS sputter depth profiles, it is shown that the oxide layer was growing with increasing temperature, and the single-crystalline Fe-10Al has a thicker formed oxide layer than poly-crystalline Fe-25Al with same oxidation treatment.

Coarsening kinetics of lamellar microstructures: Experiments and simulations on a fully-lamellar Fe-Al in situ composite

A very fine lamellar microstructure consisting of the intermetallic phases FeAl and FeAl2 forms in binary Fe-Al alloys due to the eutectoid reaction Fe5Al8 ↔ FeAl + FeAl2, which takes place in the Al range between 56.0 and 64.4 at.% at 1095 °C. A fully lamellar microstructure is obtained at the eutectoid composition 60.9 at.% Al. The initial lamellar spacing λ0 of as-cast material is 200 ± 40 nm. In this study, the kinetics of coarsening of the FeAl + FeAl2 lamellar microstructure is investigated at four different temperatures in the range 600 °C–1000 °C with holding times from 10 min to 7000 h. It is found that the increase of the lamellar spacing can be described as λ3=λ03+kt. The value obtained for the activation energy proves that the lamellar coarsening is a volume-diffusion-controlled process. Besides the experimental investigations, phase-field modeling is used to simulate the lamellar coarsening. The results are in good agreement with the experimental observations with regard to the evolution of the lamellar morphology and the value of the coarsening exponent.

Thermal performance determination of binary Fe-Al alloys at elevated temperatures

The thermal expansion and thermal diffusion properties of Fe−Al alloys were measured using dilatometric and laser flash methods. At two allotropic transformation temperatures, the length change of the rapidly solidified pure Fe by injection casting deviates from that of the pure Fe by heating-cooling processing in a dilatometer. The ferro-/paramagnetic transition and second order transition do not influence the thermal expansion behaviors of Fe−Al alloys as temperature increases from 473 to 1373 K. The thermal expansion coefficients of Fe70Al30, Fe60Al40 and Fe55Al45 alloys are apparently larger than those of Fe25Al75 and Fe90Al10 alloys. Once temperature exceeds 877 K, the thermal expansion coefficients of Fe-Al alloys are enhanced with the increase of Al content, except for Fe25Al75 alloy owing to the large packing factor of FeAl3 crystal structure. In the temperature range from 309 to 877 K, the thermal diffusivity as a function of temperature decreases in the sequence of Fe90Al10, Fe55Al45, Fe60Al40, Fe70Al30 and Fe25Al75, and their values are lower than that of pure Fe at the same temperatures. As temperature increases, the thermal diffusivity values of Fe−Al alloys increase, except that of Fe90Al10 alloy decreases in the temperature range from 294 to 979 K and then increases once temperature further increases to 1078 K because of the ferro-/paramagnetic transition. Their thermal expansion coefficient and thermal diffusivity serve as linear or polynomial functions of temperature.

The crystallographic template effect assisting the formation of stable α-Al2O3 during low temperature oxidation of Fe–Al alloys

The role of thermally grown α-Fe2O3 on the nucleation of α-Al2O3 during oxidation of binary Fe–Al alloys with 15 and 26 at.%Al at 700°C was investigated. Surface morphology of the oxide scales indicated direct nucleation of α-Al2O3 preferentially instead of conversion from metastable Al2O3 polymorphs. Oxide scale development over time was also monitored by use of synchrotron X-ray diffraction and Raman spectroscopy. The results showed that the α-Fe2O3 crystal lattice decreases in volume as oxidation progresses, which was found to be consistent with an Al3+ enrichment of α-Fe2O3 as confirmed by the change in relative intensity of α-Fe2O3 Raman peaks.

Oxidation Behavior of Binary Aluminium-Rich Fe–Al Alloys with a Fine-Scaled, Lamellar Microstructure

The oxidation behavior of binary Fe–Al alloys with an aluminum content between 58 and 64 at.% was studied by isothermal and cyclic oxidation experiments in a temperature range of 900–1000 °C. The parabolic rate constants were determined to be comparable to those of B2-ordered Fe–48Al. All investigated alloys exhibited a fine-scaled, lamellar microstructure (<300 nm lamellar spacing), which was a result of the eutectoid decomposition of the high-temperature phase Fe5Al8 into FeAl and FeAl2. None of these two phases showed a preferential oxidation. At 900 °C θ-Al2O3 formed, whereas at 1000 °C a transformation of the initially formed θ-Al2O3 to the slower growing and passivating α-Al2O3 was confirmed by grazing incidence X-ray diffraction measurements.

Defects and hyperfine interactions in binary Fe-Al alloys studied by positron annihilation and Mössbauer spectroscopies

The defects, the behavior of 3d electrons and the hyperfine interactions in binary Fe-Al alloys with different Al contents have been studied by measurements of positron lifetime spectra, coincidence Doppler broadening spectra of positron annihilation radiation and Mössbauer spectra. The results show that on increasing the Al content in Fe-Al alloys, the mean positron lifetime of the alloys increase, while the mean electron density of the alloys decrease. The increase of Al content in binary Fe-Al alloys will decrease the amount of unpaired 3d electrons; as a consequence the probability of positron annihilation with 3d electrons and the hyperfine field decrease rapidly. Mössbauer spectra of binary Fe-Al alloys with Al content less than 25 at.% show discrete sextets and these alloys make a ferromagnetic contribution at room temperature. The Mössbauer spectrum of Fe70Al30 shows a broad singlet. As Al content higher than 40 at.%, the Mössbauer spectra of these alloys are singlet, that is, the alloys are paramagnetic. The behavior of a 3d electron and its effect on the hyperfine field of the binary Fe-Al alloy has been discussed.

Mössbauer spectroscopy study of the disordering process of Fe-rich FeAlSi alloys

In this work we study systematically the influence of different Al/Si ratios on the magnetic and structural properties of mechanically disordered powder Fe75Al25−xSix, Fe70Al30−xSix and Fe60Al40−xSix alloys by means of Mossbauer spectroscopy and X-ray diffraction measurements. In order to obtain different stages of disorder the alloys were deformed by ball milling annealed (ordered) alloys during different number of hours. X-ray and Mossbauer data show that mechanical deformation induces the disordered A2 structure in these alloys. The results indicate that addition of Si to binary Fe–Al alloys makes the disordering more difficult. The study of the hyperfine fields indicates that depending on the Fe content the magnetic behaviour of these ternary alloys varies. For Fe75Al25−xSix series, the alloys have different magnetic behaviours with deformation depending on the Si content. The magnetization of the alloys with high Si content decreases with deformation, as it happens to binary Fe75Si25 and the magnetization of the alloys with low Si content increases with deformation, as it happens to binary Fe75Al25. For Fe70Al30−xSix series the mean hyperfine fields show that there are two different stages with the disordering, in a first stage the mean hyperfine fields decrease and in the second stage they increase. Finally, for Fe60Al40−xSix alloys there is a magnetic transition, from a paramagnetic ordered state to a magneticdisordered state.

The Divacancy Formation Energy for B2 FeAl Alloy Simulated with Finnis-Sinclair many-Body Interatomic Potential

The Finnis-Sinclair many-body potential was fitted for binary FeAl alloy with B2 structure. As the examination to the acquired potential function, some properties were calculated, and the results agree with the experiments well. Further, properties of point defects, such as divacancies were studied as an application.

Tetragonal magnetostriction and magnetoelastic coupling in Fe-Al, Fe-Ga, Fe-Ge, Fe-Si, Fe-Ga-Al, and Fe-Ga-Ge alloys

This paper presents a comparative study on the tetragonal magnetostriction constant, λγ,2, [ = (3/2)λ100] and magnetoelastic coupling, b1, of binary Fe100-xZx (0 &amp;lt; x &amp;lt; 35, Z = Al, Ga, Ge, and Si) and ternary Fe-Ga-Al and Fe-Ga-Ge alloys. The quantities are corrected for magnetostrains due to sample geometry (the magnetostrictive form effect). Recently published elastic constant data along with magnetization measurements at both room temperature and 77 K make these corrections possible. The form effect correction lowers the magnetostriction by ∼10 ppm for high-modulus alloys and by as much as 30 ppm for low-modulus alloys. The elastic constants are also used to determine the values of the magnetoelastic coupling constant, b1. With the new magnetostriction data on the Fe-Al-Ga alloy, it is possible to show how the double peak magnetostriction feature of the binary Fe-Ga alloy flows into the single peak binary Fe-Al alloy. The corrected magnetostriction and magnetoelastic coupling data for the various alloys are also compared using the electron-per-atom ratio, e/a, as the common variable. The Hume-Rothery rules link the e/a ratio to the regions of phase stability, which appear to be intimately related to the magnetostriction versus the solute concentration curve in these alloys. Using e/a as the abscissa tends to align the peaks in the magnetostriction and magnetoelastic coupling for the Fe-Ga, Fe-Ge, Fe-Al, Fe-Ga-Al, and Fe-Ga-Ge alloys, but not for the Fe-Si alloys for which the larger atomic size difference may play a greater role in phase stabilization. Corrections for the form effect are also presented for the rhombohedral magnetostriction, λɛ,2, and the magnetoelastic coupling, b2, of Fe100-xGax (0 &amp;lt; x &amp;lt; 35) alloys.

Structural and magnetic study of mechanically deformed Fe rich FeAlSi ternary alloys

In this work we study systematically the influence of different Al/Si ratios on the magnetic and structural properties of mechanically disordered powder Fe75Al25−xSix, Fe70Al30−xSix and Fe60Al40−xSix alloys by means of Mössbauer spectroscopy, X-ray diffraction and magnetic measurements. In order to obtain different stages of disorder the alloys were deformed by different methods: crushing induction melted alloys and ball milling annealed (ordered) alloys using different number of balls and speed. X-ray and Mössbauer data show that mechanical deformation induces the disordered A2 structure in these alloys. The results indicate that addition of Si to binary Fe–Al alloys makes the disordering more difficult. In addition, X-ray diffraction patterns show that the normalized lattice parameter variation of the disordered alloys of each composition decreases monotonically with Si content, indicating clearly that Si addition opposes the large volume increase found in FeAl alloys with deformation. The study of the hyperfine fields indicates that there is a redistribution of non-ferrous atoms around Fe atoms with the disordering; indeed, there is an inversion of the behavior of the hyperfine field of the Fe atoms. On the other hand, the magnetic measurements indicate that addition of Si to binary Fe75Al25 and Fe70Al30 alloys opposes the magnetic behavior induced by Al in the magnetism of Fe.

Mössbauer study of mechanical deformation induced order-disorder transition in Fe75AlSi alloys

We study the influence of different Al/Si ratios on the magnetic and structural properties of mechanically disordered powder Fe75Al25 − xSix alloys. The results indicate that addition of Si to binary Fe-Al alloys makes the disordering more difficult. The study of the hyperfine fields indicates that there is an inversion of the behavior of the hyperfine field of the Fe atoms surrounded by 8 Fe atoms with disordering. The magnetic and hyperfine measurements indicate that the influence of Si is opposite to the one of Al in the magnetism of Fe atoms.

The Influences of Aging on the Two Relaxation Peaks in the Air-Cooled Fe71Al29 Alloy

Binary Fe-Al alloy ingots with 29 at. %Al were prepared with commercially pure Fe and Al in a vacuum induction furnace. The specimens used in the internal friction measurement were cut using an electric sparkle machine from the ingots into a dimension 68×1.7×0.9 mm3. The heat treatment was performed in such regimes as heating temperature 900°C and holding time 1 hour in an argon atmosphere followed by furnace cooling or air cooling. The air-cooled samples were then aged at 350°C for different time to achieve the different degree of disorder. The internal fiction-temperature spectra of the air-cooled Fe71Al29alloy were measured during both heating and cooling using a computer-controlled automatic inverted torsion pendulum through free-decay and forced vibration methods. It has been found that three internal friction peaks appear during heating at 180°C termed as P1, 340°C termed as P2 and 510°C termed as P3 in air-cooled specimens, respectively. P1 and P2 peaks disappear and P3 is retained during cooling. XRD results show that the air-cooled Fe71Al29alloy is disordered and the furnace-cooled specimen possesses ordered structures. The air-cooled specimen presents similar diffraction peaks to the furnace-cooled specimen after it is heated to 650°C and then in-situ cooled to room temperature. Ageing has great influence on the P1 and P2 peaks for the air-cooled Fe71Al29alloy. The height of P1 and P2 peaks is decreased with increasing aging time, which is due to the reduction in the disorder degree of the air-cooled specimen.