FeAl Phase Research Articles

Improvement of corrosion resistance of twinning-induced plasticity steel by hot-dipping aluminum with subsequent thermal diffusion treatment

Fe–Al intermetallic layers were formed on the surface of twinning induced plasticity (TWIP) steel by hot-dipping aluminum (HDA) and subsequent thermal diffusion treatment (TDT). It was shown that the coating layer was composed of Al, FeAl3 and Fe2Al5 phases in the hot-dip state while only Fe3Al phase retained after subsequent TDT. It was found that, the corrosion resistance of TWIP steel had been significantly improved with a shift of 224 mV in the Ecorr towards anodic direction and the jcorr reduced from 26.1 μA/cm2 to 2.70 μA/cm2. Fe-Al intermetallics could lower the corrosion rate because the Al2O3 film formed during stable soak stage was slightly stable due to the higher aluminum content than matrix steel.

Effect of Mg on Microstructure and Growth Kinetics of Zn-22.3Al-1.1Si Coating

The effects of Mg on the microstructure and growth kinetics of the hot-dip Zn-22.3Al-1.1Si-x Mg (x = 0, 0.2, 0.4, and 0.6) coatings were investigated in detail. Scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction studies revealed the presence of η-Zn, α-Al, Zn/Al eutectoid, (Si), Al/Si eutectic, and Zn/Al/Mg2Zn11 ternary eutectic on the top surface of these Mg-containing coatings. Especially, a small amount of MgZn2 phase appears in top surface of Zn-22.3Al-1.1Si-0.6Mg coating. Five phases are found in the alloy layers, i e, Fe2Al5, FeAl3, τ5C, τ5H, and τ1. The addition of 0.2% Mg can delay the emergence of FeAl3 phase. When the Mg content is more than 0.2%, the outer layers of coating change from τ5C to τ5H phase. The growth of the inhibition layer is diffusion controlled for various Mg content baths. Mg improves the corrosion resistance of the Mg-containing coatings, and the Zn-22.3Al-1.1Si-0.6Mg coating possesses the highest protective properties.

Crystallization inhibition and microstructure refinement of Al-5Fe alloys by addition of rare earth elements

Hypereutectic Al-Fe alloys are extremely brittle owing to the formation of coarse Fe-rich intermetallics in the Al matrix. In this study, a Ce-rich rare-earth (RE) mixture was introduced to modify an Al-5Fe binary alloy, and the crystallization, microstructure, and mechanical properties of the alloy were investigated. The experimental results indicated that the RE addition decreased the temperature of the primary Al3Fe phase, enhanced the temperature of the eutectic transformation, and disturbed the normal crystallization process of eutectic α-Al crystals by decreasing the intrinsic growth along the preferential close-packed (111)Al plane and promoting the growths depending on the sub-closed (200)Al plane. Furthermore, RE addition refined the primary Al3Fe phases and formed Al-Ce eutectics in the as-cast state of the alloy. Fine nodules and rods of the Al4Ce and AlCe phases containing La elements were produced through homogeneous annealing via the decomposition of the Al-Ce eutectics and precipitation of REs from the RE-enriched area. Rolling further broke up the coarser primary Al3Fe phases into short rods and particles and caused the Al-Ce and Al-Fe phases to become uniformly distributed in the Al matrix. A mixed cleavage and dimple fracture was produced by RE modification, annealing, and rolling. The optimized amount of added RE mixture for improving the microstructure and mechanical properties of the alloy was 0.9 wt%.

Mechanistic understanding on the evolution of nanosized Al3Fe phase in Al–Fe alloy during heat treatment and its effect on mechanical properties

Al–1Fe (wt%) alloy containing nanosized Al3Fe phase was processed by continuous rheo-extrusion, and subjected to heat treatment at different temperatures. During heat treatment, due to the diffusion of Fe atoms, the nanosized Al3Fe phase was blunted and fragmented along the width direction, and the bluntness and fragmentation resulted in the spheroidization of Al3Fe phase. However, with the extension of heat treatment time, the spheroidized Al3Fe phase became coarse and grew along a particular plane. Then, the morphology of Al3Fe phase transformed into small plate-like. While, the plate-like Al3Fe phase with an average length of 30 µm in the as-cast Al–1Fe (wt%) alloy indicated good thermal stability during heat treatment. The different thermal stability of Al3Fe phases was attributed to their size. The large surface curvature of nanosized Al3Fe phase significantly accelerated the diffusion of Fe atoms. When the heat treatment temperature was 200 ℃ and 300 ℃, the tensile strength and elongation of the rheo-extruded Al–Fe alloy marginally decreased. The decrease of tensile strength and the decrease of elongation increased when the heat treatment temperature was greater than 400 ℃. The tensile strength and elongation of as-cast Al–1Fe (wt%) alloy were insignificantly altered. Though the loss of tensile strength of Al–Fe alloy containing nanosized Al3Fe phase was much greater, its tensile strength was always higher than the tensile strength of the as-cast Al–Fe alloy in the entire heat treatment.

Microstructural evolution during creep of lamellar eutectoid and off-eutectoid FeAl/FeAl2 alloys

The creep behavior of a fully lamellar FeAl/FeAl2 eutectoid alloy was shown to exhibit a minimum creep rate and the absence of a pronounced steady state regime. To reveal the underlying mechanisms of creep leading to the macroscopic response described above, comprehensive TEM investigation of several crept specimens were performed. In the early stages of creep, the FeAl phase primarily carries creep deformation by dislocation motion, whereas FeAl2 remains mostly plastically undeformed, except in certain locations near colony boundaries where the lamellar structure is disrupted/absent. Within the colonies, where the lamellae are intact, deformation is accommodated at the FeAl/FeAl2 interface, resulting in an increase in interface dislocations. This continues to be the case at the minimum creep rate. With further progression in creep, FeAl2 begins to participate in the process of plastic deformation in a more substantive manner through twinning and slip, while FeAl continues to plastically deform and dynamically recover. Further beyond the minimum, the lamellar structure adjacent to the colony boundaries breaks down, and these areas become the primary contributors to creep and results in a continuous loss in creep resistance. Based on these observations in the fully lamellar material, the creep response of off-eutectoid Fe-58Al and Fe-62Al as well as single-phase FeAl2 are explained.

On the Dissolution Process of Manganese and Iron in Molten Aluminum

The dissolution of Mn and Fe in liquid Al presents a challenge due to their high melting points and low diffusivity. A literature review reveals that the existing knowledge of the processes involved in the dissolution of both Fe and Mn in liquid Al is rather ambiguous. Thus, this work aimed to obtain more detailed insights into the dissolution behavior of Mn and Fe in various Al melts. The results of the Mn dissolution tests showed that three intermediate phases were involved in the dissolution process, all of which exhibited a smooth interface between Mn and the liquid. These three phases were identified as the γ2, Al11Mn4, and µ phases which grow slowly, penetrating the Mn particles. The results of the Fe dissolution tests showed that in pure Al, the Al5Fe2 phase dominates the dissolution process and penetrates the Fe particles. The addition of Ti into the molten Al alters the intermetallic compound formation by replacing Al5Fe2 by Al2Fe. The addition of Si significantly inhibited the Fe dissolution kinetics. A theoretical approach based on Ficks’ law was used to explain the experimentally obtained Mn and Fe dissolution rates. It showed that the surface area and shape of the additives significantly affected the dissolution processes.

Study of the Microstructure and Crack Evolution Behavior of Al-5Fe-1.5Er Alloy.

In this work, the microstructure of Al-5Fe-1.5Er alloy was characterized and analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) techniques. The effect of microstructure on the behavior of crack initiation and propagation was investigated using in situ tensile testing. The results showed that when 1.5 wt.% Er was added in the Al-5Fe alloy, the microstructure consisted of α-Al matrix, Al3Fe, Al4Er, and Al3Fe + Al4Er eutectic phases. The twin structure of Al3Fe phase was observed, and the twin plane was {001}. Moreover, a continuous concave and convex interface structure of Al4Er was observed. Furthermore, Al3Fe was in the form of a sheet with a clear gap inside. In situ tensile tests of the alloy at room temperature showed that the crack initiation mainly occurred in the Al3Fe phase, and that the crack propagation modes included intergranular and trans-granular expansions. The crack trans-granular expansion was due to the strong binding between Al4Er phases and surrounding organization, whereas the continuous concave and convex interface structure of Al4Er provided a significant meshing effect on the matrix and the eutectic structure.

Effect of Temperature and Zinc Coating on Interfacial Bonding between Steel and Aluminum Dissimilar Material

The application of aluminum alloys can largely reduce the weight of the vehicle body and save energy. In order of the wide use of aluminum alloy the study on the joining method of steel and aluminum dissimilar materials is very important. In order to expose the mechanism of the interface bonding between steel and aluminum dissimilar materials under the thermal and mechanical coupled loading, steel sheets were joined to 6061 aluminum sheets by a thermal and mechanical simulation system. The applied temperature and pressure were both able to be controlled precisely in the Gleeble simulation testing system. The effects of welding temperature and zinc coating on interfacial bonding were both studied. It was shown that without zinc coating a continuous interface layer between the steel and aluminum could be observed when the temperature was above 550°C. There was a FeAl3 phase existing in the interface layer with a high micro hardness. When welding temperature higher than 550°C the shear strength of the joint increased with temperature. With zinc coating a continuous interface layer could be formed above 470°C. The interface layer was mainly composed of Fe2Al5Znx and Al-Zn eutectic phases instead of the FeAl3.

Wear Inducing Phase Transformation of Plasma Transfer Arc Coated Tools during Friction Stir Welding with Al Alloy

The friction stir welding process (friction stir welding/processing, FSW/FSP) has wear problems related to stirring tools. In this study, the plasma transfer arc (PTA) method was used with stellite 1 powders (Co-based) to coat on the head of a SKD61 stirring tool (SKD61-ST1) in order to investigate the wear performance and phase transformation of SKD61-ST1 after FSW. Under the same experimental parameters, the wear data were compared with the high-speed steel SKH51 (tempering material SKH51-T and annealed material SKH51-A) and tungsten-carbide cobalt (TCC). Results showed the PTA coating was a γ-Co solidification matrix with M7C3 and M23C6 carbides. After FSW, the wear resistance of SKD61-ST1 was better than that of SKH51-A and SKH51-T and lower than that of TCC. The SKD61-ST1, SKH51-A, and SKH51-T stirring tools exhibited sliding wear after FSP, where the pin and shoulder of the stirring tool formed a phase transfer layer on the surface, and the peeling of the phase transfer layer caused wear weight loss. The main phase of the phase transfer layer of the SKD61-ST1 tool was Al9Co2. The affinity and adhesion energy of the Co-Al phase was lower than that of Fe-Al phase, and the phase transfer layer of the SKD61-ST1 tool was thinner and had lower coverage, thereby increasing the wear resistance of the SKD61-ST1 stirring tools during FSW.

Improvement of microstructure and performance for steel/Al welds produced by magnetic field assisted laser welding

In this paper, we presented joining of galvanized steel/aluminum alloy dissimilar metals using magnetic field assisted laser welding. Influence of magnetic field on weld appearance and microstructural evolution were carefully characterized using an optical microscope, scanning electron microscope and X-Ray Diffraction. The results showed that a joint with good weld quality can be achieved by magnetic field assisted laser welding. The microstructure and element distribution at interface can be effectively improved by the magnetic field effect. The interface layer mainly involved Fe2Al5 phase, FeAl3 phase, Al192.4Fe46.22 phase and a limited amount of FeAl phase and Fe3Al phase. More Fe-rich IMCs (intermetallic compounds) with better ductility and toughness can be formed, which can obviously reduce the susceptibility of hot cracking and improve the shear strength of joints. It was also found that the weld geometric features were closely associated with magnetic field variables. A method was proposed to control formation of IMCs at interface and inhibit the crack growth via adding proper magnetic field during the process of laser welding.

Bulk amorphous and nanocrystalline Al60Fe20Ti20 and Al65Fe18Ti12Ni5 alloys

Amorphous Al60Fe20Ti20 and Al65Fe18Ti12Ni5 alloys were produced by mechanical alloying. Thermal behaviour examinations revealed that the crystallisation onset temperatures are 557 °C and 539 °C for the Al60Fe20Ti20 and the Al65Fe18Ti12Ni5 alloy respectively. The crystallisation products were nanocrystalline τ2-type phases. The mechanically alloyed powders were consolidated at temperatures ensuring maintaining amorphous structure and at 1000 °C. After consolidation at 1000 °C the samples consisted of nanocrystalline τ2-type phases and Al5Fe2 phase. The hardness of the amorphous and nanocrystalline Al60Fe20Ti20 alloy is 8.04 GPa and 9.37 GPa respectively, whereas that of the amorphous and nanocrystalline Al65Fe18Ti12Ni5 alloy is 7.69 GPa and 8.79 GPa respectively. The density of the amorphous and nanocrystalline Al60Fe20Ti20 alloy is 4.113 g/cm3 and 4.20 g/cm3 respectively, while that of the amorphous and nanocrystalline Al65Fe18Ti12Ni5 alloy is 4.111 g/cm3 and 4.278 g/cm3 respectively. The hardness of the produced low-density materials is relatively high. To the best of our knowledge, such materials were produced for the first time.

Synthesis of Intermetallics in Fe-Al-Si System by Mechanical Alloying

Fe-Al-Si alloys have been recently developed in order to obtain excellent high-temperature mechanical properties and oxidation resistance. However, their production by conventional metallurgical processes is problematic. In this work, an innovative processing method, based on ultra-high energy mechanical alloying, has been tested for the preparation of these alloys. It has been found that the powders of low-silicon alloys (up to 10 wt. %) consist of FeAl phase supersaturated by Si after mechanical alloying. Fe2Al5 phase forms as a transient phase at the initial stage of mechanical alloying. The alloy containing 20 wt. % of Si and 20 wt. % of Al is composed mostly of iron silicides (Fe3Si and FeSi) and FeAl ordered phase. Thermal stability of the mechanically alloyed powders was studied in order to predict the sintering behavior during possible compaction via spark plasma sintering or other methods. The formation of Fe2Al5 phase and Fe3Si or Fe2Al3Si3 phases was detected after annealing depending on the alloy composition. It implies that the powders after mechanical alloying are in a metastable state; therefore, chemical reactions can be expected in the powders during sintering.

Microstructural Evolution of Reaction Layer of 1.5 GPa Boron Steel Hot-Dipped in Al-7wt%Ni-6wt%Si Alloy

The constituents, distribution, and characteristics of the phases formed on the coating layer of boron steel hot-dipped in Al-7wt%Ni-6wt%Si were evaluated in detail. In particular, the microstructure and phase constitution of the reaction layer were characterized. Moreover, the microstructural evolution mechanism of the phase was presented with reference to the (Al-7wt%Ni-6wt%Si)-xFe from the pseudo-binary phase diagram. The solidification layer consisted mainly of Al, Al3Ni, and Si phases. Reaction layers were formed in the order of Al9FeNi(Τ), Fe4Al13(θ), and Fe2Al5(η) from the solidification layer side. In addition, the κ (Fe3AlC) layer was formed at the Fe2Al5(η)/steel interface. From pseudo-binary phase diagram analysis, it was found that Fe4Al13(θ) can form when the Fe concentration is over 2.63 wt% in the 690 °C Al-7wt%Ni-6wt%Si molten metal. When the concentration of Fe increased to 10.0–29.0 wt%, isothermal solidification occurred in the Fe4Al13(θ) and Al9FeNi(Τ) phases simultaneously. Moreover, given that the T phase does not dissolve Si, it was discharged, and the Si phase was formed around the Al9FeNi(T) phase. The Fe2Al5(η) phase was formed by a diffusion reaction between Fe4Al13(θ) and steel, not a dissolution reaction. Moreover, Al2Fe3Si3(τ1) was formed at the Fe4Al13(θ)-Fe2Al5(η) interface by discharging Si from Fe4Al13(θ) without Si solubility. Furthermore, the Fe3AlC(κ) layer was formed by carbon accumulation that discharged in the Fe2Al5(η) region transformed from steel to Fe2Al5(η). The twin regions in the Fe4Al13(θ) and Fe2Al5(η) grain were due to the strains caused by the lattice transformation in the constrained state, wherein the phases are present between the Al9FeNi(Τ) layer and steel.

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/WSi with constituents crystallizing in the tetragonal C11 structure and TaSi/NbSi 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). 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 FeAl intermetallic compound and a disordered Fe-Al phase with 18.75 at.% Al, the -phase. Both phases were structurally and elastically quite similar but the disordered -phase lacked a long-range periodicity. To determine the interface energy in these nanocomposites, we simulated seven different distributions of atoms in the -phase interfacing the FeAl intermetallic compound. The resulting interface energies ranged from ultra low to low values, i.e., from 0.005 to 0.139 J/m. 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.

Effects of substrate surface roughness and aluminizing agent composition on the aluminide coatings by low-temperature pack cementation

In this study, a new low-temperature aluminizing process combining surface slurry pre-coating at room temperature with pack cementation process was used to prepare the iron-aluminum alloy coating. The effects of different substrate surface roughness and aluminizing agent composition on the coating surface roughness, phase structure and the thickness of each phase in the coating were studied. The results showed that the average surface roughness of the coating with 2000# substrate and 5% AlCl3 was 2.641 μm. The aluminide coatings under different experimental parameters had the same Fe3Al, FeAl, Fe2Al5 and FeAl3 phase structure. However, the thicknesses of Fe3Al, FeAl and FeAl3 phase layers decreased with the increase of AlCl3 content. Furthermore, the aluminide coating with 2000# substrate and 5% AlCl3 had a bonding strength of 67.60 MPa and outstanding thermal shock resistance. The excellent properties of the iron-aluminum alloy coating made it possible to prepare the top coat on it.

Phase diagrams and elastic properties of the Fe-Cr-Al alloys: A first-principles based study

The phase diagrams and elastic properties of the Fe-Cr-Al alloys in full-temperature and all-compositional ranges are calculated. By combining first-principles calculations and cluster variation method, binary and ternary phase diagrams are obtained. A new ternary ordered phase B32 which is different from ternary extension of binary phases appears in the ternary section around temperature of 600 K. The binary FeAl phases show an extremely high solubility for Cr, while the binary CrAl phase solid solution has a low solubility for Fe. By combining first-principles calculations and cluster expansion method, the bulk modulus, shear modulus and Poisson's ratio are calculated. The shear modulus and Poisson's ratio show a strong ordering dependency, while the ordering dependency in bulk modulus is weak. Disordered Fe-Cr alloys with a little Al solvent shows ductile property, the Al-rich corner has brittle property.

Intereaction and intermetallic phase formation between aluminum and stainless steel

Abstract The formation of intermetallic layers in reaction between 430 StainlessSteel and Al was investigated for reaction in solid/solid, solid/semi-solid and solid/liquid states. The Fe2Al5 phase was formed as a major phase in the solid-state reaction and co-existed with the τ phase (Al5.5Cr1.95Fe2.55) as precipitates in a uniform layer in the semi-solid and liquid/solid reaction states. A porous two-phase layer consisting of Fe4Al13 and δ phase (Cr2Al13) was formed in the semi-solid and liquid/solid reaction states. The microhardness distribution of the formed intermetallic layer across the interface area was investigated along with the phase formation sequence in both thermodynamical and kinetically aspects.

Effects of steel type and sandblasting pretreatment on the solid-liquid compound casting characteristics of zinc-coated steel/aluminum bimetals

Effects of zinc-coated steel type and steel surface sandblasting pretreatment in the solid-liquid compound casting of layered type steel/aluminum bimetals were investigated. The Zn coating behavior and its effects on interfacial microstructure evolution and fracture mechanism were also discussed. The aluminum fluidity in thin plate type flow channels formed by the steel insert and the mold wall primarily depended on the insert surface roughness and secondarily on the wettability. As-galvanized (GI) steel/aluminum bimetal joints showed the bonding strength of 20 MPa and more, while galvannealed (GA) steels showed poor bonding. The interfacial bonding zone consisted of most Al13Fe4, Al8Fe2Si, Al4.5FeSi intermetallic phases, as well as some Si phases. A low temperature and short time of the bonding reaction coupled with a high silicon content of the aluminum alloy suppressed the formation of Al5Fe2 phase. Oxide scales on the GA steel surface prevented the molten Zn coating from mixing with the aluminum melt. The Zn coating of GI steels was rapidly disappeared from the steel surface by the chemical affinity and surface energy-driven fluid flow as well as the diffusion, resulting in the formation of Zn-free intermetallic phases. The Zn coating of GI steels played a role in retarding the onset of bonding reaction. A long time sandblasting caused an excessive growth of intermetallic layers and the formation of Kirkendall voids on the steel side, resulting in the shift of main fracture sites and a slight decrease of the bonding strength.

Strength and Brittleness of Interfaces in Fe-Al Superalloy Nanocomposites under Multiaxial Loading: An ab initio and Atomistic Study.

We present an ab initio and atomistic study of the stress-strain response and elastic stability of the ordered FeAl compound with the D0 structure and a disordered Fe-Al solid solution with 18.75 at.% Al as well as of a nanocomposite consisting of an equal molar amount of both phases under uniaxial loading along the [001] direction. The tensile tests were performed under complex conditions including the effect of the lateral stress on the tensile strength and temperature effect. By comparing the behavior of individual phases with that of the nanocomposite we find that the disordered Fe-Al phase represents the weakest point of the studied nanocomposite in terms of tensile loading. The cleavage plane of the whole nanocomposite is identical to that identified when loading is applied solely to the disordered Fe-Al phase. It also turns out that the mechanical stability is strongly affected by softening of elastic constants and/or and by corresponding elastic instabilities. Interestingly, we found that uniaxial straining of the ordered FeAl with the D0 structure leads almost to hydrostatic loading. Furthermore, increasing lateral stress linearly increases the tensile strength. This was also confirmed by molecular dynamics simulations employing Embedded Atom Method (EAM) potential. The molecular dynamics simulations also revealed that the thermal vibrations significantly decrease the tensile strength.

Formation Procedure of Reaction Phases in Al Hot Dipping Process of Steel

This study investigated the nucleation and growth mechanism of reaction layers and phases of hot-dipped boron steel in pure Al at 690 °C for 0–120 s. In the case of a dipping time of 30 s, reaction nuclei of width 10–15 μm and height 10 μm were formed on the steel surface in the flow direction of the liquid Al. This reaction layer was formed as a mixture of θ (Fe4Al13) phase of several nm to 2 μm, θ and η (Fe2Al5) of several nm, a columnar η region, and a β (FeAl) region of 500 nm thickness at the steel interface. At the grain boundaries of ferrite, in contact with the η phase, κ (Fe3AlC) was formed. Using the calculated Fe-Al phase diagram, it was determined that when Fe was dissolved in liquid Al from the steel above 2.5 at% (0.6 wt%), the θ phase was formed. Although most of the θ phases continuously grew toward the liquid phase, the θ phase in contact with the steel was transformed into the η phase with minimal differences in composition due to the inter-diffusion of Al and Fe. It was therefore concluded that the η phase formed at the interface became a growth nucleus and grew in a columnar form toward the steel.