The aluminide coating and the aluminum oxide film formed on the surface of the coating can prevent the substrate from being oxidized or damaged in high temperature or corrosive environments [, , ]. At the same time, the iron-aluminum alloy layer can provide the active elements required for the self-healing of the aluminum oxide film. In addition, aluminum oxide films have also been used as tritium permeation barrier coatings due to their high permeation reduction coefficients [, , ]. Commonly used aluminide coating preparation methods include hot-dip aluminum plating [7,8], electroplating aluminum [9,10], coating (PC) [, , ], physical vapor deposition [ 4] and chemical vapor deposition (CVD) [, , ]. CVD and PC processes can be applied to components with complex shapes, and the resulting aluminide coatings are dense and uniform. Both approaches have great advantages.
The cladding process can be considered as a chemical vapor deposition process . The basic mechanism of the two processes to form the aluminum coating is almost the same: low-halogen aluminum diffuses to the surface of the substrate, aluminum atoms are deposited on the surface, and the aluminum coating is formed through a diffusion reaction. However, aluminizing reactions may occur during the heating and cooling stages of the infiltration process. In the CVD process, gaseous halides are generated in the evaporator and only enter the reaction chamber to react when they reach the processing temperature. The equipment cost of the CVD process is high . Therefore, a cost-effective encapsulation cementation process is a better choice.
The preparation of aluminide coatings involves complex chemical reactions, and many researchers have studied the thermodynamics of different preparation processes [14,15,18,19]. Perez et al.  studied the equilibrium composition of gaseous precursors during CVD using thermodynamic software. The results showed that AlCl, AlCl2H and AlCl2Possibly the main precursor. In addition, they found that deposition time has an effect on the type of phases in the coating. only iron2Al5phase formed in a short deposition time, and with increasing time, the FeAl3phase appears. Yang et al.  calculated the possible reactions during the aluminizing process. they found iron2Al5The phase with the smallest free energy is preferentially formed. The possible formation sequence of iron-aluminum alloys is: Fe2Al5>Iron Aluminum3>Iron Aluminum2> Iron and aluminum. Fe free energy3The Al phase is greater than zero, and the Al phase does not form when the temperature is higher than 400 °C. Furthermore, dynamics are also the focus of attention [3,13,,,,,,]. Studies have shown that aluminide coatings prepared by hot-dip aluminizing, cladding, and CVD methods follow “parabolic growth kinetics” . Yener et al.  carried out low temperature infiltration on Fe-Cr-Ni superalloy. Aluminide coating composed of FeAl and Fe3The activation energy of Al phase is 207kJ/mol. item etc. [20,21] Low carbon steels were aluminized by cladding at temperatures between 600 and 750°C. Coating is single layer iron2Al5or iron14Al86A phase with an activation energy of about 75kJ/mol. Ei-Mahallawy et al.  Hot-dip aluminizing of mild steel in a pure aluminum bath with an activation energy of 138kJ/mol. Coating contains FeAl3, iron2Al5and iron and aluminum2stage.
Due to the complexity of the reaction in the low-temperature filling and cementing process, so far there are few related thermodynamic studies. In this paper, we investigate the possible reactions occurring in the low-temperature aluminizing process used in this work by studying a similar CVD process. The formation model of aluminide coating with multilayer phase structure was established. Furthermore, low-temperature aluminizing was performed at different temperatures (600-680 °C) and times (2-4 h), and the growth kinetics were studied. The thermodynamic and kinetic studies provide a theoretical basis for the parameter selection of the aluminizing process.
Materials and methods
316L stainless steel was used as the base material. A sample with a size of φ25×1mm was polished with 150-800 mesh water sandpaper, and then ultrasonically cleaned in acetone. The aluminide coating was prepared by the combination of surface slurry precoating and low temperature aluminizing. Detailed experimental procedures and parameters have been reported in previous literature . In this study, different aluminizing processes were used for low temperature aluminizing
Morphology and Phase Structure
The cross-sectional morphology of the coating at different aluminizing temperatures and aluminizing times is shown in Figure 1. 600 ℃ aluminized 2h coating is discontinuous, and some places even have no coating. The thickness of the discontinuous aluminum coating is defined as zero. The EDS results in Table 1 show that the coating consists of Fe2Al5and iron and aluminum3stage. As the aluminizing time increases to 3h and 4h, the coating becomes continuous with a thickness of approx.
In this study, aluminide coatings were prepared by a combined process at different aluminizing temperatures and times. After aluminizing at 600°C for 2 hours, the aluminized layer is discontinuous. With increasing temperature and time, the coating becomes continuous and increases significantly in thickness. At 600°C, Fe2Al5phase is the main phase of the coating. At 650 and 680°C the coating consists of Fe3aluminum, iron aluminum, iron2Al5and iron and aluminum3stage. In addition, the surface finish
Declaration and verification
Yanhui Sun and Jian Dong designed the experiments, Jian Dong performed the experiments, Feiyu He processed the data and images, and Jian Dong wrote and revised the paper.
All authors endorse the final article
This work was financially supported byNational Natural Science Foundation of China(No.51774030).
Effect of Heat and Cryogenic Treatment on Dry Tribological Behavior of Inconel 718 Fabricated Using Laser Powder Bed Fusion
Inconel 718 (In718) is a superalloy with an FCC γ matrix reinforced with FCC γ' phase, BCT γ'' phase, δ phase (orthorhombic), etc. Each of these stages affects the mechanical strength of the part. Inconel 718 alloy fabricated using the laser powder bed fusion (LPBF) process was used in this study. To analyze the effect of post-treatment on microstructure and tribological properties, samples were subjected to standard solution heat treatment at 1100°C (2 hours) and aged (8 hours at 720°C, 10 hours at 620°C). For cryogenic treatment, soak samples in liquid nitrogen (24 h) and temper at 650 °C (2 h). From the results, it was observed that the surface hardness of the solution heat-treated samples was higher than that of the printed and conventionally prepared In718 alloys due to the presence of precipitate segregation at grain boundaries in the latter. The tribological behavior of LPBF In718 alloy under normal loads of 10, 20, 30, 40, 50 and 100N was investigated. The microhardness of the LPBF HT and LPBF CT samples was 46.7% and 29.03% higher than that of the printed LPBF samples. Worn surface analysis and counter body (wear track) using optical microscopy, 3D profilometer, SEM and Raman spectroscopy. Abrasive wear was found to be dominant in the LPBF samples, although plastic deformation and fracture were visible across the entire sample surface. The experiments were extended to conventionally prepared hot-wrought Inconel samples for comparison.
Active air aluminizing of nickel-based superalloy (IN738LC): Coating formation mechanism
2023, Surface and Coating Technology
Slurry-based reactive air aluminization (RAA) is a low-cost, high-efficiency method for forming aluminide coatings on surfaces whose final coating microstructure is affected by temperature and time. This study aimed to incorporate the RAA method into a standard heat treatment cycle for IN738LC. The painted samples were solutionized, solutionized and aged, and only aged. Different samples were prepared at different stages of aluminide formation. The formation mechanism is thoroughly discussed using FE-SEM surface topography, BSE-SEM cross-sectional observation, surface XRD characterization, and comparison of the EDS elemental composition with the Al-Ni-Cr phase diagram. A typical high-activity aluminide coating has a three-layer structure of β-NiAl and Cr-rich precipitates on the top layer, β-NiAl on the middle layer, and IDZ is formed by the RAA method. A three-stage formation mechanism is proposed in detail, including the reaction stage (interaction between molten Al and solid matrix), Al indiffusion and Ni outdiffusion at low temperature, and Al indiffusion at high temperature. After the heat treatment cycle, a protective layer of aluminum oxide of about 3 μm was detected on the sample surface. γ-aluminum2Europe3和α-Al2Europe3Formed only during aging at 845°C and solutionization of the sample surface at 1120°C.
Mechanism of Accelerated Creep Cracking of Accumulated Aluminized Coating on T92 Steel Pipe Under High Stress
2023, Surface and Coating Technology
Aluminide coatings were deposited on commercial T92 steel by infiltration. The coating exhibits a typical two-layer structure consisting of an outer FeAl layer and an inner interdiffusion layer. The creep-rupture life of coated and uncoated T92 was investigated. The results showed that the fracture life of the coated T92 was significantly reduced compared to the uncoated T92, i.e. 46% at a creep stress of 110MPa. By examining the microstructure and fracture of the coated specimens, the mechanism by which the aluminide coating reduces the creep-rupture life is elucidated. The main fracture mode of coated and uncoated T92 substrates is micropore accumulation fracture. The characteristic microstructure of aluminide coatings is responsible for premature creep rupture. The formation and brittleness of FeAl phase columnar crystals make it easy to crack during creep. The phase transformation and acicular AlN precipitation in the internal interdiffusion layer contribute to the inward propagation of cracks and the accumulation of creep damage.
Abnormal phase evolution and degradation behavior of Al-Si coating during 30,000 h aging
2022, Corrosion Science
Systematically studying the microstructural evolution and degradation mechanisms of coatings during long-term aging is beneficial not only for lifetime prediction but also for coating design. Previous studies on aluminide coatings have shown that the prepared low-temperature aluminized coatings usually have a multilayer structure with a variety of metal phases, including FeAl3, Fe2Al5, and FeAl, etc. [5,6]. Under service conditions, high-temperature aging inevitably induces complex phase transitions, which significantly alter the microstructure and properties of coatings.
The degradation behavior of silicon-modified aluminide coating aged at 650℃ for 30,000h was studied. Significant microstructural evolution, abnormal element distribution, and newly formed secondary internal oxides were observed after prolonged aging. With the aging time extending from 10,000h to more than 20,000h, interdiffusion induced the formation of α-Fe/AlNi/α-Fe structure, while AlNi precipitates gradually replaced the Fe-rich matrix as the Al source to form alumina scale on the surface. Based on the evolution of the microstructure, a new coating degradation mechanism is proposed, involving the transition of diffusion mode and oxidation mechanism.
Current-enhanced long-range diffusion of interfacial atoms to tailor coating structure and properties
2022, Surface and Coating Technology
Coating modification is an important step for some engineering materials to achieve specific performance indicators. However, most of the traditional technologies have disadvantages such as poor diffusion kinetics, degradation of substrate performance at high temperature, and complicated process. Here, taking 316 stainless steel with an aluminide coating as an example, a general approach to achieve coating structure and property tuning by electromigration is presented. α-Al after pulse current treatment2Europe3A high-resistance tritium factor layer is formed on the coating surface at a lower temperature and shorter time (about 1000 °C, a few minutes), while conventional heat treatment requires a long time and high temperature treatment (above 1200 °C for a few hours). In addition, the electrical pulses apparently accelerated the interdiffusion of atoms. Numerical simulation results show that, through the coupling of the current density gradient and the chemical potential gradient, the diffusion flux of atoms at the interface is significantly increased, which promotes the long-distance diffusion of atoms and improves the bonding force between the interfaces. The pulsed current not only optimizes the interfacial structure by accelerating atomic diffusion, but also forms α-Al2Europe3Layers formed due to the long-range diffusion of atoms, which has important implications for the research and application of aluminide tritium permeation barrier coatings.
Effect of Laser Beam Energy Density on Microstructure and Wear Resistance of Fe<inf>3</inf>Al Laser Cladding Layer
Laser cladding has emerged as one of the most effective Fe surface techniques3Aluminum coating preparation. The linear energy density (defined as η = P/v) reflects the combined effect of the power of the laser beam and the scanning speed on the quality of the obtained Fe3Aluminum coating. Here, we found that the linear energy density of the laser beam has a significant effect on the microstructure, phase and wear resistance of the resulting Fe3Aluminum laser cladding coating. Dry Sliding Wear Tests to Understand the Friction Behavior and Wear Mechanisms of Fe3Aluminum laser cladding coating. Studies have shown that Fe3The Al laser cladding coating with a linear energy density of 0.3J/mm has the optimal film quality and the best wear resistance, with an average friction coefficient of 0.45 and a specific wear rate of 0.09×10−3mm3·N−1·rice−1.
Microstructure and Phase Composition of NiCr Alloy Hot Dip Aluminized Diffusion Layer
Surfaces and Interfaces, Volume 23, 2021, Article 100988
In this paper, the results of investigations on the structure and phase composition of diffusion coatings formed on the surface of Ni80Cr20 and Ni60Cr15 alloys after hot-dip aluminizing and subsequent high-temperature heating are presented. The results showed that due to the Al plating on the surface of the NiCr alloy, a uniform coating without defects was formed. The coating consists of an Al substrate and Metallic inclusions.
It was found that heating the coating at 1100°C caused the elements to re-diffuse across its thickness and form solid solutions of Cr (in the case of Ni80Cr20 alloy) or Cr and Fe (in the case of Ni60Cr15 alloy)Available in varying degrees of Al (aluminum-rich) and Ni (nickel-rich) saturation.
at the boundary of the interfacePhases with precipitates of different compositions appear based on Cr and.At the "coating-substrate" interface, they appear as a continuous secondary reaction zone, consisting of a large amount of precipitateand(Ni80Cr20 alloy) and a small amountand(Ni60Cr15 alloy). The formation of these heterogeneous elements is due to the presence of Cr in.
protective oxideFormed on the surface of the coating due to heating. Increased exposure time leads to the formation ofspinel oxides withOn the coating surface of Ni80Cr20 alloy.
Structural characterization of heat-treated aluminized steel in different environments
Surface and Coating Technology, Volume 335, 2018, Pages 88-94
Thermal and plasma heat treatment of aluminized P91 steel in different environments such as 100% O22, 100%Ar 和 50%O2+50% Ar attempts to study microstructural defects. The heat treatment of the aluminized P91 steel consisted of normalizing at 980°C and tempering at 750°C for 0.5h and 1.5h at a pressure of 5mbar under heat treatment and plasma treatment conditions in the above environment, respectively. In the current study, a detailed study of the microstructural evaluation of the resulting diffusion coatings under different environments is reported using SEM-EDS and EBSD characterization. Further vacancy concentration, CVThe evaluation is based on precise combinations of bulk length changes and X-ray lattice parameters from XRD data. The effects of vacancy concentration, grain size and strain on the resulting coatings are presented. There is a stable α-Al2Europe3and metastable θ-Al2Europe3FeAl was confirmed by XRD analysis in heat-treated and plasma-treated samples under 100% O22and 50%O2+50% Ar environment, and a small amount of θ-Al exists in the Ar environment2Europe3Observed during plasma treatment. Plasma heat treatment below 50%O2+50% Ar environment results in coatings with stable α-Al2Europe3This is followed by the FeAl diffusion zone with the lowest vacancies, residual strain and Kirkendall porosity area fraction, and for aluminized P91 steel, the maximum length of the FeAl phase diffusion zone compared to plasma treatment with 100% Ar environment and 100% O2.
Diffusion Mechanism and Microstructure Development of Heap Aluminized Nickel-Based Alloy
Surface and Coating Technology, Volume 239, 2014, Pages 147-159
Although the aluminizing process is used extensively in industry to protect components of gas turbine blades (such as cooling channels), systematic studies and studies on the formation mechanism and chemical composition of the aluminizing layer in relation to important process parameters, including the properties of the base alloy Temperature is scarce. In this study, 4 different alloys (pure Ni, Ni–20Cr, Inconel 738 and directionally solidified CM247LC) were aluminized at three different temperatures (950°C, 1000°C, 1040°C) process, using a mixture containing a fluorine-based activator; the results were compared to those obtained with a chemical vapor deposition (CVD) aluminum plating process using the same fluorine-based gaseous precursor at 1040°C.
Microstructural characterization by SEM+quantitative EDX analysis, XRD and nanoindentation tests revealed that during the heating phase of the packaging aluminization process, Al was transported to the sample surface at temperatures too low for simultaneous significant diffusion of Ni; therefore, δ-Ni2Al3The outer layer is formed by inward Al diffusion below the alloy surface, and then its growth also continues in the isothermal stage. Therefore, the selected isothermal treatment temperature does not affect the growth mechanism, although it changes the overall thickness of the Al coating. δ-nickel2Al3Transforms to β-NiAl after subsequent vacuum heat treatment at 1120°C. In the CVD process, where the gaseous precursors are introduced only after reaching the isothermal treatment stage, Al and Ni diffuse simultaneously from the very beginning of the aluminizing process to directly form β-NiAl.
Less mobile species (heavy atoms, e.g. W) in the alloy composition hinder all diffusion phenomena, both during aluminizing and subsequent vacuum heat treatment: after aluminizing, the precipitates formed in2Al3outer layer, after vacuum heat treatment, the resulting β-NiAl layer exhibits a compositional gradient.
Growth Behavior and Properties of Zn-Al Composite Carburized Layer on Carbon Steel
Surface and Coating Technology, Volume 306, Part B, 2016, Pages 455-461
Zn-Al filler co-infiltration of medium carbon steel in a filler mixture containing Zn and Al powders with NH4Cl as an activator. The Fe-Zn layer is formed first in the coating co-cementation process. Then the Fe-Al phase nucleates locally on the surface of the Fe-Zn layer. As the reaction continues, a complete Fe-Al layer forms and grows at the expense of the Fe-Zn layer. The Fe-Zn layer is dense, but the Fe-Al layer is porous. The Fe-Al layer has better high temperature oxidation resistance, while the Fe-Zn layer has better corrosion resistance in chloride solution. The growth behavior of Zn-Al cladding on steel is discussed in detail based on the microstructural evolution and the chemical reactions associated with the co-infiltration of Zn-Al cladding.
Characterization, Growth Kinetics and High Temperature Oxidation Behavior of Aluminide Coatings Formed on HH309 Stainless Steel by Casting and Subsequent Heat Treatment
Intermetallic Compounds, Volume 120, 2020, Item 106742
In the present work, a new technique was introduced to obtain aluminide coatings through a casting process followed by heat treatment. To do this, an aluminum plate is placed on the bottom of a copper mold, and the HH309 SS melt is poured into the mold. This technology is named Cast Aluminum (CA). CA samples were heat-treated at a temperature range of 900-1050 °C for 0.5-5 h. FE-SEM, XRD, and EDS were used to characterize the microstructure, phase analysis, and chemical composition of the cast aluminum samples, respectively. The results show that (Fe,Cr,Ni)Al3and (iron, chromium, nickel)2Al5layer formed at the Al/HH309 interface. FE-SEM analysis revealed multiple layers of aluminide coating on the heat-treated sample. The coating consists of (Fe, Cr, Ni)2Al5+(Fe,Cr,Ni)Al2, (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) sublayers. Growth kinetics studies show that layer thickness increases with annealing temperature and time. The growth rate of the layer obeys the parabolic law. Activation energy of (Fe,Cr,Ni) growth2Al5+(Fe,Cr,Ni)Al2, (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers are about 203, 250 and 247 kJ/mol, respectively. Microhardness measurements show that (Fe,Cr,Ni)2Al5+(Fe,Cr,Ni)Al2, (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers have hardnesses of 820-1040, 580-710 and 380-470 HV, respectively. The oxidation resistance of cast aluminum and heat treated (CA+HT) samples in air at 1000°C was investigated. The CA+HT sample exhibited higher oxidation resistance than the uncoated sample due to the formation of protective Al2Europe3layer on the surface.
Low temperature aluminizing behavior of ferritic-martensitic steel with surface mechanically ground
Surface and Coating Technology, Volume 258, 2014, Pages 329-336
Gradient nanostructured surface layers were prepared on ferritic-martensitic (F-M) steel plates by surface mechanical abrasion treatment (SMAT). Its aluminizing behavior was studied during the filling aluminizing process and subsequent low-temperature diffusion annealing treatment. Much thicker aluminum compared to the initial sample5iron2A layer formed on the SMAT sample after filling Al plating had a growth constant about 3 times higher at 600°C. Al transformation kinetics5iron2The phase transformation to AlFe phase and α-(Fe,Al) solid solution is also enhanced during the subsequent ~700°C annealing treatment. The enhanced aluminization kinetics originate from the increased atomic diffusivity at the bulk grain boundaries and the higher concentration of vacancies in the nanostructured surface layer. With defined growth kinetics, the dual-phase aluminizing process was demonstrated to achieve a gradient surface layer with low aluminide content below the tempering temperature of F-M steels.
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