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The effect of platinum (a rare metal)–aluminide coating parameters on the tensile properties of nickel-based superalloy Rene®80 was evaluated at 871 °C. For this purpose, initial layers of platinum with different thicknesses (2, 4, 6 and 8 μm) were coated on tensile samples. Then, low-temperature high-activity (LTHA) and high-temperature low-activity (HTLA) processes were used for aluminizing. Results of microstructural evaluations using scanning electron microscope (SEM) and phase analysis by X-ray diffraction (XRD) showed a three-layer structure coating for different platinum layer thicknesses and both aluminizing processes. Increasing the thickness of the platinum layer from 2 to 8 μm led to the improvement in the final coating thickness from 91.6 to 102.1 μm in HTLA. This increase was from 128.1 to 148.6 μm in LTHA. The results of hot tensile tests at 871 °C showed a decrease in strength properties of the coated samples compared to the uncoated ones. However, HTLA and high thicknesses of the initial platinum layer showed an intense reduction. The results of fractographic evaluations about uncoated samples showed a ductile fracture. On the other hand, coated samples showed a simultaneous ductile and brittle fracture failure mechanism. But the main fracture morphology was brittle cleavage fracture which was for the HTLA. [ABSTRACT FROM AUTHOR]

Comparative studies of the heat resistance of titanium, nickel, and carbon steel with coatings based on aluminides obtained by liquid-phase aluminizing with diffusion annealing at 850, 950°C were conducted. The influence of the holding time of aluminized samples of titanium, nickel, and carbon steel at 950°C on changes in their mass and oxidation rate was determined. [ABSTRACT FROM AUTHOR]

In the article, investigation is carried out at the second stage of aluminizing, when a consistent substance containing aluminium powder (the first option) or graphite powder and aluminium powder (the second option) is applied to the surface that has undergone aluminizing at the first stage, before further electrospark alloying with an aluminium electrode, and, regardless of the drying of the consistent substance, the aluminizing process is carried out at a discharge energy of 0.52-2.6 J and a productivity of 1.0-2.0 cm2/min, while the discharge energy and productivity are chosen such that the surface roughness decreases by ≅ 3-4 times. At the second stage aluminizing for both options, when using a consistent substance that contains aluminium powder or aluminium powder and graphite powder, the microhardness of the 'white layer' and the diffusion zone are increased (to a greater degree when graphite is present in the consistent substance), the surface roughness is decreased, and the integrity of the coating is of 100%. Before practical implementation, it is recommended to carry out the aluminizing process according to the first option at the discharge energy Wр = 4.6-6.8 J, using a consistent substance containing aluminium powder and graphite powder at the second stage. [ABSTRACT FROM AUTHOR]

The structure formation and properties of the surface layers of steel parts after alloying by traditional technologies and the method of electrospark alloying (ESA) is analysed in the article. As a result of the study of the productivity of the ESA process by aluminium electrode-tool, which is one of the important parameters of the ESA technology, reserves are revealed for improving the quality of the surface layers of steel parts during aluminizing. Two options for reducing productivity in relation to the traditional are studied: the first one, when productivity is reduced by ≅ 2 times; the second one, when performance is reduced by ≅ 4 times. As established, in the first variant, when the discharge energy increases from 0.52 to 6.8 J, at the first stage of aluminizing of steel 20 and steel 40, the thickness of the 'white' layer from 20 to 75 and from 25 to 110 μm, respectively; the thickness of the diffusion zone increases from 35 to 120 and from 40 to 140 μm, respectively; the microhardness of the 'white layer' increases from 2200 to 7400 and from 2400 to 7450 MPa, respectively; the surface roughness Ra increases from 1.1 to 9.0 and from 1.0 to 8.1 μm, respectively, and the continuity increases from 80 to 100% starting with Wр = 4.6 J and from 60 to 100% at Wр = 6.8 J. In the second variant, when the discharge energy increases from 0.52 to 6.8 J, at the first stage of processing steel 20 and steel 40, the thickness of the 'white' layer increases for steel 20 from 25 to 60 μm at Wp = 4.6 J, and then it doesn't change for steel 40 from 30 to 100 μm; the thickness of the diffusion zone increases from 45 to 130, respectively; the microhardness of the 'white layer' increases from 2250 to 7300 and from 2450 to 7300 MPa, respectively; the surface roughness Ra increases from 1.3 to 9.0 and from 1.6 to 8.1 μm, respectively, and the continuity for both steel 20 and steel 40 at Wр = 0.52 J is of 95% and further increases to 100%. [ABSTRACT FROM AUTHOR]

The regularities of formation of the diffusion coatings structure on the surface of the Fe–Cr–Al alloy during hot-dip aluminizing with ultrasonic impact are studied. The phase and chemical compositions of the coatings have been established. It is shown that the ultrasonic impact leads to a change in the intermetallic phase ratio of the coatings without affecting their composition. Exposure to ultrasound promotes the formation of continuous coatings while minimizing the contact duration between the substrate and high-temperature melt. [ABSTRACT FROM AUTHOR]

The aluminide coating layer was formed on wire arc additive manufactured (WAAM) Inconel 625 (IN625) Ni-based superalloy by a pack-aluminizing process at 700 °C for 3 h. The aluminide coatings were evaluated utilizing X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and nanoindentation techniques. In addition, the oxidation performances of the aluminide coatings were compared with as-built WAAM IN625 samples based on their exposure in the open-air environment for 5, 25, and 50 h at 1000 °C. The aluminizing process provided a dense aluminide coating with a thickness of 35 µm, continuous throughout the surface. The coating layer consists of mainly NiAl, Ni 2 Al 3 , Cr 2 Al, and MoAl 5 phases and exhibited a nano-hardness of 12.85 ± 0.43 GPa. Moreover, the applied heat treatment also improved the surface hardness and elasticity modules of WAAM Inconel 625. The stability of aluminide phases (NiAl, Ni 2 Al 3) at temperatures exceeding 1000 ℃ and the formation of stable Al 2 O 3 oxide islands on the surface provided 6.63 times, 2.70 times, and 2.65 times better oxidation resistance in the aluminized samples than the as-built WAAM IN625 in the 5 h, 25 h and 50 h oxidation periods at 1000 ℃, respectively. In contrast, the increase in the oxidation time changed the oxidation mechanism of as-built WAAM IN625 from Cr 2 O 3 to Cr 2 O 3 and spinel phases such as NiCr 2 O 4, NiMoO 4, and NiO. On the other hand, the increase in the oxidation time in aluminized samples caused Kirkendall voids formation and their degradation. Therefore, it was concluded that the mechanical properties could be improved, and the oxidation resistance of these alloys could be improved with the aluminizing heat treatment. [ABSTRACT FROM AUTHOR]

The growing interest in intermetallic and metal–intermetallic materials and coatings is based on the number of favorable properties they possess, primarily mechanical. However, the lack of data on their corrosion resistance has largely limited their scope of application. In this study, the corrosion destruction mechanisms of coatings formed on substrates made of AISI 321 steel and Aluchrom W (fechralloy) were investigated. The coatings were created by alloying in an aluminum melt followed by diffusion annealing to form the ultimate intermetallic structure. Corrosion resistance was studied under cyclic exposure to a humid marine atmosphere simulator and potentiostatic tests in an aqueous NaCl solution. Corrosion destruction parameters were determined, and mechanisms for each type of coating were revealed. The conducted studies allowed us to determine the electrochemical parameters of the corrosion destruction process and its mechanisms. It was shown that the corrosion rates during potentiostating for coatings on substrates Cr15Al5 and 12Cr18Ni10Ti differed by almost twofold. Two different mechanisms of corrosion are proposed. The first is associated with the formation of Al2O3 and MgO oxide films, which at the initial stage protect only local areas of the coating surface on Cr15Al5. The second is determined by the diffusion of titanium atoms during annealing to the coating surface on a 12Cr18Ni10Ti steel substrate with the formation of TiC carbide at the grain boundaries. [ABSTRACT FROM AUTHOR]

The analysis of the influence of the parameters of electrospark alloying with an aluminium electrode on the quality (roughness, microstructure of the coating, its continuity, phase composition, and microhardness) of the aluminized layer is presented. The effect of finishing methods after aluminizing is evaluated. The heat resistance of the obtained coatings is studied. Metallographic analysis shows that the coating consists of three sections: a ‘white’ layer, a diffusion zone, and the base metal. With an increase in the discharge energy, such quality parameters of the surface layer as thickness, microhardness of both a ‘white’ layer and a transition zone, and roughness are increased. The continuity of a ‘white’ layer at the discharge energy Wp = 0.52 J is low (of 50–60%); with a subsequent increase in the discharge energy, it increases and, at Wp = 6.8 J, it is of 100%. An increase in the discharge energy during electric-spark alloying (ESA) leads to a change in the chemical and phase compositions of the layer: at low discharge energies, a layer is formed, consisting mainly of α-Fe and aluminium oxides. As Wp increases, the layer consists of iron and aluminium intermetallic compounds, as well as free aluminium, that is confirmed by the data of local x-ray microanalysis. For practical application, it is possible to recommend the process of aluminizing by the ESA method, using the modes (discharge energy in the range of 4.6–6.8 J and productivity of 2.0–3.0 cm2/min). Such process provides the formation of a ‘white’ layer with a thickness of 70–130 µm, microhardness of 5000–7500 MPa, roughness (Ra) of 6–9 µm, and continuity of 95–100%. In order to increase the thickness of the aluminized layer, it is recommended to preliminarily apply grease containing aluminium powder to the steel surface and, without waiting for it to dry, carry out ESA with an aluminium electrode. In this case, the coating continuity is of 100%, the layer thickness is of up to 200 µm, and the microhardness is of 4500 MPa. The paper presents the results of a study of the quality parameters of multicomponent aluminium-containing coatings of Al–S, Al–C–S, and Al–C–B systems. Replacing the aluminium electrode with graphite one leads to a decrease in the thickness and continuity of a ‘white’ layer, respectively, to 50 µm and 30%. In turn, the microhardness on the surface increases to 9000 MPa. The addition of 0.7 boron to the consistency substance leads to an increase in the thickness and continuity of a ‘white’ layer, respectively, up to 60 µm and 70%. The microhardness on the surface rises to 12000 MPa. In order to reduce the roughness of the surface layer and to obtain continuous coatings, it is recommended to carry out ESA with an aluminium electrode, but at lower modes.

The development of high-performance aero-engines raises the urgent requirement of coatings deposited on blades, containing both thermal barrier coatings (TBCs) on the external surface and aluminide coatings in the internal winding cooling channels. A TBC system with a low activity aluminizing process before deposition of conventional TBC was developed here to evaluate the feasibility of co-deposition technology. The thermal cyclic oxidation behavior of co-deposited “Al-modified TBC” was examined at 1100 °C and compared to that of conventional TBC. The cross-sectional microstructure evolution and elemental diffusion behavior of the modified TBC were also investigated. Even though the underlying aluminizing layer can serve abundant Al for the upper NiCoCrAlYHf layer to eliminate “Al depletion” in conventional TBC, which is a main reason for spallation, the Al-modified TBC system exhibited shorter thermal cycling life than conventional TBC. The spallation mechanism was discussed. Cracks caused by the fast growth rate of thermally grown oxide (TGO), Cr-rich precipitates resulted from the decreased γ/γ′-phase in the coating, and Re-rich precipitates originated from outward diffusion through the aluminide layer were mainly responsible for the failure.

Electrical contacts are used in electromechanical horns to make and break the circuit. Electrical contacts are generally made of tungsten refractory material due to its high melting point, high hardness, and hence good abrasion and wear resistance among all refractory metals. However, the oxidation resistance of tungsten is poor, and thus tungsten contacts used in electrical appliances undergo severe high-temperature oxidation during making and breaking electric circuit. The tungsten oxide (WO3) layer, being not very conductive with respect to thickness, can cause electrical connectivity loss and subsequently cause horn failure. In other circumstances, if oxides grow thicker, they can peel off easily due to the frequent impact during make-and-break operations, leading to a high wear rate. Thus, to improve the life of horn, aluminizing and chromizing coating treatments on tungsten contact have been developed throughes halide activated pack cementation process. The behaviour of the developed coatings under isothermal oxidation at 1000 °C for different time intervals from 2 to 100 h was studied. To evaluate the rate of oxidation, the weight gain versus time of bare and coated tungsten contacts was analysed. Macroscopic morphological analysis indicates isothermal oxidation at 1000 °C resulted into severe oxidation of bare W within 10 h and aluminized W within 25 h. On the other hand, chromized W showed negligible oxidation even after 100 h at 1000 °C. The mass gain of chromized tungsten contacts after 100 h of isothermal oxidation was 2.2 mg/cm2, which was drastically lower than the aluminized (276.9 mg/cm2) and bare (270.02 mg/cm2) tungsten contacts. The parabolic oxidation rate constant (kp in units g2 cm−4 s−1) for chromized tungsten contact was obtained as 4.7 × 10−11, which is quite low compared to aluminized and bare tungsten of 6.3 × 10−7 and 1.0 × 10−6, respectively. This suggests that the chromizing coating effectively improved the oxidation resistance of tungsten contact. [ABSTRACT FROM AUTHOR]

Chromium-molybdenum steel (16Mo3) is widely used in petroleum, gas, automotive, and construction industries due to its good oxidation resistance and mechanical properties at moderately elevated temperatures. The aim of the research was to evaluate the corrosion susceptibility of 16Mo3 steel in hot rolled and aluminized states. Aluminization was performed by diffusion pack aluminization process at 900°C/2h and 730°C/4h, respectively. Electrochemical corrosion testing included measuring open circuit potential (EOCP), linear polarization resistance (LPR), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) in potassium phosphate buffer (KH2PO4, pH = 7). Optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used for surface layer microstructure characterization before and after corrosion tests. It was demonstrated that corrosion resistance of aluminized steel increased substantially. Corrosion properties were related to the structure and properties of intermetallic phase (FeAl, FeAl2 and Fe2Al5) that formed on the surface of 16Mo3 steel.

This study investigated the characteristic properties of aluminizing, boronizing, and boro-aluminizing coatings grown on Haynes 25 superalloys and their effects on the high-temperature wear behavior. The coating processes were conducted in a controlled atmosphere at 950 °C for 3 h. Characterization studies were performed using scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction analysis, nanoindentation testing, and high-temperature wear tests. It was determined that the thickness values of aluminide, boride, and boride–aluminide coatings were 140 ± 1.50 µm, 37.58 ± 2.85 µm, and 14.73 ± 1.71 µm, and their hardness values were 12.23 ± 0.9 GPa, 26.34 ± 2.33 GPa, and 23.46 ± 1.29 GPa, respectively. The hardness of the coatings resulted in reduced wear volume losses both at room temperature and at 500 °C. While the best wear resistance was obtained in the boronized sample at room temperature due to its high hardness, the best wear resistance at 500 °C was obtained in the boro-aluminized sample with the oxidation–reduction effect of Al content and the lubricating effect of B content in the boro-aluminide coating. This indicates that the presence of aluminum in boride layers improves the high-temperature wear resistance of boride coatings. The coated samples underwent abrasive wear at room temperature, whereas at 500 °C, the wear mechanism shifted to an oxidative-assisted adhesive wear mechanism. [ABSTRACT FROM AUTHOR]

Results are provided for a study of an aluminized coating corrosion resistance on the surface of alloys of the Fe–Cr–Al system (fechrals) and austenitic corrosion-resistant steel 12Kh18N10T under conditions of a temperate climate humid atmosphere containing chlorides. Coatings are applied by immersion in a melt. A study of corrosion during variable immersion of specimens in a 3% aqueous solution of sodium chloride show that coatings of the Fe–Cr–Al system prepared by aluminizing withstand long-term exposure to the environment (corrosion rate is 0.0003 g/(m2·day). An aluminized coating on a steel 12Kh18N10T surface has twice as good corrosion resistance indicators than for a coating on a Kh15Yu5 substrate. [ABSTRACT FROM AUTHOR]

The heat resistance of titanium with coatings based on titanium aluminides deposited by liquid-phase aluminizing followed by a high-temperature hold is studied. The effect of the duration of the hold of titanium specimens in an aluminum melt and of the duration of the high-temperature exposure of the aluminized specimens on the chemical and phase composition, structure and heat resistance of the coatings is determined. [ABSTRACT FROM AUTHOR]

One of the critical issues in the application of supercritical water oxidation technology is to improve the corrosion resistance of reactor materials. Use of Al 2 O 3 coating is one of the most promising methods to address this issue. In this study, thick NiAl/Al 2 O 3 coatings on Inconel 625 substrates were prepared by a consecutive pack embedding and in-situ thermal oxidation process. The effect of aluminizing and oxidation temperature on phase structure and coating thickness is studied. Results show the diffusion of Al from the exterior to the interior of the alloy matrix to form intermetallic compounds between Al and metal elements in the matrix (Ni, Cr, Mo, etc.). Moreover, the coating thickness can reach above 300 μm at the aluminizing temperature of 950 °C. Increasing the aluminizing temperature above 950 °C will not increase the coating thickness further. After high temperature oxidation subsequently, only phases of NiAl and Al 2 O 3 were detected. The formation of Al 2 O 3 layer can be ascribed to the surface oxidization of Al. And the NiAl between the alloy substrate and Al 2 O 3 coating provides an interfacial layer that can alleviate the crack or exfoliation of ceramic coating due to the mismatching of thermal expand coefficient. The thick NiAl/Al 2 O 3 coatings prepared by aluminizing 950 °C and oxidizing at 1100 °C exhibit satisfied corrosion resistance after supercritical water test. This work would provide a significant method to develop advanced ceramics coating for the corrosion resistance of alloys. [ABSTRACT FROM AUTHOR]

Aluminizing process of titanium in aluminum melts A7 and AK12 was investigated. The influence of aluminizing regime on the structure and chemical composition of the coating on the titanium alloy VT1-0 was established. [ABSTRACT FROM AUTHOR]

The analysis of the influence of the parameters of electrospark alloying with an aluminium electrode on the quality (roughness, microstructure of the coating, its continuity, phase composition, and microhardness) of the aluminized layer is presented. the effect of finishing methods after aluminizing is evaluated. the heat resistance of the obtained coatings is studied. Metallographic analysis shows that the coating consists of three sections: a 'white' layer, a diffusion zone, and the base metal. With an increase in the discharge energy, such quality parameters of the surface layer as thickness, microhardness of both a 'white' layer and a transition zone, and roughness are increased. The continuity of a 'white' layer at the discharge energy Wp = 0.52 J is low (of 50-60%); with a subsequent increase in the discharge energy, it increases and, at Wp = 6.8 J, it is of 100%. An increase in the discharge energy during electric-spark alloying (ESA) leads to a change in the chemical and phase compositions of the layer: at low discharge energies, a layer is formed, consisting mainly of α-Fe and aluminium oxides. As Wp increases, the layer consists of iron and aluminium intermetallic compounds, as well as free aluminium, that is confirmed by the data of local x-ray microanalysis. For practical application, it is possible to recommend the process of aluminizing by the ESA method, using the modes (discharge energy in the range of 4.6-6.8 J and productivity of 2.0-3.0 cm²/min). Such process provides the formation of a 'white' layer with a thickness of 70-130 µm, microhardness of 5000-7500 MPa, roughness (Ra) of 6-9 µm, and continuity of 95100%. In order to increase the thickness of the aluminized layer, it is recommended to preliminarily apply grease containing aluminium powder to the steel surface and, without waiting for it to dry, carry out ESA with an aluminium electrode. In this case, the coating continuity is of 100%, the layer thickness is of up to 200 pm, and the microhardness is of 4500 MPa. The paper presents the results of study of the quality parameters of multicomponent aluminium-containing coatings of Al-S, Al-C-S, and Al-C-B systems. Replacing the aluminium electrode with graphite one leads to a decrease in the thickness and continuity of a 'white' layer, respectively, to 50 µm and 30%. In turn, the microhardness on the surface increases to 9000 MPa. The addition of 0.7 boron to the consistency substance leads to an increase in the thickness and continuity of a 'white' layer, respectively, up to 60 µm and 70%. The microhardness on the surface rises to 12000 MPa. In order to reduce the roughness of the surface layer and to obtain continuous coatings, it is recommended to carry out ESA with an aluminium electrode, but at lower modes. [ABSTRACT FROM AUTHOR]

This study addresses the diverse application requirements of turbine blades by utilizing a composite anti-seepage masking layer to protect the Ni-based superalloy from Al deposition during aluminizing. Simultaneously, the unmasked areas simulate blade airfoils for aluminized coating formation. Ethylene-based and glass-ceramic coatings were evaluated for anti-seepage effectiveness after aluminizing at 800 ∼ 1000 ℃. The results reveal that ethylene-based coatings prevent Al diffusion into the substrate at 800 ℃, and degradation occurs at higher temperatures. However, adding a glass-ceramic coating significantly enhances high-temperature stability and suppresses ethylene-based coating fluidity. The composited anti-seepage masking layer, with the ethylene-based and glass-ceramic coating applied twice, exhibits excellent anti-seepage masking performance on GH4169, DZ22B, and K477 superalloys, providing protection and easy removal without affecting unmasked areas. This approach improves the comprehensive performance of turbine blades, meeting the requirements of both dovetail and airfoil sections. [Display omitted] [ABSTRACT FROM AUTHOR]

The growing interest in intermetallic and metal–intermetallic materials and coatings is based on the number of favorable properties they possess, primarily mechanical. However, the lack of data on their corrosion resistance has largely limited their scope of application. In this study, the corrosion destruction mechanisms of coatings formed on substrates made of AISI 321 steel and Aluchrom W (fechralloy) were investigated. The coatings were created by alloying in an aluminum melt followed by diffusion annealing to form the ultimate intermetallic structure. Corrosion resistance was studied under cyclic exposure to a humid marine atmosphere simulator and potentiostatic tests in an aqueous NaCl solution. Corrosion destruction parameters were determined, and mechanisms for each type of coating were revealed. The conducted studies allowed us to determine the electrochemical parameters of the corrosion destruction process and its mechanisms. It was shown that the corrosion rates during potentiostating for coatings on substrates Cr15Al5 and 12Cr18Ni10Ti differed by almost twofold. Two different mechanisms of corrosion are proposed. The first is associated with the formation of Al2O3 and MgO oxide films, which at the initial stage protect only local areas of the coating surface on Cr15Al5. The second is determined by the diffusion of titanium atoms during annealing to the coating surface on a 12Cr18Ni10Ti steel substrate with the formation of TiC carbide at the grain boundaries.

Aluminized AISI 321 steels applied as heat exchange tube are generally subjected to creep–fatigue (C‐F) exposure during service. Remaining tensile and creep performance of this steel at 620°C were therefore studied under prior C‐F deformation. Results revealed that residual properties exhibit an initial increase and followed degradation with rising lifetime fraction of prior C‐F. The tensile strength and creep lifespan reach highest at 30% lifetime fraction, since dislocation cell networks are well developed and secondary nanotwins are activated at substrate. Additionally, dynamic recovery and wavy slips occurred in coatings partially accommodate local plastic deformations and inhibit defect initiations, avoiding premature material fracture. On the other hand, these networks could restrain the increase in coating thicknesses. When the lifetime fraction of prior C‐F increases to 80%, declined remnant properties are observed, which is attributed to the recovery of dislocation cells and carbide coarsening. Meanwhile, coating microcracks also accelerate steel failure. [ABSTRACT FROM AUTHOR]

Due to the distinct surface coating properties of IN-718 nickel-based alloy after boriding and aluminizing treatments, a comparative study was conducted. Optimal process parameters for both treatments were determined to be 950 °C for 10 h, based on cross-sectional analyses using scanning electron microscopy, X-ray diffraction, and electron probe microanalysis. Coating growth mechanisms were elucidated for both treatments. Electrochemical testing revealed superior corrosion resistance in the borided specimens. While nanoindentation measurements showed lower surface hardness for the aluminized specimens compared to their borided counterparts, tribological testing demonstrated superior performance of the aluminized specimens under dry sliding conditions. This enhanced wear behavior for the aluminized specimens was attributed to the lubricating effect of a plastically deformed tribolayer formed during testing. The wear mechanisms for both borided and aluminized specimens were systematically analyzed and discussed. • Boriding and aluminizing improve the corrosion and wear resistance of IN 718. • Borided specimens demonstrate lower corrosion susceptibility. • Aluminized specimens exhibit a reduced hardness gradient within the aluminizing layer. • Improved wear resistance of aluminized specimens attributed to plastically deformed layer. [ABSTRACT FROM AUTHOR]

In the present study, an equimolar HfNbTaTiZr refractory high entropy alloy was subjected to powder-pack aluminizing at 950°C for 4 h. High-temperature isothermal oxidation tests were carried out in open air on both as-cast and aluminized samples at 1000°C for the durations of 5, 25, and 125 h. Microstructural analysis and oxidation kinetics revealed the formation of a complex oxide layer consisting of (Hf,Zr)O 2 and Al 2 O 3 with AlN inclusions in the aluminized samples following oxidation. Significant cracking of the complex oxide layer occurred after the prolonged exposure of 125 h. The as-cast samples followed similar oxidation kinetics under all the conditions. However, the magnitude of mass gain for the aluminized samples was 1.18, 1.55, and 3.60 times less than the as-cast samples after 5, 25, and 125 h of oxidation, respectively. The lower mass gain in the aluminized samples indicated the formation of an adherent oxide layer on the surface of the alloy and that the molar volume of the oxide equaled or exceeded the molar volume of the alloy. As a result, the aluminized alloy was shielded from direct contact with gaseous oxygen and oxidation rate was governed by diffusion through the oxide layer. • Powder-pack aluminizing refractory HfNbTaTiZr high entropy alloy at 950 °C for 4 h. • A compact coating layer composed of AlN, Al 2 O 3 and NbZrAl 6 phases. • Powder-pack aluminizing improved the oxidation resistance of HfNbTaTiZr at 1000 °C by up to 3.6 times. • Oxygen solubility in both RHEA and aluminized RHEA led to the pest oxidation trend. [ABSTRACT FROM AUTHOR]

The study aims to investigate the effect of Al2O3 and Al additions to Nickel-base superalloys as a coating layer on oxidation resistance, and structural behavior of nickel superalloys such as IN 738 LC. Nickel-base superalloys are popular as base materials for hot components in industrial gas turbines such as blades due to their superior mechanical performance and high-temperature oxidation resistance, but the combustion gases’ existence generates hot oxidation at high temperatures for long durations of time, resulting in corrosion of turbine blades which lead to massive economic losses. Turbine blades used in Iraqi electrical gas power stations require costly maintenance using traditional processes regularly. These blades are made of nickel superalloys such as IN 738 LC(Inconel 738). Few scientists investigated the impact of Al2O3 or Al additions to Nickel-base superalloys as coating layer by using the slurry coating method on oxidation resistance to enhance the Nickel-base superalloy’s oxidation resistance. In this study, IN 738 LC is coated with two different coating percentages, the first being (10 Al+90 Al2O3) and the second being (40 Al+60 Al2O3). Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) were performed on all samples before and after oxidation. According to the results, SEM images of the surface revealed that the layer of the surface has a relatively moderated porosity value and that some of the coating layers contain micro-cracks. The best surface roughness of specimens coated with 60 % alumina+40 % aluminum was 5.752 nm. Whereas, the surface roughness of specimens coated with 90 % alumina+10 % aluminum was 6.367 nm. Results reveal that alloys with both Al2O3 and Al additions have reported a positive synergistic effect of the Al2O3 and Al additions on oxidation resistance. Moreover, the NiCrAl2O3 thermal coating has good oxidation resistance and the effective temperature of anti-oxidation is raised to 1100 °C in turn reducing the maintenance period of turbine blades [ABSTRACT FROM AUTHOR]

This article discusses the main ways of improving the operation of blast-furnace air tuyeres: increasing the service life by applying protective coatings and changing the tuyere design to intensify its cooling with water, reducing heat losses through the surface by thermal insulation, and increasing the completeness of fuel combustion in the air passage. [ABSTRACT FROM AUTHOR]

A boriding and aluminizing two-step method was used to fabricate a B–Al composite layer on the surface of pure titanium. The composite layer was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), nanoindentation, wear tests, and electrochemical experiments. For comparison, single boriding and single aluminizing treatments were performed on pure titanium. The results show that the surface morphology characteristics from the single boriding and single aluminizing treatments were transferred to the composite sample. The XRD pattern reveals that the B–Al composite layer also has XRD peaks of Al 3 Ti+TiB 2 +TiB. The SEM results revealed that the B–Al composite layer is composed of an outermost TiB 2 layer, a TiB+Al 3 Ti sublayer, an Al 3 Ti layer, and an internal diffusion layer. The corrosion resistance of the composite sample is slightly better than that of the matrix, and the hardness is approximately 3 times higher than that of the matrix. The wear type is mainly abrasive wear, and the wear resistance is improved; with increasing temperature, the wear on the surface of the sample is intensified, and the wear resistance is reduced. [ABSTRACT FROM AUTHOR]

Platinum-modified-βNiAl is a bond coat (BC) material for thermal barrier coatings (TBCs) applications applied on aeroengine hardware to reduce their surface temperatures. However, it is desirable to minimize its production and material costs by the low-cost alternatives of similar performance. It has been acknowledged that the small concentration of the reactive elements (REs), such as Zr, Hf, and Y, could tremendously enhance the oxide adhesion even in some cases better than Pt modified counterparts. The aim of this study was to design and fabricate the Zr-modified-βNiAl bond coat on CMSX-4 superalloy using an aluminizing method. Moreover, the study focused on the development of a systematic understanding of underlying mechanisms behind the beneficial effects of REs. Initially, three sets of BCs were prepared: Zr-free βNiAl (undoped), Al and Zr co-deposited in a single-step process (1SP), and Zr and Al, which were individually deposited in two processing steps (2SP): zirconizing and aluminizing. Such three sets of BCs helped to understand the processing, as well as Zr and Al effects on scale adhesion. In particular, 1SP/2SP BCs showed uniformity of Zr in the form of precipitates and networks that caused hardness enhancement. All BCs were isothermally oxidized at 1150°C for 100 hours wherein 2SP revealed the best spallation resistance, microstructural stability, and its Zr-oxide pegs were extended to substrates. In addition to the Zr effect, BC Al content was found to affect the oxide adhesion equally. Under identical Zr contents (of 1SP and 2SP = 1at %), the higher Al showed the better spallation resistance while lower Al caused the inverse effect of Zr owing to its reactive nature that was termed as over doping. Moreover, it was established that over-doping either local or into entire BC, accelerated the Al depletion that destabilized the βNiAl into γ’-Ni3Al phase. An extensive discussion is presented in the light of the observed results.

Modification of the surface is an effective approach for increasing the corrosion resistance of Mg alloys without influencing their bulk properties. Among numerous surface modification techniques, aluminizing is a low-cost and effective approach that is applied on many alloys for improved resistance to wear and corrosion. Based on the previous research, the enhancement by aluminizing can be further increased by the addition of rare-earth elements such as Y. In the current work, the effect of the addition of yttrium to the aluminizing mixture on the corrosion and tribological behavior of Mg-3%Al alloy was studied. Ball-on-disk tribometer and electrochemical testing apparatus were used to test wear and corrosion behavior and scanning electron microscopy was utilized to study the microstructures and worn surfaces. The results demonstrated that the aluminizing process is beneficial to the resistance of the alloy to wear, corrosion, and corrosive wear. The addition of Y makes the benefits larger. By adding Y, the wear volume loss decreased from 5.6×107 to 3.5×107 μm3 and the corrosive wear volume loss decreased from 4.1×106 to 3.5×106 μm3. In addition, incorporating Y in the aluminizing mixture resulted in a decrease in corrosion current from 4.59×10-2 to 6.17×10-3 μA. The improved properties are mainly attributed to the improved passivation capability of the modified surface layer. [ABSTRACT FROM AUTHOR]

The surface layer on pure TA2 titanium was manufactured using boriding and aluminizing two-step methods. High-temperature oxidation experiments were carried out at 600 °C, 700 °C, and 800 °C for 100 h. Microstructure characterization of the surface layer was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). After 1050°C-4 h boriding and 1050°C-4 h aluminizing, the composite layer from the surface to the matrix consists of a TiB 2 layer, TiB whisker + Al 3 Ti layer, Al 3 Ti layer, and diffusion layer. The Al 3 Ti layer could effectively protect the matrix, and thus, the borided and aluminized pure titanium shows excellent high-temperature oxidation resistance. [ABSTRACT FROM AUTHOR]

Steel that has been aluminized said as hot dipping aluminizing has better protection against corrosion and can protect against temperatures as high as 800°C. In hot dipping aluminizing, Steel is immersed into a molten aluminium for certain dipping time. The research aims to know the effect of preheating and dipping time on the toughness of low carbon steel. The method research was started by cutting the low carbon steel plate, according to ASTM E23 (Charpy test sample) into 16 pieces samples. Then the samples were grouped into four groups. Group-1 was initial samples. The Group-2 was directly immersed into molten aluminum 700 0C, for dipping time 5 minutes. The Group-3 was preheated at 700 oC for 30 minutes and then to be aluminized (700 oC) for dipping time 5 minutes. The Group-4 was preheated at 700 oC for 30 minutes and then to be aluminized (700 oC) for dipping time 10 minutes. Finally, all groups were tested by the Charpy test at room temperature. The results show that the aluminizing increases the toughness of low carbon steel from 228.125 KJ/m2 to 312.5 KJ/m2. The preheating process before aluminizing increases sharply the toughness of low carbon steel from 228.125 KJ/m2 to 512.5 KJ/m2. The increasing dipping time from 5-minute to 10-minute increase gradually the toughness from 512.5 KJ/m2 to 556.25 KJ/m2.

The structure and the tribological properties of steel AISI 1010 are studied after boronizing and aluminizing in powder mixtures at 850°C for 1, 3 and 5 h. The effect of the duration of the thermochemical treatment on the thickness of the diffusion layer, the surface hardness and the wear rate of steel AISI 1010 is determined. X-ray diffraction and energy dispersive analyses are performed and adhesion tests are conducted according to the Daimler Benz classification. The states of the surface layers are shown to be satisfactory both after the boronizing and after the aluminizing. [ABSTRACT FROM AUTHOR]

In this work, microstructure, mechanical properties and fracture behavior of the magnesium/steel bimetal using compound casting assisted with hot-dip aluminizing were investigated, and the interface bonding mechanism of the magnesium/steel bimetal were also analyzed. The results indicate that the magnesium/steel bimetal obtained without hot-dip aluminizing had larger gaps through the whole interface without reaction layers between magnesium and steel, leading to a poor mechanical bonding. After the steel substrate was hot-dip aluminized, an intermetallic layer along with an Al topcoat layer were formed on the surface of the steel substrate, and the intermetallic layer was constituted by Fe2Al5, τ10-Al9Fe4Si3, FeAl3 and τ6-Al4.5FeSi phases. In the case of the magnesium/steel bimetal obtained with hot-dip aluminizing, a compact and uniform interface layer with an average thickness of about 17 μm that consisted of Fe2Al5, τ10-Al9Fe4Si3, FeAl3 and Al12Mg17 intermetallic compounds was formed between the magnesium and the steel, obtaining a superior metallurgical bonding. The interface layer had much higher nano-hardnesses compared to the magnesium and steel matrixes, and its average nano-hardness was up to 11.1 GPa, while there were respectively 1.1 and 4.2 GPa for the magnesium and steel matrixes. The shear strength of the magnesium/steel bimetal with hot-dip aluminizing reached to 23.3 MPa, which increased by 8.59 times than that of the composites without hot-dip aluminizing. The fracture of the magnesium/steel bimetal with hot-dip aluminizing represented a brittle fracture nature, initiating from the interface layer. [ABSTRACT FROM AUTHOR]

A high-temperature antioxidant layer on Ti-6Al-4V alloy was prepared by a two-step method of REO-boriding followed by aluminizing (REOs are rare-earth oxides, which are Nd 2 O 3 , Gd 2 O 3 , and Tm 2 O 3). The microstructure and oxidation behavior of the layer were investigated at temperatures of 700 °C, 800 °C, and 900 °C in air. The findings unveiled that the layer significantly enhances the antioxidant capacity of Ti-6Al-4V alloy. After cyclic oxidation at 900 °C for 100 h, the average weight gain of the layer is only one fortieth of that of the matrix. TiAl 3 in the layer can react with O 2 to form a dense continuous Al 2 O 3 layer, which effectively inhibits the diffusion of O 2 to the matrix. During the cyclic oxidation, the thicker TiAl 2 , TiAl, and Ti 3 Al diffusion layers are formed sequentially between TiAl 3 layer and matrix, and TiAl 2 layer can also provide the good oxidation resistance. • REO-borided and aluminized coatings were prepared by a simple two-step process. • The coatings have excellent high-temperature oxidation resistance. • Oxygen reacts with Al 3 Ti to form a dense continuous Al 2 O 3 layer to resist oxidation. [ABSTRACT FROM AUTHOR]

To enhance the surface properties, particularly hardness and wear resistance, of AISI 321 steel, an aluminizing coating was introduced through pack cementation treatment. Microstructure characterization and wear resistance tests were conducted to elucidate the intricate relationship between the microstructure and wear behaviors. A thorough analysis on the structure and compositions revealed the incorporation of FeAl(Cr) phases, aluminum oxides, titanium compounds and B2-NiAl phases in the coating, which resulted in a remarkable 192 % increase on the surface hardness, elevating it from 226 HV to an impressive 662 HV. In comparison to the uncoated 321 steel, the aluminized counterpart exhibited a pronounced reduction in coefficients of friction (COFs), wear volume losses, and wear rates. The wear mechanisms of both the uncoated 321 steel and aluminized steel were intricately analyzed. The uncoated 321 steel endured severe adhesive and oxidation wear, characterized by pronounced plastic deformation, the presence of wear debris and flaking oxides. In contrast, the aluminized steel predominantly underwent abrasive wear, showcasing a relatively smooth worn surface and dense continuous oxides. These findings not only reveal the significant enhancement of tribological properties by aluminized coatings, but also provide valuable insights into their fundamental behavior during wear. • A ~135 μm thick Fe Al coating was introduced on the surface of AISI 321 steel. • The tribological properties were significantly enhanced after aluminizing treatment. • The coated and uncoated 321 steels exhibited different wear mechanisms. [ABSTRACT FROM AUTHOR]

In the present work, the tensile properties and microstructure of pack cementation preparing aluminized AISI 321 stainless steel with subsequent annealing treatment was investigated. The results reveal that the coatings of aluminized AISI 321 stainless steel are mainly composed of outermost Al2O3 layer, Fe-Al compound intermediate layer and Fe(Al,Cr) solid solution diffusion layer. The cross shape precipitate (NiAl) and strip precipitate (Ni3Al) are observed in Fe(Al,Cr) layer. After annealing, no new phase in aluminized coating is detected, meanwhile, the thickness of the aluminized coatings is increased and the porosity of Fe-Al layer is increased as well. In addition, the size of NiAl precipitates is decreased. The strength and plasticity of stainless steel are degraded by aluminizing treatment. However, after annealing, the aluminized steel exhibits a lower strength but a higher ductility. The crack initiation region of both aluminized steel with or without annealing treatment comprise coarse columnar grains and cleavage planes with river patterns. The fracture model of aluminized steel is cleavage fracture, while a mixed of intergranular and transgranular fracture is observed in aluminized annealed steel.

Results are presented for research on the problem of increasing the stability and durability of a diffusion aluminized coating on carbon steel 45 when held in an oxidizing atmosphere at 1100°C. A method is proposed for aluminizing from a three-layer impregnation mixture in which resorption of the aluminized layer is excluded and good oxidation resistance of the protected steel is provided. Experimental tests of an aluminized specimen made of steel 45 at a temperature of 1100°C for 200 hours showed that under the specified conditions there is 10% better oxidation resistance than for steel 12X18H10T and 1140% better than for unaluminized steel 45. [ABSTRACT FROM AUTHOR]

In this research, the formation mechanism of platinum-modified (Pt-modified) aluminide coating structure applied by out-of-the-pack aluminizing is investigated. For this purpose, the samples of superalloy were coated with 7.5 µm platinum prepared by electroplating. Then, they were heat-treated at 1080°C for 1 h in the vacuum (10−4 torr). The platinum-coated samples were aluminized using an out-of-the-pack single-step aluminizing process at 1050°C under different times and with fixed powder composition. Afterwards, the microstructures of the coatings were evaluated using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and an X-ray diffraction (XRD). The results showed that Pt-modified aluminide coating was formed at first by the inward diffusion of Al and, then, by outward diffusion of Ni. The final coating microstructure consisted of three main layers. The outermost layer of PtAl2 precipitates in a β-Ni(Pt, Al) matrix, the middle layer of Ni-rich β-Ni(Pt, Al) and the innermost layer of interdiffusion zone. [ABSTRACT FROM AUTHOR]

Chromium low alloyed steel substrate was subjected to aluminizing by hot dipping in pure aluminium and Al-Si eutectic alloy at 750°C and 650°C respectively, for dipping time up to 45 minutes. The coated samples were subjected for investigation using an optical microscope, scanning electron microscopy (SEM), Energy-dispersive X-ray analyzer (EDX) and X-ray diffraction (XRD) technique. Cyclic thermal oxidation test was carried out at 500°C for 72 hours to study the oxidation behaviour of hot-dipped aluminized steel. Electrochemical corrosion behavior was conducted in 3wt. %NaCl aqueous solution at room temperature. The cyclic thermal oxidation resistance was highly improved for both coating systems because of the formation of a thin protective oxide film in the outermost coating layer. The gain in weight was decreased by 24 times. The corrosion rate was decreased from 0.11 mmpy for uncoated specimen to be 2.9 x10-3 mmpy for Aluminum coated steel and 5.7x 10-3 mmpy for Al-Si eutectic coated specimens. The presence of silicon in hot dipping molten bath inhabit the growth of coating intermetallic layers, decrease the total coating thickness and change the interface boundaries from tongue like shape to be more regular with flatter interface. Two distinct coating layers were observed after hot dipping aluminizing in Al bath, while three distinct layers were observed after hot dipping in Al-Si molten bath. [ABSTRACT FROM AUTHOR]

ASTM P23 steel (Fe-2.25Cr-1.6W-0.1Mo in wt.%) was hot-dip aluminized and oxidized at 800∘C and 1000∘C for 20 h in air in order to determine the effect of aluminizing on the microstructure, hardness, and oxidation resistance of P23 steel. Aluminizing effectively increased the oxidation resistance of P23 steel by forming protective α -Al2O3 scales. During oxidation, outward diffusion of substrate elements and inward transport of Al and oxygen occurred simultaneously. The oxidation and interdiffusion formed voids in the coating, lowered the microhardness, and transformed the original (Al-rich topcoat)/(Al 1 3 Fe4 layer) to either (thin α -Al2O3 scale)/(Al5Fe2 layer)/(AlFe layer)/(AlFe3 layer)/(α -Fe(Al) layer) at 800∘C or (thick α -Al2O3 scale)/(AlFe3 layer)/(α -Fe(Al) layer) at 1000∘C. At 1000∘C, Fe2O3 was also formed in addition to α -Al2O3 scale, due to the enhanced outward diffusion of Fe, thus suppressing the formation of cracks in the coating. [ABSTRACT FROM AUTHOR]

This study examined the applicability of the hot-dipping aluminizing technique carried out on commercially pure titanium (Ti) as a new method for the formation of Ti-aluminides on a Ti surface. The process was carried out using pure aluminum (Al) and Al 7075 alloy in molten Al baths at 900 °C and 1000 °C for 4 h and 6 h, respectively. The microstructure, phase fraction, and composition analysis of the formed layers were examined using field emission scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction. Three different areas with different thicknesses were formed on the Ti surface by the hot-dipping aluminizing technique. The top (Al coating) layer consisted of Ti and Al elements having a higher hardness than the base metal. The second layer, formed below the Al coating, was the Ti–Al layer having the highest hardness on the surface. Below this layer the Al was diffused. As a result of the Ti aluminizing process carried out at different temperatures and durations in this study, TiAl, TiAl2, TiAl3, and Ti3Al phases were obtained. These phases positively affected the mechanical and corrosion properties. [ABSTRACT FROM AUTHOR]

Aluminum-based coatings were prepared by flame spraying. After remelting and diffusion treatment, the phase, composition, friction and corrosion resistance were analyzed experimentally by using scanning electron microscopy, X-ray diffraction, energy dispersive spectrometer, wear tester. The results indicate that the surface phase consists of Al and FeAl3, inner layer phase mainly contains FeAl 3 and Fe2 Al5. The coating has higher micro-hardness than the underlying material, with a maximum of up to HV 950. In an experiment of high-temperature oxidation at 800℃ and 900℃, the spray-aluminized samples show almost no weight gain, and the antioxidant properties of cast iron improve substantially (by dozens of times). In addition, the samples show excellent wear and corrosion resistance.

Copyright of Metallophysics & Advanced Technologies / Metallofizika i Novejsie Tehnologii is the property of G.V. Kurdyumov Institute for Metal Physics, N.A.S.U and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The paper presents the results of investigation of heat-resistant diffusion coatings on steel 08Kh17T and steel 45, produced by aluminizing and chrome aluminizing methods in powder mixtures at 900...950 °C temperature for 2...5 h. The parametric diagrams of heat resistance were plotted on the kinetic dependencies of oxidation of the samples with coatings in 800...1000 °C temperature interval. They allow evaluating endurance of protective coatings at any temperatures up to 1000 °C. [ABSTRACT FROM AUTHOR]

The paper presents the results of investigation of heat-resistant diffusion coatings on steel 08Cr7Ti and steel 45, produced by aluminizing and chromoaluminizing methods in powder mixtures at a temperature of 900-950 °C during 2-5 h. On the kinetic dependencies of oxidation of the specimens with coatings in the temperature interval of 800- l000 °C, the parametric diagrams of thermal shock resistance were plotted. They allow evaluating endurance of protective coatings at any temperatures up to 1000 °C. [ABSTRACT FROM AUTHOR]

Copyright of Metallophysics & Advanced Technologies / Metallofizika i Novejsie Tehnologii is the property of G.V. Kurdyumov Institute for Metal Physics, N.A.S.U and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A new slurry coating design incorporating a first layer of Cr microparticles at the substrate surface and a surmounting second layer of Al microparticles to enrich the substrate with Al was investigated. The microstructure of the coating was tailored by adjusting the initial thicknesses of Cr and Al layers. The intermediate layer of Cr microparticles acted as a barrier to molten Al by forming Al-Cr intermetallic compounds thereby suppressing the highly exothermic reactions usually observed between molten Al and nickel-based materials. This also significantly decreased the inward diffusion rate of Al towards the substrate at 700 °C. Annealing at 1100 °C then promoted the outward diffusion of nickel atoms from the substrate and their combination with Al from the synthesized Al-Cr intermetallic phases compounds resulting in the direct formation of β-NiAl with expelled Cr. The mechanisms of formation of the diffusion layers and the influence of Cr on the aluminizing progress are discussed. • The Cr-free Al slurry composition results in high-activity aluminide coatings. • The Cr interlayer reacts with molten Al by forming AlxCry intermetallics. • The formation of AlxCry intermetallics decreases the Al activity. • The Cr/Al ratio of the coating system has a tremendous influence on the mechanisms of formation and on the final coating microstructure. • The appropriate slurry coating design can lead to the formation of low-activity Cr-modified aluminide coatings. [ABSTRACT FROM AUTHOR]

In this work, the effects of boroaluminizing thermochemical treatments in solid medium on the microstructure and the surface properties of XC38 steel have been studied. The obtained layers have been characterized using optical and scanning electron microscopy, X-ray diffraction and Vickers microhardness. The comparative study of boroaluminized layers obtained by simultaneous and consecutive processing steps of treatment show that the much thicker diffusion layer (with 114 m) is obtained after a boriding followed by aluminizing treatment with a significant improvement of microhardness value which reach 1020 HV. [ABSTRACT FROM AUTHOR]

Abstract The mechanisms of formation of nickel aluminide coatings from alternate deposition of Al and Cr microparticles and subsequent heat treatment in Ar were studied on pure nickel. For this purpose, tailored amounts of Al and Cr water-based slurries were successively deposited on pure nickel following two different architectures (Al/Cr and Cr/Al double-layer systems). Regardless of the coating architecture, the addition of Cr microparticles was found to decrease the thermodynamic activity of Al upon aluminizing through the formation of Al x Cr y phases. This considerably limited the inward diffusion of Al towards the substrate at low temperature (e.g. 650 °C). Whereas the formation of δ-Ni 2 Al 3 was still observed with the Al/Cr double-layer system, its formation was completely suppressed with the Cr/Al one. By adjusting the composition of the deposited layers, i.e. the thickness of both Cr and Al layers, it was possible to directly form the β-NiAl phase with a further annealing at high temperature (e.g. 1000 °C for 3 h). This fostered the outward diffusion of nickel and the dissolution of synthesized Al x Cr y phases. Undissolved Cr-rich phases were also observed in the diffusion layers, which confirmed the outward growth of the coating typical of low-activity aluminizing. Highlights • Double-layer deposits (Al/Cr and Cr/Al) with tailored amounts of Al and Cr successfully elaborated by slurry on pure Ni. • The Ni substrate-Al/Cr configuration results in high-activity aluminide coatings. • The Ni substrate-Cr/Al configuration results in low-activity aluminide coatings. • The mechanisms of formation of such coatings involve the appearance of intermediate Al x Cr y intermetallics. [ABSTRACT FROM AUTHOR]

Aluminizing is a diffusion based chemical surface treatment on materials to provide excellent protection against high temperature oxidation and corrosion, but is likely to be accompanied by a degradation in mechanical properties. Laser shock peening (LSP) is a superior laser based surface strengthening technique for enhanced mechanical properties. In this study, we focus on combining these two methods on the AISI 321 stainless steel to introduce both chemical and structural gradients for superior properties. Comparative investigation on both the microstructure and mechanical properties of the as received 321 steel, aluminized 321 steel and the aluminizing + LSP processed steel indicated that: (1) A three-layer protective coating was introduced on the surface of the aluminized steel and the concentration of Al decreased gradually from the outermost layer to a certain depth, forming B2-FeAl(Cr) solid solution and Al 2 O 3 in Layer-I, B2-FeAl and DO 3 -Fe 3 Al phases in Layer-II and BCC-Fe(Cr,Al) solid solution phase in Layer-III, with B2-NiAl precipitates distributed in Layer-II and Layer-III. (2) After LSP treatment, a dislocation density gradient from the surface to the matrix, dislocation-precipitate interactions and a significant compressive stress were introduced. (3) Though the aluminized steel exhibited a decrease in the high temperature tensile properties compared to that of the as received 321 steel, the aluminizing + LSP processed steel achieved a striking enhancement. The formation of three-layer coating, dislocation density gradient, dislocation-precipitate interactions and the residual stress contribute jointly to gain the yield strength, ultimate strength and fracture elongation up to 265 MPa, 430 MPa and 37.5 % at 620 °C, respectively. • A three-layer protective coating with thickness of ∼135 μm was introduced on the surface of the aluminized steel. • A dislocation density gradient formed from the surface to the matrix after LSP. • A striking enhancement in the high temperature tensile properties was achieved. [ABSTRACT FROM AUTHOR]

The present technical solution refers to the field of electrophysical and electrochemical processing of parts, in particular, to the electroerosion alloying (EEA) of the surfaces of steel parts with aluminum (aluminizing) and sulfur (sulfidizing), and it can be used to treat the surfaces of heat-treated steel parts in order to increase their hardness, wear resistance, to prevent frictional seizure and improve the resistance to atmospheric corrosion. When aluminizing steel parts with the use of the method of electroerosion alloying (EEA) by aluminum electrode at discharge energy Wp = 0.52–6.8 J and productivity of 1.0–3.0 cm2 / min, before the EEA process by an aluminum electrode, to the surface of the part to be aluminized, there is applied a consistency substance containing sulfur and aluminum powder, and thereafter, not having waited for drying of the consistency substance, the process of aluminizing by the EEL method with an aluminum electrode is carried out, and the consistency substance should have the aluminum powder content of not more than 56 %. There have been carried out metallographic and durametric analyses of the features of the surface layers made of carbon steels after simultaneous aluminizing and sulfidizing them by the EEA method. It is shown that the structure of the layer consists of three portions, namely, a “white” layer, a diffusion zone and a base metal. Such qualitative surface layer parameters as thickness, “white” layer and transition zone microhardness values, and also roughness increase with increasing discharge energy. The “white” layer continuity for all the investigated discharge energies of Wp = 0.52, 2.60 and 6.80 J is 100 %.