OXIDATION RESISTANCE OF NANOCRYSTAL ODS ALUMINIDE COATINGS PRODUCED BY PACK ALUMINIZING PROCESS ASSISTED BY BALL PEENING

Nanocrystal ODS （oxide dispersion strengthening） aluminide coatings were produced on a stainless steel and nickel-based superalloy by the pock aluminizing process assisted by ball peening, Pure Al powders and 1% of ultra-fine Y2O3 powders were mixed by ball milling. The ultra-fine Y2O3 powders were dispersed in Al particles. Ball peening welded the Al particles onto the substrate and accelerated the formation of aluminide coating. Nanocrystal ODS aluminide coatings were produced by the outward growth at a much low temperature （below 600℃） in a short treatment time. The effects of the operation temperature and treatment time on the formation of the coatings were analyzed. SEM （scanning electron microscope）, AFM （atomic force microscope）, EDS （energy dispersive X-ray spectroscopy）, XRF （X-ray fluorescence spectrometer） and XRD （X-ray diffraction） methods were applied to investigate the microstructure of the coatings. High-temperature oxidation tests were carried out to evaluate the oxidation resistance of the ODS aluminide coatings.

Performance of the Aluminide Coating of SomeLow Alloy Steels

Two low alloy steels 0.5Cr-0.5Mo-0.25V and H85 were pack-aluminized at 900°for 4 h by using Fe-Al powder mixture containing 48% Fe, 20.6% Al- 29.4% Al2O3 and 2% NH4Cl by weight. The microhardness and oxidation resistance at 900℃ of the aluminide coatings were studied. It was found that pack-aluminizing improves the microhardness of the 0.5Cro.5Mo-0.25V steel while it reduces the microhardness of the H85 steel. Pack aluminizing highly improves the oxidation resistance after 20h exposure at 900℃ in air for the investigated steels.

Effect of Substrate Characteristics on Interdiffusion Coefficients of Ni and Al Atoms in β-NiAl Phase of Aluminide Coatings

Interdiffusion coefficients at 950℃ and 1050℃ are calculated by Wagner analysis method as a function of composition of β-NiAI phase. The β-NiAI phase is formed by pack cementation on surface of superalloy. Results of the calculation show that interdiffusion coefficients in β-NiAI phase strongly depend on the compositions and vary over several orders of magnitude. Compared with the interdiffusion coefficients in the stoichiometric β-NiAI phase, the interdiffusion coefficients in β-NiAI phase formed on superalloy is obviously small, probably due to the composition, complicated microstructure and precipitates. However, it could be seen clearly that the shapes of the diffusivity curves are very similar to each other. The similarity of the diffusion curves and the difference between interdiffusion coefficients imply that the compositions, microstructures and precipitates of superalloy have a distinctly adverse effect on the interdiffusion of Ni and Al atoms during aluminization, but do not change the essential characteristics of β-NiAI phase.

A Finite Difference Method for Determining Interdiffusivity of Aluminide Coating Formed on Superalloy

A numerical method has been developed to extract the composition-dependent interdiffusivity from the concentration profiles in the aluminide coating prepared by pack cementation. The procedure is based on the classic finite difference method (FDM). In order to simplify the model, effect of some alloying elements on interdiffusivity can be negligible. Calculated results indicate the interdiffusivity in aluminide coating strongly depends on the composition and give the formulas used to calculate interdiffusivity at 850, 950 and 1050癈. The effect on interdiffusivity is briefly discussed.

Preparation of aluminide coatings at relatively low temperatures

A method was presented to prepare aluminide coatings on metals by combining the pack aluminizing with the ball impact process. This technique applied mechanical vibration to a retort, which was loaded with pack-aluminizing powder, specimens and alloy balls. Pack aluminizing was carried out with repeated ball impact, which accelerated chemical reactions and atomic diffusion. Aluminide coatings were formed at a relatively lower temperature (below 600 ℃) and in a shorter treatment time, compared with the conventional pack aluminizing. The effects of the operation temperature and the treatment time on the formation of the coatings were analysed. The SEM, EDS and XRD analysis results show that the aluminide coatings appear to be homogeneous, with a high density and free of porosity, and have excellent adherence to the substrate. The coatings mainly consist of Al-rich phases such as η-Fe2Al5, θ-FeAl3 and ?CrAl5. Oxidation resistance was studied by high-temperature tests. The formation mechanism of the Al-coatings was also investigated. This technique provides a new approach for industrial diffusion coatings with great energy and time savings.

Effect of Microstructure on Wear Behavior of Iron Aluminide Based Coatings

An experimental study has been carried out to investigate the effect of microstructure on sliding wear behavior of Fe-Al coatings and Fe-Al/WC composite coatings produced by high velocity arc spraying (HVAS) and cored wires. After heat treatment at 300°C, 450°C, 550°C, 650°C and 800°C, the microstructure of the coatings will be changed. The changes of microstructure have obvious effects on the microhardness of the coatings, which may be the most important factor influencing the coatings sliding wear behavior. After heat treated at 450°C-650°C, increasing of the amounts of iron aluminides (including Fe3Al and FeAl ) and dispersion strengthening of Fe2\V2C and Fe6W6C will lead to a rise of microhardness of the coatings. Increasing the microhardness through heat treatment would improve the sliding wear resistance of the iron aluminide based coatings coatings.

Laser cladding of titanium alloy coating on titanium aluminide alloy substrate

A new diffusion bonding technique combined with laser cladding process was developed to join TiAl alloy to itself and Ti alloys. In order to enhance the weldability of TiAl alloys, Ti alloy coatings were fabricated by laser cladding on the TiAl alloy. Ti powder and shaped Ti alloy were respectively used as laser cladding materials. The materials characterization was carried out by OM, SEM, EDS and XRD analysis. The results show that the laser cladding process with shaped Ti alloy remedy the problems present in the conventional process with powder, such as impurities, cracks and pores. The diffusion bonding of TiAl alloy with Ti alloy coating to itself and Ti alloy was carried out with a Gleeble 1500 thermal simulator. The sound bonds of TiAl/TiAl, TiAl/Ti were obtained at a lower temperature and with shorter time.

EFFECT OF MICROSTRUCTURE OF ALUMINIDE COATING ON HIGH-CYCLE FATIGUE PERFORMANCE OF COATING-SUPERALLOY SYSTEM

The high-cycle fatigue performance of different microstructures of aluminide coating- superalloy system has been studied at 900℃.The single phase coating of coarse equiaxial grain NiAI(β)has unfavorable eJfect onJatigue life of the coating-superalloy.The fatigue life may shorten if the coating of NiAl(β)was an enrichment of coarse refractory metal grains. While an improvement can be made by dispersing numerous secondary phase particles such as extreme.fine γ′,quasi-σ-phase and others.

Effects of different pack aluminizing fillers on the aluminide coatings of Ti-6Al-4V alloy and their high-temperature oxidation behaviors

The aluminide coating process of Ti-6Al-4V alloys with different fillers(100wt.% Al_2O_3,50wt% Y_2O_3+50wt.% Al_2O_3 and 100wt.% Y_2O_3) for improvement of the oxidation resistance were investigated.The results show that the filler does not only participate in the aluminizing process,but also has much effect on the coating composition.The XRD analysis reveals that the aluminide coating with filler Al_2O_3 is predominant with TiAl_3 and TiAl phases;while the aluminide coatings with filler Y_2O_3+Al_2O_3 are predominant with Ti_3Al phase.The oxidation kinetics shows that different fillers affect greatly the oxidation resistance of aluminide coating,and the oxidation resistance of aluminized specimens with pack aluminizing filler Al_2O_3 are about 5-8 times than that of the aluminized specimens with other pack aluminizing fillers.

Kinetics Growth and Oxidation Resistance of Aluminide Coatings Deposited by the CVD Method on Re 80 Superalloy

The preliminary results of research on forming the aluminide coatings using CVD method were presented in the article. The coatings were obtained in low activity process on the surface of Rene 80 superalloy. The microstructure analysis and chemical composition analysis were performed applying different values of aluminizing process parameters. The authors present in the article the results of oxidation resistance analysis of aluminide coatings which were obtained on the surface of Rene 80 superalloy using various techniques. It was shown that the coating created during the CVD process was characterized by a good oxidation resistance at the temperature of 1100℃.

Oxidation Resistance of the Aluminide Coating Formed on Carbon Steels

Low and medium carbon steels were aluminized by the pack aluminizing technique using halideactivated pure-Al and Fe-Al packs. The effect of mixture composition, aluminizing temperatureand time and C content of the steel substrate on the structure and thickness of the aluminidelayer, and on the oxidation resistance was investigated. The optimum oxidation resistance canbe achieved with a low carbon steel substrate when the intermetallic phases Fe3Al and FeAlform the surface of the aluminide layer. In this case, the Al concentration at the surface of thealuminide coating is at least ≥15 wt pct. Formation of high Al concentration phases (FeAl3 andFe2Al5) during aluminizing should be avoided as they tend to embrittle the aluminide layer andreduce its oxidation resistance.

Formation and Characterization of Titanium Modified Aluminide Coatings on IN738LC

Up to now, the aluminide coatings used to protect industrial components at high temperature and corrosive environments have been modified by Pt, Cr, Si and Ni. In this investigation, aluminide coatings were modified by titanium and the microstructural feature and formation mechanism were evaluated. The coatings were formed on a Ni-based superalloy(IN738LC) by a two stage process including titanizing at first and aluminizing thereafter. Pack cementation titanizing performed at temperatures 950° C and 1050° C in several mixtures of Ti, A12O3 and NH^Cl. At the second stage, aluminum diffused into surface of the specimens by an industrial aluminizing process known as Elcoatl01(4 hrs at 1050° C). The modified coatings were characterized by means of standard optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-Ray diffraction methods. The results show that Ti in the coatings is mainly present in the form of TiNi and Al67Cr8Ti25. Titanium modified coatings grew with a mechanism similar to simple aluminizing; this includes inward diffusion of Al from the pack to the substrate and then outward diffusion of Ni from the substrate to the coating. The advantages and characteristics of this two-stage modified coating is discussed and the process parameters are proposed to obtain a coating of optimum microstructure.

Pack Aluminide Coatings Formed at 650℃ for Enhancing Oxidation Resistance of Low Alloy Steels

This study aims to investigate the feasibility of forming iron aluminide coatings on a commercial 9Cr-lMo (wt.%) alloy steel by pack cementation at 650 °C in an attempt to improve its high temperature oxidation resistance. Pack powders containing Al, A12O3 and a series of halide salts were used to carry out the coating deposition experiments, which enabled identification of the most suitable activator for the pack aluminising process at the intended temperature. The effect of pack aluminium content on the growth kinetics and microstructure of the coatings was then studied by keeping deposition conditions and pack activator content constant while increasing the pack aluminium content from 1.4 wt.% to 6 wt.%. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the phases and microstructures of the coatings formed and to determine depth profiles of coating elements in the coating layer. Oxidation resistance of the coating was studied at 650 °C in air by intermittent weight measurement at room temperature. It was observed that the coating could substantially enhance the oxidation resistance of the steel under these testing conditions, which was attributed to the capability of the iron aluminide phases to form alumina scale on the coating surface through preferential Al oxidation.

Zirconium Modified Aluminide Coatings Obtained by the CVD and PVD Methods

The paper presents the comparison of the structures of the zirconium modified aluminide coatings deposited on pure nickel by the CVD and PVD methods. In the CVD process, zirconium was deposited from the ZrCl3 gas phase at the 1000°C. Zirconium thin layer (1 or 7 μm thick) and aluminum thin layer (1.0, 0.7 or 0.5 μm thick) were deposited by the EB-PVD method. Deposition velocity was about 1 ?m/min. The layers obtained by the Electron Beam Evaporation method were subjected to diffusion treatment for 2 h in the argon atmosphere. The obtained coatings were examined by the use of an optical microscope (microstructure and coating thickness) a scanning electron microscope (chemical composition on the cross-section of the modified aluminide coating) and XRD phase analysis. Microstructures and phase compositions of coatings obtained by different methods differ significantly. NiAl(Zr), Ni3Al and Ni(Al) phases were found in the CVD aluminide coatings, whereas Ni5Zr, Ni7Zr2 and γNi(Al,Zr) were observed in coatings obtained by the PVD method. The results indicate that the microstructure of the coating is strongly influenced by the method of manufacturing.

Development and characterization of hot dip aluminide coated stainless steel 316L

Stainless steel(SS)grade 316L is used for orthopedic implants due to its biocompatibility;yet the effort should be done to minimize the carcinogenic and inflammatory effects related to SS 316L implants.In this research,aluminide coating of Al–Si alloy on SS 316L is characterized by using optical microscopy,energy dispersive spectroscopy(EDS),nano-indentation and corrosion testing technique.Hot dip aluminizing process is used to coat the SS 316L specimens at 765°C for 2 min immersion time.Half of the specimens are also diffusion treated in a Muffle furnace at 550°C for 4 h to produce diffused specimens of SS 316L.Microstructural examination shows the formation of flat coating/substrate interface due to Si addition.EDS analysis confirms the formation of complex intermetallic at the coating/substrate interface which finally results in increasing the hardness and corrosion resistance properties of coating.

Influence of Heat Treatment on Microstructure and Sliding Wear Behavior of Fe-Al/WC Composite Coatings

An experimental study has been carried out to investigate the influence of heat treatment at 300 ℃,450 ℃,550 ℃,650 ℃ and 800 ℃ on the microstructure and sliding wear behavior of Fe Al/WC intermetallic composite coatings produced by high velocity arc spraying (HVAS) and cored wires. The result shows, the main phases in both as sprayed and heat treated Fe Al/WC composite coatings are iron aluminide intermetallics (Fe 3Al+FeAl) and α as well as a little oxide (Al 2O 3) and carbides (WC, W 2C, Fe 2W 2C and Fe 6W 6C). After heat treated at 450-650 ℃, dispersion strengthening of Fe 2W 2C and Fe 6W 6C will lead to a rise in microhardness of the coatings. The microhardness is likely to be the most important factor which influences the sliding wear behavior of the coatings. Increasing the microhardness through heat treatment will improve the sliding wear resistance of the Fe Al/WC composite coatings.

Effect of coating repair on microstructure and mechanical properties of Ni_3Al based alloy IC6

The effect of coating repair on microstructure and mechanical properties of a directionally solidified nickel base alloy IC6 was studied. The experimental results show that after coating repair treatment, the substrate/coating interface keeps well, and the combination of coating and substrate is very good, as well as very slight mutual diffusion of alloying elements between substrate and coating occurred. Although coating repair treatment make diffusion time double, only a little amount of Mo diffused into coating, and the elements Cr and Co of NiCoCrAlYHf coating only exist in a thin layer of influence region of the substrate, which has no obvious effect on the microstructure of alloy IC6. Tensile tests at room temperature and stress rupture tests under 1?100?℃, 90?MPa of the coating repaired sample were conducted. The results show that the coating repair has no evident effect on mechanical properties. Therefore, it may be considered that when the coating is unexpectedly destroyed, it can be repaired.

Influence of Y_2O_3 on the Oxidation Behaviour of a β-NiAl Coating

A new-type of β-NiAl coating containing Y_2O_3 particles was obtained by electrolytic co-deposition and pack aluminizing techniques,and the influences of the nature of Y_2O_3 and its content on the oxidation behaviour of the coating at 1100℃ were studied.The test results indicate that the oxidation rates of the composite coatings are reduced and the scale adhesion is also improved remarkably.The content of Y_2O_3 has little influence on the oxidation rates of the composite coatings,but does obviously on the scale adhesion by modifying the configura- tion of the oxides at the scale-coating interface.The needle-like oxides penetrating into the coating show an obvious keying effect.A part of the dispersed Y_2O_3 in the as-deposited coating accumulates and forms a Y_2O_3-rich layer at the coating-substrate interface immediately after aluminizing.The layer is beneficial to reduce the degradation of the composite coating.

Microstructure and Growth Kinetics of Silicide Coatings for TiAl Alloy

In order to improve the oxidation resistance of Ti Al alloy, silicide coatings were prepared by pack cementation method at 1273, 1323, and 1373 K for 1-3 hours. Scanning electron microscopy（SEM）, energy dispersive spectrometry（EDS） and X-ray diffraction（XRD） were employed to investigate the microstructures and phase constitutions of the coatings. The experimental results show that all silicon deposition coatings have multi-layer structure. The microstructure and composition of silicide coatings strongly depend on siliconizing temperatures. In order to investigate the rate controlling step of pack siliconizing on Ti Al alloy, coating growth kinetics was analyzed by measuring the mass gains per unit area of silicided samples as a function of time and temperature. The results showed that the rate controlling step was gas-phase diffusion step and the growth rate constant（k） ranged from 1.53 mg^2/（cm^4·h^2） to 2.3 mg^2/（cm^4·h^2）. Activation energy（Q） for the process was calculated as 109 k J/mol, determined by Arrhenius＇ equation： k = k0 exp[–Q/（RT）].

Oxidation and Coatings for Power Generation： New Super Ultra Critical Steam Turbines

The steels with chromium contents between 9%and 12%wt are used for power plants with advanced steam oxidation conditions.They present good creep properties as 9%Cr steels and good oxidation resistance,at the temperatures range between 500℃～600℃.In the last years numerous investigation in development of coatings has been realized with the aim the protected them against the oxidation in order to allow operation of steam turbines at 650℃.In this study,Al-Hf protective coatings were deposited by CVD-FBR on ferritic steel HCM-12A followed by a diffusion heat treatment,have shown to be protective at 650℃under steam for at least 3000 hours of laboratory steam exposure under atmospheric pressure.Morphology and composition of coatings were characterized by different techniques, such as scanning electron microscopy(SEM),electron probe microanalysis,and X-ray diffraction(XRD). On the other hand,volatile species by mass spectrometry(MS) and thermogravimetric measurements (TG) during the initial stages oxidation of the P92 ferritic steel without and of Al and Al/Si coating by chemical vapour deposition in fluidized bed reactors(CVD-FBR) at 650℃in Ar+20%H_2O for 150 h were studied.In order to go forward to coatings design the Thermocalc code is used as base for the MSdata. TG-MS experiments were conducted in a closed steam loop in order to obtain information about the oxyhydroxides formation as reaction between coatings and steam.From those results the role of the different coating element could be established and optimized for the coating durability.An oxidation mechanism based on the TG-MS results is given.The morphology/composition and structure of the oxidized samples were also studied using SEM/EDS and XRD techniques.