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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]

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.)

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.)

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]

Abstract Hot dip aluminizing (HDA) of low carbon steel in aluminum bath containing various amounts of silicon and manganese was studied in this work. The effect of bath alloying elements on the morphology and growth kinetics of intermetallic layer during the reaction of steel substrate and aluminum bath was investigated by scanning electron microscope equipped with EDS. Results showed that Fe 2 Al 5 (η) and FeAl 3 (θ) layers are detectable when pure aluminum bath is used. In contrast, when the bath contains 6 and 12 wt% silicon, new intermetallic layer Fe 2 Al 7 Si (τ 5) has appeared adjacent to the topcoat aluminum layer. Moreover, dispersed Fe 2 Al 7 Si particles were observed in the topcoat. Presence of silicon in the compound layer had a significant effect on the growth kinetics and intermetallic layer thickness. The inhibiting effect of silicon on the growth of compound layer is possibly due to silicon placement inside the compound layer which retards the diffusion. However, in the bath with 3 wt% manganese, no changes were observed in the composition of the intermetallic layers. Phases of Fe 2 Al 5 (η) and FeAl 3 (θ) were the dominant intermetallic layers with no traces of manganese. Although, dispersed intermetallic particles of the topcoat revealed amounts of manganese. Existence of manganese in the bath indicated no significant effect on the growth rate of the layer and its morphology. Furthermore, a method for evaluating roughness intensity of the compound layer has been developed in this work. Investigations on the tongue-like morphology and roughness of the intermetallic layer revealed that the interface roughness increases linearly with the thickness of the layer. Presence of 3 wt% manganese in the aluminum bath indicated a negligible effect on the thickness and roughness of the layer. However, silicon eliminated the tongue-like morphology of the interface by retarding the rate of the growth. Highlights • Aluminizing steel by hot dipping in different Si and Mn-containing aluminum alloys • Influence of Si and Mn on the morphology and growth of the intermetallic layer • Characterization of intermetallic layers and topcoat phases by SEM-EDS • Developing a method for quantification of the interface roughness [ABSTRACT FROM AUTHOR]

The structurization of aluminum coatings on steel 20 and 40 substrates produced in different ESD modes is considered. The thickness and microhardness of ‘white’ and transition layers and the surface roughness increase and chemical and phase compositions change with higher discharge energy. The coating formed at low discharge energies mainly consists of α-Fe and aluminum oxides. Electron microprobe analysis shows that the coating produced at high discharge energies consists of iron and aluminum intermetallics and free aluminum. Compared to steel 20, the electrospark-deposited coating on steel 40 has a deeper layer with increased hardness and has greater microhardness. The surface roughness remains virtually the same. To decease roughness and increase integrity of the coatings, we recommend electrospark deposition with the same electrode (aluminum), but at lower discharge energies (Wd = 0.52 J). [ABSTRACT FROM AUTHOR]

Highlights: [•] Slurry aluminized and uncoated 9–12wt.% Cr steels exposed to KCl deposit in various atmospheres. [•] Corrosion rates and mechanisms at 560 and 610°C are studied and compared. [•] The materials are degraded by active oxidation process. [•] The coating does not provide the steels with protection against corrosion beneath KCl deposit. [ABSTRACT FROM AUTHOR]

When carbon steel is directly exposed in 99%H 2 –1%CH 4 gas mixture for plasma enhanced CVD deposition of diamond coatings, an intermediate graphite layer is preferentially formed on the substrate surface, and the substrate is subjected to a severe internal carburization attack. The diamond coatings formed easily delaminate from the steel substrate. Hot dip aluminizing treatment of the carbon steel has markedly promoted diamond nucleation, growth and interfacial adhesion. The formation of graphite intermediate phase on the substrate surface is effectively inhibited and the substrate carburization is also suppressed. The possible mechanism of this transition is discussed based on a series of analytical techniques. [ABSTRACT FROM AUTHOR]

Coatings improve the high temperature corrosion resistance of materials by enriching the subsurface zone with elements that form protective oxide scales, which increase the material lifetime. Interesting coatings are aluminum (Al) diffusion coatings formed in a pack cementation process e.g., on austenitic steels. The coating procedure may lead to the risk of crack formation within the coating because of the detrimental combination of high brittleness of the Al-rich intermetallic phases that form, the coating thickness, and the mismatch of the coefficients of thermal expansion (CTE) between the material and the coating. By means of a new coating design, Al diffusion coatings consisting of less brittle intermetallic phases with a CTE closer to that of the substrate can be applied with a controlled coating thickness. For this purpose, the required coating manufacturing parameters such as process temperature, process time, and powder composition are predicted by the presented coating design model. [ABSTRACT FROM AUTHOR]

Aluminum and titanium are deposited on the surface of steel by the pack cementation method to improve its hot-corrosion and high-temperature oxidation resistance. In this research, coatings of aluminum and titanium and a two-step coating of aluminum and titanium were applied on an AISI 304 stainless steel substrate. The coating layers were examined by carrying out scanning electron microscopy (SEM) and x-ray diffraction (XRD). The SEM results showed that the aluminized coating consisted of two layers with a thickness of 450 μm each, the titanized coating consisted of two layers with a thickness of 100 μm each, and the two-step coatings of Al and Ti consisted of three layers with a thickness of 200 μm each. The XRD investigation of the coatings showed that the aluminized coating consisted of AlO, AlCr, FeAl, and FeAl phases; the titanized layers contained TiO, NiTi, FeNi, and FeTiO phases; and the two-step coating contained AlNi, TiAl, and FeAl phases. The uncoated and coated specimens were subjected to isothermal oxidation at 1050 °C for 100 h. The oxidation results revealed that the application of a coating layer increased the oxidation resistance of the coated AISI 304 samples as opposed to the uncoated ones. [ABSTRACT FROM AUTHOR]

Abstract: Aluminizing is often used to improve steel''s resistances to corrosion, oxidation and wear. This article reports our recent attempts to further improve aluminized carbon steel through surface nanocrystallization for higher resistances to corrosion and corrosive wear. The surface nanocrystallization was achieved using a process combining sandblasting and recovery heat treatment. The entire surface modification process includes dipping carbon steel specimens into a molten Al pool to form an Al coat, subsequent diffusion treatment at elevated temperature to form an aluminized layer, sandblasting to generate dislocation network or cells, and recovery treatment to turn the dislocation cells into nano-sized grains. The grain size of the nanocrystallized aluminized surface layer was in the range of 20–100nm. Electrochemical properties, electron work function (EWF), and corrosive wear of the nanocrystalline alloyed surfaces were investigated. It was demonstrated that the nanocrystalline aluminized surface of carbon steel exhibited improved resistances to corrosion, wear and corrosive wear. The passive film developed on the nanocrystallized aluminized surface was also evaluated in terms of its mechanical properties and adherence to the substrate. [Copyright &y& Elsevier]

Aluminized coatings were produced on iron by means of two different processes: electron beam deposition under UHV of Al on iron samples previously covered with 57Fe films, and hot-dipping of iron samples in molten aluminium. Aluminized samples were submitted to thermal treatments in order to promote interdiffusion at the Fe–Al interface and favour the formation of Fe–Al intermetallic compounds of composition suitable to protect the underlying iron from oxidation. Phase composition, structure and morphology of both as deposited and thermally treated coatings were characterized by means of X-ray diffraction, Mössbauer spectroscopy and metallographic techniques. Significant differences among the effects of the Fe–Al interdiffusion occurring for Al layers produced with the two processes are pointed out and discussed. [ABSTRACT FROM AUTHOR]

Abstract: “Simple” Pt-enriched γ + γ′ coatings (16–19 at.% Al) were fabricated on René 142 and single-crystal N5 Ni-based superalloys by electroplating a thin layer of Pt followed by a diffusion treatment in vacuum at 1175 °C. By introducing a secondary short-term aluminizing step via pack cementation with NaCl activator, “enriched” γ + γ′ coatings were achieved with an increased Al content (∼22 at.% Al). The changes in composition profiles between the simple and “enriched” γ + γ′ coatings were discussed. Incorporation of reactive elements, such as Hf, into the γ + γ′ coating during the aluminizing process also was explored. The cyclic oxidation performance of the “enriched” γ + γ′ coatings was evaluated at 1100°C. [Copyright &y& Elsevier]

Abstract: Aluminium deposition on ferritic steel HCM12 A (P-122) by chemical vapour deposition in a fluidized bed reactor (CVD-FBR) has been studied. A thermodynamic study of the partial pressures of the gaseous species present in the system during the CVD, was performed using the Thermocalc software. The coating obtained was studied by XRD, SEM and EDX. The influence of the HCl input ratio on the H2/HCl reactive gas mixture was investigated. It was found that with an increase of HCl input thicker coatings consisting of the Fe–Al intermetallic phase were formed. By increasing the deposition time the thickness of the Al-layer also increases and the Fe2Al5 and FeAl3 intermetallic phases are formed. [Copyright &y& Elsevier]

Abstract: Foil aluminizing of steel, wherein Al diffuses to the base steel material by diffusion treatment after hot pressing of the aluminum foil at a lower temperature than the melting point, was performed in this study. Alloy layers formed by diffusion at temperatures ranging from 700 to 1000 °C were investigated, and their features were compared with those of hot-dip aluminized steel. In hot-dip aluminizing, an intermediate Fe2Al5 layer was formed between the aluminum layer and the base steel during aluminum coating before the diffusion treatment. In contrast, the coating layer of the foil-aluminized steel specimen after diffusion bonding of the aluminum foil consisted only of the Al layer. An Fe2Al5 layer, an FeAl layer and an Al diffused layer were formed in both the aluminized specimens subsequent to the diffusion treatment. The numbers of voids formed in the Fe2Al5 layer and at the FeAl/Al diffused layer interface of the foil-aluminized specimens are smaller than those for hot-dip aluminizing. Moreover, the FeAl and Al diffused layers are formed with a greater thickness in the foil-aluminized steel under identical diffusion conditions. The Al concentration in hot-dip aluminized steel decreased in stages from the surface to the base steel, whereas, in the foil-aluminized steel, it decreased gradually. The Fe2Al5/base steel interface in the foil-aluminized steel was thus indistinct, and cross-sectional hardness also decreased gradually. [Copyright &y& Elsevier]

Kinetics of formation of iron aluminide diffusion coatings by chemical vapor deposition (CVD) on pure iron was studied in the temperature range 1173–1373 K. AlCl3 vapor, produced by the sublimation of anhydrous aluminum chloride, was used for transporting aluminum from an aluminum source (pure aluminum) in the CVD reactor to the substrate to produce the coating. The reactor pressure was maintained at 1.33±0.133 kPa. The first step in developing a theoretical model for the coating process is to establish the kinetics of the process. This was accomplished by monitoring the weight gain and thickness as a function of time and temperature. The growth followed a parabolic rate law with the rate constant (by weight gain) steadily increasing with substrate temperature from 0.0114 mg2 cm-4 s-1at 1198 K to 0.0836 mg2 cm-4 s-1 at 1348 K. The rate constants were 4.138×10-6 and 6.554×10-6 mm2 s-1 (by thickness) at substrate temperatures 1273 and 1323 K, respectively. The activation energy for the coating process was 211±8 kJ mol-1. [Copyright &y& Elsevier]

Formation of aluminide diffusion coatings on iron, nickel and cobalt by pack cementation and chemical vapor deposition (CVD) involves a series of similar steps, the slowest among them are the transport of aluminum bearing species from the vapor phase to the substrate by gas-phase diffusion and the solid-phase diffusion of aluminum into the substrate to form the aluminide phases. The former increases the surface concentration of aluminum in the coating while the latter decreases it. Modeling of the process is based on the observation that the surface composition of the coating tends to reach a steady state value in a short time after the commencement of the process, at which stage the rates of the above two processes are equal. However, in the case of iron aluminide coatings produced by low pressure chemical vapor deposition (1.33±0.13 kPa), the rate of transport of aluminum to the substrate is much faster than the solid phase diffusion of aluminum into the iron substrate since the diffusion coefficients of the vapor species are inversely proportional to pressure and the diffusion layer thickness is reduced considerably at low pressures. Under this condition, the vapor transport is no longer a rate-determining step and the composition of the aluminide coating is decided by the kinetics of the solid phase diffusion. This model could explain the kinetics of the process, surface composition and concentration profile of the coating. [Copyright &y& Elsevier]

The process of hot dipping pure aluminum on a stainless steel (1Cr18Ni9) followed by oxidation was studied to form a surface oxide layer. The thickness of the top aluminum on the steel substrate increases with increasing aluminizing time, while the thickness of the aluminum layer in the steel decreases as the increase in dipping temperature. Lower temperature and longer time favor a thicker layer of the aluminum on the substrate. The thickness of the intermetallic layer in the steel substrate increases with dipping temperature and time. However, the higher aluminizing temperature does not appear to have a significant effect on the thickness of the intermetallic layer. The oxidation treatment of the aluminized steel at 800 °C results the formation of a top oxide layer on the steel surface, composed of α-alumina, Al4Cr and Al17Cr9. The aluminizing and oxidation treatment of the stainless steel creates about 120 μm thickness of top oxide layer which has an extremely sound adherency to the steel substrate and a greatly improved properties of thermal shock withstanding, high temperature oxidation resistance and anti-liquid aluminum corrosion. [Copyright &y& Elsevier]

A new powder liquid coating method is proposed for the aluminization of Fe. Mixed powder slurries of Al + Ti or Al + Al2O3 are pasted onto Fe specimens, and the specimens are then dried and heated in a vacuum. Unlike hot dipping or powder pack cementation, this technique can be used to aluminize specimens selectively without the need for special equipment or halides. The amount of Al adhering to the substrate is determined by the Al–Ti reaction or coalescence of molten Al in Al2O3 powder during heat treatment. The Al concentration profile of the modified layer can be controlled by adjusting the powder mixing ratio or heat treatment conditions. The properties of the modified layer are analyzed using a new formulation, where the diffusion equation is treated numerically with consideration of the concentration dependence of the interdiffusion coefficient. The calculated profiles are stable and in good agreement with the experimental data. [Copyright &y& Elsevier]