Aluminum-rich Layer Research Articles

Improving Hot Corrosion Resistance of Cobalt Based Superalloy (Superco-605) Using HVOF Sprayed Oxide Alloy Powder Coating

Superco-605 is a cobalt based superalloy used widely in the hot section of gas turbines. During their operation, they are exposed to high temperature and molten salt environment, causing damage to the component due to hot corrosion. In this investigation, Superco-605 alloy was coated using a fused oxide alloy powder, \( 10\,\% {\text{Al}}_{ 2} {\text{O}}_{ 3} + {\text{CoCrAlTaY}} \) , using high velocity oxy-fuel process. Coating thickness was in the range of 260–280 µm and it was lamellar in nature. Uncoated and coated samples were subjected to cyclic hot corrosion tests at 800 °C in presence of molten salt environment of \( {\text{Na}}_{ 2} {\text{SO}}_{ 4}{+}50\% {\text{V}}_{ 2} {\text{O}}_{ 5} \). Resistance to hot corrosion was investigated using thermogravimetric analysis, visual observations, X-ray diffractometry, scanning electron microscopy and energy dispersive X-ray spectroscopy. Both samples developed an oxide scale at the top. The uncoated Superco-605 exhibited intense spalling during hot corrosion cycles. The oxide scale on the coated substrate was thin compared to total thickness of the coating. There was a chromium depleted region close to the oxide top scale. An alumina rich layer at the interface of coating-substrate prevents penetration of oxygen into the substrate material. Based on the experimental results, it is concluded that the fused oxide alloy powder is effective in reducing hot corrosion tendency of Superco-605 alloy.

The role of aluminum distribution on the local corrosion resistance of the microstructure in a sand-cast AM50 alloy

Abstract Site-specific analytical electron microscopy was performed on a corroded sand-cast AM50 alloy. Areas close to partially divorced eutectic were the regions with less corrosion damage. The corrosion product layer in these areas consisted of a columnar section of predominantly amorphous MgO. At the alloy interface, an aluminum-rich layer was identified. Electron energy-loss spectroscopy suggests this layer is metallic in character. The corrosion product film on the primary α-Mg grains possessed a bi-layer morphology: a thin columnar film and a thicker, porous sub-layer. The formation of the Al-rich layer depends on the Al content in solid solution at a specific location.

A Combined Hot Dip Aluminizing/Laser Alloying Treatment to Produce Iron-Rich Aluminides on Alloy Steel

In the present study, a two-step surface treatment was employed to produce an iron-rich aluminide surface layer on 9Cr1Mo steel. In the first step, the steel specimen was immersed in a 1073 K (800 °C) aluminizing bath for 90 seconds to obtain an Al-rich iron aluminide/Al double layer. Then, the aluminum-rich layer and some portion of the substrate material underneath were melted using a pulsed Nd:YAG laser with different pulse energies in the range of 4 to 8 J. The surface and cross section of the alloyed layers were studied using optical microscopy, scanning electron microscopy, an electron microprobe, and X-ray diffraction analyses. The results showed that after laser remelting of the hot dipped aluminized steel at a pulse energy of 7 J, an integrated layer of FeAl replaced the primary layer.

Advanced glass etching method exhibiting the controllable etch stop using metal etchant

The limitations of conventional glass etching methods are a bottleneck in the further development of glass as a substrate, which is critically important in micro- and nanoelectromechanical systems. In this paper, we describe a new wet etching method for Pyrex glass using a metal etchant. An aluminum pattern is deposited on the Pyrex and then subjected to thermal and electrostatically induced interdiffusion to form an aluminum-rich layer of controllable thickness in the glass, which can then be etched with an aluminum etchant. Because the underlying glass acts as an etch stop, no etching mask is required and the etching depth is effectively controlled by the electric field strength. This method can be used for fabricating glass nanochannels and nanodepth structures, and also in processes incompatible with hydrogen fluoride.

On the Influence of a Heat Treat for an Aluminizing Progress Based on Al Microparticles Slurry for Model Ni and Ni20Cr. Experimental and Theoretical Approaches.

The use of thermal barrier coating systems allows superalloys to withstand higher operating temperatures in aeroengine turbines. Aiming at providing oxidation protection to such substrates, an aluminum-rich layer is deposited to form the α-Al2O3 scale over which a ceramic layer (i.e. YSZ layer) is applied to provide thermal insulation. A new approach is now being investigated within the FP7 European project « PARTICOAT », in which a single step process is employed by applying micro-sized aluminum particles. The particles are mixed in a binder and deposited by brushing or spraying on the substrate surface. During a heat treatment, the particles sinter and oxidize to form a top coat composed of hollow con-joint alumina spheres and simultaneously, an Al-rich diffusion zone is formed in the substrate. For a better understanding of the diffusion / growth processes, preliminary tests were carried out on pure nickel and Ni20Cr model alloys prior to further application on commercial superalloys. The effect of the heat treatment on the coating characteristics (number of layers, thickness, composition, homogeneity, etc.) was particularly investigated to emphasize the mechanisms of diffusion governing the growth of the coatings. The establishment of the diffused layers occurred very readily even at intermediate temperatures (650 and 700°C). However, the layers formed did not match perfectly with the thermodynamic modeling because of the quick incorporation of Ni into molten Al at intermediate temperatures (650°C). In contrast, at higher temperatures (700 and 1100°C) the phases predicted by Thermocalc are in good agreement with the observed thickness of the diffused layers. The incorporation of Cr as an alloying element restrained Al ingress by segregation of Cr even at very low temperatures aluminizing temperatures (625°C).

The effect of an alumina counterface on friction reduction of CuO/3Y-TZP composite at room temperature

Abstract The friction behavior of CuO/yttria-stabilized tetragonal zirconia (3Y-TZP) composite in dry sliding against alumina at room temperature has been investigated. The results show that an alumina counterface has a crucial role on the frictional behavior when sliding against CuO/3Y-TZP composite in comparison with other counter materials. Pure 3Y-TZP shows high friction and wear under the same conditions. It is found that the friction reduction behavior is dependent on the sliding test conditions such as load and humidity. A thin aluminum-rich layer less than 200 nm thick on the contact surface during the low friction situation has been found by various analyzing techniques including interference microscopy, micro-Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microcopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The induced change of contact conditions and interfacial chemical reaction between CuO and alumina to form the phase CuAlO2 increase the wear of alumina and accelerates the formation of an aluminum-rich surface layer. The presence of such a layer in the contact is beneficial for reducing friction. After a certain sliding distance, the coefficient of friction shifts from a low value to a high value due to a change in the dominating wear mechanism. This transition is shown to be caused by a different composition and thickness of the interfacial layer.

Evolution of microstructures of galvanised and galvannealed coatings formed in 0·2 wt-% aluminium–zinc bath

The present investigation examines the evolution of the microstructures of galvanised steel during the galvannealing process with a special reference to the formation and breakdown behaviour of aluminium rich inhibition layer at the substrate/coating interface. The interstitial free steel was galvanised in a molten zinc bath with 0·2 wt-% aluminium content. The bath temperature and strip entry temperatures were 460 and 480°C respectively. Aluminium of 0·2 wt-% was chosen to retard the formation of iron–zinc intermetallic compounds in the coating. The as dipped galvanised interstitial free steel was annealed above the melting point of zinc in a salt bath at 480°C for a period ranging from 1 to 60 s. After annealing, the as galvanised coatings exhibited microstructural changes as a function of test time. The present work highlights the possible mechanisms of nucleation of iron–zinc intermetallic compounds during galvannealing treatment of steel galvanised in aluminium containing zinc bath.

Effect of Elevated Strip Entry Temperature on the Formation of the Aluminum Rich Inhibition Layer at the Substrate Coating Interface During Continuous Galvanizing

During continuous hot dip galvanizing, the use of higher line speeds and thicker gauge strip often results in the steel strip entering the bath at a temperature significantly higher than that of the zinc bath. Studies on the effect of elevated strip entry temperature on the galvanizing reactions are limited. This investigation evaluates the microstructures of galvanized steel produced in a zinc bath containing 0.2 wt% dissolved aluminum essentially to study the effect of elevated strip entry temperatures on the formation of an aluminum rich inhibition layer at the substrate-coating interface. It is concluded that the inhibition layer consists of the orthorhombic Fe2Al5Znx (0<x<1) phase and the morphology of the interfacial layer progresses from a fine crystalline to coarse grain structure with increasing strip entry temperature. Higher strip entry temperature facilitates the formation of zeta (ζ) phase in the coatings. It is also observed that the overall inhibition layer thickness increases with increasing strip entry temperature.RésuméLors de la galvanisation en continu, l'utilisation de vitesses plus élevées de ligne et d'une bande plus épaisse a souvent pour résultat que l'acier en bandes entre dans le bain à une température significativement plus élevée que celle du bain de zinc. Les études de l'effet de la température d'entrée élevée de la bande sur les réactions de galvanisation sont limitées. Cette investigation évalue les microstructures de l'acier galvanisé produites dans un bain de zinc contenant 0.2% en poids d'aluminium dissout, essentiellement pour étudier l'effet des températures d'entrée élevées de la bande sur la formation d'une couche d'inhibition riche en aluminium à l'interface substrat-revêtement. On conclut que la couche d'inhibition consiste de la phase orthorhombique Fe2Al5Znx (0<x<1) et que la morphologie de la couche d'interface progresse d'une structure de grain cristalline fine à grossière avec l'augmentation de la température d'entrée de la bande. Une température d'entrée plus élevée de la bande facilite la formation de la phase zêta (ζ) dans les revêtements. On observe également que l'épaisseur globale de la couche d'inhibition augmente avec l'augmentation de la température d'entré se de la bande.

Effect of Elevated Strip Entry Temperature on the Formation of the Aluminum Rich Inhibition Layer at the Substrate Coating Interface During Continuous Galvanizing

During continuous hot dip galvanizing, the use of higher line speeds and thicker gauge strip often results in the steel strip entering the bath at a temperature significantly higher than that of the zinc bath. Studies on the effect of elevated strip entry temperature on the galvanizing reactions are limited. This investigation evaluates the microstructures of galvanized steel produced in a zinc bath containing 0.2 wt% dissolved aluminum essentially to study the effect of elevated strip entry temperatures on the formation of an aluminum rich inhibition layer at the substrate-coating interface. It is concluded that the inhibition layer consists of the orthorhombic Fe2Al5Znx (0<x<1) phase and the morphology of the interfacial layer progresses from a fine crystalline to coarse grain structure with increasing strip entry temperature. Higher strip entry temperature facilitates the formation of zeta (ζ) phase in the coatings. It is also observed that the overall inhibition layer thickness increases with increasing strip entry temperature. Résumé Lors de la galvanisation en continu, l'utilisation de vitesses plus élevées de ligne et d'une bande plus épaisse a souvent pour résultat que l'acier en bandes entre dans le bain à une température significativement plus élevée que celle du bain de zinc. Les études de l'effet de la température d'entrée élevée de la bande sur les réactions de galvanisation sont limitées. Cette investigation évalue les microstructures de l'acier galvanisé produites dans un bain de zinc contenant 0.2% en poids d'aluminium dissout, essentiellement pour étudier l'effet des températures d'entrée élevées de la bande sur la formation d'une couche d'inhibition riche en aluminium à l'interface substrat-revêtement. On conclut que la couche d'inhibition consiste de la phase orthorhombique Fe2Al5Znx (0<x<1) et que la morphologie de la couche d'interface progresse d'une structure de grain cristalline fine à grossière avec l'augmentation de la température d'entrée de la bande. Une température d'entrée plus élevée de la bande facilite la formation de la phase zêta (ζ) dans les revêtements. On observe également que l'épaisseur globale de la couche d'inhibition augmente avec l'augmentation de la température d'entré se de la bande.

レーザー加熱を用いた鉄の局所表面アルミニウム粉末被覆

A local surface coating using Al and Ti powders on Fe substrate is successfully performed by a laser heating method. A high hardness and high oxidation resistance can be achieved by this method. The thickness of aluminum rich layer can be controlled by the amount of the Al and Ti powder mixture paste. Concentration of aluminum at the coating layer increases with increasing laser scanning speed. Pre-heating at 573K for 10.8 ks improves the uniformity of the coating.

The corrosion of intermetallic alloys in liquid zinc

We studied the static isothermal corrosion of three intermetallic alloys: Fe 3Si, Fe 3Al and Ni 3Al alloy in liquid zinc at 450 °C compared with 316 stainless steel in this paper. The three intermetallic alloys showed a lower resistance to corrosion than 316 stainless steel in liquid zinc. The intermetallics exhibited one or two regular corrosion layers, and the corrosion process was controlled by the dissolution mechanism in liquid zinc although the respective corrosion processes were different. There was no preferential corrosion. The respective corrosion processes were discussed below. An aluminum-rich layer was formed on the surface of 316 stainless steel and Fe 3Si alloy, and acted as a diffusion barrier to zinc, while aluminum itself did not diffuse into the solid matrix. The elements of solid matrix (316 stainless steel and Fe 3Si alloy) diffused through the aluminum-rich layer into liquid zinc. Chromium in Fe 3Al alloy and 316 stainless steel dissolved preferentially from the corrosion layer into liquid zinc. Silicon dissolved preferentially from Fe 3Si alloy matrix into the corrosion layer. The interface between the solid Ni 3Al alloy and liquid zinc remained almost smooth and was independent of microstructure. The boundaries between solid phases (γ and γ′) played no perceptible role in the dissolution of Ni 3Al alloy.

Oxidation behavior of in situ synthesized (TiB + TiC)/Ti–Al composites

In the present study, oxidation behaviors of TiB and TiC particle reinforced titanium matrix composites (TMCs) were performed in air at 823–923 K. The oxidation kinetics follows approximately a parabolic rate law. The oxidation rates decrease gradually as the oxidation proceeds. The oxide scales formed on TMCs were characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD), X photoelectron spectrometry (XPS), and transmission electron microscopy (TEM). The oxygen coefficients in oxide and solid solution were estimated from microhardness measurements. The oxide scale formed was composed of an outer rutile layer, an intermediate alumina-rich layer, and a rutile-rich layer. The oxide scales formed exhibited excellent spallation resistance under all testing conditions. No scale cracking or spallation could be observed, implying that growth and thermal stresses generated during heating and cooling periods had been effectively released. The mechanisms of the improvement on spallation resistance are discussed based on microstructure studies.

Structure of superhard nanocrystalline (Ti,Al)N layers deposited by reactive pulsed magnetron sputtering

Stoichiometric Ti 1 − x Al x N layers with 0.55 < x < 0.57 have been deposited by adjusting working point and pulse times for reactive pulsed magnetron sputtering of aluminum and titanium target. By variation of the substrate bias voltage between floating potential and 100 V, the hardness of the layers is drastically raised up to 38 GPa. The aim of the present work is the identification of structural reasons for the achievement of these very high hardness values. XRD investigations with grazing angle of incidence have revealed that the layers consist of two phases with predominantly cubic rock salt and minor amounts of hexagonal wurtzite structure, respectively. The structure of the layer with the maximum hardness has been additionally investigated by cross-section TEM. It can be shown that the microstructure consists of repeatedly interrupted columns with a lateral size of about 25 nm. Within these columns, globulitic nanocrystallites with a grain size between 6 and 12 nm are present. In addition the TEM investigations have revealed that the layer is composed of alternating aluminum rich and titanium rich layers with a period of about 2.9 nm. It is concluded that the maximum hardness values are mainly caused by the presence of this defined superlattice structure, which hinders the formation and movement of dislocations. By GD-OES depth profiling of the chemical composition it can be shown that the layers possess a good oxidation, by the formation of a thin aluminum oxide passivation layer.

Aluminizing Behaviors of Vacuum Plasma Sprayed MCrAlY Coatings

AbstractThe objective of this study is aluminide overlay coatings of MCrAlY sprayed by a vacuum plasma spraying (VPS) process for the protection against high-temperature corrosion and oxidation of gas turbine components. Diffusion coating processes have been applied for many years to improve similarly the environmental resistance by enriching the surface of nickel-based superalloys with chromium, aluminum, or silicon element. Recently, aluminizing of MCrAlY coatings is used for improving further the high-temperature oxidation resistance. However, the aluminizing properties of plasma-sprayed MCrAlY coatings, which have an important effect on the coating performance, have not been clarified. In this study, five kinds of plasma-sprayed MCrAlY (CoCrAlY, CoNiCrAlY, CoNiCrAlY+Ta, NiCrAlY, and NiCoCrAlY) coating were selected for pack-aluminizing tests. The as sprayed and the heat-treated (1393 K, 2 h, argon cooled and 1116 K, 24 h, argon cooled) MCrAlY specimens were Al-Cr-Al2O3-NH4Cl pack-aluminized at 1173, 1223, and 1273 K for 5, 10, and 20 h, respectively. The experimental results showed that the aluminizing process formed the aluminum rich layers of NiAl or CoAl phase. It also indicated that the thickness of the aluminum rich layer showed a parabolic time-dependence in all MCrAlY coatings. The order of reaction diffusion rate was NiCoCrAlY=NiCrAlY&amp;gt;CoNiCrAlY&amp;gt;CoNiCrAlY+Ta&amp;gt;CoCrAlY. There was a tendency that the reaction diffusion rate by aluminizing increased with increasing nickel content in the MCrAlY coatings and the reaction diffusion rate of as sprayed MCrAlY coatings is faster than that of the heat-treated MCrAlY coatings.

High Temperature Oxidation of TiAlN Thin Films for Memory Devices

TiAlN film is promising candidate as an electric barrier layer for integration of high permittivity oxides and ferroelectrics in advanced memory devices. TiAlN thin films of various compositions were prepared on poly-Si substrates by reactive magnetron sputtering. The power applied to Al target was changed to obtain the various atomic ratio of Al in TiAlN films and the oxidation behaviors of films were studied in the temperature range of 650 ∼800 using rapid thermal annealing (RTA) system. The atomic ratio of Al in TiAlN films affected the microstructure and oxidation behavior of films. The aluminum-rich oxide layer was formed on the surface of film due to the outward diffusion of Al ions during oxidation. Protective layer formed on the surface is a AlTiNO layer and this oxide layer played a role as the barrier against oxygen diffusion. Oxidation resistance of films increased with increasing the Al contents and TiAlN films were comparatively stable even at 800 . TiAlN films are very attractive barrier material for memory devices application.

Improvement of oxidation behavior of a Ti-48Al-2Cr alloy by a nitridation treatment

The effect of a nitridation treatment for 10 hr at 800°C on the oxidation resistance of a Ti–48Al–2Cr (at.%) alloy in air at 800°C was evaluated. Results prove that nitridation decreases by about 40% the total mass gain of nonnitrided material, although the oxidation mechanism is the same for both materials. The oxidation can be divided into two stages. The formation of a nonprotective mixed alumina–rutile scale during the transient stage results in a high oxidation rate. A further decrease in the oxidation rate arises from the establishment of an external alumina-rich layer during the steady stage. The main difference between the scale developed on both materials is the continuous nature of the nitride layer present in the nitrided material during the entire exposure. The thin continuous nitride layer formed during the nitridation treatment acts beneficially as a diffusion barrier, preventing oxygen dissolution in the α2-Ti3Al phase during the transient stage. Furthermore, the oxygen gradient through the oxide scale is kept low, because no oxygen is removed at the scale–alloy interface.

The effect of silicon on the near-surface microstructural evolution during high temperature heat treatment of a TiAl based alloy

The near surface microstructural evolution of a Ti–45Al–2Nb–0.4Mn alloy, heat treated at 1350°C was investigated. The specimens were encapsulated in quartz capsules prior to the heat treatment and it was observed that during the heat treatment Si evaporated from the capsule wall and deposited on the surface of the specimen. It was also found that deposition of Si on the outer surface of the specimen caused Ti to diffuse outwards to form a titanium silicide layer at the surface, resulting in the formation of an aluminum-rich layer. However, no loss of any alloying element from the specimen was observed. It is, therefore, suggested that Si-rich coating could be appreciably effective in preventing the loss of aluminum during high temperature exposure of TiAl based alloys. It was also observed that the aluminum-rich layer, which formed near the surface of the specimen during the heat treatment, consisted of single phase γ and the interior bulk transformed to massive γ. The region between the aluminum-rich layer and the massively transformed bulk consisted primarily of lamellar structure with fine interlamellar spacing and small amounts of primary γ-phase along the grain boundaries.

Oxidation behaviour of Al-alloyed ZrSi 2 at 700°C

The oxidation resistance of ZrSi 2-4 at.% Al prepared by reaction sintering was evaluated in air at 700°C. Two different stages were distinguished. In the first regime the oxidation rate remains high whereas the second one is characterised by a marked decrease on the oxidation rate. Passage from the first to the second stage, associated with a transition stage, is determined by the formation of a continuous protective alumina-rich layer at the scale/alloy interface. An oxidation mechanism is proposed to explain the different stages occurring through the oxidation. Grain boundaries contribute to the fast inward oxygen transport into the alloy, leading to internal oxidation of non-reacted silicon and aluminium during the reactive sintering stage of the processing. Oxidation resistance could be improved by increasing the grain size of the alloy and by using an appropriated processing method addressed to minimise or suppress the presence of silicon and aluminium particles at ZrSi 2 grain boundaries.

Oxidation resistance improvement of Ti-50Al intermetallics by pre-oxidizing and subsequent polishing

The oxidation resistance of Ti-50Al intermetallics has been improved by pre-oxidizing at 800°C×48 h in air and subsequently polishing the outer TiO 2 layer of the oxide scale. The cyclic oxidation resistance’s at 800°C of the per-oxidized and polished specimens are substantially improved and the oxidation kinetics follow the parabolic rate law. This means that the outermost alumina-rich layer of the pre-oxidized and polished specimens is protective against oxidation at 800°C in air. This characteristic is attributed to the compressive stress relief associated with the removal of the outer TiO 2 layer. The stress relief will alleviate cracking or rupture of the scales and simultaneously reduce the stress-assisted diffusion in the scale. At the same time, the catalytic effect of outer rutile titania may also play a detrimental role in the oxidation resistance of TiAl at high temperature.

Identification of the high-temperature impact/friction of aeroengine blades and cases by micro Raman spectroscopy

The impact-friction of the tip of a blade coated either with a NiCoCrAlYTa alloy or with a Ni/BN–Al deposit and accelerated to high linear speeds (300–500 m/s) has been studied in high temperature gases (∼1000 °C) on NiCrFeMo super-alloy substrates coated with a stabilized zirconia (8% at Y 2O 3). Electron/optical microscopy, X-ray diffraction and Raman microspectroscopy were used to identify the phases present in the debris. Raman surface and section analysis were performed on the deposits and debris in order to get a step by step description of the impact-friction process and comparison was made with model spinel compositions prepared by a chemical route. The formation of a NiO-rich transient liquid lubricant enhancing the formation of a well-defined groove was also established. A more homogeneous and regular contact was obtained for blade tips covered with a mixture of Ni/BN and an aluminium-rich layer.