Al Depletion Research Articles

Microstructure evolution of vacuum plasma sprayed CoNiCrAlY coatings after heat treatment and isothermal oxidation

The microstructure of a CoNiCrAlY coating vacuum plasma sprayed onto an aluminized MARM002 Ni-base superalloy was investigated in the as-deposited condition, after heat treatment to promote adhesion, and after oxidation in air for 100 h at 1100 °C. The as-sprayed coating comprised three main phases: the solid solution γ-(Ni, Co), the intermetallic compound β-(Ni, Co)Al and the intermetallic compound γ′-(Ni, Co)3Al that grew as small precipitates inside γ grains. γ′ phase was present even in the as-sprayed condition because of Al diffusion into the coating from the aluminized substrate during manufacture at temperatures of 800–1000 °C. The microstructure was fine-scale and complex with β phase within γ grains, at γ grain boundaries and large β grains surrounded by γ matrix. After heat treatment, the microstructure comprised β grains typically of 3-μm diameter randomly distributed throughout the γ matrix. There was little or no Al depletion of the coating because of Al diffusion from the substrate aluminized layer. Diffusion of C from the substrate induced the formation of Cr23C6 and M5Y (where M was predominantly Ni and Co) in the as-sprayed coating; with M5Y transforming to Y2O3 during heat treatment due to internal oxidation. After oxidation, there was a loss of Al despite the aluminized layer, because of the formation of a thermally grown oxide (TGO) layer on the upper surface of the coating, mainly comprising Al2O3 with some Cr2O3. There was a corresponding reduction in β phase fraction and a substantial reduction in the substrate aluminized layer thickness. Some β grains persisted close to the TGO at grain boundaries, together with Y2O3 and Cr23C6 particles.

Study of the oxide scale integrity on Ni‐base alloy CMSX‐4 during isothermal and thermal‐cycling exposure

AbstractThe excellent isothermal high‐temperature corrosion resistance of the single crystalline Ni‐base superalloy CMSX‐4, which is due to external alumina formation, is substantially reduced under thermal‐cycling conditions. Particularly, tapered edges exhibit a high cracking and spallation rate leading to premature onset of breakaway oxidation in combination with strong internal corrosion attack. To characterize these effects, specimens with different geometries were prepared, i.e., rectangular‐shaped and wedge‐shaped specimens. Measurements under isothermal and thermal‐cycling conditions at a temperature of 1100°C in air were carried out using a self‐designed thermobalance. By using wedge‐shaped specimens the conditions for spallation, Al depletion and eventually, breakaway oxidation in combination with internal corrosion (Al2O3, TiN and AlN) could be correlated with the change in thickness of the specimen and the duration of the high‐temperature dwell time. As compared to rectangular‐shaped specimens, wedge geometries revealed a much higher susceptibility to oxide spallation and consequently, a higher rate of aluminium depletion. An evaluation of the surface fraction affected by oxide spallation and an analysis of the spalled oxidation products were carried out in order to determine the role of dwell time duration as well as the effect of the number of thermal cycles.

Polymer-bound aluminium salen complex as reusable catalysts for CO 2 insertion into epoxides

Two polymeric aluminium salen complexes in where the backbones are either a partially crosslinked polystyrene [(Al(salen)/PS)] or poly(ethylene glycol bismethacrylate) [(Al(salen)/PEA)] have been synthesised and used for the carbon dioxide insertion into epoxides to form cyclic carbonates. The catalytic activity of these polymers is similar to that of the unsupported aluminium salen complexes, and the polymeric catalysts can be easily separated from the reaction mixture and reusable in consecutives runs. The activity and reusability of the polymeric salen complex depends on the nature of the polymer: PEA being a polymer with a high oxygen content in the backbone enhances the initial activity as compared to PS, but Al(salen)/PEA exhibits lower stability as compared to Al(salen)/PS and a Al depletion occurs upon use. The presence of nucleophiles such as N-methylimidazole or N, N-dimethylaminopyridine in excess increases the catalytic activity of the polymeric Al(salen) catalyst. Also polymeric nucleophiles have been found to be suitable reusable co-catalysts for this reaction.

First-principles study of boron segregation to the edge dislocation inB2-ordered FeAl

Density functional theory is employed to study the segregation of boron in [100](010) edge dislocation core in $B2$-ordered FeAl. We demonstrate that the observed boron atmosphere accompanied with large Al depletion in the compressed region of dislocation core cannot be ascribed to the single substitution of boron for Al or simple interstitial states. Instead, our calculation results suggest that (i) boron can segregate to the dilated region as interstitials, which is consistent with the classical Cottrell theory, and (ii) the boron interstitials possibly cluster together via the bridges of B substitution (for Al) in both the dilated and the compressed regions. Our theoretical study may be helpful for accounting for the recent experimental observations [Blavette et al., Science 286, 2317 (1999) and Annu. Rev. Mater. Res. 33, 215 (2003)].

Surface and bulk investigation of ZSM5 and Al-MCM-41 using synchrotron XPS, XANES, and hexane cracking

We present a comparative study of ZSM5 and Al-MCM-41 catalysts using spectroscopic and chemical techniques. The analysis of conventional and synchrotron XPS spectra of these catalysts reveals the presence of a topmost surface-related Si peak in addition to the bulk peak. XANES results suggest structural modification upon heating Al-MCM-41 at 500 °C. Depth-resolved XPS data show Al depletion from the surface of Al-MCM-41 in contrast to surface enrichment of Al in ZSM5. These surface modifications could be one of the reasons for the weak acidity of Al-MCM-41 in chemical reactions such as hexane cracking at different temperatures.

Al-rich Precipitation in CoNiCrAlY Bondcoat at High Temperature

A thermal barrier coating (TBC) is applied on a surface of a gas turbine blade to provide a thermal barrier and oxidation resistant properties for the components. The ability to resist oxidation of the coating arises from the self-healing, protective Al2O3 scale on top of the bondcoat, which is formed during service. However, if Al depletion occurs within the bondcoat, the protective scale will lose its self-healing ability, and hence, its oxidation-resistant property. This paper investigated the depletion of Al within the bondcoat by studying the microstructure of the bondcoat on a gas turbine blade after it has been in 4000 h service at 1200 °C. The results showed that Al depletion had occurred at different levels throughout the turbine blade. In the area where Al depletion had not yet occurred, precipitation of an Al-rich phase was detected. Most of the Al was contained within this phase, leaving only small amount of Al in the surrounding matrix. A well-defined boundary was observed between the depleted and non-depleted regions.

Focused ion beam assisted analysis of the oxidation of a NiAl coating on pure Ni

Discs of (pure nickel 15mm diameter and 4mm thickness) have been polished to a 1 μm finish, aluminised using a standard pack CVD process to give a β-NiAl coating and then the surface of this coating polished to a 1 μm finish again. Samples have then been isothermally oxidised in a muffle furnace for 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 and 1024 hours at 950°C, and subjected to surface and cross-sectional microscopy and analysis. Scales were also cross-sectioned and imaged using Focused Ion Beam Milling. The oxidation rate was determined to be 2:6×10−13 g2 cm−4 s−1 and for times less than 512 hours the predominant alumina phase was θ alumina. Al depletion effects were observed due to both oxidation and interdiffusion between the Ni substrate and the NiAl coating. Using these effects, it has been shown that tensile strains of up to 9% can arise and these strains readily explain the formation of intrefacial elliptical pores observed by FIB cross sectioning. FIB sectioning also indicated that different coating grains exhibit different oxide-coating interfaces which arise due to different grain orientations. The faceting of the coating-oxide interface and the development of cuboidal faceted coating structures beneath spalled oxide is thought to be due to substructure development (sub-grain development or recrystallisation) arising from the plastic strains generated by Al depletion effects.

Influence of processing variables on the properties of dc magnetron sputtered NiAl coatings containing Hf additions

In this study a series of NiAl–0.1Hf coatings were deposited onto Ni-base superalloy (CMSX-4) substrates at various substrate temperatures and working gas pressures, using direct current (dc) magnetron sputtering. Postdeposition microstructural and chemical analyses using scanning electron microscopy, transmission electron microscopy, and electron probe microanalysis techniques showed the as-deposited coatings to consist of single phase columnar NiAl grains. It was additionally observed that the structural morphology of the coatings could be altered by changing the substrate temperature and Ar pressure. The general trends were in accord with the structure zone model proposed by Thornton [J. A. Thornton, Annu. Rev. Mater Sci. 2, 239 (1977)], however the temperature ranges for each microstructure occurred at lower temperatures. Annealing in argon at 1273K for 1h resulted in some grain growth and in the formation of a small interdiffusion zone between the coating and substrate. After oxidation at 1323K, the β phase gradually transformed to γ′ due to Al depletion caused by oxidation and interdiffusion. Coatings deposited at higher substrate temperatures were observed to have larger grains and to exhibit higher resistance to oxidation. This improvement is attributed to a lower volume fraction of grain boundaries in those coatings.

Deposition of NiAl coating for improvement of oxidation resistance of cold-rolled Ni 3Al foils

β-NiAl, an important intermetallic compound in Ni–Al systems, together with γ′-Ni 3Al, has been used as a coating material for Ni-base superalloys, because of its excellent oxidation resistance. In this study, NiAl-based coating through RF magnetron sputtering was selected for deposition on Ni 3Al thin foils which were cold-rolled up to 96% without any intermediate annealing steps. The thickness of the coating on Ni 3Al foils is restricted below 10 μm, which is much thinner than general coatings on bulk alloys. Insufficient coating thickness can induce rapid Al depletion in the NiAl coating layer. However, the addition of 3 at.% Ti was very effective in retarding atomic diffusion and Al depletion in the coating layer. The binary Ni–50Al coating did not form a stable α-Al 2O 3 oxide during cyclic oxidation tests at 1000 °C. Moreover, rapid Al diffusion between the coating layer and substrate was observed. Ni–47Al–3Ti–0.1Y was then selected as a modified coating composition. The Ni–47Al–3Ti–0.1Y coating showed superior oxidation resistance due to stable α-Al 2O 3 formation and effective retardation of Al diffusion from the coating layer. In addition, Ni-oxide and α-Al 2O 3 coexisted in the bare Ni 3Al foil without any coating, and the bare Ni 3Al foil exhibited more stable oxidation behavior than the bulk Ni 3Al alloy.

Deposition and corrosion resistance of HVOF sprayed nanocrystalline iron aluminide coatings

The microstructure and corrosion properties of nanocrystalline Fe–40Al coatings obtained by thermal spraying of milled powder were investigated. The coatings were sprayed under similar high-velocity oxy-fuel (HVOF) conditions and were varied by the size of the starting feedstock powder. The coatings have complex microstructure consisting essentially of a mixture of well-flattened splats and non-fully melted powder particles within which an equiaxed nanometer-scale structure is retained. Amorphous Al 2O 3 and nanocrystalline Fe-rich oxides together with Fe 3Al (resulting from Al depletion and reaction in the flame) were present at intersplat boundaries. The amount of these phases and porosity, as well as the presence of unmelted powder particles, has been quantified. It is shown that the feedstock powder size has a strong effect on the coating hardness by modifying the amount of hard unmelted powder particles. The electrochemical response of the coatings shows the same general type of active–passive–transpassive behaviour than the microcrystalline bulk Fe–40Al but with poorer corrosion resistance parameters. Analysis of corrosion damage shows a prevalent localized attack at intersplat boundaries or around unmelted powder particles, probably enhanced by galvanic phenomena, that is likely responsible for the poorer corrosion properties of the coatings. To a lesser extent, corrosion takes place by a more global form of attack within splats containing ultrafine grains. If the amount of unmelted powder particles controls the overall hardness of the coatings, it appears to have limited direct effect—if any—on the corrosion behaviour. Thus, the hardness/corrosion balance can be optimized by a good selection of powder size feedstock.

Progressive Stages in the Formation of Kaolin Minerals of Different Morphologies in the Weathering of Plagioclase

AbstractHalloysite and kaolinite occur in gneissic weathering profiles developed under a temperate climate in the Leucogia area of NE Greece. X-ray diffraction analyses of samples from three profiles of partially weathered gneisses have revealed that kaolinite becomes more abundant than halloysite with increasing intensity of the decomposition processes. To resolve the genetic relationship between halloysite and kaolinite, microtextures were examined by scanning electron microscopy using polished thin-sections and freshly exposed fracture surfaces, and their chemical compositions were determined using an analytical scanning electron microscope fitted with energy dispersive and wavelength dispersive spectrometers. Different morphological forms of halloysite and kaolinite were detected within the same rock mass. In the earliest stage of weathering, spheroidal aggregates consisting of microcrystalline halloysite are formed on the plagioclase surface. With progressive weathering, spheroidal halloysite converts to tubular halloysite. As weathering advances, tubular halloysite converts to platy halloysite, which in turn converts to kaolinite. Halloysite and kaolinite may coexist in the upper parts of the profiles. Electron microprobe analyses of spheroidal, tubular and platy halloysite and kaolinite show that the chemical composition of the various forms indicate a progressive Fe enrichment and Al depletion with advancing kaolinization, from VIFe0.08VIAl3.70 a.p.f.u. in spheroidal halloysite through VIFe0.29VIAl3.50 in platy halloysite to VIFe0.53VIAl3.32 in newly formed kaolinite. Final-stage, book-type kaolinite approaches its ideal chemical composition (VIFe0.04VIAl3.95). The mineralogy and composition of the kaolin minerals and the development of the various morphological forms of halloysite and kaolinite in different parts of the profiles are attributed to the chemistry of the ambient solutions. However, other physical and microenvironmental conditions (e.g. time and space available) also seem to have strong influence on the precipitation processes producing the morphological variations.

Influence of Cryomilling on the Morphology and Composition of the Oxide Scales Formed on HVOF CoNiCrAlY Coatings

Commercially available, gas-atomized CoNiCrAlY powder was cryomilled to produce powder with nanocrystalline grains. The cryomilled powder and conventional gas-atomized powder were thermally sprayed using the HVOF process to prepare two coatings with fine-grain (∼15 nm) and coarse-grain (∼1 μm) microstructure, respectively. The two coatings were isothermally oxidized in air at 1000° C for up to 330 hr. The morphology and composition of the oxide scales formed on the two coatings were compared with each other. The results indicate that, while a fine-grain microstructure can promote the formation of a pure alumina layer on the coating by increasing the Al diffusion rate toward the surface, it can also accelerate the Al depletion by increasing the Al diffusion rate toward the substrate, which results in the formation of non-alumina oxides after long-term oxidation. The mechanisms governing the oxide formation are discussed in terms of atomic diffusion and thermodynamic stability.

Cyclic oxidation behavior of Fe-40Al sheet

The cyclic oxidation behavior of a 200 μm thick Fe-40at.%Al sheet was investigated at 1373 K in air for up to 63 cycles. A compact and adherent α-Al2O3 scale formed on the sheet. Scale cracks with tensile crack features developed due to thermal stress, but these were repaired by forming new α-Al2O3 scale beneath the cracks. Simultaneously, the creep strain of Fe-40Al sheet was also observed to increase with the number of oxidation cycles. The EPMA results show that no obvious Al depletion zone formed in the sheet, and that the Al content in the substrate decreased from the original 39.2 to 37.6 at.% after 63 cycles. The Al content of a commercial 50 μm thick Fe-20Cr-11Al (at.%) foil decreased from the original 11.1 to 1.1 at.% after 63 cycles, with no oxide scale cracks development.

Oxidation resistance of diffusion coatings formed by pack-codeposition of Al and Si on -TiAl

Oxidation resistance of the aluminide and silicide diffusion coatings pack-deposited on γ-TiAl were studied in air over the temperature range of 800 and 850°C for up to 4596 h. The oxidation kinetics of the coatings was monitored by intermittent weight gain measurement at room temperature. The XRD and SEM/EDS techniques were used to identify the oxide scales formed during the oxidation process and to assess the thermal stability of the coatings at the oxidising temperatures. It was revealed that the TiAl3 coating underwent preferential Al oxidation to form the Al2O3 scale in the early oxidation stage, which resulted in Al depletion and formation of TiAl2 in the subsurface of the coating. The Al depletion could not be sufficiently compensated by Al diffusion from the inner layer of the coating and eventually, in the late oxidation stage, led to the Ti oxidation and formation of the TiO2 phase in the scale. The preferential Si oxidation was the main oxidation mechanism for the coatings with an outer silicide layer and an inner TiAl3 layer with the formation of SiO2 as the stable oxide scale. The thermal stability of the coatings over the temperature range up to 850°C was discussed in relation to the high-temperature stability of diffusion couples of different coating layers.

Intermetallic FeAl based coatings deposited by the electrospark technique: corrosion behavior in molten (Li+K) carbonate

In the strongly corrosive environment of the molten carbonate fuel cells (MCFC) the protection of the wet-seal areas of the bipolar plates is accomplished by aluminum diffusion coatings. The coating layer is usually produced by depositing metallic aluminum onto a stainless steel surface with the ion vapor deposition (IVD) technique followed by a diffusion annealing treatment in order to transform the as-deposited Al into a corrosion resistant FeAl-based intermetallic surface alloy. In an attempt to find cheaper alternatives, in this work we report the results obtained depositing directly a layer of FeAl intermetallic alloy onto a 316L austenitic stainless steel by using the electrospark deposition (ESD) technique. ESD can apply metallurgical bonded coatings in ambient conditions without the need of post-deposition annealing treatments. Structure, chemical composition and morphology of the FeAl coating has been analyzed and compared to a standard IVD aluminized coating. By electrochemical measurements and long-term immersion tests it is inferred that the corrosion resistance of the electrospark coating is comparable to that of the IVD coating. Some sign of coating degradation after 1000 h immersion in the (Li+K) carbonate mixture at 650 °C was due to coating microcracking and Al depletion.

Atomistic simulations of the bulk effects of Mg on the cohesion of the Ni 3Al– x at.% Mg grain boundary

Monte Carlo simulation with the energetics described by the embedded atom method (EAM) has been employed to calculate not only grain boundary concentration profiles of Ni, Mg and Al, but also dependence of the grain boundary cohesion on the Mg bulk concentration, for the Ni 3Al– x at.% Mg (100%Ni\\100%Ni)[001]/∑5(210)/36.87° grain boundary, at equilibrium. We suggest that the bulk effects of Mg on the grain boundary include the primarily positive effect on the Ni enrichment at the grain boundary, i.e. the Mg atoms induce Ni atoms to substitute into Al sites, the secondarily positive effect on it, i.e. the Mg atoms substitute into Al sites, and the negative effect on it, i.e. the Mg atoms substitute into Ni sites. For the Ni 3Al– x at.% Mg grain boundary, at the equilibrium, the calculations show that when x (the Mg bulk concentration) increases from 0.1 to 0.5, the Mg enrichment increases, both the Ni enrichment and the Al depletion maximizes at x=0.2. The calculations also show the best cohesion of the grain boundary at x=0.2.

Local chemical and structural gradients in the creep deformed superalloy SC16

The local chemical concentrations of the elements in the nickel-base single crystal superalloy SC16 were examined by electron microscopy and atom probe tomography after standard heat treatment and creep deformation of 0.5% at 1223 K along the [0 0 1] direction. Creep deformation led to directional coarsening of the γ′ precipitates. The atom probe measurements were carried out along the [0 0 1] and [0 1 0] directions. Enrichment of Al and slight depletion of Ti in the γ′ precipitate near the γ′/γ interface were observed after creep deformation, whereas in the γ phase both these elements were depleted. An enrichment of Cr was detected near the interface in γ channels perpendicular to the applied stress. Small γ′ precipitates (size ∼3 nm) were observed by atom probe in the γ-matrix indicating non-equilibrium state of the alloy.

Martensitic transformation in CVD NiAl and (Ni,Pt)Al bond coatings

The martensitic phase transformation in single-phase β-NiAl and (Ni,Pt)Al coatings was investigated. After isothermal exposure to 1150 °C for 100 h, the β phase in both types of coatings was transformed to a martensite phase during cooling to room temperature. Martensitic transformation was also observed in the (Ni,Pt)Al bond coat with and without a YSZ top layer after thermal cycling at 1150 °C (700 1-h cycles). The transformation took place due to Al depletion in the coating from the formation of the Al2O3 scale and interdiffusion between the coating and superalloy substrate. The effects of the martensitic transformation on coating surface stability (‘rumpling’) via volume changes during the phase transformation are discussed with regard to TBC failure.

Thermodynamics of Gas‐Phase Parasitic Reactions in AlGaN MOVPE Growth

A thermodynamic analysis of the system Al–Ga–N–C–H was performed to explain the influence of gas-phase parasitic reactions on the growth of AlGaN by metalorganic vapour phase epitaxy (MOVPE). It is shown that the relatively high stability of trimeric amide [DMAlNH2]3 can lead to the Al depletion in the gas phase, and the Al/Ga ratio in the gas phase near the substrate is lower than the one in the initial gaseous mixture. Qualitative agreement was found between a number of experimental results and the thermodynamic prediction of the influence of pressure and the initial gas phase composition on the solid AlGaN composition.

Al–Sn thin films deposited by pulsed laser ablation

Al–Sn thin films were deposited at room temperature on Si substrate by pulsed laser ablation (PLA) of Al–Sn alloy targets in a high-vacuum (10 −5 Pa) chamber. To our knowledge, it is the first time that Al–Sn alloy is deposited as thin films by pulsed laser deposition. Different diagnostic techniques were used to deduce the morphology and the composition of the films obtained. The films were planar and well adhered to their substrates, even though a strong Al depletion effect was observed indicating a noncongruent laser ablation regime. The origin of the aluminum deficit in the deposited films seems to be due to the thermal ablation process of the targets.