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MCrAlY (M = Ni, Co) coatings are commonly used on gas-turbine components as oxidation resistant overlay coatings and bondcoats for thermal barrier systems. The present work focuses on the effect of the aluminizing process on the CoNiCrAlY coating microstructure. In the as-received condition the outer part of the coating consisted mostly of β-(Ni,Co)Al with interspersed precipitates of Cr-rich carbide and Cr-rich boride precipitates. Formation of σ-CoCr was observed at the interface between the β-layer and the inner initial CoNiCrAlY microstructure. Scanning electron microscopy (SEM) combined with energy and wavelength-dispersive X-ray spectroscopy (EDX/WDX) was employed to characterize the aluminized CoNiCrAlY coating. Phases were then identified by electron backscatter diffraction (EBSD). Detailed microstructural studies of the coating were corroborated with the help of coupled thermodynamic-kinetic calculations to model the aluminizing process. The calculations were performed with the in-house developed code employing the commercially available thermodynamic and kinetic databases (ThermoCalc). The mechanisms of the observed microstructural changes were elucidated with the help of the modelling results.

Three model alloys, Co-10Ni-20Cr-15W, Co-30Ni-20Cr-15W and Ni-20Cr-15W (all in wt.%) were investigated in Ar-50%H2O at 700 and 750 °C for up to 3000 h reaction. The results showed the formation of thin chromia scales on the sample surfaces in all cases. For the Co-base alloys, accompanied by the Cr2O3 formation, a chromium depletion zone was detected underneath the oxide scale along with Co3W with some amount of dissolved Cr and Ni. This kind of Co3W was enriched along the oxide/alloy interface. The formation of this intermetallic phase was considered to follow nucleation and subsequent growth based on morphology and composition analyses. Increasing nickel content reduced the amount of Co3W formation. For the nickel-base alloy Ni-20Cr-15W, no intermetallic phase was detected. The formation of the Co3W intermetallic was discussed based on phase transformation induced by chromium depletion. The effect of nickel on this phenomenon was also discussed according to thermodynamic phase equilibrium analysis.

MCrAlY (M = Ni, Co) coatings are commonly used on gas-turbine components as oxidation resistant overlay coatings and bondcoats for thermal barrier systems. In the present work the microstructural f...

Using alloy 230 as an example, a generalized oxidation lifetime model for chromia-forming Ni-base wrought alloys is proposed, which captures the most important damaging oxidation effects relevant for component design: wall thickness loss, scale spallation, and the occurrence of breakaway oxidation. For deriving input parameters and for verification of the model approach, alloy 230 specimens with different thicknesses were exposed for different times at temperatures in the range 950–1050 °C in static air. The studies focused on thin specimens (0.2–0.5 mm) to obtain data for critical subscale depletion processes resulting in breakaway oxidation within reasonably achievable test times up to 3000 h. The oxidation kinetics and oxidation-induced subscale microstructural changes were determined by combining gravimetric data with results from scanning electron microscopy with energy dispersive X-ray spectroscopy. The modeling of the scale spallation and re-formation was based on the NASA cyclic oxidation spallation program, while a new model was developed to describe accelerated oxidation occurring after longer exposure times in the thinnest specimens. The calculated oxidation data were combined with the reservoir model equation, by means of which the relation between the consumption and the remaining concentration of Cr in the alloy was established as a function of temperature and specimen thickness. Based on this approach, a generalized lifetime diagram is proposed, in which wall thickness loss is plotted as a function of time, initial specimen thickness, and temperature. The time to reach a critical Cr level at the scale/alloy interface of 10 wt% is also indicated in the diagrams.

In the present work, the oxidation behavior of two commercial Ni-base superalloys, CMSX-4 and CM247, in synthetic air with and without 2 vol.% SO2 at 1050°C has been studied. The corrosion reactions in the presence of SO2 could not be explained simply in terms of the contents of the main scale‐forming alloying elements, Cr and Al. The far better resistance of CMSX-4 is related to the formation of a rather pure and dense alumina scale after a very short period of transient oxidation. Rapid development of an alumina scale prevents access of molecular SO2 to the metal surface thus effectively suppressing internal sulfidation. In contrast, CM247 with a similar Al-content formed an Al-rich oxide scale with local intrusions and/or inhomogeneities caused by the underlying alloy microstructure, which deteriorated its resistance to internal sulfidation and resulted in rapid failure in synthetic air + 2% SO2.

The high temperature air oxidation of the two conventionally cast Ni-based superalloys Rene 80 containing 92 wt ppm boron and CM 247 containing 110 wt ppm of boron was studied at temperatures between 850 and 1050 °C and times ranging from 1 to 100 h. Analysis by GD-OES and SEM/WDX/EBSD revealed the surface scale on Rene 80 after exposure at 1050 °C to consist of Cr2O3 with TiO2 in the outer part and BCrO3 and CrTi2O5 in the inner part. Internal Al2O3 precipitates were present below the external scale. After 1 h exposure of CM 247 an inner dense alumina scale formed over eutectic alloy regions containing coarse γ′ precipitates. Over dendritic regions a Cr-rich oxide scale with BCrO3-precipitates formed initially and aluminum was internally oxidized. After longer oxidation times an alumina sub-scale grew from the internal precipitates, and oxidation of the initially present depleted alloy zones resulted in formation of outer Ni and Co rich oxides. In both alloys, boron depletion in the bulk alloy occurred due to formation of the mentioned boron rich oxide. The extent of depletion was substantially higher for the chromia forming Rene 80 than for CM 247. In the latter material the boron depletion was suppressed after the formation of the dense, inner alumina scale.

An accurate procedure for predicting oxidation-induced damage and lifetime limits is crucial for the reliable operation of high-temperature metallic components in practical applications. In order to develop a predictive oxidation lifetime model for Ni–Cr alloys, specimens of wrought NiCrW alloy 230 with different thicknesses were cyclically oxidized in air at 950–1050°C for up to 3000 h. After prolonged exposure, two types of carbides as well as a Cr-rich nitride (π-phase) precipitated in the γ-Ni matrix. The oxidation-induced loss of Cr from the alloy resulted in the formation of subscale zones, which were free of the Cr-rich carbide and nitride but also of the Ni-W rich M6C. The width of the M6C-free zone was smaller than that free of the Cr-rich precipitates. Thermodynamic and diffusion calculations of the observed time- and temperature-dependent Cr depletion processes identified that back diffusion of C occurred which resulted in an increased volume fraction of M23C6 in the specimen core. With increasing time and temperature, the amount of π-phase in the specimen core increased. The subscale depletion of the initially present Cr-nitrides and the formation of Cr-nitrides in the specimen center is believed to be related to a mechanism which is qualitatively similar to that described for the Cr carbide enrichment. However, with increasing time and decreasing specimen thickness, N uptake from the atmosphere becomes apparent. As a result, the precipitates present in the specimen center eventually consisted almost exclusively of nitrides.

The conventionally cast Ni-base superalloy Rene 80 containing 90 wt ppm boron was oxidized in synthetic air at 850–1,050 °C for times up to 100 h. Extensive microstructural studies using GD-OES and SEM/WDX/EBSD revealed formation of a porous chromia scale containing TiO2 precipitates at its outer surface and semi-continuous layers of BCrO3 and CrTi2O5 at its base. The development of B-rich oxide in the scale is related to its high thermodynamic stability and the high alloy boron diffusivity. Its enrichment in the oxide scale resulted in B-depletion from the alloy matrix, which was extremely rapid at 1,050 °C, draining the boron content of a 2 mm thick alloy coupon in 100 h. After 100 h exposure at 950 °C, the B-content dropped below 40 wt ppm, but only minor depletion was found after 100 h at 850 °C. Predictions of B-depletion based on calculation of its diffusion from a thin, flat sheet were in good agreement with experiment at 1,050 °C. Precipitation of B-containing compounds (borides) within the alloy is shown to account qualitatively for slower B-outdiffusion at lower temperatures.

Modern gas turbine materials are subjected to increasing operating temperatures in service environments, which contain high concentrations of gaseous species originating from fuel impurities, e.g. sulfur. In the present study, the oxidation behavior of two commercially available nickel-base alloys, Rene 80 and PWA 1483, was investigated for up to 500 h duration in synthetic air and synthetic air containing 2% SO 2 at 1050 °C. In order to investigate the mechanisms of SO 2 attack, specimens of the two alloys after different oxidation times were characterized using a number of surface analytical techniques such as scanning electron microscopy, XRD, and glow discharge optical emission spectrometry. The corrosion reactions in the presence of SO 2 were found to be strongly alloy dependent and could not be explained simply in terms of the contents of the main scale-forming alloying elements, Cr and Al. Severe internal sulfidation was observed underneath the chromium-rich oxide scale on Rene 80, leading to breakaway oxidation after longer exposure times. For PWA 1483 sub-scale formation of a near-continuous alumina occurred, which resulted in substantial suppression of sulfidation. Consequently, no breakaway oxidation of PWA 1483 was observed up to 500 h exposure at 1050 °C in synthetic air with 2% SO 2 . The formation of a dense alumina sub-scale on PWA 1483 is believed to be correlated with the Ta-addition in this material. It is proposed thatTa can tie up Ti by forming a mixed oxide of the type TiTaO 4 . In this way, it prevents the incorporation of Ti into the chromia scale, which can lead to enhanced scale growth.

The high temperature air oxidation of the two conventionally cast Ni-based superalloys Rene 80 containing 92 wt ppm boron and CM 247 containing 110 wt ppm of boron was studied at temperatures between 850 and 1050 °C and times ranging from 1 to 100 h. Analysis by GD-OES and SEM/WDX/EBSD revealed the surface scale on Rene 80 after exposure at 1050 °C to consist of Cr2O3 with TiO2 in the outer part and BCrO3 and CrTi2O5 in the inner part. Internal Al2O3 precipitates were present below the external scale. After 1 h exposure of CM 247 an inner dense alumina scale formed over eutectic alloy regions containing coarse γ′ precipitates. Over dendritic regions a Cr-rich oxide scale with BCrO3-precipitates formed initially and aluminum was internally oxidized. After longer oxidation times an alumina sub-scale grew from the internal precipitates, and oxidation of the initially present depleted alloy zones resulted in formation of outer Ni and Co rich oxides. In both alloys, boron depletion in the bulk alloy occurred due to formation of the mentioned boron rich oxide. The extent of depletion was substantially higher for the chromia forming Rene 80 than for CM 247. In the latter material the boron depletion was suppressed after the formation of the dense, inner alumina scale.

In order to increase the efficiency and decrease the costs of power generation, gas turbines have to be able to operate using a wide range of alternative fuels, such as crude oil, biogas or unclean syngas. Many of the alternative fuels contain substantial amounts of contaminants, especially sulphur. This means that gas turbine components will be exposed to environments in which not only oxidation but also sulphidation will be an issue. Such components are typically coated with protective coating systems, including MCrAlY-coatings and thermal barrier coatings. However, during service, after the long term exposure to cyclic temperature coatings may crack or spall, thereby allowing direct access of the hot gases to the superalloys surface. Therefore the oxidation resistance of the Ni-base superalloy to oxidizing/sulphidizing atmospheres becomes an important factor for the lifetime of the turbine components. In the present work the oxidation behaviour of Ni-base superalloys in high pO2, SO2-containing environments have been studied. For this purpose, screening tests of commercial alloys commonly used in stationary gas turbines, were performed to establish the sensitivity of alloys to the presence of SO2 in the test gas (synthetic air). Based on the screening test results, commercial Ni-base superalloys with similar Cr and Al contents were chosen to determine which parameters affect the material resistance against the enhanced attack by SO2. In the first part of the thesis, the behaviour of alloys with 12-14% Cr and 3-4% Al, namely PWA 1483 and Rene 80, in synthetic air is compared with that in synthetic air + 2% SO2 at 1050°C. The second part of the present investigation concerns the behaviour of Ni-base superalloys with 5-6% Al and 6-8% Cr. For the latter comparison two alloys were chosen: CMSX 4 and CM 247. The results of the present work clearly show that Ni-base alloys such as PWA 1483 and Rene 80, as well as CMSX 4 and CM 247 exhibit tremendous differences in resistance to SO2 attack in high pO2 gas, in spite of possessing similar Cr and Al contents. The overall corrosion behaviour in the SO2 containing gas critically depends on the other alloy constituents. The far better resistance of PWA 1483 and CMSX 4 in the SO2-containing environment is shown to be related to the rapid development of a dense alumina scale, which prevents access of molecular SO2 to the metal surface and effectively supresses sulphidation. In contrast, a porous chromia based scale formed on Rene 80. The latter type of surface scale allows easy molecular access of SO2, which eventually results in breakaway oxidation triggered by formation of internal chromium sulphide. The formation of fast growing porous chromia scale on Rene 80 was attributed to the Ti addition of 5 wt. %, which increases the growth rate of the Cr2O3-scale by p-type doping thereby suppressing the formation of a protective alumina scale. Furthermore, Ta addition to Ti containing Ni-base alloys, such as in PWA 1483, was found to promote external alumina scale formation by forming a mixed Ti/Ta oxide compound, hence preventing the enhancement of chromia growth by Ti incorporation. The effect of Ti and Ta on the scale formation was verified by using model alloys of the same base composition Ni-9Co-14Cr-3Al. Owing to a high Al-content of 5.6 wt. % CMSX 4 formed rather pure alumina scale after relatively short period of transient oxidation. In contrast, CM 247 with the same Al-content formed an Al-rich oxide scale with high amounts of Hf and B-containing oxides, which compromised its resistance to sulphidation and resulted in rapid failure in synthetic air + 2% SO2. Using model alloys it was shown that the addition of B is detrimental for the oxidation resistance of Ni-base alloys with 8 wt. % Cr and 5-6 wt. % Al. The formation of Al/B mixed oxides can explain why after short time exposure the Al2O3 formation was locally hindered, thereby allowing transport of SO2 and enhanced formation of internal Cr-sulphide. The reason for the rapid enrichment of B within the scale is that it forms a thermodynamically very stable oxide combined with much faster boron diffusivity in the Ni-rich matrix as compared to Al and Cr. The rapid incorporation of boron into the oxide scale apparently resulted in boron depletion from the alloys. This was indicated by specimens exposed for 500 h to synthetic air which showed continuous Al2O3 scale formation for all three studied model materials.