Bruce A Pint

Experimental observations in support of the dynamic-segregation theory to explain the reactive-element effect

The addition of reactive elements can have a significant effect on the oxidation behavior of alumina- and chromia-forming alloys. A model has been developed to explain the effects associated with the addition of reactive elements that is based on the segregation of reactive-element ions to scale grain boundaries and the metal-oxide interface. Reactive-element ions use these interaces as pathways for diffusion from the metal substrate to the gas interface of the scale. The driving force for this outward diffusion is the oxygen potential gradient across the scale. Doping of the scale grain boundaries results in scale growth primarily by inward oxygen diffusion, while doping at the metal-oxide interface slows the growth of interfacial voids and thus improves scale adhesion.

18O/SIMS characterization of the growth mechanism of doped and undoped α-Al2O3

Sequential oxidation experiments at 1200°C and 1500°C using16O and >95%18O-enriched environments were conducted on undoped and Y- and Zr-doped β-NiAl and FeCrAl alloys. After oxidation, samples were analyzed by SIMS sputter depth profiling. At 1200°C, a clear pattern was established where the undoped α-Al2O3 was found to grow by the simultaneous transport of both Al and O. Zr-doped α-Al2O3 was found to grow mainly by the inward transport of oxygen. The profiles in all cases indicate O diffusion primarily by shortcircuit pathways. Results at 1500°C (only on β-NiAl) indicated a similar behavior but were less conclusive. Y and Zr were found to segregate to the oxide grain boundaries at 1200°C and 1500°C. The segregation of these dopants is believed to impede the cation transport in the α-Al2O3 scale and thereby change the oxidation mechanism.

The oxidation mechanism of θ-Al2O3 scales

Abstract A nearly single-phase θ-Al2O3 scale has been grown on two FeCrAl alloys, according to a time-temperature-transformation relationship for the formation of external alumina scales. SIMS sputter depth profiles of the 18 O 16 O distribution consistently indicate that θ-Al2O3 scales grow primarily by the outward transport of Al. This mechanism is consistent with the observed scale microstructure and is significantly different from tracer profiles for α-Al2O3 scales.

Influence of Sulfur, Platinum, and Hafnium on the Oxidation Behavior of CVD NiAl Bond Coatings

The influences of S, Pt, and Hf on the oxidation behavior of chemical vapor deposition (CVD) NiAl bond coatings on high-S and low-S superalloys were investigated. Cyclic and isothermal-oxidation testing at 1150°C revealed that alumina-scale adherence to NiAl coatings was very sensitive to substrate S impurities. Scale spallation, as well as the growth of voids at the oxide–metal interface, increased as S increased. However, Pt-modified coatings were not sensitive to S, and did not form voids at the oxide–metal interface. Transmission-electron microscopy (TEM) revealed that alumina scales formed on (Ni,Pt)Al had Hf ions (from the superalloy) segregated to grain boundaries, whereas Hf was not detected in the alumina scale formed on NiAl coatings. Results suggested that the detrimental influence of S on scale adherence to NiAl is primarily related to void growth, which is eliminated or significantly reduced in Pt-modified coatings. A simple model relating void growth to excess vacancies and sulfur segregation is proposed.

The reactive element effect in commercial ODS FeCrAI alloys

Two commercial oxide dispersion strengthened alumina-forming FeCrAl alloys, Inco alloy MA956 and Kanthal alloy APM, were studied in order to look at the effect of reactive elements on their oxidation behaviour. MA956 has a distribution of Y2O3−Al2O3 particles, while APM has a ZrO2—AI2O3 distribution. Isothermal oxidation at 1000°C and 1200°C showed a reduced oxidation rate for both alloys compared to that of an undoped FeCrAl alloy. In short-term cyclic tests at 1200°C, both alloys exhibited excellent scale adhesion. Using scanning transmission electron microscopy with X-ray energy dispersive spectroscopy, both Y and Zr, respectively, were found to segregate to the oxide grain boundaries and the metal-scale interface after oxidation at 1000°C and 1200°C. These experimental observations are discussed with regard to a new theory to explain the reactive element effect.

The effect of various oxide dispersions on the phase composition and morphology of Al2O3 scales grown on β-NiAl

A series of oxide-dispersedβ-NiAl alloys were oxidized in order to explore the effect of various cation dopants on the ϕ-α phase transformation in the Al2O3 scale and the effect of phase composition on the scale microstructure. Larger ions such as Y, Zr, La, and Hf appeared to slow theϕ- toα-Al2O3 phase transformation, while a smaller ion, Ti, appeared to accelerate the transformation.

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

Abstract 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.

On the formation of interfacial and internal voids inα-Al2O3 scales

Microstructural observations were used as the basis for a discussion of the formation and growth of voids in alumina scales. Reactive-element additions to alloys and alloy desulfurization appear to inhibit the growth of interfacial voids, thus improving scale adhesion. This phenomenon is analyzed in terms of surface energies. In addition, a model is proposed for the formation of large internal voids in α-Al2O3 scales. These voids appear to be too large to form as a result of vacancy coalescence and are more frequently observed in scales not doped with a reactive element. The model is based on a growth mechanism where inward and outward growing ridges at scale grain boundaries eventually seal off and form internal voids.

Effect of composition on the oxidation and hot corrosion resistance of NiAl doped with precious metals

Cast NiAl alloyed with Cr, Pt, Pd, Ir and Ru was tested in 1-h cycles at 950°C under hot corrosion conditions and at 1150°C in oxygen. For comparison, Hf-doped NiAl variants and a cast NiPtAl alloy resembling the composition of commercial aluminide coatings were included. Cr was the only element that reduced hot corrosion attack of NiAl significantly. However, at higher temperatures, addition of Cr to Hf-doped NiAl accelerated the alumina scale growth rate and promoted spallation of the oxide scale. The results from initial detailed characterization indicate that rejection of chromium at the metal-oxide interface gives rise to the formation of chromium-rich precipitates in the alloy, which apparently modify its oxidation behavior. This suggests that for NiAl-based substrates, hot corrosion resistance and exceptional scale spallation resistance may be mutually incompatible goals.

Comparison of thermal expansion and oxidation behavior of various high-temperature coating materials and superalloys

Abstract The thermal expansion mismatch between a metallic substrate and its external oxide scale generates a strain on cooling that is a primary cause of spallation of protective oxide scales. This study compares thermal expansion behavior and cyclic oxidation performance of the two major composition classes of high-temperature commercial coatings for protection of single-crystal superalloys. The thermal expansion of cast MCrAlY (M = Ni and/or Co) alloys and cast aluminides (NiAl, (Ni,Pt)Al and Ni3Al) was measured at temperatures up to 1300°C and compared to that of a single-crystal Ni-base superalloy. The tendency for scale spallation from each alloy was evaluated by cyclic oxidation testing at 1150°C. The coefficients of thermal expansion for the aluminides were lower than those of the MCrAlY-based alloys at all temperatures and scale adherence to the Hf-doped aluminides was generally superior. Scale adherence to the various compositions of MCrAlY-type alloys did not directly correlate to their thermal expansion behavior or substrate strength. For both types of materials, the presence of a reactive element (Y,Hf, etc.) had no detectable effect on thermal expansion but a major effect on scale adherence. There was no obvious influence of Al content on the thermal expansion of β phase Ni–Al compositions. The addition of Pt resulted in a lower average thermal expansion for hyperstoichiometric (Ni,Pt)Al at temperatures above 930°C, but this effect was not observed in hypostoichiometric (Ni,Pt)Al.