Effects of Dopant Additions on the High Temperature Oxidation Behavior of Nickel-based Alumina-forming Alloys

Abstract

Nickel-based alloys continue to play a crucial role for high temperature applications due to their excellent mechanical properties. Despite their high strength, toughness, and creep resistance, these alloys are vulnerable to aggressive oxidation and corrosion upon exposure to air at high temperature, limiting their application space. To promote oxidation resistance, nickel is typically alloyed with aluminum, which enables the formation of a stable, continuous layer of Al2O3 on the alloy surface. A protective Al2O3 layer should develop quickly and then grow slowly at temperature to effectively limit metal loss. Minor alloying elements additions are well-known to reduce Al2O3 scale growth rate. Even small reductions in the growth rate can make a substantial difference in extending the lifetime of a high temperature component. Yet many of the mechanisms by which dopants reduce oxidation rate remain unclear. To develop accurate predictive models of oxidation rate and design highly oxidation resistant alloys for a range of applications, an understanding of the fundamental effects of dopants on oxidation is critical. The current work aims to elucidate the mechanisms by which common dopants, known to improve oxidation resistance of commercial alloys, modify α-Al2O3 growth. Model NiAl and NiCrAl alloys doped with Ti, Y, and Si were oxidized at temperatures from 950-1200 °C, and high-resolution characterization techniques were used to probe the resulting alumina scale microstructure and chemistry. In particular, the effects of minor alloying additions on alumina scale development in the early stages of oxidation and Al/O transport through an established alumina scale after longer exposures were investigated. During the early stages of alumina scale development, metastable Al2O3 phases form first and grow quickly before transforming to α-Al2O3 via a nucleation and growth process. Y additions did not significantly change the transformation rate, while Si and Ti additions were found to accelerate the overall transformation to α-Al2O3 on NiAl by different mechanisms. The addition of Ti decreased the incubation time for nucleation of α-Al2O3, while Si additions accelerated the metastable to α-Al2O3 transformation rate. These findings can inform the development of alloys that quickly form protective α-Al2O3 even at moderate (~800-1000 °C) temperatures, where rapid metastable Al2O3 growth can quickly degrade thin-walled components. Once a fully α-Al2O3 scale is formed, dopants affect the transport of Al and O along the α-Al2O3 grain boundaries, thus altering the α-Al2O3 growth rate. Ti, Y, and Si had different impacts on the individual magnitudes of Al and O fluxes through the scale, resulting in different oxidation rates. Differences in Al and O fluxes can also affect the evolution of α-Al2O3 scale microstructure, that can then impact other aspects of scale growth such as stress development. Accordingly, the new knowledge gained from this work can be used to design alloys that not only have very slow oxidation rates, but develop more adherent scales and longer lifetimes. Through its systematic approach, this work examined specific aspects of the complex oxidation process and clarified the mechanisms by which different dopant elements can affect scale growth. This new information can be used for targeted alloy design and modelling of scale growth processes.