Abstract
Over a decade ago, γ′-strengthened Co-base alloys were introduced as potential replacement for conventional Ni-base Superalloys. Insufficient resistance against high-temperature oxidation restricts the number of possible applications. The present study contributes to the understanding of elementary mechanisms such as material transport during extensive oxide scale formation on γ/γ′ Co-base alloys to explain their inferior oxidation behaviour. A clear dependency of the scale growth kinetics on W content and oxidation temperature is demonstrated by thermogravimetry and subsequent analysis of cross-sections. By means of electron backscattered diffraction (EBSD), the evolution of microstructures in the outer oxide layers were examined depending on the oxidation temperature. Sequential exposure of samples in 16O2- and 18O2-containing atmospheres proved counter-current material transport. The combination of focused ion beam (FIB) and secondary ion mass spectroscopy (SIMS) visualised the formation of new oxide phases mainly on the outer and inner interface of the oxide scale. An elaborate review of available transport paths for oxygen is given during the discussion of results. All experimental findings were combined to a coherent explanation of the inferior oxidation resistance of this relatively new class of high-temperature materials at temperatures above 800 °C.
Highlights
Conventional Fe, Ni- or Co-base alloys proved their suitability for various technical applications, especially during operation in harsh conditions at elevated temperatures
The above presented findings that can be directly correlated to material transport during oxidation of Co-base model alloys with the nominal composition Co9AlxW (x = 8, 9, 10 at.%) in the transient stages of scale growth between 800 °C and 900 °C are discussed in detail
The results obtained after two-stage oxidation experiments revealed similar transport mechanisms for all considered ternary Co-base model alloys between 800 °C and 900 °C
Summary
Conventional Fe-, Ni- or Co-base alloys proved their suitability for various technical applications, especially during operation in harsh conditions at elevated temperatures Due to their good mechanical performance, Ni-base Superalloys were unchallenged for decades as alloys that are used for turbine blades in sections of gas turbines where they withstand aggressive service conditions.[1] The strengthening by a cuboidal L12 (γ′) phase that is coherently embedded in the (A1) matrix phase ensures the outstanding resistance against creep at high temperatures.[2,3] In general, adequate durability in oxidising atmospheres is achieved by the addition of Al and Cr, to establish the formation of protective alumina and/or chromia scales.[3,4] The systematic description of processes, such as material transport, which are essential for the growth of continuous, diffusion-limiting Al2O3 and Cr2O3 layers on various alloys, started over 50 years ago.[5,6,7] Wagner[5,8] repeatedly demonstrated that diffusion processes on the reaction front between oxide and substrate need to be elucidated, to explain or predict the growth of barrier layers in multi-element alloys. All acquired results on relevant elementary processes are supplemented by classical model predictions from literature
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