Abstract
The oxidation resistance of novel γ/γ’-strengthened Co-base superalloys is clearly outmatched by their Ni-base counterparts within the high-temperature regime. Therefore, surface modification strategies to foster protective alumina growth seem auspicious. This study elucidates the impact of fluorination and shot-peening on protective alumina formation at 900 °C for a quaternary Co-base model alloy (Co-Al-W-Ta system) which is well known for an exceptionally low inherent oxidation resistance. Time-resolved isothermal gravimetric analysis (TGA) in synthetic air, detailed electron microscopic analysis, and X-ray diffraction (XRD) were used. For polished samples, no pronounced enhancement of oxidation resistance could be obtained by halogenation. However, in case of shot-peened samples (halogen-free), an increased tendency for alumina formation is found compared to polished surfaces. The very early stages of oxidation were identified to be especially crucial with respect to sustainable protective scale growth. Most noteworthy is the observation of a strong synergistic effect derived by a combination of halogenation and shot-peening, leading to significantly increased oxidation resistance.
Highlights
Gas turbine blades have to simultaneously withstand highest mechanical loads and temperatures within harsh environments [1, 2]
The present study focuses on a quaternary Ni- and Cr-free Co-base model alloy of the Co-Al-W-Ta system, showing an exceptionally low oxidation resistance in polished surface state which could be improved by fostering alumina growth after applying shot-peening and heating in oxygen deficient atmosphere [18]
Among the polished samples (Fig. 1a), the oxidation resistance of the halogen-free sample is the worst, whereas it is slightly enhanced for samples of higher F-intensity
Summary
Gas turbine blades have to simultaneously withstand highest mechanical loads and temperatures within harsh environments [1, 2]. In 2006, Sato et al laid the cornerstone for Co-base superalloys by discovering an ordered intermetallic phase (γ’) that exhibits L 12 crystal structure in the ternary Co-Al-W system This γ’-phase (Co3(Al,W)) can precipitate coherently within the face-centred cubic (fcc) matrix (y), resulting in a two-phase microstructure which is highly comparable to that of Ni-base superalloys [9, 10]. Even though quite remarkable mechanical properties could be achieved since 2006, the oxidation resistance of Co-base superalloys at temperatures exceeding 800 °C still undoubtedly ranks behind their Ni-base counterparts and needs to be increased for serious competitiveness [5, 12,13,14,15,16]. According to Wagner’s model, both a sufficient concentration and diffusivity of Al and Cr within the alloy can be identified as crucial to achieve this goal [17]
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