Protective Alumina Scale Growth at 900 °C for a Ni- and Cr-Free Co-Base Model Alloy with γ/γ’-Microstructure: Synergistic Effects by Combining Shot-Peening and Halogenation

S. Hagen, K. Beck, D. Kubacka, H.-E. Zschau, M.C. Galetz, E. Spiecker, S. Virtanen

Oxidation of Metals 97 (2022), 209-226, DOI: 10.1007/s11085-021-10087-y

Hagen2021

Investigations of oxide scales grown on halogen-free shot-peened samples during oxidation at 900 °C in synthetic for 2 min (a and c) and 30 min (b and d) duration. Positions of elemental distribution maps (STEM-EDX) depicted in c and d are marked by white rectangles within a and b (STEM-HAADF).

Reprinted with permission from Springer according to the terms of the Creative Commons CC BY license.

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.

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