High-Temperature Oxidation Behavior of a Novel Co-Base Superalloy
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High-Temperature Oxidation Behavior of a Novel CoBase Superalloy STE´PHANE A.J. FORSIK, ALBERTO O. POLAR ROSAS, TAO WANG, GIAN A. COLOMBO, NING ZHOU, SAMUEL J. KERNION, and MARIO E. EPLER A new polycrystalline c¢-strengthened cobalt-base superalloy with improved oxidation resistance up to 1100 C is presented. Based on the Co-Al-W-Ni-Cr-Ti system, the chemistry was optimized via computational thermodynamics, and an 18 kg ingot was successfully melted under vacuum and processed. During cyclic oxidation trials at 800 C, 1000 C, and 1100 C, the Co-base superalloy gained significantly less mass per surface area than the benchmark Waspaloy tested in similar conditions. X-ray diffraction and EDS analysis of the oxidized surface showed that a continuous layer of Al2O3 forms between the substrate and the external oxides, providing the necessary oxidation resistance. https://doi.org/10.1007/s11661-018-4736-7 The Minerals, Metals & Materials Society and ASM International 2018
I.
INTRODUCTION
SINCE the discovery of the c¢ phase (L12 structure) in the Co-Al-W system in 2006,[1] c¢-strengthened cobaltbase superalloys have been the subject of intense research as they represent a promising replacement for conventional superalloys for high-temperature applications. A dispersion of c¢-Co3(Al,W) precipitates in a c-Co matrix provides them with a yield strength exceeding in some cases that of their nickel counterparts above 900 C.[2] The higher melting temperature of pure cobalt also opens up the possibilities of pushing their upper operating temperature above that of nickel-base superalloys. Ease of manufacturability, sufficient mechanical properties, and microstructural stability are among the requirements for this new alloy system to successfully compete with existing Ni-base alloys. Oxidation resistance at operating temperature is crucial to maintaining structural stability and relies on the ability to form a continuous layer of either chromia (Cr2O3) or alumina (a-Al2O3) scales to slow down the diffusion of reactive species and protect the substrate. When exposed to high temperature, Co-base superalloys usually form multiple layers of oxides. Brittle and porous cobalt oxides form the outer layer while the
STE´PHANE A.J. FORSIK, ALBERTO O. POLAR ROSAS, TAO WANG, GIAN A. COLOMBO, NING ZHOU, SAMUEL J. KERNION and MARIO E. EPLER are with the R&D Department, Carpenter Technology Corp., P.O. Box 14662, Reading, PA 19612. Contact e-mail: [email protected] Manuscript submitted March 15, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
middle layer is composed of a mixture of CoO, Co3O4, spinels, and W-rich oxides such as CoWO4 or NiWO4.[3] Depending on the chemistry, alumina or chromia forms an internal layer at the oxide/metal interface that prevents further oxidation. Chromia is less effective at slowing down diffusion[4,5] and has a tendency to form volatile CrO3 above 1000 C[6] which limits the use of chromia-forming superalloys at high temperature, whereas alumina scales remain stable since there are no vapor species in the
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