Oxidation-Related Microstructural Changes at a Crack Tip in Waspaloy After Elevated-Temperature Dwell-Fatigue Testing
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n has a profound and deleterious influence on the life of Ni-based superalloys used in high-temperature turbine engines. Numerous studies have shown that the debit in life, under sustained load for long periods of time as well as under time-dependent cyclic loading in air, is attributed to diffusion of oxygen to the grain boundary (GB) causing brittle metal oxides to form and premature intergranular fracture to occur.[1–7] While microstructural studies conducted in samples tested under static oxidation condition have provided useful information on the types of oxides formed, these studies do not truly reflect the crack-tip oxidation environment under cyclic loading conditions, which is a dynamic process where the oxides are formed and broken constantly by the advancing crack. The type, sequence, and kinetics of oxide formation at and ahead of the crack tip primarily depend on alloy composition and test
G.B. VISWANATHAN and MICHAEL J. MILLS are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210. Contact e-mail: [email protected] DAVID E. MILLS is with the RollsRoyce Corporation, Indianapolis, IN 46206 Manuscript submitted July 1, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
conditions. Depending on the alloy composition, earlier studies have reported buildup of spinels of Ni(O), Fe(O), and Co(O) oxides on the exposed surfaces, and a thermodynamically stable subsurface Cr2O3 oxide passive layer closer to the base metal in association with cracks in samples tested under cyclic loading condition.[3–6] On the other hand, only a handful of studies[8–14] that has paid close attention to the region immediately ahead of the crack tip where the local microstructure, including the presence c¢ precipitates, carbides, and borides play a major role in the oxidation process. The purpose of this study is to explore the oxidation processes associated with intergranular fatigue-failure in Waspaloy,[15] a relatively simple c/c¢ superalloy (nominal composition: Ni-58.3, Cr-19.5, Co-13.5, Mo-4.3, Ti-3.0, Al-1.3, C-0.03), and second-phase particles are largely absent at GBs. In this study, a systematic analysis of GB oxidation microstructure spanning from an open crack to the regions far ahead of the crack-tip oxide intrusion, typically up to 3000 to 5000 nm along the same GB has been investigated in detail. The dwell-fatigue crack growth tests were conducted on notched standard KB Bar specimen at Rolls-Royce, Indianapolis at 704 C with a 900-second dwell. The crack investigated is a secondary crack present following specimen failure. This study benefits from the advances in aberration scanning transmission electron microscopy coupled with highsensitivity energy-dispersive spectroscopy (EDS) enabling a clear distinction between oxidation layers as well as details of segregation patterns both within individual layers and ahead of the oxide intrusion – features that are critical for understanding the mechanism of oxidation. Site-specific thin foils were extracted by focused ion be
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