Grain Boundary Serration in Nickel-Based Superalloy Inconel 600: Generation and Effects on Mechanical Behavior
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Grain Boundary Serration in Nickel-Based Superalloy Inconel 600: Generation and Effects on Mechanical Behavior YUANBO T. TANG, ANGUS J. WILKINSON, and ROGER C. REED Grain boundary serration in the superalloy Inconel 600 was studied. Two microstructural variants, one with nonserrated and the other with serrated grain boundaries were generated by altering the heat-treatment conditions, while keeping other aspects of the microstructure unchanged. The effect on the creep response between 700 °C and 900 °C was measured, and the different failure modes and accumulated damage were quantified using high-angular resolution electron backscatter diffraction analysis in the scanning electron microscope and also by X-ray computed tomography. It is found that serration plays a more crucial role in the high-temperature/low-stress regime when an intergranular cracking mechanism involving cavitation is operative; here it plays a role in improving both creep life and creep ductility. Any effect of serration is less prevalent at low temperatures where transgranular failure is dominant. https://doi.org/10.1007/s11661-018-4671-7 Ó The Author(s) 2018
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INTRODUCTION
SERRATION of grain boundaries in metals and alloys, for example the nickel-based superalloys of the type studied here, is intriguing for various reasons. First, there is complexity arising from the inapplicability of the generally accepted picture of smooth or gently curved grain boundaries, as observed on the microscale. Second, there is the fundamental question of why grain boundary serration arises in the first place. Third, effects of serration on the mechanical behavior of these materials are not well understood. What are the underlying fundamental effects which cause it? At present, no truly unequivocable explanation exists. Detailed experimentation involving carefully controlled measurements and high-resolution characterization is needed. Serration of grain boundaries in nickel alloys was first reported by Larson et al.[1] and Miyagawa et al.[2] in 1976; slow cooling from above the c0 dissolution temperature was employed. Since then, attempts have been made to elucidate the formation mechanism, details of heat treatment and the role of alloy
YUANBO T. TANG and ANGUS J. WILKINSON are with the Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. Contact e-mail: [email protected] ROGER C. REED is with the Department of Materials, University of Oxford and also with the Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK. Manuscript submitted January 23, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
composition.[3–11] The study of Koul & Gessinger on the Inconel 738, Nimonic 115 and Nimonic 105 alloys,[3] involving an impressive combination of experimentation and modeling, identified the role of c0 migration on serration. The role of the c0 phase has since been confirmed by a number of researchers.[6,12–15] But what happens in the absence of the c0 phase? Alloys containing very few or no c0 -forming e
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