Comparison of Methods for Quantification of Topologically Close-Packed Phases in Ni-Based Superalloys
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POLYCRYSTALLINE nickel-based superalloys are used for hot section components in turbine engines, and therefore experience long periods of time at elevated temperatures during service. These conditions promote the formation of brittle, intermetallic topologically close-packed (TCP) phases that can be detrimental to alloy performance.[1] Mechanisms by which TCP phases have been reported to compromise mechanical performance include interfacial decohesion at precipitate interfaces,[2] brittle fracture of TCP precipitates accelerating crack propagation,[3] and stress concentration at precipitates causing microcrack generation.[4] Historically, TCP phase formation has been a problem for single crystal nickel-based superalloys due to their high refractory contents and c¢ volume fractions. However, new generations of engines present more demanding operational environments for polycrystalline alloys, requiring the development of novel alloys that contain higher refractory element contents and c¢ volume A. S. WILSON, K. A. CHRISTOFIDOU and H. J. STONE are with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK. Contact e-mail: [email protected] A. EVANS is with the Bundesanstalt fu¨r Materialforschung und -pru¨fung (BAM), 8.5 - Micro-NDT, Unter den Eichen 87, 12205 Berlin, Germany. M. C. HARDY is with the Rolls-Royce plc, PO Box 31, Derby DE24 8BJ, UK. Manuscript submitted May 24, 2019. Article published online October 15, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
fractions. As such, these alloys are becoming increasingly prone to detrimental TCP phase formation. This has motivated several recent studies to investigate TCP phase formation in polycrystalline alloys.[3,5,6] Such precipitation must generally be avoided, which necessitates balancing the requirement of improved mechanical performance and environmental resistance with the need to avoid compromising the microstructural stability.[7] Key to the assessment of thermal stability during alloy development is the ability to accurately quantify the amount of TCP precipitation that occurs, allowing direct comparison of performance between alloys. A number of different possible methods for performing this quantification can be found in the literature, the first of which is areal analysis. This involves calculating the average area fraction of a particular phase across a large number of images that are representative of the alloy’s microstructure, and has previously been used to quantify the precipitation of TCP phases.[8] However, there are various problems with this technique, the most significant of which is the requirement to be able to consistently distinguish the phase of interest from other phases present in the sample. This is not trivial for TCP phases as one type of TCP phase can be similar in morphology and composition to other TCP phases, as well as certain carbides and borides, which makes it difficult to distinguish them using scanning electron microscopy (SEM). Selected area diffraction in the transmission e
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