Formation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades
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INTRODUCTION
AS superalloys have excellent mechanical strength, phase and surface stability, and resistance to creep, corrosion, and oxidation under relatively severe mechanical stresses at elevated temperatures close to their melting point,[1–4] they have found widespread application in gas turbine engines for jet propulsion and electricity generation. Newly developed superalloys are continually sought to be used in the hottest parts of the engine because the efficiency of the engine, fuel economy and reduction of emissions, can be improved by higher operation temperatures.[4] The strengthening mechanism for these superalloys is mainly by solid solution hardening and the precipitation of an intermetallic phase.[1] Solid solution hardening is achieved by the addition of different soluble elements, such as Cr, Mo, W, and Re via the inhibition of dislocation movement and the decrease of stacking fault energy in the crystal lattice, which leads to the inhibition of cross slip of dislocations.[3] Precipitation hardening is obtained through the additions of Al, Ti, and Nb which have limited solubility in the alloy matrix. During heat treatment, a supersaturated solid solution generates finely distributed precipitates of gamma prime (c¢) phase which inhibits dislocation movement. Superalloys are one of the most compositionally complex alloys developed, sometimes containing more than ten alloying elements, and through their component manufacture they are subjected to multiple steps of KEEHYUN KIM is with the School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK. Contact e-mail:[email protected] and PAUL WITHEY is with the School of Metallurgy and Materials, University of Birmingham, and also with Rolls-Royce plc, PO Box 31, Derby DE24 8BJ, UK. Manuscript submitted November 4, 2016. Article published online March 13, 2017 2932—VOLUME 48A, JUNE 2017
melting, casting, and heat treatment. As a result, they are subject to numerous metallurgical phenomena, such as melting, solidification, homogenization, aging, precipitation, transformation, coarsening, microsegregation, and chemistry variation. Therefore, these complexities can make it difficult to understand exactly the strengthening mechanism of Re in superalloys. The distribution of Re clusters, about 1 nm across, in the c phase, which was observed by field ion microscopy and atom probe, may induce this strengthening mechanism,[5,6] while there are other studies showing no clustering of Re in superalloys, which was confirmed by extended X-ray absorption fine structure (EXAFS), atom probing, and first-principles density functional theory calculations.[7,8] Depending on the analysis method or equipment, different interpretations can be made. In addition, as analysis techniques and equipments develop, new findings can be made even in these widely investigated alloy systems. One of these recent findings is the formation of an intermediate layer containing Re-rich particles in Ni-based single-crystal turbine blades along grain boundaries.[9] In single-crystal
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