Characterization of Phase Chemistry and Partitioning in a Family of High-Strength Nickel-Based Superalloys
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INTRODUCTION
NICKEL-BASED superalloys are widely used in the high-pressure section of gas turbine engines due to their exceptional oxidation resistance and strength at temperatures up to and exceeding 1000 C.[1,2] Commercial and environmental drivers seek to increase turbine operating temperatures, requiring further alloy optimization in order to satisfy the increasing demands on strength and oxidation resistance. Complex trade-offs have to be considered in order to retain these properties alongside fatigue resistance, creep resistance, and phase stability during the optimization process.[3] Much of the high-temperature strength of these alloys come from the precipitation of fully coherent ordered FCC c¢ phase precipitates.[4] These precipitates retain strength at high temperatures via their unique ability to resist deformation by promoting cross slip of partial dislocations onto locked planes.[5] This deformation mechanism is heavily influenced by c¢ volume fraction and chemistry, as c¢ forming elements such as Ti, Ta, and Nb have been shown to increase strength by raising the stacking fault energy at anti-phase boundaries (APB) found in the ordered precipitates.[6,7] However, there are limits M.T. LAPINGTON, M.P. MOODY and P.A.J. BAGOT are with the Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK. Contact e-mail: mark.lapington@materials. ox.ac.uk D.J. CRUDDEN is with the Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK. R.C. REED is with the the Department of Materials, University of Oxford and also with the Department of Engineering Science, University of Oxford. Manuscript submitted November 28, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
regarding the addition of c¢ forming elements, as high c¢ volume fraction has been shown to have detrimental effects on processing and fatigue resistance,[8,9] while addition of Nb and Ti can adversely affect c¢ phase stability due to their potential to form deleterious secondary phases such as d (Ni3Nb)[10] or g (Ni3Ti).[11,12] Ti has also been previously shown to reduce oxidation resistance in model Ni-Cr-Ti polycrystalline alloys.[13,14] Optimization of Ti and Nb levels is therefore necessary in order to find an appropriate balance between high-temperature strength, oxidation resistance, and phase stability. Recent advances in computational optimization techniques have utilized thermodynamic modeling to predict phase chemistries and physical properties based on composition alone, resulting in major cost and time savings. The Alloys-by-Design (ABD) process developed by Reed et al.[15] utilizes a set of merit-indices to sort through these compositions looking to balance the trade-offs mentioned above. This ABD process has been modified for application to polycrystalline superalloys by Crudden et al.,[16] which has been used to create a family of superalloys to investigate composition-property relationships, with a focus on the effect of Ti and Nb on microstructure and phase chemistry as they relate to oxid
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