Study into the Role of Nickel Vapor on Surface Modification of a Third-Generation Single-Crystal Superalloy
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Study into the Role of Nickel Vapor on Surface Modification of a Third-Generation Single-Crystal Superalloy DIMITRA SPATHARA, DUNCAN PUTMAN, and NILS WARNKEN A third-generation single-crystal (SX) superalloy was heat treated at 1350 C for 24 hours while being exposed to nickel vapor. It is observed that a layer of 60-lm thickness deposited onto the alloy. This prevented the formation of TCP phases and the occurrence of melting, which were reported in previous studies. The interdiffusion between the alloy and deposited layer shows strong indications of cross-diffusion effects, as shown from the composition profile measurements and simulations. https://doi.org/10.1007/s11661-018-4712-2 The Author(s) 2018
I.
INTRODUCTION
SINGLE-CRYSTAL (SX) nickel-based superalloys, due to their superior resistance in creep and fatigue properties, have undoubtedly contributed to the impressive evolution of energy efficient modern jet engines.[1–4] Industrial processing involves casting followed by solution heat treatment, during which pronounced surface modification has been reported for the Re-rich third-generation Ni-based superalloys.[5] It was suggested that this could be the result of sublimation—the direct transition from solid into the gas state—of alloying elements at high temperature and high vacuum. The idea that a Ni source can act as an inhibitor to the reaction of sublimation was first suggested by Wang et al.[5] The current work aims to explore this idea further and study the effect on the microstructure beneath the surface of a third-generation SX superalloy after it has been exposed to Ni vapor at high temperature. Simulations are used to rationalize the observations. II.
EXPERIMENTAL AND SIMULATION METHODS
A. Heat Treatment Experiments Experiments were performed to allow Ni to sublimate from a source of pure Ni and to resublimate on a sample of SX superalloy. This used the fact that the
DIMITRA SPATHARA and NILS WARNKEN are with the School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Contact e-mail: [email protected] DUNCAN PUTMAN is with the Global Manufacturing Centre, Rolls-Royce plc., Derby DE24 8BJ, UK. Manuscript submitted March 15, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
activity of Ni in the superalloy is lower than the activity in pure Ni, which is, by definition, equal to one. The SX Ni-based superalloy CMSX-10K* has *CMSX-10 is a registered trademark of Cannon-Muskegon Corporation, 2875 Lincoln St, Muskegon, MI.
been subjected to a 24-hour heat treatment at 1350 C, surrounded by Ni foil, to provide a source for Ni vapor, wrapped in such a way that the foil was not in contact with the surface of the alloy sample. The nominal alloy composition can be found in Table I. A cylindrical bar of 8-mm diameter and 65-mm length, already solution heat treated and primary aged, was polished mechanically by surface reduction of around 0.5 mm to eliminate any modification that may have been present from previous processing. Subsequently, the sample was cross secti
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