Segregation and Phase Transformations Along Superlattice Intrinsic Stacking Faults in Ni-Based Superalloys
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Segregation and Phase Transformations Along Superlattice Intrinsic Stacking Faults in Ni-Based Superalloys T.M. SMITH, B.D. ESSER, B. GOOD, M.S. HOOSHMAND, G.B. VISWANATHAN, C.M.F. RAE, M. GHAZISAEIDI, D.W. MCCOMB, and M.J. MILLS In this study, local chemical and structural changes along superlattice intrinsic stacking faults combine to represent an atomic-scale phase transformation. In order to elicit stacking fault shear, creep tests of two different single crystal Ni-based superalloys, ME501 and CMSX-4, were performed near 750 C using stresses of 552 and 750 MPa, respectively. Through high-resolution scanning transmission electron microscopy (STEM) and state-of-the-art energy dispersive X-ray spectroscopy, ordered compositional changes were measured along SISFs in both alloys. For both instances, the elemental segregation and local crystal structure present along the SISFs are consistent with a nanoscale c¢ to D019 phase transformation. Other notable observations are prominent c-rich Cottrell atmospheres and new evidence of more complex reordering processes responsible for the formation of these faults. These findings are further supported using density functional theory calculations and high-angle annular dark-field (HAADF)-STEM image simulations. https://doi.org/10.1007/s11661-018-4701-5 The Minerals, Metals & Materials Society and ASM International 2018
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INTRODUCTION
NI-BASED superalloys are frequently used in the hot section of jet turbine engines due to their high strength and excellent high-temperature properties.[1–4] Currently, new research is exploring on how to improve these properties as demand for more efficient turbine engines requires ever-increasing operating temperatures. As these temperatures are increased, athermal deformation modes, such as anti-phase boundary (APB) shearing, begin to transition to mechanisms involving diffusional processes.[5] For example, in the temperature regime between 600 C and 800 C, reorder-mediated c¢ precipitate shearing modes become prevalent during creep.[6–8] These modes include superlattice stacking faults (SSF) and deformation twinning. One of these important mechanisms which requires improved understanding is the formation of superlattice intrinsic stacking faults (SISFs). T.M. SMITH and B. GOOD are with the NASA Glenn Research Center, 21000 Brookpark Road, Cleveland OH 44135. Contact e-mail: [email protected] B.D. ESSER, M.S. HOOSHMAND, G.B. VISWANATHAN, M. GHAZISAEIDI, D.W. MCCOMB, M.J. MILLS are with the Center for Electron Microscopy and Analysis, The Ohio State University, Columbus OH 43212. C.M.F. RAE is with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK. Manuscript submitted March 12, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The first models for the formation of SISFs inside a c¢ precipitate were presented by Kear et al.[9,10] In the Kear models, to explain the formation of SISFs without nearest neighbor violations which would form by the shear of a6 h112i Shockley partials and a2 h
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