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Abstract

Introduction

A new disk superalloy has been developed by NASA to improve high-temperature creep performance utilizing the recently discovered local phase transformation strengthening mechanism. Creep tests were performed at 760 °C and 552 MPa, to approximately 0.3% plastic strain, a regime where the formation of c′ shearing modes such as superlattice extrinsic and intrinsic stacking faults are active. The new alloy exhibited superior creep performance over the current state-of-the-art superalloys, ME3 and LSHR. High-resolution characterization confirmed the formation of the strengthening η phase along superlattice extrinsic stacking faults and v phase along superlattice intrinsic stacking faults. In addition, creep deformation analysis via scanning transmission electron microscopy appears to show a significant reduction in microtwin formation as compared to LSHR and ME3. This improvement in creep performance was also accompanied by an improvement in both room temperature and high-temperature strength. Keywords




Superalloys Local phase transformations Super-X EDX




Creep

T. M. Smith (&)
T. P. Gabb
J. Stuckner
L. J. Evans NASA Glenn Research Center, 21000 Brookpark Rd., Cleveland, OH 44135, USA e-mail: [email protected] K. N. Wertz Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433, USA A. J. Egan
M. J. Mills Department of Materials Science and Engineering, Center for Electron Microscopy and Analysis, the Ohio State University, Columbus, OH 43212, USA




Polycrystalline Ni-base superalloys are the principal alloy system utilized for turbine disks in the hot section of jet engines [1]. To date, commercial turbine disks are limited to operating at temperatures up to 700 °C [1, 2]. Therefore, the high-temperature behavior of disk alloys presents a limitation to increasing the operating temperature of turbine engines, which is required to improve efficiency and emissions. These alloys achieve their superior high-temperature strength through coherent-ordered face-centered cubic (FCC) Ni3Al c′ precipitates which can make up over 50% of the volume of the alloy [3–5]. At temperatures below 700 ° C, creep deformation occurs by dislocation looping or shearing of the c′ precipitates [6]. Above 700 °C, creep deformation transitions to more diffusion-mediated processes via formation of superlattice intrinsic or extrinsic stacking faults (SISFs and SESFs) [7]. These shearing modes require atomic reordering of the precipitate lattice to progress [8, 9]. Indeed, the first direct observation of solute segregation along a stacking fault was made by Viswanathan et al. in 2015, where c-formers Co and Cr were discovered to be segregated along SISFs in the disk alloy ME3 [10]. Subsequent studies have since confirmed the presence of c-former segregation (Co, Cr, and Mo) along superlattice stacking faults (SSFs) as well as microtwins and unit ½ dislocations [11–16] involved in shearing the c′ precipitates. It is believed the segregation of c-formers along the stacking faults reduce the occur