Segregation to creep-induced planar faults in Ni-base SX superalloys

Zhongmin Long1, Christian Dolle 1, David Bürger2, Yolita Eggeler 1*

1Laboratory for Electron Microscopy, Karlsruhe Institute of Technology, Karlsruhe, Germany

2 Institute for Materials, Ruhr-Universität Bochum, Bochum, Germany

*yolita.eggeler@kit.edu

Ni-base single-crystal (SX) superalloys find application in turbine blades for gas engines due to the high-temperature and high-stress strength originating from the coherent γ/γ’ microstructure. It is well-known at sufficiently high stresses, two 1/2<101> dislocation families can react and dissociate in the γ channels. This allows leading 1/3[-1-12] Shockley partial gliding on {111} planes to cut into γ’ particles where they create planar faults [1]. We study the segregation behaviours across the planar faults by performing the [11-2](111) creep shear experiments, to intentionally activate the slip system [112ത](111) with the highest Schmid factor of 1 where the resolved shear stress is exactly equal to loading stress. The creep specimens are interrupted after 1% and 2% shear strain under 250 MPa at 750°C. The resulting microstructure is investigated using analytical scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDXS) focussing on structural, physical, and chemical details of the local deformation

Figure 1.  STEM results were obtained from a 750 °C/250 MPa shear creep experiment interrupted after 1%. Two-beam contrast STEM images: (a) g = (-1 ̅11). (b) g = (111 ̅). High-resolution STEM micrographs of planar faults showing an SESF (c) and a SISF (d). (e) a SISF region for EDXS mapping (left) and alloying concentration profiles (right).

Numerous stacking faults are observed after 1% creep strain. The typical triangular shape of the corner region of a small central γ’ particle shows the TEM foil is cut along the (111) plane, where four planar faults (referred to as f1, f2, f3, f4) lying on {111} planes are in full contrast with g = (-111), in Fig. 1a. Fig. 1b presents the specimen perpendicular to the (111) plane with the [1-10] direction parallel to the electron beam, sharp bright lines indicate edge-on stacking faults under g = (11-1). Figure 1c and 1d show weak and bright Z-contrasts for the superlattice extrinsic (SESF) and intrinsic (SISF) stacking faults at atomic resolution, respectively, indicating that they may have different segregation behaviours. Fig. 1e presents the high-resolution SISF (left) across which the chemical distribution was measured by EDXS and the concentration profiles (right). Cr and Co are enriched across the SISF while it contains less Ni and Al. The alloy element Re has not segregated to the SISF, which is partly in agreement with theoretical predictions [2]. 2% strain sample with longer creep time will also be investigated to find out how strain and time affect the segregation behaviours.

References
[1] Eggeler, Y. M. et al. Annu. Rev. Mater. Res., 2021, 51, 209–240.
[2] Zhao, X. et al. Comput. Mater. Sci, 2022, 202, 1–8.
[3] Authors gratefully thank Christian Kübel, Di Wang and Yuting Dai from the KIT KNMFi (Electron Microscopy and Spectroscopy Laboratory, KIT) for allowing to use of the KNMFi facility for aberration-corrected STEM and EDXS micrographs, and acknowledge the DFG priority program SFB‐TR 103.