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

Abstract text (incl. figure legends and references)

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 dislocation families with different Burgers vector can react and dissociate into two partial dislocations in γ channels. This allows the leading 1/3[-1-12] Shockley partial dislocation continuously gliding on {111} planes to cut into γ" precipitates where they create planar faults [1]. We study the segregation behaviours of alloying elements across the planar faults by performing the [11-2] (111) creep shear experiments, to intentionally activate the slip system [11-2] (111) with the highest Schmid factor of 1 where the resolved shear stress is exactly equal to loading stress. The creep-deformed specimens are interrupted after 1% and 2% shear strain under 250 MPa at 750°C. The resulting microstructure is investigated using conventional transmission electron microscopy (TEM), analytical scanning TEM (STEM) with energy-dispersive X-ray spectroscopy (EDXS) focussing on structural, physical, and chemical details of the local deformation.

Fig. 1 presents the specimen perpendicular to the (111) plane with the [1-10] direction parallel to the electron beam. Numerous stacking faults (SF) are observed after 1% and 2% creep strains. Fringe contrasts under two-beam conditions indicate inclined stacking faults, shown in Fig 1a and 1d, where the 2% strain sample has more planar faults within one γ" precipitate indicating a higher density of planar faults in the 2% sample. High-resolution STEM micrographs illustrate the superlattice extrinsic nature of stacking faults (SESF) in the 1% and 2% strained samples, see Fig1b and 1e. The chemical distributions across SESF are measured by EDXS and the corresponding concentration profile of 1% and 2% samples are shown in Fig. 1c and 1f respectively. Both samples show almost similar segregation tendency, which is that γ forming elements Cr, Co and Re are enriched across the SESF while γ" alloying elements Ni and Al are depleted, which is partly in agreement with theoretical predictions [2]. For these measurements all microscope parameters and sample thickness for EDXS analysis are kept the same to quantitatively find out how creep strain and time affect the evolution of segregation.

Figure 1.

(S)TEM micrographs of the [1-10] foil normal sample show typical planar faults in Ni-base SX superalloys crept at 750 °C/250 MPa to creep strains of 1% (upper row) and 2% (lower row). TEM bright filed images present fringe contrast of inclined stacking faults under two-beam conditions in (a) g = (00-2) and (d) g = (002) excitation condition. High resolution STEM micrographs indicate the superlattice extrinsic nature of edge-on stacking faults of (b) 1% and (e) 2% strain specimens. Alloying concentration profiles across SESF of (c) 1% and (f) 2% from corresponding EDXS mappings.

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 (Electron Microscopy and Spectroscopy Laboratory, KIT) for HRSTEM micrographs and EDXS results, and acknowledge the DFG priority program SFB‐TR 103.