Abstract text (incl. figure legends and references)

Single crystal nickel-base superalloys are an important class of high-temperature materials used as blade substrate material in industrial gas turbines and jet engines. Their excellent creep properties are due to a high volume fraction of L1_2 y'-precipitates embedded in a FCC y-matrix forming a coherent y/y'-microstructure [1]. With increasing creep strain dislocations from the y-channels cut into the y'-precipitates forming complex planar faults, which show a different chemical composition than its hosting y'-phase [2]. An in-depth understanding of these diffusion-assisted microstructural deformation mechanisms is needed for further practical improvement of the next generation of nickel base superalloys [3].
In the present study TEM/STEM-methods are used to investigate the evolution of creep-induced solute segregation to planar faults and their formation in y'-precipitates in single crystal ERBO1 nickel base superalloy double shear creep specimens [4]. The specimens were creep deformed at 750°C and 250 MPa and interrupted upon 1% plastic deformation. For a detailed characterization of the planar faults, TEM thin foils were prepared with a foil normal that allows to analyze the planar faults in edge-on [1-10] and in-plane [111] configurations.
First preliminary results of planar faults in edge-on configuration show planar fault traces in single or multiple y'-precipitates for 1% strain states, see Fig. 1. The planar faults are formed by partial dislocations which can be found attached to the faults within the precipitates (see arrow 1) and at the y/y'-interface (see arrow 2). Applying the g.b/R effective invisibility criterion (Fig. 1c) shows that with an excitation condition using the reciprocal space vector g(-1-1-1) the planar faults are out of contrast indicating a displacement in the [11-2] direction. With respect to the crystallographic orientations of the single crystal double shear creep specimen and the derived highest resolved shear stress and Schmid factor for the shear system [11-2](111) this displacement vector is expected.
For the in-plane configuration partial dislocations that form planar faults while cutting through the y'-precipitate can be observed, see Fig. 2. Arrow 1 marks one cutting event where dislocations have cut into a y'-precipitate leaving a planar fault in their wake, indicated by a visible change in contrast. At the same time these dislocations extend into the y-channel and seem to split up, marked by arrow 2 in the WBDF image (Fig. 2b), additional LACBED and HR-STEM analysis is needed to prove this.
To assess the evolution of solute segregation to the planar faults and gain a better understanding of the y'-precipitate cutting processes the specimens will be further evaluated. Future investigations will include a characterization of the intrinsic and extrinsic nature of planar faults and their atomic stacking sequence with HR-STEM. Furthermore, a detailed burgers vector analysis of the dislocations interacting at the y/y'-interface is planned.

The authors acknowledge the DFG priority program SFB‐TR 103.

[1] R.C. Reed - Superalloys, Cambridge University Press 2006
[2] Y.M. Eggeler et al. - Acta Materialia 113. 2016. 335-349
[3] Y.M. Eggeler et al. - Annu. Rev. Mater. Res. 2021. 51:209–40
[4] D. Bürger et al. - Crystals 2020. 10. 134