Fig. 1. Schematic of creep ruptured specimens to conduct SEM and TEM studies. SEM images were taken on longitudinal sections parallel to the tensile axis (σt), while TEM studies were performed on transverse sections normal to stress axis. Regions with 1 mm (Ⅰ), 6 mm (Ⅱ) and 11 mm (Ⅲ) distance to fracture surface were investigated.

Fig. 2. SEM images of heat-treated specimens before the creep test: (a) SEM image on transverse section of S1 showing the typical cruciate dendritic structures; (b) dendritic structures in S2. The γ/γ′ microstructures in dendrite core (D) and interdendritic region (ID) are properly distinguished.

Fig. 3. Quantitative EDX dot-matrix results of heat-treated S1 and S2: (a) bar charts of number distributions of points in different ranges of absolute segregation coefficients ζi for each element; (b) relative segregation coefficients ζR of each element.

Fig. 4. Creep data of S1 and S2 collected at 1373 K and 137 MPa: (a) curve of displacement x as a function of time t. The inset provides a close sight of the initial stage curves; (b) evolution of area shrinkage Ψ as a function of distance to fracture surface.

Fig. 5. Macroscopic features in regions Ⅰ, Ⅱ and Ⅲ of crept S1 and S2: (a-c) SEM images of S1; (d-f) SEM images of S2. The black and white arrows roughly differentiate the dendrite cores (D) and interdendritic regions (ID). The stress axis of σt is denoted by the right dashed arrows. Local enlargements of areas marked by white dashed boxes are given in the insets of (c) and (e).

Fig. 6. Evolution of volume fraction of TCP precipitates (fTCP) as function of distance to fracture surface of S1 and S2.

Fig. 7. Rafted structures in regions Ⅰ, Ⅱ and Ⅲ of S1 and S2: (a-f) SEM images in dendrite cores (D) and interdendritic regions (ID) of S1; (g-l) SEM images in corresponding regions of S2. The stress axis of σt is denoted by the right dashed arrows.

Fig. 8. Quantitative data of the rafted structures in regions Ⅰ, Ⅱ and Ⅲ of S1 and S2: (a) evolution of volume fraction of γ′ rafts (fγ′) as the function of distance to fracture surface. The dashed line of x = 0 denotes the data of heat-treated materials; (b) evolution of termination ratio R as the function of distance to fracture surface. The dashed horizontal line of R = 1 denotes the start of topological inversion. The black arrow points out the data of 0 for heat-treated materials.

Fig. 9. Dislocation substructures in regions Ⅰ, Ⅱ and Ⅲ of S1 and S2: (a-f) STEM images in dendrite cores (D) and interdendritic regions (ID) of S1; (g-l) STEM images of S2. The rod-like items marked by white arrows are TCP precipitates which are enveloped by zones of γ′ phase. The STEM images were taken under rigid beam direction parallel to [001].

Fig. 10. Dislocation networks in region Ⅲ of S1 and S2: (a-b) STEM images in dendrite core (D) and interdendritic region (ID) of S1; (e-h) STEM images of S2. The STEM images were taken under rigid beam direction parallel to [001].