Fig. 1. Heat treatments performed in this study: (a) standard heat treatments; (b) solution heat treatments.

Fig. 2. SEM images of the η phase with different distribution and volume fraction in (a) FTS and (b) FRS samples.

Fig. 3. Dark-field TEM images of the η phase and the γ′-free zones, and the corresponding SAED patterns in (a, b) FTS sample and (c, d) FRS sample. The different widths of γ′PFZs could be obviously observed.

Fig. 4. Dark-field TEM images of the spherical γ′ precipitates and the corresponding SAED patterns in (a-c) FTS sample and (d-f) FRS sample. The primary γ′ phase could not be observed in FRS sample.

Fig. 5. SEM images of η precipitates in RS sample solution-treated at 960 °C for (a) 6 h, (b) 14 h and (c, d) 24 h. (d) High-magnification SEM image and the elemental maps of the granular and plate-shaped phases highlighting the disappearance of the needle-like η phase.

Fig. 6. Bright-field TEM images and the corresponding SAED patterns of η precipitates in RS alloy solution-treated at 960 °C for (a, d) 1 h, (b, e) 6 h and (c, f) 8 h, respectively. The growth and dissolution behaviors of η phase can be observed.

Fig. 7. (a) An HRTEM image of the η/γ-matrix interface along the [011_]γ and [21_1_0]η zone axis and the corresponding fast Fourier transform (FFT) patterns; (b) the enlarged image of the red rectangle in (a). The presence of SF with Shockley partial dislocations of b = $\frac{1}{6}{{\left[ 211 \right]}_{\gamma }}$ can be found at the γ/η interface. The sample was aged for 1 h at 960 °C.

Fig. 8. (a) An HRTEM image of the η/γ-matrix interface along the [011_]γ and [21_1_0]η zone axis, and (b) inverse FFT image of (a); (c, d) the enlarged images of the red rectangles in (a); (e, f) the FFT patterns of the yellow rectangles in (a). The presence of many edge dislocations of b = $\frac{1}{4}{{\left[ 0001 \right]}_{\eta }}$ can be found in the η precipitate. The sample was solution-treated for 6 h at 960 °C.

Fig. 9. (a) The mechanism of $\frac{1}{6}{{\left[ 211 \right]}_{\gamma }}$ Shockley partial dislocation in FCC structure altering the stacking sequence of close packed plane; (b) the transformation mechanism of FCC-γ→HCP-η by a systematic passage of SFs with Shockley partial dislocations; (c) the transformation mechanism of HCP-η→FCC-γ by the climbing of $\frac{1}{4}{{\left[ 0001 \right]}_{\eta }}$ edge dislocations.

Fig. 10. Bright-field TEM images of MC and Laves phases in RS samples solution-treated at 960 °C for (a) 6 h, (b) 8 h, (c) 14 h and (d) 24 h, respectively. The Lave precipitates can be found to form around the MC carbides.

Fig. 11. Bright-field TEM images and corresponding electron diffraction patterns of (a-d) MC and (e-h) Laves phase in RS samples solution-treated at 960 °C for 24 h. The MC carbides and Laves phase can be found without specific orientation relationship.

Fig. 12. (a) Bright-field TEM image and (b) elemental maps from EDX spectrum images showing chemical segregation between MC and Laves phase; (c) elemental distributions from EDX line scanning across the MC precipitates highlighting the Cr segregation around the MC precipitates.

Fig. 13. Microhardness of RS 718Plus alloy over different solution times.

Fig. 14. Schematic diagrams of the evolution of secondary phases in RS 718Plus alloy during solution treatment at 960 °C. The secondary phases consist of the η phase, the MC carbides and the Laves (C14) phase.