Fig. 1. The microstructure evolution near grain boundaries after aging at 800 °C for (a) 20 h, (b) 200 h, (c) 500 h and (d) 1000 h.

Fig. 2. (a, b) SEM images showing the γ′ precipitation free zones (PFZ) along the two sides of grain boundaries after aging at 800 °C for 1000 h. (c, d) TEM images showing the γ′ phase being absorbed by η.

Fig. 3. (a) TEM image showing the nanoscale γ′ phase in grains after solution treatment, which was confirmed by the superlattice diffraction spots in (b). (c) The evolution of γ′ phase after aging at 800 °C for different time, with arrows indicating the γ′ phase with a rounded cubic morphology.

Fig. 4. (a) Schematic illustration showing the randomly distributed γ′ phase in matrix in 3D space, ES-γ′: the extra-small γ′ phase. (b) Top view of the white plane in (a) and the sections of γ′ phase after cutting. (c) A typical TEM image of the γ/γ′ structure, arrows indicating the extra small sections excluded from counting. (d) The cube of γ′ radius with respecting to aging time at 800 °C.

Fig. 5. (a) Engineering stress-strain curves, (b) tensile strength, yield strength and (c) elongation of K4750 alloy after aging at 800 °C for different time.

Fig. 6. The dislocation configuration of the fractured specimens after aging at 800 °C for 2 h. (a) Well-defined slip bands travelling across the γ′/γ structure toward two directions. (b, c) Enlarged slip bands consisting paired dislocations, Δ: the spacing between two dislocations. (d) An illustration showing the structure of one set of dislocation pair framed in (c), τ: the applied shear stress, Dl: the leading dislocation, Dt: the trailing dislocation, APB: the anti-phase boundary.

Fig. 7. (a-c) The dislocation configuration of the fractured specimen after aging at 800 °C for 20 h. (d) An illustration showing the structure of one set of dislocation pair framed in (c).

Fig. 8. (a) Sheared-off segments of γ′ phase in the fractured specimens after aging at 800 °C for 300 h. (b) An illustration of (a).

Fig. 9. Orowan loops in the fractured specimens after aging at 800 °C for 300 h.

Fig. 10. The dislocation configuration of the fractured specimen after aging at 800 °C for 1000 h. (a, b) Paired dislocations in the same γ′ phase. (c) γ′ segments after dislocations shearing and (d) Orowan loops after dislocations by-passing.

Fig. 11. The evolution of deformation mechanisms as a function of aging time and γ′ diameter. WCD: weakly coupled dislocations, SCD: strongly coupled dislocations.

Fig. 12. (a) Calculated CRSS as a function of γ? diameter with different γAPB values from 0.1 to 0.3 J m-2. (b) Strength evolution as a function of γ? diameter: experimental data associated with theoretical curves (γAPB taken as 0.175 J m-2) including the contribution of several strengthening components, WQ: the yield strength after water quenched.

Fig. 13. The fractographies and longitudinal microstructures of the fractured specimens after aging at 800 °C for (a-c) 2 h, (d-f) 20 h, and (g-i) 1000 h.

Fig. 14. Microstructures near the fracture area for specimens after aging at 800 °C for (a, b) 2 h, (c, d) 20 h, and (e, f) 1000 h. The inverse pole figure (IPF) maps and the kernel average misorientation (KAM) maps showing stress concentration at grain boundaries and interdendritic areas.

Fig. 15. Microstructures near the fracture area for specimens after aging at 800 °C for 1000 h. The band contrast (BC) image, IPF and KAM maps show the stress concentration at (a-c) the dispersed MC carbides in interdendritic areas and (d-f) the continuous carbides along grain boundaries.