Fig. 1. TEM images of γ′ phase in K4750 alloy with (a) 0B, (b) 0.007 wt. % B and (c) 0.010 wt. % B after aging at 800 ℃ for 1000 h.

Fig. 2. SEM images of microstructure in K4750 alloys with (a) 0B, (b) 0.007 wt. % B and (c) 0.010 wt. % B after the full heat treatment (the diffraction spot of MC carbide in 0B alloy is contained in (a)); the microstructure near grain boundaries and enlarged view of degraded MC carbides of K4750 alloys with (d and g) 0B, (e and h) 0.007 wt. % B and (f and i) 0.010 wt. % B after aging at 800 ℃ for 1000 h.

Fig. 3. EPMA images of K4750 alloy with (a-e) 0B and (f-j) 0.010 wt. % B after aging at 800 ℃ for 1000 h; the distribution maps of (b and g) C, (c and h) Cr, (d and i) Ti and (e and j) Nb.

Fig. 4. (a) TEM image, (b) selected area diffraction pattern and (c) EDS spectra of needle-like η phase in K4750 alloy without B after aging at 800 ℃ for 1000 h.

Fig. 5. SEM images of M23C6 carbides in K4750 alloy with (a and d) 0B, (b and e) 0.007 wt. % B and (c and f) 0.010 wt. % B after the full heat treatment (a-c) and after aging at 800 ℃ for 1000 h (d-f).

Fig. 6. (a and c) SEM images of continuous M23C6 carbides in 0B alloy after aging at 800 ℃ for 1000 h; (b) TEM image of needle-like η phase inset corresponding SAED; EDS maps show the element distribution of image (c): (d) Cr, (e) Ti and (f) Ni.

Fig. 7. (a) SIMS image of grain boundary in K4750 alloy with 0.010 wt. % B after aging at 800 ℃ for 1000 h, (b) B-map and (c) the distribution of B element along the blue line in image (a).

Fig. 8. (a) The stress rupture life and (b) the ductility of K4750 alloys with various B contents at 750 ℃/380 MPa after aging at 800 ℃ for 1000 h.

Fig. 9. The fracture morphology of K4750 alloys with (a) 0B, (b) 0.007 wt. % B and (c) 0.010 wt. % B after stress rupture testing at 750 ℃/380 MPa.

Fig. 10. The longitudinal microstructures of K4750 alloys with (a-c) 0B and (d-f) 0.010 wt. % B after stress rupture testing at 750 ℃/380 MPa.