Fig. 1 Cross-sectional OM micrographs of N263 TIG weld joint: macro appearance of weld joint a; microstructure of b base metal, c HAZ, d weld metal

Fig. 2 Microstructure characterizations of N263 base metal: a SEM image; b TEM image and SAD pattern of M23C6 carbides at the grain boundary; cγ′ particles in γ matrix; d EDS results of M23C6 carbide in b

Fig. 3 EDS analysis results of MC carbides in the base metal. MC can be expressed as (Ti, Mo)C

Fig. 4 Microstructure characterizations of the weld metal: a dendritic columnar grains; b SEM image of precipitated particles; c MC and M23C6 carbides; d EDS results of MC carbide in c; e TEM image of precipitated particles; f coherent γ′ particles showing strain contrast and lines of no contrast

Fig. 5 Morphologies of γ′ particles in the weld metal during thermal exposure at 750 °C for a 0 h, b 1000 h, c 2000 h, d 3000 h

Fig. 6 Curves of γ′ particle size d3, d2, and d versus thermal exposure time t under different models: a LSW model; b TIDC model; c non-integer temporal exponent model

Fig. 7 Microstructure evolution after 1000 h thermal exposure at 750 °C: a SEM graphs of interdendritic precipitates in the weld metal; bη laths and lined γ′ particles are found around the decomposed MC carbide; c no η laths appear away from MC carbide

Fig. 8 MC carbides decompose after being exposed for 3000 h and transform to γ′ particles and η laths. a SE and b BSE images showing the small γ′ particles on the MC carbide. c-f EDS elemental maps of C, Ti, Mo, and Cr

Fig. 9 Schematic illustration of MC carbides decomposition: MC + γ → M23C6 + γ′ + η

Fig. 10 Equilibrium phase diagrams of N263 through thermodynamic calculation. γ′ phase is metastable and η phase may precipitate during long-term thermal exposure

Fig. 11 Characterizations of η phase in the weld metal after being exposed for 3000 h: a chemical etching grooves of η phase at the grain boundary and emerged γ′ free zones; bη phase around a decomposed MC carbide; c TEM image and SAD pattern of η phase

Fig. 12 Impact toughness results of base metal and weld metal after thermal exposure at 750 °C: a impact toughness values; b, c weld metal fracture without thermal exposure, d, e weld metal fracture after thermal exposure for 3000 h

Fig. 13 Microstructures of fractured impact specimen in the weld metal after thermal exposure for 3000 h: a the whole fracture; b intergranular rupture; c precipitates at the grain boundary hinder and deflect the crack propagation