Fig. 1. OM images showing a low porosity for as-built samples: (a) XY plane in low magnification, (b) YZ plane in low magnification, (c) XY plane in high magnification and (d) YZ plane in high magnification.

Fig. 2. (a) One-step heat treatment schedule and (b) two-step heat treatment schedule proposed in the current work.

Fig. 3. CALPHAD-based thermodynamic calculated equilibrium step diagram for IN738LC alloy based on the composition characterized in Table. 1.

Fig. 4. EBSD IPF maps of as-built IN738LC sample showing (a) representative 3D reconstructed grain structure, (b) grain structure in XY plane, (c) grain structure in YZ plane and (d) corresponding pole figures from YZ plane.

Fig. 5. (a) BSE image, (b) BF-TEM micrograph and (c) SAED revealing bright secondary particles observed in the as-SLMed sample, (d) HAADF-STEM image and (e-k) corresponding STEM-EDX element maps showing the relative chemical composition of this bright phase. SAED pattern was taken along <001>Ni zone axis.

Fig. 6. EBSD-IPF maps showing grain structure of heat-treated samples with (a) one-step heat treatment at 1230 °C/2 h and two-step heat treatment at (c) 850 °C/24 h + 1230 °C/2 h, (e) 950 °C/24 h + 1230 °C/2 h, (g) 1050 °C/24 h + 1230 °C/2 h; (b, d, f, h) corresponding {100} pole figures, respectively.

Fig. 7. Grain width distribution measured from the heat-treated samples in Fig. 6.

Fig. 8. BSE images showing grain boundary phases (marked by white arrows) of samples subject to the first-step heating of (a) 850 °C/24 h, (b) 950 °C/24 h, (c) 1050 °C/24 h.

Fig. 9. (a) BF-TEM micrograph and (b) SAED revealing dark secondary particles observed in the sample after first-step heat treatment of 950 °C/24 h, (c) HAADF-STEM image and (d-j) corresponding STEM-EDX element maps showing the relative chemical composition of this grain boundary phase. SAED pattern was taken along <001>Ni zone axis.

Fig. 10. Plot of average grain size as a function of heating time for the two investigated heating schedules (one-step heat treatment at 1230 °C and two-step heat treatment of 950 °C/24 h + 1230 °C) showing different grain growth behaviours.

Fig. 11. Grain structure of the heat-treated samples before the creep test: EBSD IPF maps showing the grain size and morphology for the sample subject to (a) SHT, (b) 950 °C/24 h + 1230 °C/10 h+ SHT.

Fig. 12. Microstructure of the heat-treated samples before the creep test: BSE images showing the strengthening γ' phases and grain boundary MC carbides for the samples subjected to (a, c) SHT and (b, d) 950 °C/24 h + 1230 °C/10 h+ SHT.

Fig. 13. (a) High-temperature (850 °C) creep curves for heat-treated SLM-IN738LC specimens under two stress testing conditions. (b) Plot of secondary creep rate vs stress for the above creep test.

Fig. 14. Misorientation angle distribution showing the grain boundary misorientation relationship observed in (a) as-SLMed sample and (b) 950 °C/24 h sample.

Fig. 15. BSE images showing carbides of (a) as-built sample, (b) 950 °C/24 h sample after heating to 1230 °C.

Fig. 16. SE images showing intergranular fracture mode for crept samples under (a) SHT condition, (b) 950 °C/24 h + 1230 °C/10 h+ SHT condition and their corresponding crack distribution (c), (d), respectively.

Fig. 17. BF-STEM micrograph revealing dislocation substructure observed in crept (200 MPa) samples under (a) SHT condition, (b) 950 °C/24 h + 1230 °C/10 h+ SHT condition. Micrographs were taken along <110>Ni zone axis.