Fig. 1. (a) Schematic diagram of the mockup of 316L-308L/309L-SA508 nozzle/safe-end weld joint. (b) Metallurgical morphology of 308L weld metal in three dimensions. (c) TEM image of microstructure of 308L weld metal showing M23C6 carbides along the δ-ferrite/austenite phase boundary. (d) SAED pattern of carbide and austenite. (e) Dimension of the CERT specimen. The sampling positions for metallurgical and TEM observations are indicated in the 308L weld cladding in (a).

Fig. 2. Crack size frequency distributions of the 3-dpa irradiated specimen with different strains: (a) 2% strain, (b) 3% strain, (c) 5% strain.

Fig. 3. Slip lines formed on the 3-dpa irradiated specimen strained to 3% and their interactions with grain boundary (G.B.) and phase boundary: (a) slip lines transmit from one austenite grain to an adjoining one by changing their directions, (b) slip lines terminate at G.B., (c) slip lines induce a step at the cracked G.B., (d) slip lines directly pass through the δ-ferrite and phase boundary without changing directions, as indicated in the circled regions.

Fig. 4. Cracks on the 3-dpa irradiated specimen with 3% strain: (a) G.B. crack, (b) phase boundary (P.B.) crack, (c) G.B. crack propagates to form P.B. crack, (d) oxide film rupture and the resultant transgranular cracks, (e-g) G.B. cracks terminate at δ-ferrite.

Fig. 5. Lathy faulted planes in austenite in the 3-dpa irradiated specimen with 3% strain: (a) TEM-DF image, (b) SAED pattern, (c) HRTEM image and the corresponding FFT patterns, (d) TEM-DF image, (e) TEM-BF image.

Fig. 6. Cross-sectional TEM observation of the interaction between slip lines and the grain boundary in the 3-dpa irradiated specimen with 3% strain: (a) TEM-BF image showing dislocation channel and channel expansion, (b) SAED pattern of the circled region in channel expansion, (c) TEM-DF image of Frank dislocation loops obtained by using the rel-rod method, (d) TEM-BF image corresponding to (c). The FIB sampling position is indicated in the SEM image inserted in (a). The rel-rod used for TEM-DF image is circled in the SAED pattern inserted in (c).

Fig. 7. Cross-sectional TEM observation and TKD analysis of the interaction between the slip line and phase boundary in the 3-dpa irradiated specimen with 3% strain: (a) TEM-BF image and SEAD pattern of dislocation channel in austenite, (b) TEM-DF image showing atomic plane rotation in δ-ferrite and lathy faulted planes in austenite, (c) TKD BC image, (d) phase map, (e) crystal orientation map, (f) KAM map. The FIB sampling position is indicated in the SEM image inserted on the left bottom of (a).

Fig. 8. Cross-sectional TEM observation of slip lines for 3% strained specimen: (a) SEM image indicating FIB sampling position, (b) TEM-BF image showing atomic plane rotation in δ-ferrite, (c) STEM-HAADF image showing cross-sectional morphology, (d) TEM-BF image showing dislocation channel in austenite, (e) TEM-BF image showing lathy faulted planes in austenite. The regions corresponding (b), (d) and (e) are marked in (c) with different colors.

Fig. 9. Relationship between deformation microstructures and irradiation defects in austenite in the 3-dpa irradiated specimen with 3% strain: (a) under-focus TEM-BF image of lathy faulted planes and voids in austenite, (b, c) TEM-DF images of lathy faulted planes and Frank dislocation loops in austenite, (d) under-focus TEM-BF image of voids in a dislocation channel in austenite. The SAED pattern for the TEM-DF image in (c) is inserted in the upper left corner.

Fig. 10. Cross-sectional observation of crack 1# along the grain boundary: (a) STEM-HAADF image and element mapping of the crack tip, (b, c) TEM-BF image and SAED pattern of the deformation twin on one side of the crack, respectively, (d) HRTEM image of the deformation twin, (e-h) FFT patterns corresponding to regions 1, 2, 3 and 4 in (d), respectively.

Fig. 11. Cross-sectional observation of crack 2# along the grain boundary: (a) STEM-HAADF image and element mapping of the crack, (b) TEM-BF image of the crack showing dislocation channel on one side of the crack tip. The FIB sampling position is shown in the SEM image inserted in (b).

Fig. 12. Cross-sectional TEM observation of crack 3# along the grain boundary: (a) SEM image showing the FIB sampling position, (b) STEM-HAADF image of the cross-sectional morphology of the crack, (c) O map, (d) Fe map, (e) Cr map, (f) Ni map, (g) Mn map, (h) S map.

Fig. 13. STEM-BF images and element mappings of the cracks which (a) initiate and propagate only along the phase boundary and (b) originate from the grain boundary cracking. The FIB sampling positions of the cracks are inserted in the STEM-BF images.

Fig. 14. Schematical illustrations of the propagation mechanism of IASCC cracks along austenitic grain boundaries (a) in matrix interior and (b) close to specimen surface.