Fig. 1. (a) Variations in the austenite grain size and area fraction of δ-ferrite across the thickness of plate, and (b) dimensions of a compact tension (CT) specimen (unit: mm).

Fig. 2. Microstructures of (a, c) FF samples and (b, d) CF samples: (a, b) solution treated condition and (c, d) accelerated aged condition.

Fig. 3. TEM images near the grain boundary in FF specimens after accelerated-aging treatment at 750 °C for 10 h. (Inset refers to the corresponding selected electron diffraction patterns of M23C6 carbide and the right-hand grain)

Fig. 4. EBSD images of CF sample after aging at 750 °C for 10 h: (a) IPF, (b) phase map, and (c) corresponding pole figures of two phases marked in (b).

Fig. 5. Variation of fatigue crack growth rate (da/dN) with the stress intensity range (ΔK) in the Paris region.

Fig. 6. Fatigue crack growth paths in (a) FF and (b) CF samples in the solution treated condition: (c) Detailed phase map marked in (b), and (d) KAM map marked in (c). (e) The main crack path characteristics near the δ-ferrite, and (f) microstructure ahead of crack tip.

Fig. 7. Fatigue crack growth paths in (a) FF and (b) CF samples after aging at 750 °C for 10 h. (c) Main crack path morphologies near the decomposed δ-ferrite, and (d) detailed microstructure near the decomposed δ-ferrite ahead of crack tip.

Fig. 8. ECCI image through the crack path in solution treated CF sample.

Fig. 9. Nanoindentation results of the austenite and δ-ferrite in the solution treated and accelerated aged samples: (a) load- displacement curves; (b) the hardness and modulus of different phases.

Fig. 10. Schematic illustrations about the role of δ-ferrite in the crack propagation processes in (a, b) solution treated and (c, d) accelerated aged CF sample.