Fig. 1 OM images of (a) entire joint, (b) BM, and (c) NZ; TEM images of (d) BM, and (e) NZ.

Fig. 2. Crystallographic characters of fully lamellar structure in NZ: (a) TEM bright field, (b) high-resolution TEM of α/β interface along [$2\bar{1}\bar{1}0$]α // [$11\bar{1}$]β direction, (c) Fourier transformation taken from high-resolution TEM in α/β interface.

Fig. 3. Typical twins in NZ: (a) Bright field, (b) high-resolution TEM for selected area in (a) along [$1\bar{1}10$] dirention, (c) Fourier transformation from high-resolution twin in (b).

Fig. 4. Backscattered electron (BSE) images of BM and NZ after annealing at 900 °C for (a, d) 5 min, (b, e) 60 min, and (c, f) 180 min.

Fig. 5. EBSD BC/All-Euler map and misorientation angle distribution of BM after annealing at 900 °C for (a, d, g) 5 min, (b, e, h) 60 min, and (c, f, i) 180 min.

Fig. 6. EBSD BC/All-Euler map and misorientation angle distribution of NZ after annealing at 900 °C for (a, d, g) 5 min, (b, e, h) 60 min, and (c, f, i) 180 min.

Fig. 7. Aspect ratio distribution of NZ after annealing for (a) 0 min, (b) 5 min (c) 60 min, (d) 180 min, (e) 300 min, and (f) spheroidization with annealing time.

Fig. 8. (a) Thickness versus annealing time, and (b) coarsening behavior under static annealing, for BM and NZ.

Fig. 9. (a, d, g, j) EBSD maps for typical lamellae under static annealing; (b, e, h, k) corresponding point to point misorientation along marked line, and (c, f, i, l) corresponding pole figure.

Fig. 10. TEM morphology of NZ after static annealing for the holding time of (a) 5 min, (b) 60 min, and (c) 300 min at 900 °C.

Fig. 11. Spheroidization mechanism of lamellae in NZ under static annealing: (a) initial state, (b) merging process of lamellar edges, (c) orientation of adjacent lamellae changing to be the same, (d) formation of a complete spheroidized grain.

Fig. 12. Superplastic behavior of BM and NZ after annealing for different time: (a) elongation, (b) initial peak flow stress, and (c) macroscopic tensile specimens after fracture.