Fig. 1. (a) Thermal expansion curve matching with the derivative curve of Ti-5321 alloy during continuously heating process [14]; (b) HEXRD patterns of the interrupted samples at 390 ℃, 570 ℃ and 690 °C.

Fig. 2. TEM analysis of S-390 recorded along <011>β zone axis: (a) Bright-field image of S-390 and the corresponding SAD pattern (inset). (b) Dark-field image of α phase corresponding to the red circle in (a). (c) and (d) Dark-field image of ω phase corresponding to the white circle in (a). (e) HRTEM image showing the β, ω and α regions. (f) HRTEM image showing the adjacent parallel acicular α regions. (g) Three parallel profiles along the <111>β direction inside the β region from the rows marked 1, 2 and 3 in (e). (h) Three parallel profiles along the <111>β direction inside the ω region from the rows marked 4, 5 and 6 in (e).

Fig. 3. TEM analysis of S-570 recorded along <0001>α zone axis: (a) Bright-field image showing the striated α plates. (b) Corresponding selected area electron diffraction (SAD) pattern. The α reflections elongate along the same direction of the striated α plate, marked by the yellow double arrow in (a) and (b). (c) HRTEM image showing the internal structure of α phase with coherent α/α interface. Satellite spots corresponding to the lattice misfit inside the striated α are clearly visible in the FFT pattern inset. (d) The intensity profile along the striated region marked by the red rectangle in (c), showing the fluctuation of interplanar spacing. (e) The α/β interface showing typical BOR of α/β phase, with a two-dimension schematic inset.

Fig. 4. TEM (a) and BSE (b) microstructure of S-690 showing string-like α precipitations. The EBSD observation of S-690: (c) invers pole figure (IPF) showing string-like α precipitations and small α laths with the same orientation, (d) {0001}α, {$10\bar{1}2$}α, {110}β and {111}β pole figures (PF) illustrating the BOR between α phase and β matrix.