Fig. 1 TEM observations of $\left\{ {10\overline{1} 2} \right\} <10\overline{1} \overline{1}>\alpha$ twinned sample at different temperatures during in situ heating: a room temperature, b 600 °C, c 880 °C, d 950 °C; (a-1, b-1, c-1, and d-1) bright field TEM images, (a-2, b-2 and c-2) the corresponding SAED patterns of the circled regions at the twin boundary in the BF images along the $\left[ {1\overline{2} 10} \right]_\text{m} //\left[ {1\overline{2} 10} \right]_\text{t}$ zone axes, (d-2) the key diagram of the SAED pattern of the circled region in d-1. The red, blue, green, and yellow hexagons in d-2 represent four FCC-Ti variants

Fig. 2 SAED patterns of $\left\{ {11\overline{2} 2} \right\} <11\overline{2} \overline{3}>$ twinned sample during in situ heating: a–c SAED patterns along the $\left[ {\overline{1} 100} \right]_\text{m} //\left[ {\overline{1} 100} \right]_\text{t}$ zone axis during heating at a room temperature, b 600 °C, c 980 °C; d the key diagram of the FCC-Ti diffraction pattern in c

Fig. 3 a Schematic diagram of the orientation relationship between α phase and FCC-Ti, b rotation relationship of the four sets of SAED patterns of FCC-Ti in Fig. 1d-2, c, d schematic diagrams of the orientation relationship between αmatrix/αtwin and FCC-Ti in $\left\{ {10\overline{1} 2} \right\} <10\overline{1} \overline{1}>$ twinned sample

Fig. 4 a SAED patterns of $\left\{ {10\overline{1} 2} \right\} < 10\overline{1} \overline{1} >$ twinned sample at 600 oC, b the key diagram of the intermediate state diffraction pattern in a, c schematic illustration of HCP-Ti → FCC-Ti phase transition

Fig. 5 Cohesive energy variation of HCP-Ti, BCC-Ti, and FCC-Ti caused by different content of interstitial atoms (O, N, and C) with inset showing the cohesive energy difference of FCC-HCP and BCC-HCP