Fig. 1 Optical microstructure of Ti5321 alloy after solution-treated at 900 °C for 90 min followed by water quenching

Fig. 2 Detailed holding temperatures of CHT cycles

Fig. 3 Schematic illustration of CHT process

Fig. 4 a Volume fraction of α phase after different CHT cycles; b length, width and aspect ratio of α phase after different CHT cycles

Fig. 5 a XRD profiles of the solution-treated, CHT-9, CHT-37, CHT-47 and CHT-100 specimens; b an enlarged view of 38°-42° in a

Fig. 6 SEM micrographs of Ti5321 after different CHT cycles: a CHT-9; b CHT-37; c CHT-47; d CHT-100

Fig. 7 EBSD analysis of Ti5321 after different CHT cycles: a1-d1 inverse pole figure (IPF) maps of α phase; a2-d2 aspect ratio frequency distribution plots and cumulative distribution curves of α phase. a CHT-9; b CHT-37; c CHT-47; d CHT-100. GB means grain boundary

Fig. 8 EDS analyses of Ti5321 after different CHT cycles: a1-a4 SEM images of CHT-9, CHT-37, CHT-47 and CHT-100 specimens, respectively. b-i EDS line scanning results of different elements along the direction of the white arrows in a1-a4

Fig. 9 TEM-EDS analysis of Ti5321 in CHT-47 specimen: a bright-field (BF) TEM images; b selected area electron diffraction (SAED) pattern of the cycled region b in a; c dark-field TEM image of α phase; d EDS mapping corresponding to the squared region d in a. Highlighted colors represent element enrichment

Fig. 10 Micro-hardness of the specimens after different CHT cycles

Fig. 11 Schematic diagrams of α phase spheroidization during CHT: a α phase spheroidization process with boundary splitting mechanism; b α phase spheroidization process with termination migration mechanism

Fig. 12 TEM-EDS analysis of Ti5321 in CHT-9 specimen: a, c bright-field (BF) TEM images; b EDS mapping corresponding to the squared region b in a; d EDS mapping corresponding to the squared region d in c. Highlighted colors represent element enrichment