Fig. 1. TEM characterizations of δ-hydride and γ-hydride precipitations in hot-forged commercially pure Ti. (a) TEM image of a δ-hydride lamella and a γ-hydride lamella coprecipitated in one grain of the hot-forged sample observed along [112-0] HCP zonal axis. (b, c) The corresponding SAED patterns of the δ-hydride and γ-hydride lamellas in (a), respectively. (d, e) Plasmon peaks of the δ-hydride and the γ-hydride in EELS, respectively. (f) Schematic diagram of the orientations of the δ-hydride and γ-hydride lamellas in the HCP matrix.

Fig. 2. TEM characterizations of γ-hydride precipitations in the electropolished α-Ti sample. (a) TEM image of the γ-hydride lamellas (blue arrows) in the electropolished α-Ti sample. Three variations of the γ-hydride with 120 degrees to each other were observed along [0001] HCP zonal axis. (b-d) SAED patterns of three γ-hydride variations corresponding to the areas marked by circles 1, 2 and 3 in (a), respectively. (e) Plasmon peaks of the α-Ti and the γ-hydride in EELS, respectively. (f) An enlarged TEM image of one γ-hydride lamella. (g-i) A series of SAED patterns obtained by tilting the sample from the zonal axes of [001] FCT to [114]FCT and then to [112]FCT.

Fig. 3. TEM characterizations of δ-hydride precipitations in the electropolished α-Ti sample. (a) TEM image of numerous δ-hydride lamellas (yellow arrows) in the electropolished samples seeing along $<11\bar{2}0>_{HCP}$ zonal axis. (b) The corresponding SAED pattern obtained from the yellow circle area in (a). (c) Plasmon peaks of the α-Ti and the δ-hydride in EELS, respectively. (d) An enlarged TEM image of one δ-hydride lamella with the corresponding SAED pattern (inset).

Fig. 4. Characterizations of a twinned δ-hydride lamella in the electropolished α-Ti sample. (a) HRTEM image of a twinned δ-hydride lamella with twin boundary indicated by a yellow dashed line. (b) The corresponding SAED of the twinned δ-hydride lamella showing the {111} twin relationship. (c) An enlarged HRTEM image of the area marked by the green dotted rectangle in (a), showing the atomic structures of the α-Ti/δ-hydride interfaces and the {111} twin boundary. (d-f) The corresponding FFT patterns of the selected squares marked as 1, 2 and 3 in (c), respectively.

Fig. 5. The nucleation and growth process of γ-hydride lamellas. (a) A TEM image of γ-hydride lamellas in the electropolished sample seeing along [0001] HCP zonal axis. The inset is the SAED pattern of the area indicated by a blue circle. (b-d) HRTEM images of three γ-hydrides at nucleation and growth process, respectively.

Fig. 6. Schematic illustration of the transformation mechanism of α-Ti to γ-hydride. (a) The HAADF-STEM image of the α-Ti/γ-hydride boundary seeing along [0001] HCP zonal axis. (b-d) The atomic diagrams of the process how the phase boundary gets 2-layers step migration. (e-g) Flow charts of the γ-hydride growth mechanism.

Fig. 7. Schematic illustration of the transformation mechanism of α-Ti to δ-hydride. (a) TEM image and the corresponding SAED (inset) of a δ-hydride lamella formed by electrochemical polishing. (b) HAADF-STEM image of the α-Ti/δ-hydride boundary seeing along <11 2- 0> HCP zonal axis. (c-f) The atomic illustration for the δ-hydride phase transformation. The larger solid dots are FCC arranged Ti atoms, while hollow circles are HCP arranged Ti atoms. The smaller dots are H atoms. All the blue atoms are on the Layer-1, and all the red atoms are on the Layer-2.