Fig. 1. SEM micrographs of the powder mixture compact heated at 595 °C for different durations: (a) 15 min; (b) 35 min; (c) 3 h; (d) 36 h. The inserts are the typical microstructures of one reinforcing particulate, and the regions marked with red rectangular are the sampling positions of TEM foils.

Fig. 2. SEM images of the reaction product in the specimen heated at 595 °C for different durations: (a-a2) 15 min; (b-b2) 35 min; (c-c2) 3 h; (d-d2) 36 h. The first column images display the whole morphologies of the reaction product, the second column display the morphologies of the inner compact structure, and the third column display the morphologies of the outer jagged structure.

Fig. 3. XRD patterns of the specimen heated at 595 °C for different durations.

Fig. 4. TEM analysis results of the specimen heated at 595 °C for 15 min: (a) Bright-field TEM image; (b) and (c) HRTEM images of the area marked by arrows b and c in (a) respectively, inserts are corresponding FFT patterns; (d) Magnified image of the area labeled by red box in (a); (e) HRTEM image of the plate marked by arrow e in (d); (f) Mixed images of HAADF image and EDS mappings of Al and Si of the area A labeled in (d), white arrows indicate SFs; (g) FFT pattern of the area labeled in (e); (h)-(j) SAED patterns obtained from the area labeled by red circle in (a).

Fig. 5. TEM results of the specimen heated for 35min: (a) Bright-field TEM image; (b) Magnified bright-field image of particle A marked in (a); (c)-(e) SAED patterns obtained from the area labeled in (a).

Fig. 6. TEM results of the specimen heated at 595 °C for 3 h: (a) Bright-field TEM image of the specimen; (b)-(c) Mixed images of HAADF image and EDS mappings of Al and Si of the area labeled in (a); (d)-(f) SAED patterns obtained from the area labeled by red circle in (a); (g) Magnified bright-field image of the particle B marked in (a), insert is the corresponding SAED patterns.

Fig. 7. TEM analysis results of the region labeled red box in Fig. 6g: (a) HRTEM image; (b) and (c) Magnified HRTEM images of the regions labeled by A and B in (a) respectively; (b1) and (b2) FFT patterns of the regions b1 and b2 labeled in (b); (b1’) and (b2’) Theoretical diffraction patterns of τ2 along [100] and [001] zone axis respectively; (c1) FFT pattern of region c1 labeled in (c); (c1’) Theoretical diffraction patterns of τ1 along [010] zone axis.

Fig. 8. TEM results of the specimen heated for 36h: (a) Bright-field TEM image of the sample; (b)-(d) SAED patterns obtained from the area labeled in (a).

Fig. 9. Chemical compositions of τ1 and τ2 phases.

Fig. 10. Schematic diagrams of the morphologies of τ1 and τ2 crystals: (a) Morphology of τ1 crystal predicted by BFDH method; (a1) Cross-section of τ1 crystal along (010) plane; (a2) Cross-section of τ1 crystal along (100) plane; (b) Morphology of τ2 crystal predicted by BFDH method; (b1) Cross-section of τ2 crystal along (100) plane; (b2) Cross-section of τ1 crystal along (001) plane.

Fig. 11. TEM results of Ti/A356 powers interface in the specimen heated at 595 °C for 15 min: (a) HAADF image of a local area on the specimen shown in Fig. 4a; (b) and (c) EDS mapping of the Mg and O elemental distribution; (d) Bright-field TEM image of the area marked in (a); (e)-(f) HRTEM images of Film B and Film A marked in (d), inserts are corresponding FFT patterns.

Fig. 12. Schematic of different structures: (a) Structure model of C49 TiSi2; (b) Arrangement of C49 TiSi2 along [100] zone axis; (c) Formation mechanism of C49 TiSi2 along [001]; (d) Structure model of τ1; (e) Arrangement of τ1 along [010] zone axis; (f) Formation mechanism of τ1 along [010] zone axis.

Fig. 13. Formation mechanisms of the micro-steps on the short sides of the hexagonal cross-section for defective τ2 crystals.

Fig. 14. Bright-field TEM images of defective τ2 crystals in the specimen heated for 3 h.

Fig. 15. Microstructures of reaction product in the specimens heated for different durations: (a) 6 h; (b) 8 h.

Fig. 16. Schematic for formation process of reaction products during heating of Ti-A356 mixed powder compact at 595 °C. (a) Local breakage of Al2O3/TiO2 dual-layered oxide film; (b) Dissolution of Ti powder and precipitation of τ1 and τ2 crystals; (c) Formation of reaction products with two morphologies; (d) Growth and coarsening of τ1 and τ2 crystals; (e) Coarsening of large τ1 crystals by exhausting the small τ2 crystals in the inner compact structure; (f) Formation of large τ1 plate agglomerate.