Fig. 1. Schematic of the temperature gradient furnace and experimental setup in micro-CT.

Fig. 2. Temperature-time curves at different positions of furnace/sample and variation of temperature gradient for 2D radiographic experiment.

Fig. 3. Overall observation of directional solidification of sheet-like sample: (a) whole liquid phase at 0 min; (b) dendritic growth stage at ~6 min; (c) cellular growth stage at ~36 min; (d) planar growth stage at ~100 min; (e) range of temperature gradient correspond to three growth stages.

Fig. 4. Dendritic to cellular growth transition during directional solidification at: (a) 6 min; (b) 19 min; (c) 25 min; (d) 36 min; (e) range of temperature gradient at dendritic growth stage; (f) two dendritic coarsening mechanisms: (A) proposed by Kattamis [45] (coarsening by dissolution at tips and deposition in the concave space between two arms); (B) proposed by Mortensen [46](coarsening by tips joining of dendrites, leading to entrapped liquid).

Fig. 5. Cellular to planar growth transition during directional solidification at: (a) 37 min; (b) 19 min; (c) 25 min; (d) 36 min; (e) range of temperature gradient corresponding to cellular to planar growth stage.

Fig. 6. (a) and (b) planar growth in the latter part of directional solidification at 97 and 115 min respectively; (c-e) the remelting process of a protrusion in front of S/L interface; (f) a quenched planar interface of Al-Cu alloy; (g) range of temperature gradient corresponding to planar growth stage.

Fig. 7. 3D in-situ observation of S/L interface morphology transition from dendritic to cellular during directional solidification: (a1-d1) 3D morphology of interface; (a2-d2) longitudinal section; (a3-d3) transverse section located at ~1 mm below the S/L interface (indicated with yellow dashed line in a2-d2) at 26, 51, 70, 90 min, respectively.

Fig. 8. Evolution of the interface growth velocity with solidification time.