In-situ observation of solid-liquid interface transition during directional solidification of Al-Zn alloy via X-ray imaging

The morphological instability of solid/liquid(S/L) interface during solidification will result in different patterns of microstructure. In this study, two dimension(2 D) and three dimension(3 D) in-situ observation of solid/liquid interfacial morphology transition in Al-Zn alloy during directional solidification were performed via X-ray imaging. Under a condition of increasing temperature gradient(G), the interface transition from dendritic pattern to cellular pattern, and then to planar growth with perturbation was captured. The effect of solidification parameter(the ratio of temperature gradient and growth velocity(v), G/v) on morphological instabilities was investigated and the experimental results were compared to classical "constitutional supercooling" theory. The results indicate that 2 D and 3 D evolution process of S/L interface morphology under the same thermal condition are different. It seems that the S/L interface in 2 D observation is easier to achieve planar growth than that in 3 D, implying higher S/L interface stability in 2 D thin plate samples. This can be explained as the restricted liquid flow under 2 D solidification which is beneficial to S/L interface stability. The in-situ observation in present study can provide coherent dataset for microstructural formation investigation and related model validation during solidification.