Multi-scale dendritic patterns sequentially superimposed in a primary semi-solid matrix

The principle and control for solidification present crucial challenges. Through in situ and real time ob- servation on the solidification of M50 bearing steel, here we discovered that the base of conventional dendritic arms is constituted by a primary semi-solid matrix. Due to the superimposition of temperature fluctuations from the large to the subtle scale, multi-scale dendritic patterns will sequentially emerge and evolve in the primary semi-solid matrix. These findings redefine the essence of multi-scale dendrites as the multi-scale segregation patterns in the primary semi-solid matrix, reconstitute the time and space sequence in the formation of multi-scale dendrites, and reveal the superimposition of temperature fluc- tuation as the driving principle. These new understandings will fundamentally influence the solidification field. These results also indicate important engineering applications, such as designing multi-scale den- drites, controlling the dendritic segregation and eliminating the detrimental eutectics.