This paper presents recent progress of in situ observation for the microstructure evolution during solidification. Nucleation and fragmentation of dendrite arms are important issues for controlling microstructure during solidification. However, there are few studies on in situ observation of nucleation and fragmentation in metallic alloys. Time resolved X-ray imaging technique has been developed to observe solidification of metallic alloy systems in situ. Fragmentation of dendrite arms often occurred at the root after growth velocity was reduced for the Sn-13 at.-%Bi alloys and the Al-15 mass%Cu alloys. In the Al-15 mass%Cu alloys, both of nucleation and fragmentation contribute to formation of grain structure. The result suggested that fragmentation should be considered for controlling grain structure.
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