Ultrasound processing of metal alloys is an environmental friendly and promising green technology for\nliquid metal degassing and microstructural refinement. However many fundamental issues in this field\nare still not fully understood, because of the difficulties in direct observation of the dynamic behaviours\ncaused by ultrasound inside liquid metal and semisolid metals during the solidification processes. In this\npaper, we report a systematic study using the ultrafast synchrotron X-ray imaging (up to 271,554 frame\nper second) technique available at the Advanced Photon Source, USA and Diamond Light Source, UK to\ninvestigate the dynamic interactions between the ultrasonic bubbles/acoustic flow and the solidifying\nphases in a Bi-8%Zn alloy. The experimental results were complimented by numerical modelling. The\nchaotic bubble implosion and dynamic bubble oscillations were revealed in-situ for the first time in\nliquid metal and semisolid metal. The fragmentation of the solidifying Zn phases and breaking up of the\nliquid-solid interface by ultrasonic bubbles and enhanced acoustic flow were clearly demonstrated and\nagreed very well with the theoretical calculations. The research provides unambiguous experimental\nevidence and robust theoretical interpretation in elucidating the dominant mechanisms of microstructure\nfragmentation and refinement in solidification under ultrasound
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