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rasonic waves inside liquid media have been widely used in industry, e.g., in ultrasonic cleaning, sonochemistry, and medical treatment. In the past few decades, extensive laboratory results have demonstrated that applying ultrasound waves into solidifying liquid alloys can lead to the refinement of alloy microstructures.[1–3] Although ultrasound-induced grain refinement has been shown effective in many metallic alloy systems, almost all previous research have interpreted the mechanism of grain refinement based on post-mortem microstructural characterization of the solidified alloys and empirical correlation, if any, between the measured grain size and the input ultrasonic power.[4,5] A very recent high-speed imaging study of ultrasonic treatment of a solidifying organic transparent alloy revealed that the shock wave emitted from imploding bubbles can fracture the growing dendrites,[6] increasing the grain multiplication effect that leads to the enhancement of grain refinement. However, in situ and real time studies of the fundamentals of how the highly dynamic ultrasonic waves and the ultrasonic bubbles interact with the liquid metal, the semisolid and solid phases nucleated during solidification have not been reported mainly due to the difficulties in studying the bubble dynamics in the opaque liquid metal. In this paper, we report a number of in situ imaging studies of the dynamic behavior of ultrasonic bubbles in a Sn-13 wt pct Bi and a Bi-8 wt pct Zn alloy using the ultrafast X-ray phase contrast imaging (PCI) facility housed at the Advanced Photon Source (APS), Argonne National Laboratory, US. The real-time imaging studies JIAWEI MI, Senior Lecturer, and DONGYUE TAN and TUNG LIK LEE, Ph.D. Students, are with the School of Engineering, University of Hull, East Yorkshire, HU6 7RX, U.K. Contact e-mail: [email protected] Manuscript submitted October 1, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B

are complemented by a numerical simulation of the bubble dynamics using the classical Gilmore model.[7] The experiments were carried out at the beamline 32-ID-B of APS, and the detailed description of the experiment can be found in References 8, 9. The undulator gap was set to 14 to 18 mm with the X-ray energy of 5.017 to 7.758 keV. The transmitted X-rays were converted into visible light by a fast scintillator crystal, and the signals were projected 45 deg to a CCD camera (Photron, Inc.). Images were recorded with a spatial resolution of 1 lm/pixel, and the fields of view depend on the acquisition rate due to the limited readout speed of camera. The image acquisition rate can reach up to 271,560 frames per second (fps), and it is the fastest X-ray imaging beamline so far in the world. Sn-13 wt pct Bi and Bi-8 wt pct Zn alloys were used in the studies because of their low melting temperatures. The cyclic ultrasonic acoustic pressures introduced into the liquid metal are calculated using the Helmholtz equation (Eq. [1]) and finite element-based commercial software Comsol Multiphysics as detailed in References 8, 9   ðx=CÞ2 1 rPa ¼