A High-Speed Imaging and Modeling Study of Dendrite Fragmentation Caused by Ultrasonic Cavitation
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TRODUCTION
SUBJECTING liquid metallic alloys to an ultrasonic field with a sound intensity above a certain threshold can produce refined, nondendritic microstructures, as shown in Al-,[1,2] Mg-,[3,4] and Ni-based alloys and steels.[5,6] The grain refinement effect can occur when ultrasound is applied to the alloy melt at a temperature above its liquidus temperature or between the liquidus and solidus temperatures during solidification. The links between the resulting grain size of an alloy solidified in this manner and the various ultrasonic parameters, i.e., frequency, intensity, power density, etc. applied during solidification, have been studied by postmortem microstructural analysis[7–9] and have provided some useful experimental correlations. The proposed mechanisms for grain refinement caused by ultrasound generally fall into three categories: (1) the promotion of homogeneous grain nucleation by increasing the alloy melting point and therefore leading to a melt undercooling at locations immediately adjacent to any imploding ultrasonic cavities or bubbles (when cavities are filled with vapor or gas nucleated in an ultrasonic field with the sound intensity above a certain threshold[1]) because of the transient high pressure generated during implosion[10,11]; (2) the enhancement of heterogeneous nucleation by ‘‘activating’’ insoluble impurity particles in the melt, e.g., oxide inclusions, by increasing wettability with the melt, DA SHU, Associate Professor, and BAODE SUN, Professor, are with the State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] JIAWEI MI, Lecturer and Royal Society Industry Fellow, is with the Department of Engineering, University of Hull, East Yorkshire HU6 7RX, UK. PATRICK S. GRANT, Cookson Professor of Materials, is with the Department of Materials, University of Oxford, Oxford OX1 3PH, UK. Manuscript submitted September 27, 2011. Article published online May 16, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
reducing agglomeration, etc.[1,5]; and (3) the multiplication of grains by fragmentation of growing dendrites to seed the formation of new grains ahead of the solidification front. In reality, one or all of the mechanisms may come into play at different stages of solidification. Given the potency of the grain refinement effect (up to an order of magnitude) and the potential technological implications for the casting industry, increasing the currently limited mechanistic understanding of ultrasound in liquid and semisolid metals is important. In situ observation of dendritic microstructure evolution in real time is likely to be helpful to reveal the relative importance of each ultrasonic parameter at different stages of solidification. Direct observation of solidification, usually comprising dendritic growth, can be realized by optical imaging of transparent alloys, and this approach has been widely used to investigate the dynamics of dendrite evolution under different thermal regimes, mechanical stirring/ shearing
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