Influence of Pressure Field in Melts on the Primary Nucleation in Solidification Processing
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Influence of Pressure Field in Melts on the Primary Nucleation in Solidification Processing MILAN RAKITA and QINGYOU HAN It is well known that external fields applied to melts can cause nucleation at lower supercoolings, fragmentation of growing dendrites, and forced convection around the solidification front. All these effects contribute to a finer microstructure of solidified material. In this article, we analyze how the pressure field created with ultrasonic vibrations influences structure refinement in terms of supercooling. It is shown that only high cavitation pressures of the order of 104 atmospheres are capable of nucleating crystals at minimal supercoolings. We demonstrate the possibility of sononucleation even in superheated liquid. Simulation and experiments with water samples show that very high cavitation pressures occur in a relatively narrow zone where the drive acoustic field has an appropriate combination of pressure amplitude and frequency. In order to accurately predict the microstructure formed by ultrasonically assisted solidification of metals, this article calls for the development of equations of state that would describe the pressure-dependent behavior of molten metals. DOI: 10.1007/s11663-017-1029-2 Ó The Minerals, Metals & Materials Society and ASM International 2017
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INTRODUCTION
ULTRASONIC vibrations have a complex influence on how materials solidify, especially those which crystallize, such as metals[1,2] and various organic and inorganic compounds.[1] One of the most significant features of ultrasonically assisted solidification is the increased nucleation rate at lower supercoolings, which is analyzed in this article. The resulting microstructure consists of equiaxed grains instead of dendrites.[3–7] This is very significant because finer equiaxed grains lead to better mechanical properties.[2,3,5] One should add to this list the acceleration of other rate processes, such as degassing,[7,8] then thermal diffusion, dross removal, surface cleaning and passivation,[9] and accelerated rate of chemical reactions.[8,10] The main contribution to ultrasonically induced nucleation comes from cavitating bubbles, which create high pressures during their collapse and rebound. High pressures cause two competing effects in the liquid phase: increase in the freezing temperature and heating due to compression. As an illustration, Figure 1 shows
MILAN RAKITA is with the School of Engineering Technology, Purdue University, West Lafayette, IN. QINGYOU HAN is with the School of Engineering Technology, Purdue University, Knoy Hall of Technology, Room 129, 401 N. Grant Street, 47907-2021. Contact e-mail: [email protected] Eighth International Conference on Physical and Numerical Simulations of Materials Processing—ICPNS 2016 symposium. Manuscript submitted December 21, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
the variation of the equilibrium freezing point of water as a function of pressure, together with adiabatic heating with initial temperatures of 273 K and 263.15 K (0 °C and 10 °C). The ice-water equil
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