Magnetic Effects on Microstructure and Solute Plume Dynamics of Directionally Solidifying Ga-In Alloy
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https://doi.org/10.1007/s11837-020-04305-2 2020 The Author(s)
SOLIDIFICATION BEHAVIOR IN THE PRESENCE OF EXTERNAL FIELDS
Magnetic Effects on Microstructure and Solute Plume Dynamics of Directionally Solidifying Ga-In Alloy ANDREW KAO PETER D. LEE
,1,5 NATALIA SHEVCHENKO,2 SHENGYA HE,2,3 ,4 SVEN ECKERT,2 and KOULIS PERICLEOUS 1
1.—Centre for Numerical Modelling and Process Analysis, Old Royal Naval College, University of Greenwich, Park Row, London SE109LS, UK. 2.—Helmholtz-Zentrum Dresden-Rossendorf, Institute of Fluid Dynamics, Bautzner Landstrasse 400, 01328 Dresden, Germany. 3.—State Key Laboratory of Advanced Special Steel & School of Materials Science and Engineering, Shanghai University, Shanghai 200444, People’s Republic of China. 4.—Mechanical Engineering, University College London, London WC1E 6BT, UK. 5.—e-mail: [email protected]
The effects of applying a 0.2-T transverse magnetic field on a solidifying Ga25 wt%In alloy have been investigated through a joint experimental and numerical study. The magnetic field introduced significant changes to both the microstructure and the dynamics of escaping high-concentration Ga plumes. Plume migration across the interface was quantified and correlated to simulations to demonstrate that thermoelectric magnetohydrodynamics (TEMHD) is the underlying mechanism. TEMHD introduced macrosegregation within the dendritic structure, leading to the formation of a stable ‘‘chimney’’ channel by increasing the solutal buoyancy in the flow direction. The resulting pressure difference across the solidification front introduced a secondary hydrodynamic phenomenon that subsequently caused solute plume migration.
INTRODUCTION During solidification of alloys, partitioning of elements leads to the formation of a solute boundary layer in the vicinity of the liquid–solid interface. This paper considers the behavior of alloys, such as industrial nickel-based superalloys, where the density of the solute may be lighter than that of the bulk liquid. This leads to strong buoyancy forces in this solute layer directed back towards the bulk liquid, causing the formation of solute plumes that emanate from the interface. Under certain conditions, the escaping solute can form stable channels, or chimneys, that when fully solidified become a defect known as ‘‘freckles’’.1–4 The aim of this work is to investigate the effects of applying a magnetic field on such systems, with the long-term goal of
(Received May 22, 2020; accepted July 22, 2020)
using magnetic fields to tailor the microstructure and control the solute segregation so as to enhance the properties of cast components. During solidification, inherent thermoelectric currents are formed due to the variation in composition by phase and thermal gradient. These currents interact with an externally applied magnetic field, generating a thermoelectric Lorentz force (TEF) that drives flow through a phenomenon known as thermoelectric magnetohydrodynamics (TEMHD). The TEMHD effect, also known as thermoelectric magnetic convection (TEMC), h
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