Numerical Prediction of the Accessible Convection Range for an Electromagnetically Levitated Fe 50 Co 50 Droplet in Spac
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INTRODUCTION
METAL processing frequently involves melting and solidification. The microstructure of metallic products varies significantly depending on the solidification paths. For some compositions, such as an iron-cobalt system, if the alloy undercools below the critical temperature which varies as a function of composition, it can solidify in a two-step manner, known as double recalescence.[1–3] The metastable body-centered cubic (BCC) phase nucleates in the liquid, and the stable face-centered cubic (FCC) phase follows. Double recalescence yields more globular and more homogeneous grains which exhibit better tensile ductility and fatigue strength, but worse fatigue crack propagation property and fracture toughness. These properties can possibly tailored by controlling the phase selection during solidification. One of the important parameters that affect the microstructure is the time duration between the two recalescence events, which is referred to as the delay time.[4] Generally, the solidification front speed of the stable FCC phase is faster than that of the metastable phase by an order(s) of JONGHYUN LEE, formerly Postdoctoral Associate with the Department of Mechanical Engineering, Tufts University, 200 College Avenue, Medford, MA 02155, is now Research Assistant Professor with the Department of Mechanical and Industrial Engineering, University of Massachusetts, 160 Governors Drive, Amherst, MA 01003. XIAO XIAO, Graduate Research Assistant, and DOUGLAS M. MATSON, Associate Professor, are with the Department of Mechanical Engineering, Tufts University and also with the Department of Mechanical Engineering, University of Massachusetts, 160 Governors Drive, Amherst, MA 01003. Contact e-mail: [email protected] ROBERT W. HYERS, Professor, is with the Department of Mechanical Engineering, University of Massachusetts. Manuscript submitted July 9, 2014. Article published online September 17, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
magnitude.[5] If one wants to obtain globular grains by inducing double recalescence, enough delay time must be secured such that the whole region of the sample is swept by the metastable phase and then followed by the stable phase. Therefore, the science related to the delay time must be understood for successful tailoring of the material properties during solidification. It has been hypothesized that the delay time is a strong function of the convection in the melt.[5] Hanlon et al.[4,6] tested the hypothesis by investigating the deformation of metastable dendrites due to melt convection. Their numerical results show that the deformation of the tips of neighboring dendrites by drag forces due to convection causes the collision of the secondary dendritic arms. This collision can possibly shorten the delay time significantly by providing the preferred nucleation site for the stable phase. Testing the hypothesis experimentally is a challenging task due to the nature of the problem. In order to investigate the influence of the convection on the delay time, i.e., resulting microst
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