On the Coupling Mechanism of Equiaxed Crystal Generation with the Liquid Flow Driven by Natural Convection During Solidi
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, MENGHUAI WU, and ANDREAS LUDWIG are with the Department of Metallurgy, Montanuniversitaet Leoben, Franz-JosefStr. 18, 8700 Leoben, Austria. Contact e-mail: [email protected] Manuscript submitted February 3, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Coupling mechanism at the origin of CET: 1-2 strong flow running through the mush transporting out dendrite fragments (white dots); 3-4 equiaxed growth and drag of the downward flow. If the vortex is sufficiently stable, the horizontal configuration can lead to freckle appearing; a) vertical solidification front; b) horizontal solidification front. https://doi.org/10.1007/s11661-018-4489-3 The Author(s) 2018. This article is an open access publication
I.
INTRODUCTION
THE mechanism of how equiaxed grains originate can be explained by two theories: heterogeneous nucleation or fragmentation. The nucleation consists primarily in the creation of nuclei, and secondly, their growth. When the temperature falls below the melting temperature, low enough that an interface between solid and liquid can be created, a nucleus is formed. Most nucleation models postulate that nucleation is a thermally activated process.[1] Once the nucleus reaches the critical size Rc , it can continue to grow. The decreasing atoms’ mobility at reduced temperatures contributes to the nucleation rate via the atomic variation frequency and the probability of capturing a new atom. The nucleation rate increases rapidly with rising undercooling. In the nucleation mechanism of equiaxed crystal formation, the flow plays an important role during the crystal growth, but is of no significance in nuclei creation phase. Through its interaction with the thermosolutal field, the flow impacts the size of the undercooled region, which needs to be sufficiently large to allow equiaxed crystal growth. In addition, flow also controls the transport of the nuclei. The generation of equiaxed crystals by fragmentation inside the mushy zone entails the fulfillment of three conditions. First the production of dendrite’s fragments in the mushy zone should take place. Secondly, a flow strong enough to penetrate the mush and carry these dendrite fragments in the bulk. The transformation of the fragments into equiaxed crystals is the last condition, i.e., the existence of a sufficiently large undercooled region ahead of the columnar front. Dendrite fragmentation could occur for different reasons and in many circumstances. Paradies et al.[2] performed experiments under forced flow convection (around 10 cm s1) in superheated SCN—acetone melt, under different system parameters (cooling rate, temperature difference between the chill walls, melt flow rate). The fragmentation rate (number of fragments counted per mm2 and per second) correlated with a higher velocity melt near the mushy zone. Small fragments less than 0.2 mm were used to measure the velocity melt near the mushy region (0.2 mm beyond the longest dendrite and 0.4 mm into the mushy region). The increase in fragmentation rate correlated with the direction of the fl
