Effects of Partition Coefficients, Diffusion Coefficients, and Solidification Paths on Microsegregation in Fe-Based Mult
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s a common defect phenomenon in continuous casting slab. Solute microsegregation is significant in that it produces solute-enriched liquid of low melting point in the form of interdendritic films, resulting in the compositional inhomogeneities, cracks, nonequilibrium phase, and the formation of inclusion, which lower the mechanical properties of the final product. Microsegregation is a significant problem in many metallic alloy systems.[1–4] Microsegregation is caused by the redistribution of solute during solidification, as solute is generally rejected into the liquid. Its fundamental cause is the difference of the thermodynamic equilibrium solubility of alloy elements in the different phases that coexist in
YUNWEI HUANG, MUJUN LONG, PENG LIU, DENGFU CHEN, HUABIAO CHEN, LINTAO GUI, TAO LIU, and SHENG YU are with the Laboratory of Metallurgy and Materials, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, P.R. China. Contact email: longmujun@ cqu.edu.cn Manuscript submitted March 2, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
the mushy region during solidification.[1] The solubility levels are affected by the following three factors. (1) Partition coefficients: The equilibrium partition coefficient, defined as the ratio of solute composition in the solid to that in the liquid at equilibrium during solidification,[3] is an expression of the relative affinity of a solute for the solid and liquid solvent phases. By defining the thermodynamic limit for solute distribution, it plays an important role in the redistribution of solute during solidification. The liquid in the vicinity of the solid/liquid interface is either enriched in (if kef < 1) or depleted of (if kef > 1) solute. As pointed out by Nastac and Stefanescu,[5] calculated microsegregation is closely related to the value of the distribution ratio used. Therefore, it is necessary to have an accurate value of this parameter for process modeling. (2) Diffusion coefficients: Diffusion coefficients have a direct effect on the solute diffusion transport within the solid phase. If the diffusivity is high, any distributing is removed as it occurs, and equilibrium freezing takes place. On the other hand, if the solute diffuses slowly in the solid phase, some of the segregation will still be present because of the inability of solid-state diffusion to fully return the composition to its equilibrium constant level after solidifi-
cation is complete. Clyne and Kurz (CK)[6] proposed that the effects of dendrite tip undercooling, extensive arm coarsening, dendrite geometry, and incomplete mixing in the liquid on solute distribution are subordinate to those of solid-state diffusion. The final liquid fraction achieves a solute concentration, CLf, which depends not only on the partition coefficient at the solid/liquid interface but also on the diffusion coefficients in the solid phases. (3) Solidification paths: During the solidification process of molten steel, the different solute contents directly determine the different paths of solidification, which are closely related t
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