Morphology evolution of abnormal tertiary dendrite by diffusion-controlled remelting/resolidification in directionally solidified Sn-Mn peritectic alloy

Although the growth of tertiary dendrite is also important in influencing the properties of alloys, the analysis on it is relatively scarce since the growth of it is always assumed to catch up to the tips of growing primary dendrite. Distinct from this, the abnormal tertiary dendrites which grow towards lower temperature has been observed on thicker secondary branches during upward directional solidification of Sn-40at.%Mn peritectic alloy. The experimental result shows that the dendritic structure is influenced by not only the temperature gradient zone melting (TGZM) but also the Gibbs-Thomson (G-T) effects which can induce diffusion-controlled remelting/ resolidification processes. Further examination on this abnormal tertiary dendrite arm shows its gradual decomposition during directional solidification. Analysis indicates that the decomposition of it is closely related with these remelting/resolidification processes. The remelting rate at the back surface of the thicker secondary branch is found to be lower than that at the tip of abnormal tertiary dendrite. However, the remelting process is first finished on the back edge of thicker secondary branch where no abnormal tertiary dendrite exist due to the shorter length there. A diffusion-controlled analytical model incorporating peritectic reaction and both effects is established to analyze the decomposition process, and the Ivantsov's equation-based solutions have been introduced, which agrees reasonably well with the experimental observation.