Dendrite coarsening in directionally solidified Sn-36 at.%Ni peritectic alloy in the presence of the migration of secondary dendrite arms

During directional dendritic solidification of Sn-36 at.%Ni peritectic alloy, secondary dendrite arm migration caused by temperature gradient zone melting (TGZM) has been observed. Experimental and theoretical analyses demonstrate that melting/solidification of primary and peritectic phases caused by TGZM and peritectic reaction leads to acceleration of the coarsening process of secondary dendrite. The coarsening process has been characterized in terms of specific surface area (S_V) of secondary dendrites. By analyzing the variation of S_V with solidification time, it is found that although the coarsening process is restricted by peritectic reaction in isothermal condition, this retard of coarsening by peritectic phase is weakened by the TGZM effect in non-isothermal condition. The kinetics of peritectic reaction which is found to be crucial to determine the coarsening process is characterized by the reaction constant f which ranges from 0.3 to 0.7 with increasing growth rates.