Liquid morphology during diffusion-controlled liquid migration in thermal stabilization of directional solidification

Abstract

Thermal stabilization is crucial in directional solidification since the required melt concentration for the following directional growth is obtained through it. In the thermal stabilization stage, a mushy zone where the microstructure changes as thermals stabilization proceeds is formed. The melt concentration difference in liquid droplets in it can lead to diffusion-determined remelting/resolidification phenomena in the mushy zone in the imposed temperature gradient. This is also known as liquid migration by temperature gradient zone melting (TGZM) which can influence the melt concentration at the initial directional growth interface. However, in addition to the axial temperature gradient, a radial one is always present, especially in a high axial temperature gradient. Besides, although the liquid droplet was assumed to be elongated along the axial direction due to the difference in the remelting and resolidification rates, it has not been confirmed yet. Thus, the liquid migration in the presence of both axial and radial temperature gradients was analyzed in the Sn–Ni peritectic system (L + Ni3Sn2→Ni3Sn4) when an axial temperature gradient Ga of 40K/mm is imposed in this work. A diffusion-controlled analytical model was built to describe the liquid migration in the presence of both temperature gradients. Both the analytical prediction and experimental observation confirm that the morphology change of liquid droplets through its migration is not obvious by the TGZM effect. Instead, the growth of liquid droplets with increasing temperatures during their migration is more attributed to the conglomeration of them due to solute enrichment at the solid-liquid interface.