Concentration and fluid flow effects on kinetics, dendrite remelting and stress accumulation upon rapid solidification of deeply undercooled alloys

Upon free solidification of a deeply undercooled melt, the solute diffusion in both the interface and the bulk liquid is far from equilibrium, and the concentration and the fluid flow may also play an important role in the solidification. Thus, under such conditions, assumptions about local equilibrium, ideal dilute solution solidification and solidification free of fluid flow can no longer be valid. In the present work, first, in order to reveal the concentration effect, we compared the results of the non-equilibrium dendrite growth models describing the solidification of dilute and non-dilute undercooled melts. It was found that under local non-equilibrium conditions, the predicted results of the non-dilute solution model and the dilute solution model are to a certain extent different from each other at the intermediate undercooling range. It was also found that the concentration effect could substantially decrease the relaxation effect. Second, we considered the effect of fluid flow on the rapid solidification of an undercooled melt. It was found that the fluid flow would affect not only the size of the dendrite tip radius, mainly at the small undercooling range, but also the dendrite growth velocity at the intermediate undercooling range. In particular, fluid flow makes the sizes of the dendrite tip radius at high undercooling close to those predicted by the dilute model. Thus, fluid flow could make the non-dilute solution exhibit dilute solution solidification behaviors. It was also found that fluid flow reduces the relaxation effect to some extent. Considering the fluid flow effect, we used an extended chemical superheating model to predict the dendrite remelting phenomenon of the non-dilute melt. The model predicted that the dendrite remelting phenomenon would abruptly disappear once the dendrite growth velocity exceeded the solute diffusion velocity in a bulk undercooled melt. Third, considering the fluid flow effect, we used the concept of equivalent undercooling and a recently developed physical model to calculate the stress accumulation during rapid solidification. The results of this new model could explain well the stress-induced dendrite breakup mechanism of grain refinement at high undercooling.