Evidences of grain refinement by dynamic nucleation and by remelting in undercooled metals

Grain refinement exists generally in the rapid solidification of undercooled molten metals and semiconductors. Investigations about it have been wildly performed for about forty years. However, its mechanism has been uncertain and contradictory up to date. For example, experiments show that grain refinement is brought about by dynamic nucleation mechanism in pure Ni, but by re-melting mechanism in Ni₇₀Cu₃₀ alloy. It is uncertain which is the real mechanism in the Ni-based metallic system or whether there is more than one mechanism co-existing in the system. Even more, since a little amount of impurity was doped into pure Ni for preserving the microstructures of undercooled specimen after rapid solidification, the influence of solute on grain refinement in pure metals throws doubt in whether grain refinement occurs at all in pure metals.; To gain a clear understanding of what happened in metallic system and what was the influence of solute on grain refinement, this project investigated systematically grain refinement in metallic systems Ni_100-xB _x (x = 0, 0.10, 0.2, 0.30, 0.45, 0.50, 0.55, 0.60 at.%) by means of fluxing technique.; In systems Ni_100-xB_x (x = 0.25, 0.30, 0.45, 0.50, 0.55, 0.60 at.%), two grain refinement transitions are discovered: the first one sites in the regime of low undercooling and the second one in that of high undercooling. They co-exist in every system. The microstructures of the as-solidified specimens show that the first grain refinement is brought about by re-melting of dendrites and the second by dynamic nucleation. The characteristic microstructures of the two mechanisms are found in every system and illustrated by those of a typical system Ni_99.45B_0.55. With the decrease of concentration of solute B, the first grain refinement is seriously affected so that the features of the transition disappear almost in such a System Ni_99.75B_0.25. Whereas, the second grain refinement is not influenced at all, except a change of the value in critical undercooling ΔT*. In Ni_99.90B_0.10 and pure Ni systems, no grain refinement is displayed by the microstructures of the specimens because of serious grain growth at elevated temperature following the rapid solidification.; The specific volume of the systems Ni_100-xB_x (x = 0, 0.10, 0.25, 0.30, 0.45, 0.50, 0.55, 0.60 at.%) was measured with the change of undercooling. Two sharp transitions of specific volume were found in systems Ni_100-xB_x (x = 0.25, 0.30, 0.45, 0.50, 0.55, 0.60 at.%) but one that in Ni_99.90B_0.10 and pure Ni systems. Comparing the transitions of specific volume with those of grain refinement in the systems Ni_100-xB_x (x = 0.25, 0.30, 0.45, 0.50, 0.55, 0.60 at.%), it is found that the characters of these two kinds of transitions are agreed completely. Therefore, the volumetric transition in Ni_99.90B_0.10 and pure Ni systems uncovers the facts that grain refinement process indeed took play in these two systems during rapid solidification and was brought about by dynamic nucleation.