On the mechanism of grain refinement in undercooled molten metals.

摘要

It was shown that the microstructures of an undercooled specimen depends on the initial bulk undercooling defined as DT=Tm-Tk where Tm is its thermodynamic melting temperature and Tk is the kinetic crystallization temperature. One example is the phenomenon of grain refinement in a highly undercooled metallic melt, which describes a sudden decrease in the average grain size of the undercooled specimen in a narrow undercooling range deep in the undercooling regime.; Over the years, many proposals have been put forward to explain the underlying grain refinement mechanism. They include dynamic nucleation, dendrite remelting, and dendritic break-up by interdendritic fluid flow. However, direct experimental support of these models is not available.; The objective of this study is to understand the mechanism of grain refinement of highly undercooled molten metals. It was found that there are two different mechanisms, one for nominally pure metal and the other one for alloys. In pure Ni system, grain refinement of highly undercooled Ni is brought about by dynamic nucleation and cavitation is the origin of dynamic nucleation. On the other hand, grain refinement in highly undercooled CU30Ni 70 is brought about by a multiplication of novel dendrite which is unstable against remelting. The formation of the novel dendrite is studied through Fe-Ni alloys.*; *Originally published in DAI Vol. 59, No. 9. Reprinted here with corrected author name.