Grain refinement and grain coarsening of undercooled Fe-Co alloy

A bulk Fe-30 at. %Co alloy melt is undercooled in molten glass slag to investigate microstructure evolution under various undercoolings. Within achieved undercoolings, there are two grain refinement processes and one coarsening process: the first refinement transition from coarse dendrite to granular crystals occurred in low undercooling range, the second one was the refinement of directional fine dendrite to sphere-like particles in a medium undercooling, and the last was grain coarsening with the increase of metastable dendrite cores in a large undercooling range. The mechanisms of both grain refinements and grain coarsening process are discussed. Grain refinements are both caused by dendrite remelting, whereas grain coarsening results from metastable phase dendrite cores, which increase interface energy of the system and lead to transfer of grain boundary and coalition of grain and conduce grain coarsening finally.