Growth kinetics and grain refinement mechanisms in an undercooled melt of a CoSi intermetallic compound

The melt of a congruently melting CoSi non-stoichiometric intermetallic compound was undercooled and solidified spontaneously. The recalescence processes were recorded by a high speed camera to measure the growth velocities and the as-solidified microstructures were analyzed by electron back-scattering diffraction (EBSD) to show the grain refinement mechanisms. The monotonous increase of the growth velocity with undercooling indicates that the effect of disorder trapping is insignificant. The microstructure changes from coarse dendrites to refined equiaxed grains and then to coarse dendrites. Spontaneous grain refinement at low undercooling can be described well by both the dendrite fragmentation model and the chemical superheating model. The formation of coarse dendrites instead of refined equiaxed grains at high undercooling is ascribed to the considerable decrease of the dendrite remelting fraction. A transition from dendrites to dendritic seaweeds occurs at high undercooling. Regarding that the formation of dendritic seaweeds at high undercooling does not lead to spontaneous grain refinement, it cannot be the origin of grain refinement even in the case that dendritic seaweed splits is possible to assist the grain refinement process. The current work could be helpful for controlling the non-equilibrium microstructures and improving the mechanical properties of intermetallic compounds.