Evolution of solidification microstructure in undercooled Co 80 Pd 20 alloys

Applying fluxing method, Co₈₀Pd₂₀ alloy melts were undercooled up to 340 K. Evolution of the solidification microstructure of the undercooled alloys was investigated. Within the range of the achieved undercooling (ΔT), two grain refinement events were observed. The first event happens in the range of 50 K<ΔT<265 K, while the second event takes place at ΔT>280 K. Based on experimental investigations and theoretical calculations, the mechanisms of grain refinements were illuminated. The first grain refinement event is induced by the remelting effects involved in the post-recalescence period, resulting in the formation of a spherical grained microstructure. In the narrow transition range of 265 K<ΔT<280 K, the microstructure is occupied by a mixture of coarse dendrites with equiaxed grains. When ΔT is higher than 280 K, which is rather above the hypercooling limit, recrystallisation of the rapidly solidified solid happens and leads to the formation of an equiaxed grained microstructure.