Applying fluxing method, hypercooling was achieved in Co80Pd20 alloys. The rapidly solidified microstructures and substructures of hypercooled Co80Pd20 alloys, subjected to rapid quenching and natural cooling after recalescence, have been studied. Transmission electron microscopy was used to investigate the substructures such as dislocations in the as solidified microstructures. X-ray diffraction line profile analysis was used for illuminating and determining the microstrain due to rapid solidification in the as solidified microstructures. In the undercooling range studied, the microstrain in the rapidly solidified structures increases with increasing undercooling. Quenching immediately after rapid solidification is helpful to preserving the microstrain in the microstructures.
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 (Δ T), two grain refinement events were observed. The first event happens in the range of 50 K<Δ T<265 K, while the second event takes place at Δ T>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<Δ T<280 K, the microstructure is occupied by a mixture of coarse dendrites with equiaxed grains. When Δ T 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.
Applying the fluxing method, hypercooling was achieved in Co80Pd20 alloys. After recalescence, the rapidly solidified microstructures of hypercooled Co80Pd20 alloys were subjected to rapid quenching. Applying transmission electron microscopy, high densities of substructures, e.g. dislocations and subgrains, and migration of high angle grain boundaries suggest the occurrence of partial recrystallisation in the hypercooled Co80Pd20 alloys.
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