Abstract
The effect of undercooling on grain structure is investigated in pure nickel, Ni75Cu25, and DD3 singlecrystal superalloy by employing the method of molten salt denucleating combined with thermal cycling. Meanwhile, a comparison of factors that may be related to structure formation is performed and the difference in the refined structure between Ni75Cu25 alloy and DD3 single-crystal superalloy is explained. Only one grain refinement occurs at the critical undercooling in pure nickel, whereas two take place at both low and high undercoolings in Ni75Cu25 and DD3 single-crystal superalloy melts. The first grain refinement at low undercoolings mainly originates from dendrite remelting driven by the chemical superheating produced in recalescence, and the second one at high undercoolings is due to the recrystallization process as a result of the high stress provided in the rapid solidification after high undercooling. Dislocation morphology evolution in as-solidified structure is also provided by the transmission electron microscopy (TEM) technique to further verify the recrystallization mechanism.
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