Solidification of undercooled molten Ni-based alloys

The effect of undercooling on grain structure is investigated in pure nicke l, Ni75Cu25, and DD3 single-crystal superalloy by employing the method of m olten salt denucleating combined with thermal cycling. Meanwhile, a compari son of factors that may be related to structure formation is performed and the difference in the refined structure between Ni75Cu25 alloy and DD3 sing le-crystal superalloy is explained. Only one grain refinement occurs at the critical undercooling in pure nickel, whereas two take place at both low a nd high undercoolings in Ni75Cu25 and DD3 single-crystal superalloy melts. The first grain refinement at low undercoolings mainly originates from dend rite 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) t echnique to further verify the recrystallization mechanism.