Effect of bulk undercooling on microstructure transformation mechanism of rapidly solidified nickel alloys

The microstructural evolution and mechanism of grain refinement during rapid solidification of Ni–Cu alloys were studied by molten glass purification and cyclic superheating. The maximum undercooling has achieved 320 K for Ni65Cu35, 295 K for Ni55Cu45 and 280 K for Ni50Cu50 alloys. Two mechanisms were found to be responsible for the grain refinements under small undercooling and large undercooling, respectively. The grain refinement at small undercooling is mainly caused by dendrite superheating and remelting, whereas that for large undercooling is mainly due to recrystallization occurred at high angle grain boundary and high proportion of twin boundaries. Meanwhile, almost no dislocations and other defects were observed in the substructure of the alloy with large undercooling.The average microhardness was found a sharp decrease in the alloy with large undercooling, which indicates that the plastic strain accumulated in the microstructure formed under large undercooling is completely dissipated in post-recalescence, and the deformation energy can offer the driving force for recrystallization in the microstructure.