Rapid solidification microstructure evolution and grain refinement of deeply undercooled nickel alloys

Abstract

High undercoolings of Ni75Cu25, Ni70Cu30 and Ni65Cu35 alloys were achieved by combined methods of cyclic superheating and molten glass purification. The average grain size of the undercooled alloys was decreasing with increasing Cu atomic percent. In the obtained undercooling ranges, it could be found that there were two grain refinement events at low undercoolings and high undercoolings. The grain refinement at low undercoolings was caused by dendrite remelting, while high-angle grain boundary and annealing twins indicated that recrystallization had occurred at high undercoolings. Applying TEM method, it was found that many dislocations and twins were present in the substructure at high undercoolings. The microhardness of Ni65Cu35 alloy with undercooling was sharply decreased at the critical undercooling confirmed that the stress accumulated in the microstructure at high undercoolings was consumed due to recrystallization process.