Solidification Of Undercooled Ni_54.6Pd_45.4 Alloy

Undercooling technique make it possible for a bulk alloy melt to rapidly solidify even if the cooling rate is not too high. If the alloy melt can be undercooled up to such a degree that the system cannot be reheated above the solidus temperature during the adiabatic recalescence, i.e. has been hypercooled, the entire liquid will fully solidify rapidly. As slow solidification is absent, effects of superheating and ripening on the disintegration of the primary solid can be excluded from the analysis of the structure formation. To examine the solidification structure of the hypercooled alloy is therefore considerably helpful for revealing the crystal growth behavior and structure evolution during solidification.Bulk Ni54.6Pd45.4 alloy melts were undercooled by the glass fluxing technique in combination with cyclical superheating, and hypercooling was realized in this alloy. Metallographs of the Ni54.6Pd45.4 alloy at various undercooling were investigated systematically. The recrystallization process in the solid formed at high undercooling was investigated by quenching the sample at different cooling stages.With increasing undercooling, the microstructure of the single phase alloy with an equilibrium solidification temperature range undergoes two grain refinements, one of which occurs in a low-undercooling range, and the other above a high critical undercooling. For the melts without equilibrium solidification temperature, such as Ni54.6Pd45.4, however, grain refinement only occurs at high undercooling. Developed dendritic substructures exist in the refined grains at large undercooling, and they are correlated across the grain boundary.The structure evolution driven by recrystallization in the bulk undercooled Ni54.6Pd45.4 alloy was investigated by quenching the sample at different cooling stages. It is found that recrystallization of the solid beyond the critical undercooling for grain refinement starts at the end of rapid solidification, and develops during the slow solidification and the subsequent cooling process. At the initial stage of recrystallization, nuclei successively form in the sample, leading to a decrease in grain size. At the later stage, annexation of grains dominates the recrystallization, due to which the grain size increases. As undercooling prior to solidification increases, the deformation degree in the rapidly solidified dendrite net rises, and the recalescence temperature as well as the duration at high temperature decreases. Consequently the grain size of the recrystallization structure decreases.