Solidification Of Undercooled Ni-Pd Alloys

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 Ni-Pd alloy melts were undercooled by the glass fluxing technique in combination with cyclical superheating, and hypercooling was realized in some alloys. Metallographs of the Ni-Pd alloys at various undercooling were investigated systematically. The electron backscattered diffraction (EBSD) technology was applied to investigate the grain boundary misorientations of the solidification structure. The solute distribution within the crystal grain was analyzed by energy dispersive X-ray. The recrystallization process in the solid formed at high undercooling was investigated by quenching the sample at different cooling stages. On the basis of the experimental results and theoretical analyses, crystal growth and structure formation during solidification of the Ni-Pd alloys were discussed, and the influence factors were revealed.The enthalpy of fusion of the Ni-Pd alloy decreases with the increasing solute content. As a result, the Ni_100-xPd_x(x=25, 45.2 and 45.4)alloys were undercooled beyond their hypercooling limits in the experiment. It is demonstrated that crystals grow in a dendritic form in the hypercooled alloy melts, and solute redistribution still exists in the hypercooled solidification.With increasing undercooling, the microstructure of the 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 Ni_54.6Pd_45.4, Ni and Pd, however, grain refinement only occurs at high undercooling. In the refined grain at low undercooling, there is only one or two dendrite arms observed and the Pd content at the grain boundary is always higher than inside. Developed dendritic substructures exist in the refined grains at large undercooling, and they are correlated across the grain boundary. The Pd content at the grain boundary is not always larger than inside.When EBSD was applied to study the grain boundary misorientation of the refined grains, it is found that the grain-refined microstructure at low undercooling presents an nearly statistical distribution of grain boundary misorientation, and no twins were found. In the grain-refined microstructure at high undercooling, most of the grain boundaries have large misorientation angles, and twins were found. As a substructure, the dendrite in a refined grain at high undercooling exhibits no misorientation among its arms It is proposed that the grain refinement at low undercooling results from dendrite remelting driven by chemical superheating, and the grain refinement at high undercooling is from recrystallization. Experimental results demonstrate that the occurrence of the grain refinement at low undercooling is associated with the equilibrium solidification temperature range of an alloy. The fact that the grain refinement at low undercooling still occurs in the Ni_54.8Pd_45.2 alloy with an equilibrium solidification range of 5 K indicates that the lower limit of solidification temperature range for grain refinement may be very low.Recrystallization at high undercooling starts at the end of rapid solidification. The average grain size in the recrystallization structure first decreases to a minimum and then increases with the recrystallization proceeding. This indicates occurrences of the first and second recrystallization processes. A larger undercooling, a severer solute partition and a smaller solute diffusion coefficient will lead to a smaller grain size in the crystallization structure.