Microstructure refinement mechanism of undercooled Cu55Ni45alloys

Different undercooling degrees of Cu55Ni45 alloy were obtained by the combination of molten glass purification and cyclic superheating, and the maximum undercooling degree reached 284 K. The microstructure of the alloy was observed by metallographic microscope, and the evolution of microstructure was studied systematically. There are two occasions of grain refinement in the solidification structure of the alloy: one occurs in the case of low undercooling, and the other occurs in the case of high undercooling. Electron backscatter diffraction (EBSD) technology was used to analyze the rapid solidification structure under high undercooling. The features of flat polygonal grain boundary, high proportion of twin boundary, and large proportion of large angle grain boundary indicate recrystallization. The change in microhardness of the alloy under different undercooling degrees was studied by microhardness tester. It was found that the average microhardness decreased sharply at high undercooling degrees, which further confirmed the recrystallization of the solidified structure at high undercooling degrees.

Grain refinement mechanism of rapidly solidified nickel alloys

The solidification microstructure evolution of Ni-25 at.% Cu alloys under different undercooling degrees were studied by the cladding method and cyclic superheating method. Two grain refinement phenomena were observed in the obtained undercooling. In the low undercooling condition, dendrite remelting is the main reason for grain refinement in the recalescence process, while in the high undercooling condition, the stress accumulated in the recalescence process leads to recrystallization in the later stage of recalescence. Under the condition of high undercooling, the solidification structure is composed of complete equiaxed grains with relatively uniform grain size, which indicates that grain boundary migration occurs during grain growth.

Grain Refinement and Solidification Velocity of Supercooled Melts

Using a high speed thermal imaging camera, solidification velocity of undercooling is accurately determined for a highly undercooled Ni70Cu30 binary concentrated single phase alloy. It is observed that the velocity of solidification is first continuously and smoothly increasing at the low and mediate undercoolings then increasing discontinuously to a maximum value at the high undercooling range. Thus, the solidifcation velocity-undercooling relation has an abrupt discontinuity, which is caused by nonequilibrium diffusion of solutes in the liquid side near the advancing solid liquid interface. Grain refinement of high undercooling is directly induced by recrystallization, which was in clear contrast with the remelting induced dendrite fragmentation in conventional alloys.

Effects of High-Density Electric Current Pulse on the Undercooling of Fe-B Eutectic Alloy Melt

The solidification microstructure of Fe-B eutectic alloy under high undercooling and high-density electric current pulse (ECP) was investigated with the technique of molten glass slag purification combined with cyclical superheating and the ECP treatment. The effects of high-density ECP on the undercooling of Fe-B eutectic alloy melt were analyzed by the DSC method. The analysis results showed that the solidification microstructure of Fe-B eutectic alloy under ECP was similar to that obtained by the high undercooling technique. The undercooling obtained under two experimental conditions was basically the same, proving that the high undercooling of the metallic melt could be realized by the ECP.