Solidification microstructure evolution and grain refinement mechanism under high undercooling of undercooled Ni90Cu10 alloys

The maximum undercooling of about 240 K of Ni90Cu10 alloys was achieved by using molten glass purifcation and cyclic superheating methods.The solidification structure of the undercooled Ni90Cu10 alloy under different undercooling was observed by optical microscope. The microstructure evolution of the undercooled Ni90Cu10 alloy under different undercooling was systematically studied, and the grain refinement structure was obtained under low undercooling and high undercooling,respectively. High-speed camera was used to capture the solidification changes of the solid-liquid interface, and it can be seen that the morphology of the solidification front was also different in different undercooling intervals.Moreover, with the difference of initial undercooling, the starting position of solidification nucleation point in the whole undercooled melt is also different.The driving force of recrystallization mainly comes from the deformation energy accumulated during the recalescence of undercooled melt. It was confirmed by optical microscopy and electron back scattering diffraction analysis that recrystallization under high undercooling is the main mechanism of grain refinement.