Nonequilibrium solidification velocity, recalescence degree and grain refinement of highly undercooled Ni-based single-phase alloys

Abstract

The solidification velocity-undercooling relationship of infinite miscible solid solution Ni-Cu alloys was established. The research shows that the growth velocity follows four stages of “exponential growth—platform period—linear growth—platform period” within the undercooling range. The solute trapping occurs in the linear growth stage with a discontinuity velocity jump which makes the migration interface smooth and solidification uniform. According to the recalescence degree-undercooling function, the relationship among solute composition, non-equilibrium solidification degree and grain refinement was established. It is found that higher recalescence degree enhances the non-equilibrium solidification effect and the primary solid fraction formed is higher, which makes it easier for grain refinement to occur at high undercooling. By using EBSD characterization, the obvious misorientation and high-density twin boundaries were observed in the crystal structure at high undercooling. The microhardness decreases sharply after critical undercooling, and HRTEM analysis shows that the structural orientation of adjoining grains is totally different, which can provide comprehensive and reliable evidence for recrystallization mechanism.