The Investigation Of Rapid Solidification In Undercooled Ni-base Alloy And Subsequent Solid-state Transition Process

In this paper, a systematic investigation in purification process and micropurification mechanism of Ni-Sn alloy and Co Cr Cu Fe Ni-Sn high entropy alloy have firstly been performed, by employing glass slag purification incorporated with cyclic superheating techniques. The macro and micro structure evolution of the two undercooled alloys with undercoolings and the influence of post-recalescence process were investgated systematically by SEM, XRD and HRTEM methods. With a series of theoretical models, the mechanism of recrystallization at large undercoolings and the recrystal grain growth in post- recrystallization process were analyzed in theory. The main conclusions were as follows:1. Three times microstructure and morphology transitions of undercooled Ni-Sn alloy has happened in the obtained undercooling 36 K<ΔT<290 K. When 36 K≤ΔT<75 K, the solidification structure was made of staggered dendritic structure; When 36 K≤ΔT<75 K, the solidification structure was made of granular dendrite; When 145 K≤ΔT<200 K, the solidification structure was made of directional dendrite; When ΔT≥200 K, the solidification structure was made of granular grain.2. The direct evidence of recrystallization with high undercoolings(ΔT≥200 K) has been found, such as large-angle grain boundary and lattice defects. Combined with the BCT model, the classical nucleation theory and other models confirm that the granulation phenomenon of Ni-Sn alloy at large undercoolings is due to recrystallization which caused by plastic deformation within the organization.3. The process of grain growth and solute segregation accompanying recalescence of large undercooled Ni-Sn alloy has been calculated by thermal kinetic model of grain growth, and it is found that the fitting results are in good agreement with the experimental results. It is confirmed that the grain growth process in the later stage of recrystallization is affected by the coupling effect of recalescence, solute rejection and solute segregation. The decrease of grain boundary energy and the enhancement of thermal stability is due to the segregation of solute atoms into the grain boundaries in the process of rapid solidification.4. The Co Cr Cu Fe Ni-Sn high entropy alloy which in the range of 0-260 K undercoolings has been prepared by this experiment. Three phases like FCC1, FCC2, and FCC3 have been found, and the phase structure remains stable at difference undercoolings. It has been found that liquid phase-separation phenomenon caused by the segregation of Cu rich phase with the increasing of undercoolings, and until ΔT≥70 K the typical microstructure delamination phenomenon has been formed.