Structural Evolution And Metastable Phase Formation Of Ni-Si Eutectic Alloy Under High Undercooling/Superundercooling Conditions

In this paper, the high undercooling was achieved, in bulk eutectic Ni-Si alloy melts by the combination of molten-glass and cyclic superheating, and the undercoolings are controlled by stimulating the nucleation with a nickel needle at top of the sample. Based on the classical nucleation theory and transient nucleation theory, the competitive nucleation between the metastable and stable phases has been calculated, and the critical undercooling of forming metastable phase is obtaioned. The relation of microstructure evolution vs undercooling in Ni-Si eutectic alloy was systematically investigated. The directional growth under different undercooling was analyzed on the foundation of free dendrite growth theories (LKT/BCT). The process of microstructure evolution and metastable phase formation was analyzed with chemical method, SEM, energy spectrum (EDS) and X-ray diffraction technique. The main results are as follows:(1) A large undercooling level up to 550k (0.386Te) was firstly achieved in bulk eutectic Ni₇8.6Si₂1.4 melt by the combination of molten-glass and cyclic superheating under argon shield.(2) With the increase of undercooling, the sequence of the metastable phases formation in the undercooled Ni₇8.6Si₂1.4 eutectic melt is: β2 -Ni₃3Si Ni₃1Si₁2 Ni₃Si₂ andNi₃.04Si₀.96, and these metastable phases can be kept in room-temperature.(3) Directional columnar crystals structure is obtained at the undercooling of 72K and 195K, respectively. Based on the (BCT) model, the growth process of directional columnar crystals is analyzed and the relationship of the dendrite growth velocity, dendrite tip radius, thermal undercooling, solutal undercooling, curvature undercooling and interfacial kinetic undercooling vs. various initial undercooling is obtained.(4) When undercoolings exceed 390K, the morphology of a (Ni) phase transits from the non-faceted phase to faceted phase that is found for the first time. The abnormal phenomenon is explained with new crystals growth theory and experiment result.(5) When alloy melts was hyper-undercooling solidification, the highly fine microcrystalline structure was obtained. The refinement mechanism of crystal grain under hyper-undercooling is discussed. The sudden refinement of crystal structure is determined by the high nucleation rate under large undercooling.(6) Based on the classical nucleation theory and transient nucleation theory, the calculation results of competitive nucleation (between stable phase and metastable phases, metastable phase and metastable phases) indicate that it would occur competitive nucleation in Ni₇8.6Si₂1.4 alloy melts with the increase of undercooling. However, the calculation results of transient nucleation theory are more agreeable with the results of experiments. The fact indicates that the dynamics is more precise than thermodynamics for predicting the competitive nucleation in undercooled melts.