Thermophysical properties and rapid solidification mechanism of liquid Zr₆₀Ni₂₅Al₁₅ alloy under electrostatic levitation condition

Abstract

In this study, the thermophysical properties and rapid solidification mechanism of highly undercooled liquid Zr₆₀Ni₂₅Al₁₅ alloy were investigated through the electrostatic levitation technique. The maximum undercooling of this alloy has attained 316 K (0.25<i>T</i>_L). Both density and surface tension displayed a linear relationship with temperature, while viscosity demonstrated an exponential temperature dependence. When alloy undercooling was less than 259 K, two significant recalescence events were observed during solidification, corresponding to the formation of pseudobinary (Zr₆Al₂Ni + Zr₅Ni₄Al) eutectic and ternary (Zr₆Al₂Ni + Zr₅Ni₄Al + Zr₂Ni) eutectic. The growth velocity of the binary eutectic phase gradually increased with undercooling and reached the maximum value at an undercooling of 259 K. In contrast, once undercooling surpassed 259 K, a single recalescence event occurred, leading to the independent nucleation of all three compound phases from alloy melt and the rapid growth of a ternary anomalous eutectic structure. Notably, the growth velocity of the ternary eutectic phase exhibited a gradual decline with the continued increase in undercooling. This diminishing trend of the growth velocity suggests, hypothetically, that further increasing the undercooling might entirely suppress crystal growth dynamically at a threshold of 385 K. With classical nucleation theory and the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model, the onset of crystallization for the three phases was calculated, leading to construct a time-temperature-transformation (TTT) diagram. This diagram elucidates the competitive nucleation among the three phases in the undercooled melt. Both theoretical and experimental evidences revealed that Zr₆Al₂Ni phase primarily nucleated at lower undercooling levels, whereas the simultaneous nucleation of all three phases becomes feasible at higher undercooling.