Eutectic growth mechanism and heat transfer dynamics of ternary Cr-Ni-Fe alloy during triple containerless processing

The rapid solidification of ternary Cr_49.5Ni_40.5Fe₁₀ alloy was realized by triple containerless processing techniques including free fall, falling quench and direct quench in order to systematically investigate its eutectic growth mechanism under different heat transfer conditions. Based upon Newtonian cooling model and classical heat transfer equation, the thermal information inside alloy droplets was obtained to derive the cooling rate, temperature difference and temperature gradient. After containerless solidification, the macroscopic segregation of primary δ phase was remarkably reduced owing to the microgravity effect and the spherically symmetrical temperature field within alloy droplets. Three typical eutectic structures were observed in the alloy droplets. With the increases of cooling rate, temperature difference and temperature gradient, the microstructures of alloy droplets evolved from coarse lamellar eutectic to refined anomalous eutectic and finally transformed to coarse anomalous eutectic because of the independent growth of δ and γ phases. Especially, under falling quench condition, a non-uniform eutectic microstructure appeared, which was refined gradually from the surface to the center of alloy droplets. This was attributed to the sudden changes of these three thermodynamic parameters once the alloy droplets were rapidly quenched into liquid coolant.