Competitive Growth Kinetics Between Primary Dendrites and Pseudobinary Eutectics Within Liquid CoCrFeNiSn Quinary Alloy

Bulk liquid CoCrFeNiSn quinary alloy has been undercooled by up to 297 K (0.2T_L) for exploring its rapid solidification kinetics and associated crystalline orientation features. The constituent phases of this alloy were determined as a FCC c-Co₃Cr₃Fe₃Ni solid solution and a hexagonal g-(Ni₂Co, Cr, Fe)₃Sn₂compound. At small undercooling primary c phase dendritic growth prevailed and interdendritic g compound appeared as a minor phase. Once liquid undercooling exceeded a threshold of 216 K, the eutectic growth between c and g phases was induced and led to the formation of refined lamellar eutectic structures. As a comparison, solidification analyses based on small alloy droplets were also conducted to investigate the crystallization behaviors under the conditions of larger undercooling and cooling rate, which revealed a similar microstructural transition from coarse primary dendrite to refined dendrite and finally into lamellar eutectic structure. In particular, complete eutectic growth could be achieved when droplet diameters further decreased below 97 lm, and lamellar eutectic structures became dominant uniquely. As undercooling increased further, remarkable grain refinement was accomplished in both conditions. Electron backscatter diffraction (EBSD) characterizations demonstrated that refined c phase dendrites exhibited distinct polycrystalline structure and contained a number of twin crystals. For lamellar structures, eutectic growth mode would result in the preferential orientation relationship of {110}_c//{0001}_gand h332i_c== h1120i_gbetween the two constituent phases.