Microstructure formation and transition of undercooled bulk Ni70.2Si29.8 eutectic alloy melt were investigated by melt fluxing,cyclical overheating and cooling under high-frequency vacuum melting.The maximum undercooling of the alloy melt amounted to 428 K.Scanning electron microscope(SEM),energy-dispersive X-ray spectroscopy(EDS) and optical microscopy techniques(OM) were adopted to investigate the microstructure and identify the phase composition.The cooling curves of eutectic alloys upon solidification which were subjected to different undercoolings were described and compared.The complex microstructure evolution was observed in the as-solidified samples with the increase of undercooling.Surprisingly,an extremely fine microstructure was achieved at the max undercooling of 428 K,and the lamellar distance of about 50-100 nm was observed.Based on the solution entropy of eutectic phases,the microstructure transition with the undercooling was analyzed.Calculated results showed that the microstructure transition process was ascribed to solution entropy of transition,i.e.,the complex microstructure evolution was attributed to a transition from faceted-faceted(FF)→faceted-nonfaceted(FN)→nonfaceted-nonfaceted(NN) eutectic systems concurring with increased undercooling. Microstructure formation and transition of undercooled bulk Ni70.2Si29.8 eutectic alloy melt were investigated by melt fluxing, cyclical overheating and cooling under high-frequency vacuum melting. The maximum undercooling of the alloy melt amounted to 428 K. Scanning electron microscope (SEM) , energy-dispersive X-ray spectroscopy (EDS) and optical microscopy techniques (OM) were adopted to investigate the microstructure and identify the phase composition. The cooling curves of eutectic alloys upon solidification which were subjected to different undercoolings were described and compiled. complex microstructure evolution was observed in the as-solidified samples with the increase of undercooling. Surprisingly, an extremely fine microstructure was achieved at the max undercooling of 428 K, and the lamellar distance of about 50-100 nm was observed.Based on the solution entropy of eutectic phases, the microstructure transition with the undercooling was analyzed. Calculated results showed that the m icrostructure transition process was ascribed to solution entropy of transition, ie, the complex microstructure evolution was attributed to a transition from faceted-faceted (FF) → faceted-nonfaceted (FN) → nonfaceted-nonfaceted (NN) eutectic systems concurring with increased undercooling.