Solidified morphology and mechanism of bulk nanocrystalline Fe80P13C7 alloy at isothermal undercooling

The molten Fe80P13C7 alloys can achieve a large undercooling up to 320 K by fluxing technique. With the help of fluxing technique, the molten Fe80P13C7 alloys can be solidified at different undercooling (ΔT) through isothermal undercooling experiment. It is indicated that the microstructure of the solidified Fe80P13C7 alloy specimens is refined significantly with the increasing undercooling and the grain size is about 20 μm, 10 μm, 200 nm and 70 nm for ΔT=50 K, 150 K, 250 K and 320 K, respectively. The solidification morphologies of the solidified Fe80P13C7 alloy specimens under different undercooling are quite different. When ΔT=50 K, it presents a traditional solidification microstructure under a undercooling condition, composed of the primary dendrite and anomalous eutectic within the dendrites. When ΔT=150 K, a cell-like solidification morphology can be found, which can be proposed to be formed based on the nucleation and growth of spinodal decomposition mechanism. When ΔT=250 K, there is a strong direction of the solidification under an optical micrograph, two zones can be divided, and the microstructure of each zone presents a network which results from a liquid spinodal decomposition. When ΔT=320 K, the microstructure presents a random network completely. Microhardness test shows that the hardness of the solidified specimens increases with the undercooling.