Microstructure evolution of undercooled iron-copper hypoperitectic alloy

Fe₅₀Cu₅₀ hypoperitectic alloy has been undercooled and rapidly solidified by glass fluxing technique and drop tube processing. It is found that in both procedures metastable phase separation occurs if this alloy melt is undercooled to a certain extent. In the glass fluxing experiment, the obtained maximum undercooling is 81 K, and the sample is macroscopically separated into one upper Fe-rich part and one lower Cu-rich part. These two parts subsequently exhibit secondary phase separation, which is characterized by Cu-rich blocks precipitated in the Fe-rich part and Fe-rich spheres dispersed in the Cu-rich part. During drop tube processing, a 'dendritic-partially separated-entirely separated-mixed structure' microstructural evolution takes place in the rapidly solidified particles. Macroscopic phase separation appears as the central Fe-rich part and surrounding Cu-rich part, while secondary phase separation is characterized by Cu-rich spheres distributed in the Fe-rich part and Fe-rich spheres scattered in the Cu-rich part. The primary γ-Fe dendrite growth is mainly controlled by solute diffusion and conspicuous solute trapping has taken place. As for the coalescence of separated Fe-rich droplets, the Stokes motion is dominant in glass fluxing experiment, whereas in drop tube it is weakened and the effect of Marangoni migration becomes evident.