Synergic effects of time dependence and thermodynamic driving on metastable phase separation of liquid Fe50Cu50 alloy

The synergic effects of time dependence and thermodynamic driving on the metastable phase separation of liquid Fe50Cu50 hypoperitectic alloy were explored with three kinds of experimental methods including differential scanning calorimetry (DSC), laser heating, and drop tube. The calculated incubation time indicated that the secondary Cu-rich liquid phase kept the priority to nucleate when alloy undercooling exceeded 28 K. The cooling rates in three kinds of experiments covered six orders of magnitude from 3x10-1 to 6x105 K/s, and resulted in wide range of phase separation time and globule migration velocity. The extent of phase separation was determined by the globule migration distance in the phase separation time. Under 0.33 and 0.83 K/s slow cooling rates in DSC experiments, liquid phase separation was dominated by Stokes motion, and extended phase separation time led to more complete macrosegregation. At a higher cooling rate of 1500 K/s in laser heating experiment, the enhanced Marangoni migration resulted in distinctive macrosegregation in short phase separation time. Once liquid phase separation occurred under microgravity state in drop tube experiment, the phase separation time was the crucial factor dominating microstructure evolution. Core-shell macrosegregation formed in medium size alloy droplets with sufficient phase separation time, while dispersed structure appeared in small droplets with reduced phase separation time. Peritectic structure arose again due to the extremely short phase separation time in tiny alloy droplets.