Metastable phase formation in undercooled Fe–Co melts

Soft magnetic Fe–Co alloys display the primary fcc phase solidification for >18 at.% Co in conventional near-equilibrium solidification processes. Undercooled Fe–Co melt drops within the composition range of 70–50 at.% Fe have been investigated with the electromagnetic levitation technique. The solidification kinetics was measured in situ using a high-resolving Si photodiode. The melt drops were undercooled up to 263 K below the liquidus temperature and subsequently quenched onto a chill substrate in order to characterise the solidification sequence and the microstructure. The transition from stable fcc phase to primary metastable bcc phase has been observed after reaching a critical undercooling level which increases with rising Co content. The growth velocity drops obviously after transition to metastable bcc phase formation. Moreover, parabolic flight experiments were performed in order to study the phase selection under reduced gravity conditions. The investigation of the solidification events yielded a much smaller critical undercooling and an increased life time of the metastable bcc phase.