VERIFICATION OF METASTABLE PHASE-DIAGRAMS BY IN-SITU MEASUREMENTS ON UNDERCOOLED MELTS

The electromagnetic levitation technique combined with pyrometric meth ods have been applied to verify the metastable phase diagram of Fe-Cr- Ni alloys by in situ observation of phase selection processes in under cooled melts. The temperatures of levitated drops prior to solidificat ion were determined by a two-color pyrometer and the recalescence beha vior of the undercooled melt was recorded from a high-speed photosensi ng device with a sampling rate of 1 MHz. Fe69Cr31-xNix alloy melts wit h different Cr/Ni ratios were investigated for undercooling levels up to 320 K. The transition from single- to double-recalescence behavior was found beyond a critical undercooling level for primary austenitic alloys with Cr/Ni <1.5. For the first time, this gives direct evidence of metastable phase formation in this alloy system from ill situ obse rvations. The dendrite growth velocity displayed a sudden drop at the critical undercooling level, confirming the metastable phase formation . The intermediate arrest temperature of the double-recalescence event fits well with the calculated metastable liquidus line derived from a subregular solution model for the Gibbs free energy.