Verification of metastable phase diagrams by in situ measurements on undercooled melts

The electromagnetic levitation technique combined with pyrometric methods have been applied to verify the metastable phase diagram of Fe-Cr-Ni alloys by in situ observation of phase selection processes in undercooled melts. The temperatures of levitated drops prior to solidification were determined by a two-color pyrometer and the recalescence behavior of the undercooled melt was recorded from a high-speed photosensing device with a sampling rate of 1 MHz. Fe69Cr31-xNix alloy melts with 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 observations. 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.