NUCLEATION AND METASTABLE PHASE-FORMATION IN UNDERCOOLED FE-CR-NI MELTS

The non-equilibrium solidification behaviour of undercooled Fe-Cr-Ni melts is analysed theoretically with respect to the competitive formation of b.c.c. and f.c.c. phases. Nucleation as well as dendrite growth processes in undercooled melts are considered. Thermodynamic modelling of homogeneous nucleation as a function of the melt undercooling revealed that the b.c.c. phase may be promoted even in primary f.c.c. type solidification alloys. The homogeneous nucleation temperatures for different cooling conditions are calculated as functions of the alloy composition. Containerless solidification experiments, electromagnetic levitation melting as well as drop-tube solidification experiments of gas atomized droplets, confirmed the predicted tendency of metastable b.c.c. phase formation. In order to prevent the decay of metastable phases by solid state transformations, high cooling rates of samples were achieved by applying the new technique of liquid metal quenching of as-solidified levitated drops. Calculated dendrite growth velocities agree reasonably with experimentally determined values up to a critical undercooling of approximately 150 K. For selected alloy compositions, transitions between the fastest dendrite growth modes as a function of undercooling from b.c.c. to f.c.c. and vice versa were derived from dendrite growth theory. The implications of the present results for rapid solidification processing of stainless steels are discussed briefly.