NUCLEATION AND PHASE SELECTION IN UNDERCOOLED FE-CR-NI MELTS .2. CONTAINERLESS SOLIDIFICATION EXPERIMENTS

The solidification behavior of undercooled Fe-Cr-Ni melts of different compositions is investigated with respect to the competitive formatio n of delta-bcc (ferrite) and gamma-fcc phase (austenite). Containerles s solidification experiments, electromagnetic levitation melting and d rop tube experiments of atomized particles, show that delta (bcc) soli dification is preferred in the highly undercooled melt even at composi tions where delta is metastable. Time-resolved detection of the recale scence events during crystallization at different undercooling levels enable the determination of a critical undercooling for the transition to metastable bcc phase solidification:in equilibrium fcc-type alloys . Measurements of the growth velocities of stable and metastable phase s, as functions of melt undercooling prior to solidification, reveal t hat phase selection is controlled by nucleation. Phase selection diagr ams for solidification processes as functions of alloy composition and melt undercooling are derived from two types of experiments: X-ray ph ase analysis of quenched samples and in situ observations of the recal escence events of undercooled melts. The experimental results fit well with the theoretical predictions of the metastable phase diagram and the improved:nucleation theory presented in an earlier article. In par ticular, the tendency of metastable delta phase formation in a wide co mposition range is confirmed.