Solidification of undercooled Fe-Cr-Ni alloys: Part III. Phase selection in chill casting
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IN two recent articles,[1,2] the authors described thermal behavior and structural evolution in the solidification of undercooled Fe-Cr-Ni alloy droplets. Melting and solidification were carried out using electromagnetic levitation and subsequent gas cooling. Both thermal behavior and solidification structures showed that primary bcc solidification was dominant in the undercooled Fe-Cr-Ni alloys, even in the primary fcc alloys. It was concluded that the phase selection was determined at the nucleation stage due to the ease of bcc nucleation from the undercooled melts. Similar results and conclusions have been recently reported by Volkmann et al.[3,4] In contrast to those findings, some solidification processes produce preferential fcc solidification in similar alloy compositions. Vitek and co-workers[5,6] investigated the solidification of several commercial stainless steels using laser beam melting and splat quenching techniques. They found that primary fcc solidification became favored in primary bcc alloys with increasing cooling rates. Elmer et al.[7] examined the effect of solidification rate on the structure of Fe-25 wt pct Cr-Ni ternary alloys using an electron beam melting technique, and confirmed these results. It was shown that fcc dendrites selectively grew from the ‘‘matching substrate’’ even if the ‘‘substrate’’ consisted of an fccbcc duplex structure. A similar tendency was observed in laser beam surface melting[8,9] and in capacitor discharge welding.[10] Mizukami et al.[11] investigated the casting of type 304 stainless steel on copper chill, and found metastaTOSHIHIKO KOSEKI, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, is Senior Researcher, Oita R&D Laboratory, Nippon Steel Corporation, Oita 870, Japan. MERTON C. FLEMINGS, Toyota Professor of Materials Processing, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted September 11, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
ble fcc solidification to occur at the chill surface. The prevalence of fcc solidification in chill casting and substrate quenching was also reported in Fe-Ni alloys.[12,13] In the case of laser and electron beam welding, it is obvious that the phase selection is controlled by growth, since there is no nucleation barrier for solidification. Vitek et al.[5] presumed that the melt ahead of growing dendrites was undercooled below the T0 for liquid/fcc, so that fcc growth became feasible. Bobadilla et al.[8] attempted to explain the preferential fcc growth by the use of the Kurz–Giovanola– Trivedi (KGT) model,[14] a dendrite-tip model for constrained growth. The tip temperature of an fcc dendrite was estimated to be higher than that of a bcc dendrite with increasing growth rates, even in a primary bcc alloy composition. They therefore concluded that fcc growth is dominant over bcc growth at high growth rates in the presence of a positive temperature gradient ahead of t
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