Solidification of undercooled Fe-Cr-Ni alloys: Part I. Thermal behavior
- PDF / 848,500 Bytes
- 9 Pages / 598 x 778 pts Page_size
- 3 Downloads / 207 Views
I.
INTRODUCTION
PHASEselection and microstructural stability in ferrous alloys are issues of theoretical and practical interest in rapid solidification research. Selection between body-centered cubic (bcc) (ferrite) and face-centered cubic (fcc) (austenite) during rapid solidification has been widely studied, particularly in the Fe-Ni alloy system beginning with the work by Cechv] in Fe-29.5 wt pct Ni powders. Later, the potential for metastable bcc solidification was indicated theoretically in hyperperitectic Fe-Ni alloys undercooled below the liquidus line extension of the bee phase, t2] An excellent summary of previous experimental work on phase selection in Fe-Ni alloys is given in Reference 3. Despite many studies, actual experimental evidence of metastable bcc solidification is limited.V.4J In addition, nearly all of the evidence has been obtained in solidification of fine powders.t3] Thoma and Perepezko[3] point out that the metastable bcc is readily retained in the final structure of rapidly cooled fine powders but not in the final structure of slowly cooled bulk specimens, thus making it difficult to develop a full understanding of the solidification process in bulk Fe-Ni alloys. They considered both the solidification path and subsequent solid-state transformations and concluded metastable bcc solidification occurred in specimens of a range of different sizes in their study. We seek in this work to present direct evidence of that metastable bcc solidification in alloys whose equilibrium structure is fcc. Munitz and AbbaschiantS] quenched bulk specimens of hyperperitectic Fe-Ni alloys from the undercooled liquid state. Dendrite cores were found to be enriched in nickel when the specimens were undercooled below the To for fcc TOSHIHIKO KOSEKI, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, is Senior Researcher, Steel Research Laboratories, Nippon Steel Corporation, Futtsu, Chiba 293, 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 November 1, 1994. METALLURGICALAND MATERIALS TRANSACTIONS A
but above the TOfor bcc. On the other hand, the enrichment was not found when the specimens were undercooled below the TO for bcc. They concluded that the former solute-enriched dendrite cores resulted from the partitionless solidification of primary fcc, and the latter were due to the primary metastable bcc solidification. In other words, metastable bcc solidification was considered to be attainable when the melt specimens were undercooled below the To for bcc. However, since nucleation occurred during quenching, actual nucleation temperatures and subsequent thermal behavior were not measured. In some undercooling experiments,t6,7,81double (two-step) recalescence behavior was observed in hyperperitectic FeNi alloys. It was proposed that the double recalescence was caused by metastable bcc so
