Solidification of undercooled Fe-Cr-Ni alloys: Part II. Microstructural evolution
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I.
INTRODUCTION
IN a recent article,tq the authors described thermal behavior during solidification of five undercooled Fe-Cr-Ni alloys. Compositions of the alloys investigated are shown on the ternary phase diagram (liquidus surface) of Figure l(a) and on the 70 wt pct Fe isopleth of Figure l(b). "Double recalescence" was found in that workV] to occur in primary fcc alloys 1 and 2 when the melt undercooling prior to nucleation exceeded some given amount. It was concluded that metastable bcc phase formed as primary phase during the first recalescence, and stable fcc during the second recalescence. In alloy 3, near the eutectic line, double recalescence was not observed. It was nonetheless concluded that the primary phase was again metastable bcc, since a second recalescence implying the bccto-fcc transformation occurred after the first recalescence was complete, during subsequent cooling. Alloys 4 and 5 showed a similar second recalescence on cooling when the temperature dropped from that where bcc was the stable phase to the region where fcc was the stable phase. At the higher undercoolings in alloys 3, 4, and 5, the bcc to fcc transformation appeared to be massivelike. In this article, microstructures of these five alloys are presented confirming and extending the preceding conclusions. Microstructures described are from samples from which the thermal measurements of the previous article were made and from samples which were rapidly quenched at various stages of the solidification process.
T. KOSEKI, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, is Senior Researcher, Steel Research Laboratories, Nippon Steel Corporation, Chiba 293, Japan. M.C. FLEMINGS, Toyota Professor of Materials Processing, is wtth the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted March 6, 1995. 322~-VOLUME 27A, OCTOBER 1996
II. SOLIDIFICATION BEHAVIOR AND MICROSTRUCTURE OF Fe-Cr-Ni ALLOYS IN CONVENTIONAL SOLIDIFICATION PROCESSES WITH LITTLE MELT UNDERCOOLING Prior to describing the experimental results, the solidification behavior and resulting microstructures of Fe-Cr-Ni alloys in conventional solidification processes are briefly reviewed. It is useful for the interpretation of and comparison with rapidly solidified structures obtained in this study. Solidification of austenitic stainless steels, i.e., Fe-based Fe-Cr-Ni alloys, has been studied over the last two decades in conventional casting and welding processes and is classified into the four modes mentioned subsequently, depending on the primary phase and subsequent solid-state transformation,t3~1 They are (a) fully fcc solidification (A mode); (b) primary fcc, plus eutectic reaction (fce + bcc) at the terminal stage (AF mode); (c) primary bcc, plus eutectic reaction (bcc + fcc) at the terminal stage (FA mode); and (d) fully bcc solidification (F mode). Primary fcc solidification is favored by an increase in fcc forming elements su
