Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part II. Containerless solidification experiments
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I.
INTRODUCTION
CONTAINERLESS processing methods avoiding any contact of the liquid specimen with a solid wall are capable of establishing large undercooling levels in metallic melts.[1] This will allow metastable phases to form upon solidification of the samples in the same manner as in rapid solidification processes. The Fe-Cr-Ni alloy system, which represents the basis for the technically important stainless steels, is very convenient for investigations of phase selection in undercooled melts. The phase diagram and the thermal properties of the Fe-Cr-Ni alloy system are well established.[2] The primary solidification mode changes from the d-ferritic (bcc) to the g-austenitic (fcc) if the atomic fraction ratio Ni/Cr ratio exceeds '0.5 for nearequilibrium solidification conditions.[2] This has been confirmed for stainless steel welding processes.[3] Under rapid solidification conditions, phase selection is not solely determined by the alloy composition but on process parameters, too. In droplet processing techniques, the metastable d-ferrite phase seems to be promoted.[4,5,6] Kelly et al.[4] and Wright et al.[5] observed a change of the solidified phase in centrifugally atomized 303- and 304-type stainless steels from fcc for large particles to bcc for small particles. The same tendency was reported by MacIsaac et al.[6] for the solidification of 316-type austenitic stainless steel droplets embedded in a glassy matrix--a solidification technique allowing large undercooling levels but smaller cooling rates than atomization. Preliminary experiments on the phase se-
lection in levitated Fe69Cr312xNix samples with different Ni to Cr ratios showed that, in a wide composition range, metastable bcc solidification occurs if a critical undercooling level is exceeded, even at low cooling rates of the order of 10 K/s.[7] In Part I of this series of articles,[8] the preferred formation of the metastable bcc phase in undercooled Fe-Cr-Ni melts was predicted from a nucleation theory that improved previous theoretical attempts[4,9,10] by considering the effect of a finite liquid-solid interface thickness and nucleus composition deviations. The main focus of the present article is the experimental verification of the predicted phase selection in undercooled Fe69Cr312xNix melts for a wide range of compositions x by direct observation of the temperaturetime characteristics during the recalescence of levitated samples. The time-resolved recalescence profiles determined with a sampling rate of 1 ms also permit the measurement of the growth velocity of the primary phase as a function of melt undercooling. The comparison of the growth rate of stable and metastable phases allows predictions concerning the phase selection mechanism. As-solidified samples were immersed into a liquid-metal pool in order to minimize postsolidification transformations, and the stable and metastable phases were identified by X-ray diffraction (XRD). The results are compared with drop tube solidification experiments performed with identical alloy composition
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