Experimental Evidence of the Effect of Alloying Additions on the Stagnant Stage Length During Cyclic Partial Phase Trans
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nt decades, kinetics of the austenite-to-ferrite transformation in steel have been widely studied because of their practical importance in advanced steel design and production.[1–6] Despite abundant effort, the fine details of alloying element partitioning behavior[7–17] at the local migrating interface are still not well understood. Recently, a cyclic phase transformation experimental approach was applied to study the growth kinetics of the austenite-to-ferrite transformation in the Fe-0.023C0.17Mn (all in wt pct).[18] A distinct ‘‘stagnant stage,’’ during which the degree of transformation does not vary, but the temperature changes, was observed. It was further proven by high-temperature laser scanning confocal microscopy that the stagnant stage is indeed due to the interface temporarily being rendered immobile instead of incubating nucleation.[19] The stagnant stage was well described by the local equilibrium (LE) model,[20,21] while the paraequilibrium (PE) model[22,23] was not capable of describing the stagnant stage, although PE condition was thermodynamically expected at the imposed experimental conditions. In a following paper,[24] using the LE model, the kinetics of the cyclic HAO CHEN, Postdoc Researcher, and SYBRAND VAN DER ZWAAG, Professor, are with the Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands. Contact e-mail: [email protected] ROMAN KUZIAK, Professor, is with the Institute for Ferrous Metallurgy, ul. K. Miarki 12, 44-100 Gliwice, Poland. Manuscript submitted August 29, 2013. Article published online October 16, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
phase transformations were simulated for a series of ternary Fe-0.02C-xM and quaternary Fe-0.02C-0.2MnxM alloys. It was predicted that the length of the stagnant stage increases linearly with the increasing concentration of alloying element M (except Co), and that the effects of substitutional alloying elements are additive in the quaternary alloys. Stagnant stage is of great interest both fundamentally and practically. From a fundamental point of view, the stagnant stage can be used as an effective tool to discriminate the existing phase transformation models, and to check whether alloying elements locally partitioning or not at the migrating interfaces. From a practical point of perspective, it can be applied to suspend ferrite formation upon cooling during the steel processing. In order to better understand the stagnant stage, in the current study, dedicated cyclic experiments are performed in a series of Fe-C-xMn, Fe-C-Mn-xNi, and Fe-C-Mn-xCo alloys to investigate the effect of alloying additions on the stagnant stage length. The materials studied here are a set of Fe-0.1C-xMn (x = 0.25, 0.5, 1.0, all in wt pct), Fe-0.1C-0.25 Mn-xNi(x = 0.25, 0.5, 0.75, all in wt pct) alloys, and two Fe-C-Mn-Co alloys (Fe-0.13C-0.17Mn-0.67Co and Fe-0.066-0.15Mn-1.4Co). A carefully tuned Ba¨hr 805A dilatometer is used to measure the dilation of the specimen (10 mm in length and 5 mm in diameter) d
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