Discussion on the Alloying Element Partition and Growth Kinetics of Proeutectoid Ferrite in Fe-C-Mn-X Alloys
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In a recent study of ferrite growth from deformed Fe-0.1C-1.5Si-3Mn austenite,[1] it was shown that alloying element partitioning occurred in a different manner from the component ternary alloys. More specifically, Si was partitioned at temperatures ~150 C below Ae3, whereas in Fe-C-Si ternary alloys, it is partitioned only immediately below Ae3.[2] Nearly 2 decades ago, Tanaka et al.[3] discussed synergistic effects of alloying elements on the growth of ferrite in Fe-C base quaternary alloys. The term ‘‘synergistic’’ means the deviation from the sum of the effects of alloying elements in ternary alloys. They attributed the effects to coupled solute drag (CSDE),[4] that is, enhanced solute drag due to carbon or the addition of the second alloying element. The CSDE is associated primarily with the growth mode controlled by carbon diffusion and is presumably insignificant in partitioned growth due to sluggish boundary motion. The difference in alloy element partitioning in higher order alloys is possibly the manifestation of another synergistic effect, and thus, alloy partitioning in quaternary alloys is worth studying
R. WEI, Graduate Student, is with the Graduate School of Science and Engineering, Ibaraki University, Hitachi 316-8511, Japan. M. ENOMOTO, Professor, is with the Department of Materials Science and Engineering, Ibaraki University. Contact e-mail: enomotom@ mx.ibaraki.ac.jp Manuscript submitted June 23, 2011. Article published online September 23, 2011 3554—VOLUME 42A, DECEMBER 2011
in detail. In this report, we apply a method of calculating alloy partitioning employed in the previous study[1] to reanalyze data in three Fe-C-Mn-X alloys, the chemical compositions of which are shown in Table I, to assess the capability of the method by comparing with DICTRA. In addition, significant differences are noted in the local condition at the boundary or growth mode of ferrite from those presented previously in these alloys. In an excellent review of the theory of diffusioncontrolled growth in multicomponent alloys, DeHoff[5] presented graphical construction of an interfacial tieline, albeit diffusional interactions between solutes were not incorporated. Then, Tanaka et al. extended Coates equations[6] to calculate local equilibrium interfacial tie-lines and parabolic growth rate constants in quaternary alloys fully taking into account the interactions and diffusivities of all solutes. The procedure of solving Coates equations involves multiple regression analysis of a/c phase boundary composition, which takes much time and often causes a numerical problem in higher order alloys. In contrast, if one ignores the differences in diffusivity between alloying elements (Table II), which are several orders of magnitude smaller than that of carbon, he can readily calculate the concentration of alloying elements at the boundary in ferrite and austenite. The method uses a carbon-component ray in the calculation of the interfacial tie-line in not only negligibly-partitioned local equilibrium (NPLE) but also partitioned local equilibrium (P
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