Simulation of Nucleation of Proeutectoid Ferrite at Austenite Grain Boundaries during Continuous Cooling
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OFFERMAN et al.[1–5] have reported an in-situ observation of ferrite nucleation and growth kinetics during continuous cooling in three Fe-C-Mn-Si steels, each containing 0.21, 0.35, and 0.45 pct C, 0.5 to 0.7 pct Mn and 0.2 to 0.3 pct Si. The experiment was conducted by three-dimensional X-ray diffraction (3DXRD) microscope. The number of ferrite nuclei as well as ferrite volume fraction was determined from the diffraction spots and integrated intensity of ferrite diffraction rings. The major conclusions drawn by these authors was that the value of VW, the volume of a critical nucleus in Wulff space,* was approximately two orders *Offerman et al. defined a factor W, which consists of the cube of nucleus/matrix interfacial energy and the geometrical factor representing the critical nucleus shape. It is the numerator of the activation energy of nucleation DG* = W/(DGV)2, where DGV is the driving force for nucleation. According to the well-known Wulff theorem,[22] the activation energy of nucleation is expressed as DG* = 4VW/(DGV)2. Hence, VW(=W/4) is used throughout this article.
of magnitude smaller than that reported from isother-
M. ENOMOTO, Professor, is with the Department of Materials Science and Engineering, Ibaraki University, Hitachi, 316-8511 Japan. Contact e-mail: [email protected] J.B. YANG, Research Associate, formerly with the Department of Materials Science and Engineering, Ibaraki University, is with Institute for Materials Research, Tohoku University, Sendai, 980-8577 Japan. This article is based on a presentation given in the symposium entitled ‘‘Solid-State Nucleation and Critical Nuclei during First Order Diffusional Phase Transformations,’’ which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati, Ohio under the auspices of the TMS/ASMI Phase Transformations Committee. Article published online March 26, 2008 994—VOLUME 39A, MAY 2008
mal nucleation kinetics in Fe-C and Fe-C-X alloys,[6–9] where X is a substitutional alloying element. Aaronson et al.[10] discussed this finding extensively with respect to the methods of experiment and data analysis. In the discussion, omission of the overlap of solute diffusion fields associated with ferrite particles (so-called soft impingement), assumption of constant activation energy among different types of nucleation site over the temperature range of cooling, and possible failure of discriminating ferrite particles of similar spatial orientations as separate ones were proposed to substantially influence their conclusion. In a reply to the discussion and a follow-on study, Offerman et al.[2,5] asserted that these were only of negligible importance. The present authors conducted a preliminary analysis of these results using a pillbox critical nucleus model, and reported that the rate controlling diffusivity, soft impingement, and decrease in available nucleation sites with the progress of transformation have to be fully taken into account in the analysis of experimental observations.[11] In response to these propositions, Offerman et al.[12]
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