In Situ 3D Neutron Depolarization Study of the Transformation Kinetics and Grain Size Evolution During Cyclic Partial Au
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INTRODUCTION
THE kinetics of the austenite-to-ferrite (c-a) and the ferrite-to-austenite (a-c) transformations in low-alloyed steels have attracted extensive attention due to their practical importance and scientific challenges.[1–5] During the austenite-to-ferrite transformation, the ferrite H. FANG is with the Fundamental Aspects of Materials and Energy Group, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands and also with the Novel Aerospace Materials Group, Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands. Contact e-mail: [email protected] S. VAN DER ZWAAG is with the Novel Aerospace Materials Group, Faculty of Aerospace Engineering, Delft University of Technology and also with the School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China. N.H. VAN DIJK is with the Fundamental Aspects of Materials and Energy Group, Faculty of Applied Sciences, Delft University of Technology. Manuscript submitted February 14, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
first nucleates at the preferred nucleation sites and subsequently grows into the austenite grains. As observed with synchrotron X-ray diffraction, ferrite nucleation occurs in a certain temperature (or time) range, where new nuclei continuously form until a maximum density is reached.[6] Once nucleated, the growth of a ferritic grain, i.e., the interfacial migration, is controlled by interfacial mobility and diffusion of solute elements in the vicinity of the moving interface. To explore the effect of the alloying elements M (=Mn, Ni, Co, etc.) on interfacial migration in Fe-C-M steels, extensive studies have been performed using conventional isothermal or continuous heating and cooling experiments.[7–10] However, in such experiments where nucleation and interfacial migration take place simultaneously, the impossibility to determine the nucleation rate during the entire transformation process unavoidably leads to nonnegligible uncertainties in the derivation of the interfacial mobility and investigating the effect of the alloying elements. To avoid the effect of nucleation on the transformation kinetics, the concept
of cyclic partial austenite-ferrite transformation, where the temperature is varied cyclically within the c/a two-phase region, was recently proposed.[11] This cyclic approach has proven to be more informative in studying the effect of interfacial mobility and alloying elements on the rate of the interfacial migration as a result of the (assumed) absence of new nucleation events from the moment of the first inverse transformation cycle. This assumption is physically realistic and has been verified ex-situ by 2D metallographic cross sections.[12] A large number of dilatometric cyclic partial phase transformation measurements[11,12] and various modeling approaches such as DICTRA,[11] 1D mixed-mode modeling,[13] and 1D phase-field modeling[14,15] have been used to study the effect of alloying element M on the aust
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