An Overview of the Cyclic Partial Austenite-Ferrite Transformation Concept and Its Potential
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NTRODUCTION
The kinetics of the austenite-ferrite transformation in low-alloy steels has been of great interest for several decades due to its great importance in the development of modern automotive and general purpose construction steels,[1–11] and an overview of the recent developments in the field has been provided in.[12] The austenite-to-ferrite transformation involves ferrite nucleation and migration of the austenite-ferrite interface surrounding the ferrite nucleus into the parent austenite grain.[13,14] The kinetics of the interface migration has been found to be controlled by the combination of the diffusion of alloying elements and the actual austenite-ferrite interface mobility, e.g., the mixed-mode growth.[15,16] The effects of alloying elements on migrating interfaces (ALEMI) have been discussed extensively elsewhere.[17] Two classical models have been explored extensively: (i) the Local Equilibrium (LE) model,[18–20] in which the interface is assumed to migrate under full local equilibrium with partitioning of both C and M (M=Mn, Ni, Mo, et al.). Depending on the alloy composition and the transformation temperature, the rate of interface migration is determined either by carbon diffusion or M diffusion; (ii) the Paraequilibrium (PE) model[21,22] in which it is assumed that there is no partitioning of M at the migrating interface, and thus interface migration is purely controlled by carbon diffusion. Several phenomenological models based on Cahn’s solute drag theory[23] or Hillert’s Gibbs energy dissipation theory[24] have been developed to model the transition
HAO CHEN, Assistant Professor, is with the Key Laboratory for Advanced Materials of Ministry of Education, School of Materials Science and Engineering, Tsinghua University, Beijing, China, and also with the Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629HS Delft, The Netherlands. Contact e-mail: [email protected] SYBRAND VAN DER ZWAAG, Professor, is with the Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS, Delft, The Neterlands. Manuscript submitted December 13, 2015. Article published online October 25, 2016 2720—VOLUME 48A, JUNE 2017
between the PE and LE conditions.[25–29] Dedicated experiments, e.g., conventional isothermal or continuous heating/cooling experiments,[30–32] decarburization experiments,[33,34] and gradient experiments,[35] have been designed to investigate the effects of alloying elements on the migrating interfaces. Despite abundant efforts, the effects are not yet fully understood, and it is still one of the most challenging questions in the field of solid-state phase transformations.[17] The austenite-ferrite interface mobility is generally determined by fitting transformation models to the conventional experimental transformation curves invariably starting from a 100 pct parent phase and ending in a 100 pct new product phase.[36–40] However, as the actual nucleation rate during such conventional experiments plays an important role but cannot be
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