Predicting the Effect of Mo, Ni, and Si on the Bainitic Stasis
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kinetics of the bainitic transformation in steels has been widely studied for many decades both from an experimental and a theoretical point of view because of its great importance in steel design and production. Despite abundant efforts, the mechanism of bainitic transformation is still highly controversial, and there are many unresolved issues.[1–14] One of the most important unresolved issues in the field is Transformation Stasis (TS) phenomenon (also called the incomplete bainite transformation (IBT) phenomenon[2,12,15,16]), which is defined as: the isothermal bainite transformation temporarily ceases before the fraction of bainite reaches the fraction of ferrite predicted for the paraequilibrium condition,[17,18] and the transformation will only continue once until the carbides start to form substantially and consume carbon from the austenite. Until now, there have been two competing views on the physical origin of TS phenomenon: (i) Diffusionless approach.[2,4] The TS phenomenon is attributed to the fact that the bainitic transformation is essentially diffusionless. According to this model, the isothermal transformation should cease when the free energy of austenite phase equals that of the bainitic ferrite; (ii) Diffusion school,[15,16] in which it was deduced that the stasis was possibly due to solute drag effect at the migrating austenite/bainitic ferrite interface. Over the past decades, many experiments were performed to validate these two views, as both schools seem to be capable of describing the experiments to their own satisfaction, there is no clear proof for either theory yet. Recently, much effort has been paid to select proper alloys to discriminate the existing views and figure out HAO CHEN, formerly Postdoctoral Researcher with the Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629HS, Delft, The Netherlands, is now Postdoctoral Fellow with the University of British Columbia, Vancouver, BC, Canada. Contact e-mail: [email protected], [email protected] SYBRAND VAN DER ZWAAG, Professor, is with Faculty of Aerospace Engineering, Delft University of Technology. Manuscript submitted October 12, 2013. Article published online March 18, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
the physical origin of TS. In Reference 19, the TS phenomenon was studied in a series of Fe-C-Mn-Si and Fe-C-Mn alloys with different Mn concentrations, which clearly discriminated the predictive power of both models regarding the effect of alloying element composition on the stasis. A Gibbs energy balance approach (GEB),[19,20] in which dissipation of Gibbs energy due to diffusion inside interface is assumed to be equal to the available chemical driving force, successfully described the TS phenomenon in the Fe-C-Mn-Si and Fe-C-Mn alloys. The GEB model predicted that during the isothermal bainitic transformation, there is a sharp growth mode transition from a fast mode without diffusion of Mn inside the austenite–bainite interfaces to a sluggish mode with diffusion of Mn inside interfaces. The TS is
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