Transformation character of ferrite formation by a ledge mechanism under a mixed-control model

Abstract

A mixed-control model was developed to study the transformation character of ferrite formation by a ledge mechanism. A numerical two-dimensional diffusion-field model was combined to describe the evolution of the diffusion field ahead of the migrating austenite/ ferrite interface. The calculation results show that the bulk diffusion-controlled model leads to a deviation from experimental results under large solute supersaturation. In the mixed-control model, solute supersaturation and a parameter Z together determine the transformation character, which is quantified by the normalized concentration of carbon in austenite at the austenite/ferrite interface. By comparing with experimental data, the pre-exponential factor of interface mobility, M ₀, is estimated within the range from 0.10 to 0.60 mol·m·J⁻¹·s⁻¹ for the alloys with 0.11wt%–0.49wt% C at 700–740°C. For a certain Fe-C alloy, the trend of the transformation character relies on the magnitude of M ₀ as the transformation temperature decreases.

Keywords

iron carbon alloys / modeling / ferrite / ledge mechanism / diffusion / interfaces

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