Thermodynamic mechanism of the magnetic-field-induced carbon content variation in bainitic ferrite during super bainite transformation

[Display omitted] •The magnetic moment of bainitic-ferrite arises from the interaction of the Fe-C bond length and the position of Fe atom.•The carbon atom in the octahedral interstices affects the magnetic moment of the nearby iron atom.•Negative magnetic free energy changes lead to an enrichment of the paraequilibrium carbon concentration at the α/γ interface. The experimental results demonstrate the decrease in carbon content of bainitic ferrite under a 12 T high magnetic field. A hybrid approach combining electron probe microanalyzer (EPMA), first-principles calculations and Weiss molecular-field theory was used to evaluate the relationship between the carbon content and magnetism in bainitic ferrite. The calculated results demonstrate that the origins of the magnetic structural characteristics are controlled by interactions amongst the various sites of iron atoms and Fe-C bond distances. The negative magnetic Gibbs free energy changes increased the transformation driving force between the ferrite (α) and austenite (γ) phases, leading to an increased paraequilibrium carbon concentration at the α/γ interface, leading to an increase in the carbon diffusion in the austenite near the interface. As a consequence, the carbon atoms in bainitic ferrite decrease which is consistent with the experimental results.