On the transition between grain boundary ferrite and bainitic ferrite in Fe–C–Mo and Fe–C–Mn alloys: The bay formation explained

The transition between grain boundary ferrite (GBF) and bainitic ferrite (BF) formation in Fe–C–Mo and Fe–C–Mn alloys has been investigated from a diffusional point of view by a so called Gibbs energy balance approach based on solute drag theory, in which the chemical driving force is assumed to be equal to energy dissipation due to alloying elements diffusion inside the interface. The GEB approach predicted that with decreasing temperature the transformation is dominated by GBF and BF formation, respectively, which provided a rationale for the transition between GBF and BF formation. The presence of transformation bay phenomenon in Fe–C–Mo alloys was well captured by the GEB approach, and the bay phenomenon was found to be due to strong solute drag effect of Mo. No bay phenomenon was predicted in Fe–C–Mn alloys. The effect of Mo and Mn on bainite start temperature has also been well predicted by the GEB approach.

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