Morphological Stability of δ-Ferrite/γ Interphase Boundary in Carbon Steel

The morphological changes of the delta-ferrite/gamma interphase boundary have been observed in situ with a high-temperature confocal scanning laser microscope (HTCSLM) during delta/gamma transformations (delta -> gamma and gamma -> delta) of Fe-0.06 wt pct C-0.6 wt pct Mn alloy, and a kinetic equation of morphological stability of delta-ferrite/gamma interphase boundary has been established. Thereafter, the criterion expression for morphological stability of delta-ferrite/gamma interphase boundary was established and discussed, and the critical migration speeds of delta-ferrite/gamma interphase boundaries are calculated in Fe-C, Fe-Ni, and Fe-Cr alloys. The results indicate that the delta-ferrite/gamma interphase boundary is very stable and nearly remains absolute planar all the time during gamma -> delta transformation in Fe-C alloy. The delta-ferrite/gamma interphase boundary remains basically planar during delta -> gamma transformation when the migration speed is lower than 0.88 mu m/s, and the interphase boundary will be unstable and exhibit a finger-like morphology when the migration speed is higher than 0.88 mu m/s. The morphological stability of delta-ferrite/gamma interphase boundary is primarily controlled by the interface energy and the solute concentration gradient at the front of the boundary. During the constant temperature phase transformation, an opposite temperature gradient on both sides of delta-ferrite/gamma interphase boundary weakens the steady effect of the temperature gradient on the boundary. The theoretical analysis of the morphological stability of the delta-ferrite/gamma interphase boundary is coincident with the observed experimental results utilizing the HTCSLM. There is a good agreement between the theoretical calculation of the critical moving velocities of delta-ferrite/gamma interphase boundaries and the experimental results.