In Situ 3D Neutron Depolarization Study of the Transformation Kinetics and Grain Size Evolution During Cyclic Partial Austenite-Ferrite Phase Transformations in Fe-C-Mn Steels

Abstract

We have analyzed the evolution of the ferrite fraction and average ferrite grain size during partial cyclic austenite-to-ferrite and ferrite-to-austenite phase transformations in an Fe-0.25C-2.1Mn (wt pct) steel using three-dimensional neutron depolarization (3DND). In the 3DND experiments, the ferrite fraction is derived from the rotation angle of the neutron polarization vector, and the average grain size is determined from the shortening of the polarization vector. From these, the number density of ferrite grains is derived, which indicates that grain nucleation is negligible during partial cycling in the intercritical regime and that all transformation kinetics can be attributed to growth processes only. In the multiple successive cyclic partial transformations, the interfacial migration rate was found to be sluggish due to Mn partitioning. The transformation kinetics determined with 3DND was compared to the predicted behaviors for diffusion-controlled simulations under local equilibrium and para-equilibrium interfacial conditions. It was found that the simulation predictions under local equilibrium only qualitatively capture the transformation kinetic with a difference of one order of magnitude in the variation in the ferrite fraction during cycling. The cyclic behavior of this Fe-0.25C-2.1Mn (wt pct) steel shows that the austenite-ferrite interface indeed migrates back and forth during cycling, while at the same time, there is a gradual increase in both the ferrite fraction and the average ferrite grain size over subsequent cycles. The intrinsic cyclic behavior is only visible after subtracting the effect of the progressive interfacial migration into austenite. The present study demonstrates the advantage of 3DND in studying partial cyclic phase transformations over conventional experimental approaches.

Direct TEM observation of α/γ interface migration during cyclic partial phase transformations at intercritical temperatures in an Fe-0.1C −0.5Mn alloy

“…During a cyclic partial phase transformation experiment the temperature is cycled between two temperatures, T 1 and T 2 , both within the aþg phase region [14]. Using this approach, the effect of nucleation, which is difficult to measure experimentally [14], can be excluded from an analysis of the kinetics as the transformation only progresses through the migration of already existing interfaces [13,21]. Secondly, in a CPPT experiment it is much easier to unambiguously allocate the interface kinetics to the local interface conditions present, which in itself is a function not only of the steel composition (in particular the Mn concentration as Mn has a high partitioning coefficient and can affect the effective interface mobility significantly [15]) and also of the thermal history.…”