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

Abstract

The kinetic behaviour of austenite/ferrite interfaces in a low carbon e 0.5 mass% Mn containing steel during Cyclic Partial Phase Transformation (CPPT) experiments has been investigated using hot stage Transmission Electron Microscopy (TEM). Individual interfaces were observed to display behaviour typical of CPPT experiments as recorded in macroscopic dilatometry experiments and demonstrated i) the "normal", ii) inverse transformations and iii) a stagnant stage in which the interface migrates at a very low velocity as a result of the interface passing through a Mn enriched zone due to the preceding transformation. The length of the stagnant stage determined from the TEM observations shows excellent agreement with that measured from dilatometry and kinetic modelling, whilst the distance migrated from the interface shows some disparities which are primarily attributed to differences in assumptions about grain geometry and nucleation. No special interface features were observed when the interface changed direction and passed through the previously Mn-enriched zones. General observations on the interaction of the transformation interface with microstructural features are also reported.

Detailed In Situ Hot Stage Transmission Electron Microscope Observations of the Localized Pinning of a Mobile Ferrite-Austenite Interface in a Fe-C-Mn Alloy by a Single Oxidic Particle

“…During the stagnant stage, which has been attributed to a significant interfacial enrichment of Mn, [33,34] the interface was observed to maintain a more consistently straight morphology like that seen in the present experiments. [31] 2D phase field simulations of the d-ferrite-to-austenite transformation by Sato et al [4] provide a useful point of comparison with the present study. While there are some differences, there is qualitative agreement between the observed behavior and the simulated behavior as the pinning effect developed.…”

“…Previous in situ TEM studies of the austenite-to-ferrite transformation [28,31] showed that during the normal transformation, there can be local variations in the morphology of the interface varying between curved and nearly straight. During the stagnant stage, which has been attributed to a significant interfacial enrichment of Mn, [33,34] the interface was observed to maintain a more consistently straight morphology like that seen in the present experiments.…”

“…Mompiou et al [30] studied martensite-austenite boundary migration in a Fe-20Ni-5.4Mn alloy between 400 and 600°C observing interfacial dislocations in both normal and lateral mode interface migration. Nutter et al [31] performed cyclic partial phase transformation heat treatments between 790°C and 840°C on a Fe-0.1C-0.5Mn steel, observing an evolving morphology in interface migrating in the normal mode and making kinetic measurements, maximum measured velocities during the austenite-to-ferrite transformation were 100 to 1000 nms À1 . Both Onink et al and Nutter et al found overall transformation kinetics that were consistent with calculation or bulk experiment.…”

“…Both Onink et al and Nutter et al found overall transformation kinetics that were consistent with calculation or bulk experiment. This absence of direct observations is not surprising given the inherently short interaction time of less than 2 seconds between an austenite-ferrite interface moving at a usual velocity exceeding 100 nms À1 [31] and regular pinning particles with a typical diameter of 200 nm.…”

“…In the present study, which originated from its parent study on the austenite-ferrite interface velocity during cyclic partial phase transformations in a Fe-0.1C-1.0Mn model alloy for 2nd and 4th generation automotive steels, the (absolute) austenite-ferrite interface velocity typically varies between <10 and 600 nm/s, depending on the (temperature) distance between the cooling-heating inversion points. [31] Under such slowly moving interface conditions the pinning of the moving interface by a single (oxide) particle can be captured with sufficient spatial (~20 nm) and temporal (0.16 seconds) resolution in in situ TEM experiments at low magnification.…”