Direct TEM Observation of the Movement of the Austenite-Ferrite Interface in Steels

The solid-state phase transformation from austenite to ferrite is one of the key transformations in determining the final microstructure of steels, and therefore is an important factor in the versatility of steels. An improved understanding of this process can therefore lead to greater control over final properties in steels.
This thesis presents the results of cyclic partial phase transformation (CPPT) experiments using in situ hot stage TEM in order to directly observed the interface behaviour during thermal cycling in the α + γ austenite phase region, for Fe-C-Mn alloys with up to 0.33 wt% C and 1.5 wt% Mn. Hot stage TEM experiments of this type have not been reported in the literature before and are able to provide new information which is not available from other techniques commonly used to carry out CPPT experiments such as dilatometry.
The observation of individual interfaces makes it possible to observe not just the kinetics of the transformation but also the interface morphology and its interaction with microstructural features. Individual interface behaviour could be classified into the three stages expected during CPPT from bulk experiments: the normal transformation, the inverse transformation and the stagnant stage. The TEM observations show that sluggish interface migration continued during the stagnant stage, without the interface becoming completely static for any significant period of time. This migration distance was measured to be <500 nm, making this difficult to detect using other techniques. Furthermore, the observations showed that the interface often displayed an evolving morphology during the normal transformation, but reverted to a more stable, often straight or smoothly curved morphology during the stagnant stage.
TEM observations also showed good reversibility between cycles with the interface migrating along approximately the same path during thermal cycling, with only minor changes. Interaction with microstructural features was also observed, including the segmentation of the interface due to nearby twin boundaries and the pinning of the interface by foreign particles.
Finally, the results of in situ TEM observations of the growth of bainite are presented. Two growing bainite plates were observed and the lengthening and thickening rates
3 measured. Lengthening occurred in a series of discrete steps with periods of relatively slow or no growth interspersed with periods of rapid lengthening. In contrast it was observed that the subsequent thickening of the plates was a continuous process.