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