In situ observation of austenite-ferrite interface migration in a lean Mn steel during cyclic partial phase transformations

High-temperature laser scanning confocal microscopy (HT LSCM) has been applied to investigate the austenite-ferrite interface migration during cyclic phase transformations in situ in a Fe-Mn-C alloy. It has been found that during the cyclic phase transformations the transformation proceeds via the migration of existing austenite-ferrite interfaces. The interfaces migrate in a retraceable way. For the first time, the so-Called stagnant stage has been observed directly. The new in situ observations show that the interface migration rates for interfaces in different grains are comparable with each other prior to soft impingement, while the equilibrium migration distances for different interfaces can be quite different, depending on the local grain size. The average interface velocities as measured by HT LSCM are in very good agreement with the velocities derived from dilatometric data, and those are predicted by a local equilibrium transformation model.