An in situ transmission electron microscopy study of interface growth during martensitic transformation in an Fe-Ni-Mn alloy

Highlights: → Movement of martensite interfaces was investigated using in situ heating in the TEM. → Martensite interfaces moved by a ledge mechanism rather than advancing uniformly. → Martensite ledges vary from atomic dimensions to several tens of nanometers in height. → Martensite lath dissolution occurred preferentially on one side. - Abstract: Frame-by-frame analysis of the austenite/lath martensite interface during in situ heating using transmission electron microscopy was used to provide direct information on the mechanisms of the martensite interface motion in an Fe-20Ni-5.5Mn (wt.%) alloy. When the temperature was increased to ∼550 deg. C, the tip of the lath martensite receded slightly, and then decomposed into ledges on only one side of the lath. With further time at 550 deg. C, the ledges migrated in a start-stop fashion; the highest velocity observed was 0.79 μm s-1. When the temperature was increased to ∼580 deg. C, the interface on the mobile side of the lath preferentially receded within certain transformation twins formed during the earlier quenching treatment. Based on these experimental observations, it appears that the austenite may form an array of parallel twins during the martensitic transformation, which coalesce to form a lath shape. This lath then thickens in one direction only to establish the final morphology by a ledge mechanism displaying start-stop growth behavior.