In Situ Observation Revealing the Critical Role of Defect Recovery in Austenite on Carbon Diffusion and Bainite Transformation Kinetics

This study employs in situ transmission electron microscopy heating to comprehensively characterize the dynamic transformation of retained austenite (RA) to bainite and its underlying mechanisms in medium‐carbon, medium‐alloy steels. Critically, the distinct phase transformation kinetics between blocky and filmy austenite morphologies originate primarily from their divergent capabilities for low‐temperature recovery of transformation‐induced dislocations. Under consistent heating, blocky austenite undergoes rapid bainitic transformation (<3 min between 290 and 320 °C). This rapid kinetics stems from effective dislocation recovery, which restricts carbon diffusion primarily to the lattice pathway, resulting in only modest enrichment. In stark contrast, filmy austenite exhibits severely constrained recovery, maintaining a high dislocation density that provides rapid pipe diffusion channels. This significantly enhances carbon enrichment within the austenite, thereby substantially prolonging the transformation incubation time (>30 min). The direct in situ observations reveal a governing principle for RA within a martensitic matrix during low‐to‐medium temperature tempering: the dislocation density within the RA dictates carbon diffusion kinetics into the austenite, while the resultant carbon enrichment determines the thermal stability of the RA.