In-Situ Heating Observation of Precipitation Behavior with Dislocation Emission in Nitrogen-Added Austenitic Heat-Resistant Steels

This study examines the precipitation behavior of Cr2N and the associated dislocation emission mechanisms in nitrogen-added austenitic stainless steels during isothermal aging at 973 K. Microstructural development was characterized by in-situ heating high-voltage electron microscopy (HVEM), which enables to direct observation of relatively thick specimens (∼1 µm). TEM analyses identified Cr2N as the dominant precipitates in the steels aged at 973 K, which emitted prismatic loops and slip dislocations from their interfaces. Atomically-resolved TEM analysis clarified that dislocations were preferentially emitted from the semi-coherent interfaces between Cr2N and the matrix, whereas emissions from the incoherent interfaces were rarely confirmed. In-situ TEM observations demonstrated the emission of both slip dislocations and prismatic loops from the Cr2N interfaces, consistent with eigenstrain-based Ashby-model. Furthermore, in-situ observations revealed the migration of misfit dislocations, suggesting a transition of the semi-coherent interface toward incoherent interfaces, which is expected to influence both dislocation emission behavior and coarsening kinetics of Cr2N.