Ex-situ and in-situ S/TEM study of bainitic ferrite nucleation and martensite transformation for various heat treatment scenarios

• The bainitic ferrite nucleation was observed using in-situ S/TEM. • L12-Kappa carbide was revealed either in the in situ and ex-situ S/TEM studies in the bainitic structure instead of the B2-NiAl phase expected from the theoretical calculations. • The dynamic recovery of dislocations during the austenite-to-ferrite transformation at 670 °C led to the formation of numerous sub-grains and twin boundaries in the austenite. • Kappa carbide is expected to be the product of an ordering reaction of a NiAl-enriched FCC phase formed from the spinodal decomposition of the austenite. Dynamics in matter and processes observed using in-situ microscopy methods provide an immense capability for direct revealing phenomena in the nanoscale. As a part of this work, Fe-7.9Ni-2.8Al-0.77Cu-0.4C wt.% bainitic steel (named BainNiAlCu), designed for (Ni, Al)-rich particles formation, was investigated using ex-situ and in-situ Scanning Transmission Electron Microscopy (S/TEM), dilatometry and X-ray diffraction (XRD). The bainitic and martensitic transformation was conducted inside a microscope column. The progress of austenite-to-ferrite transformation in the annealing step at 670 °C has generated dislocations in the adjacent austenite to accommodate the volume misfit between ferrite and austenite. The dynamic recovery of these dislocations has introduced numerous sub-grains and twin boundaries in the austenite, which act as nucleation sites for bainitic ferrite accelerating significantly this transformation during isothermal annealing at 250 °C. On the other hand, the B2-NiAl phase expected from the theoretical calculations was not revealed either in the in-situ or ex-situ microstructural studies, nor in the XRD patterns. The bainitic structure contained Kappa carbide resulted from ordering reaction of a NiAl-enriched FCC phase formed from the spinodal decomposition of the austenite, which would suppress the formation of B2 precipitates at 650 °C. The insight into the nucleation of bainitic ferrite considering various factors was revealed.