Atomic Resolution Scanning Transmission Electron Microscopy of TiC Precipitates in Low Carbon Ferritic Steels

Controlling the precipitation of alloy carbides with sizes of a few nanometers in ferritic steels is one of the important issues for designing of high strength steels. In this study, the interfacial atomic structures of nanometer-sized titanium carbides (TIC) in 0.05C-0.5Mn-0.1Ti-3Al (mass%) ferritic steels are investigated using scanning transmission electron microscopy. Plate-like TiC precipitates satisfying Baker-Nutting orientation relationship with the ferrite matrix are observed. High angle annular dark field scanning transmission electron microscopy with atomic resolution reveals the arrangement of Ti atomic columns in TiC and Fe atomic columns in the ferrite matrix. The TiC platelet with similar to 8 nm in length and similar to 1 nm in thickness has a coherent planar interface, which length is over the transition size from the lattice mismatch model. The semi-coherent TiC with similar to 14 nm in length and similar to 4 nm in thickness has the ledges with misfit dislocations on the planar interface. The lattice spacing of TIC along the coherent planar interface is found to be smaller than the lattice spacing of the semi-coherent broad interface or the value calculated from the lattice constant of bulk TiC.