Structure and stability of hcp iron carbide precipitates: A first-principles study.

Hexagonal close-packed (hcp) iron carbides play an important role in steel processing and in steel products. The recent discovery of novel ultrafine (2-5 nm) iron carbide (ε'-Fe2+xC) precipitates in TRIP steel sheds a new light on the hcp family of carbides. Here we present a first-principles study on the relative stability, and the electronic, magnetic properties of the ε'-Fe2C phases. Different stackings of Fe-sheets and orderings of C atoms were investigated and compared with experimental data and with Jack's model. We find very favorable formation enthalpies for these new members of the hcp family, and we present a first-principles-refined model for the crystal structure of the ultrafine Fe(C) precipitates. These findings are useful for the characterization of nano-sized iron carbide precipitates, for understanding their role in the microstructure of steels, and for the design of novel steels having even more desirable properties.