Atom Probe Tomography Study of Homogeneous and Heterogeneous Precipitation of Niobium Carbides in a Model Ferritic Steel

The precipitation of the NbC phase in a ferritic low carbon microalloyed steel have been investigated since the early stages using atom probe tomography (APT). Homogeneous precipitation from the random solid solution is studied since the first minutes of ageing at 873 K. A (Fe;Nb)C-type metastable phase form prior to the precipitation of the equilibrium stoichiometric carbide NbC (NaCl structure), a semi-coherent phase that has a high misfit with the ferritic matrix. On the other hand, heterogeneous precipitation along dislocation lines is also observed by computerised field ion microscopy and APT. The results obtained are compared with conventional transmission electron microscopy studies and Monte Carlo simulations of heterogeneous precipitation recently obtained