Characterization of NbC and (Nb,Ti)N nanoprecipitates in TRIP assisted multiphase steels

Multiphase steels utilising composite strengthening may be further strengthened via grain refinement or precipitation by the addition of microalloying elements. In this study a Nb microalloyed steel comprising martensite, bainite and retained austenite has been studied. By means of transmission electron microscopy (TEM), we have investigated the size distribution and the structural properties of (Nb,Ti)N and NbC precipitates, their occurrence in the various steel phases, and their relationship with the Fe matrix. (Nb,Ti)N precipitates were found in ferrite, martensite, and bainite, while NbC precipitates were found only in ferrite. All NbC precipitates were found to be small (5 20 nm in size) and to have a face centred cubic crystal structure with lattice parameter a = 4.36 ± 0.05 A. In contrast, the (Nb,Ti)N precipitates were found in a broader size range (5 150 nm) and to have a face centred cubic crystal structure with lattice parameter a = 8.09 ± 0.05 A. While the NbC precipitates were found to be randomly oriented, the (Nb,Ti)N precipitates have a well-defined Nishiyama Wasserman (N-W) orientation relationship with the ferrite matrix. An analysis of the lattice mismatch suggests that the latter precipitates have a high potential for effective strengthening. DFT calculations were performed for various stoichiometries of NbCX and NbXTiYNZ phases and the comparison with experimental data indicates that both the carbides and nitrides are deficient in C and N content.