Atomic Scale Characterisation of GP Zones in the Fe-Nb-(C-N) System

In this paper, the first experimental evidence of GP zones in a very dilute Fe-Nb-C-N model high-strength low alloy (HSLA) were demonstrated, where the precipitation sequence is started from a random solid solution under the simple action of temperature during ageing. Combining high-resolution transmission electron microscopy (HRTEM), field-ion microscopy (FIM) and atom-probe tomography (APT), the GP zones were shown to be one monolayer in thickness, and that their composition is essentially NbN based. This result demonstrates - similarly to what is observed in Al based alloys - that GP zones can also act as the onset of phase transformation sequence in steels. This result is also of particular importance for the class of HSLA steels, the mechanical properties of which are commonly optimized by a fine dispersion of metallic carbo-nitrides. Taking advantage of this new precipitation mechanism leads to new perspectives for improving their mechanical properties