Atomic-scale imaging and analysis of single layer GP zones in a model steel

The first experimental evidence of single-layered NbN GP zones in steel was shown through the combined use of advanced atomic-scale resolution techniques. A model low-alloyed steel was developed that contained very low, controlled amounts of solute species. Field ion micrographs were obtained using neon as imaging gas at a pressure of 10 -3 Pa and a base temperature of 40 K. The bright contrast of the GP zone indicates that this latter has a higher cohesive energy than the ferrite matrix. The bright spots corresponding to the solute atoms in the GP zones form platelets, clearly evidenced in the reconstructed volume. GP zone is preferentially retained at the specimen surface, causing ion trajectory aberrations, and thus biasing the 3D reconstructions. The amount of nitrogen atoms being equal to the one of niobium, the local chemistry in the GP zone is assumed to be stoichiometric niobium nitride (NbN).