Ordering of Primary Carbonitrides in an Austenitic Steel Revealed by Transmission Electron Microscopy and Atom Probe Tomography.

We reveal for the first time an ordering phenomenon of a type of carbonitrides in a Super304H austenitic steel via the techniques of transmission electron microscopy and atom probe tomography. Solution-treated Super304H austenitic steel samples containing a few primary carbonitrides were aged at 923 K for 5000 h. The results show that the primary carbonitrides in the Super304H steel are non-stoichiometric compounds enriched in Nb and with a NaCl-type structure. The crystal structure of the Nb-rich carbonitrides is in a disordered state in the solution-treated steel, while in an ordered state in the aged steel. This observation suggests the occurrence of a disorder⁻order structural transition in the primary carbonitrides during long-term aging. We found that such a disorder⁻order structural transition is accompanied by carbon diffusion from the primary carbonitrides into the austenitic matrix. The ordering phenomenon is discussed based on the long-range ordering of structure vacancies in the non-metal lattice of the superstructure, providing new insights for the understanding of ordering in non-stoichiometric compounds.