The characterization of a nanoscale MnNi cluster in thermally aged reactor pressure vessel steels

Abstract

Abstract The MnNi-rich clusters or precipitates (MNPs) are one of the most important nanofeatures resulting in irradiation-induced embrittlement of reactor pressure vessel (RPV) steels. In this study, the microstructure and mechanical properties of the thermally-aged RPV steel were investigated by microhardness and tensile test, x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT), revealing the presence of a novel nanoscale B2 MnNi phase. The formation of these B2 MnNi phases was dominated by the thermodynamic of the RPV-steel system, and high-density dislocations could boost their formation and growth. Meanwhile, for samples aged for 2 h at 723 K, these ordered phases result in a yield-strengthening increment of 190 MPa. Although aged-induced ordered phases have a clear difference in terms of the lattice structure, nucleation rate, and thermal stability compared to irradiation-induced MNPs, the discovery of these B2 ordered phases helps us to establish a more comprehensive understanding of the nature and formation mechanism of MNPs produced by neutron irradiation.