Characterization of Precipitates in 316 Stainless Steel Neutron-Irradiated at ∼390°C by the Combination of CDF-TEM, EF-TEM, and HR-TEM

Abstract

In this work, the combination of Conventional Dark Field (CDF), Energy Filtered (EF), and High Resolution (HR) Transmission Electron Microscopy (TEM) enabled fine scale characterization of the precipitate microstructures in type 316 Stainless Steel (SS). This steel was irradiated beforehand in the Phénix fast reactor at a temperature of about 390°C and up to a dose of 39 dpa. Four types of precipitates were identified, namely M6C (η), M23C6 (τ), Ni3Si (γ′), and M6Ni17Si7 (G). The results emphasize the importance of establishing the correlation between the data obtained by various analysis methods, especially with the presence of nanoscale precipitates of different phases. Indeed, the electron diffraction patterns taken at the low index zone axis from different grains of the matrix showed that the low total diffracting volume of these precipitates makes their reflections relatively weak and the reflections arising from the different phases and variants are difficult to index. These weak and numerous reflections prevent full characterization of the different types of precipitates using only the technique of indexing of diffraction pattern. CDF-TEM enabled imaging of the morphology and the spatial distribution of different precipitates and determination of their density and size. Unlike the technique of energy dispersive spectroscopy of X-ray photons, EF-TEM is insensitive to the radioactivity of 316 SS. It can be routinely used to image the relative proportion of chemical species present in different phases and their relative spatial distribution around cavities also induced by irradiation. In spite of some disturbances in the lattice of precipitates related with this radiation, HR-TEM confirms the value of their lattice parameters given by electron diffraction patterns and the association of some precipitates with cavities observed by means of Conventional Bright Field-TEM and EF-TEM.