Characterisation of oxide dispersion-strengthened steel by extended X-ray absorption spectroscopy for its use under irradiation

An oxide dispersion-strengthened (ODS) steel (Fe, Cr, Al) has been investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy with respect to the dispersoids. To assess the response of this material to irradiation calculations with the code stopping and range of ion into matter (SRIM) were also performed. The samples were investigated in the as received condition. Scanning Electron Microscopy (SEM) observations show that the material is homogeneous apart from (Ti, Al)-rich precipitates. Transmission electron microscopy (TEM) investigations show the yttria nano-particles still well crystallised even after the mechanical and thermal treatments for the ODS preparation. The size of the yttria (Y2O3) nano-particles was in the range of 15–70 nm, finely dispersed in the ferritic matrix. The yttria nano-particles present a Y absorption edge that can be used to analyse the atomic environment of Y. In fact the EXAFS analysis shows that the yttria particle structure resembles monoclinic. The paper discusses the atomic environment of Y assuming potential transformation cubic → monoclinic during thermal treatments. However, model (FEFF) calculations of the EXAFS spectra demonstrate presence of distortions in the lattice that is neither cubic nor pure monoclinic. The SRIM results show that the yttria nano-particles appear more transparent than the steel matrix with regard to the H or He irradiations. The structure of the dispersed yttria nano-particles is therefore expected to remain rather unaltered under irradiation conditions. Long term ageing conditions together with irradiation may, however, lead to changes in matrix and dispersoids which are expected to be well characterized by EXAFS according to our results.