Non Destructive Characterization of Oxide Dispersion Strengthened Steels

Various complementary NDT methods have been used for the characterization of oxide dispersion strengthened steels. Complex microstructure of these materials requires multi technique approach in order to better understand the effects of different nanostructures in specific environmental conditions. All techniques used in the experiment, namely positron annihilation lifetime spectroscopy (PAS), small angle neutron spectroscopy (SANS), thermoelectric power (TEP) and Barkhausen noise (BN) measurements, are sensitive to spectrum of defects in nano/micro metric range. Present report discusses feasibility of application of individual techniques on investigation of ODS materials and some degradation mechanisms. In particular, 475°C embrittlement of high-Cr ODS was studied. Precipitation of Cr-rich phase, responsible here for severe degradation of material properties, was experimentally observed by SANS as a new scattering type of ~ 2nm scattering centres in the microstructure of thermally aged materials. The same process was observed in TEP as the increase of relative Seebeck coefficient and the new precipitates were found to act as a pining site for magnetic domain wall movement by the Barkhausen noise measurement. Although the precipitation of α’ phase does not change positron lifetime spectra directly, our experiments shown their non trivial effect on the lifetime parameters. Substantially different behaviour of vacancy type defects was observed in the embrittled high-Cr ODS materials, than in the 9%Cr ODS aged at the same conditions. Present results provide important guidelines for future irradiation experiments and application of the NDT on the material characterization.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen