2019, Vladimirov, P. V., Borodin, V. A., Бородин, Владимир Алексеевич

The paper deals with the first principles simulation of formation energies and migration barriers of self point defects, including vacancies, di-vacancies and single interstitial atoms, in metallic yttrium. The vacancy formation energy in yttrium is shown to be relatively high (similar to 1.8 eV), whereas the migration barriers are very similar for the jumps inside the basal planes and between basal planes, being equal to similar to 0.65 eV. The sum of these numbers reasonably reproduces the experimental values of the self-diffusion activation barriers. The vacancy pairs at the first nearest neighbor separation (divacancies) have binding energy of similar to 0.2 eV, which is only weakly sensitive to the divacancy orientation in the lattice, whereas vacancy pairs at the second and third nearest-neighbor separations are energetically unfavorable, suppressing the dissociation of divacancies. Together with the noticeably lower divacancy migration barriers with respect to single vacancies, this makes divacancies efficient mediators for mass transfer in Y. Among multiple possible configurations of a single interstitial, only the basal octahedral one is found to be the true energy minimum, while all the other considered possibilities are either unstable, or saddle points on the potential energy surface. This is in contrast to other hcp metals, where several metastable interstitial configurations often coexist. The lowest migration barriers for single interstitial diffusion along the basal plane and between planes are practically equal, similar to 0.4 eV, implying isotropic diffusion of interstitials in yttrium. Overall, the predicted properties of point defects in yttrium are in line with the general trends for hcp metals with the cla ratio below root 8/3.

2019, Owusu-Mensah, M., Jublot-Leclerc, S., Gentils, A., Baumier, C., Borodin, V. A., Бородин, Владимир Алексеевич

© 2019 Elsevier B.V.ODS steels are ferritic-martensitic steels reinforced with (Y,Ti) oxide dispersions to enhance the creep and radiation resistance at elevated temperatures. Their conventional fabrication is achieved by ball milling followed by high-temperature consolidation. An alternative approach of ion beam synthesis has been suggested recently to study the early precipitation stages of oxide nanoparticles. To clarify the details of Ti-based oxide nanoparticle precipitation, Ti+ and O+ ions were implanted into high-purity Fe-10 wt%Cr thin foils at room temperature and subjected to thermal annealing. Nano-size oxide particles and larger surface oxide islands with pronounced Cr enrichment were observed after in situ Transmission Electron Microscopy (TEM) annealing at 600 °C and were identified as a mixed iron-chromium spinel. The features observed after ex situ annealing at 800 °C were also identified as iron-chromium oxide, but with a certain titanium enrichment. The observations thus suggest that titanium plays no major role in the early stages of oxide precipitation.