Conventional Oxide dispersion strengthened steels are characterized by thermally stable, high density of Y-Ti-O nanoclusters, which are responsible for their high creep strength. Ti plays a major role in obtaining a high density of ultrafine particles of optimum size range of 2-10 nm. In Al-containing ODS steels developed for corrosion resistance, Y-Al-O clusters formed are of size range 20 -100 nm, and Ti fails in making dispersions finer in the presence of Al. Usage of similar alloying elements like Zr in place of Ti is widely considered. In this study, binding energies of different stages of Y-Zr-O-Vacancy and Y-Al-O-Vacancy complexes in the bcc Iron matrix are studied by firstprinciple calculations. It is shown that in all the stages of formation, Y-Zr-O-Vacancy clusters have higher binding energy than Y-Al-O-Vacancy clusters and hence in ferritic steel containing both Zr and Al, Y-Zr-O-Vacancy clusters are more stable and more favored to nucleate than Y-Al-O-Vacancy clusters. The bonding nature in each stage is analyzed using charge density difference plots for the plausible reason for higher stability of Y-Zr-O-Vacancy clusters.

Ab initio molecular dynamics simulations using VASP was employed to calculate threshold displacement energies and defect formation energies of Y4Zr3O12 δ-phase, which is the most commonly found phase in newly developed Zr and Al-containing ODS steels. The Threshold displacement energy (Ed) values are determined to be 28 eV for Zr3a PKA along [111] direction, 40 eV for Zr18f atoms along [111] direction and 50 eV for Y recoils along [110] direction.Minimum Ed values for O and O' atoms are 13 eV and 16 eV respectively. The displacement energies of anions are much smaller compared to cations, thus suggesting that anion disorder is more probable than cation disorder. All directions except the direction in which inherent structural vacancies are aligned, cations tend to occupy another cation site. The threshold displacement energies are larger than that of Y2Ti2O7, the conventional precipitates in Ti containing ODS steels. Due to the partial occupancy of Y and Zr in the 18f position, the antisite formation energy is negligibly small, and it may help the structure to withstand more disorder upon irradiation. These results convey that Zr/Al ODS alloys, which have better corrosion resistance properties compared to the conventional Ti-ODS alloys, may also possess superior radiation resistance.

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