Quantitative Assessment of Vacancy Defect Populations in Self-Ion Irradiated Molybdenum
Microstructural evolution–driven degradation governs material properties and is closely linked with defect behavior. Quantitatively characterizing defects and their evolution is essential for elucidating the underlying degradation mechanisms. To this end, the defects were introduced at room temperature using self-ion irradiation for damage levels ranging from 0.01 to 2 displacements-per-atom. The depth distribution of vacancy defects were characterized by means of a variable-energy positron beam and compared with simulation results. Quantitative analyses of vacancy in the most damaged regions were carried out by combining positron annihilation spectroscopy with a simulated annealing algorithm parametrized with a positron trapping model and first-principles calculations. The defect size distribution -from single vacancy to vacancy clusters- was assessed at each damage levels, providing insights into the quantification of early-stage vacancy defects. Our results revealed that at 0.01 dpa, nearly all vacacncy defect exists as isolated single vacancies. The proportion of isolate single vacancies gradually drops to ~20% with increasing damage level, reaching a steady state (> 0.5 dpa). Meanwhile, clusters consisting of four or more single vacancies account for ~12 % of the total vacancy defects. when the damage level exceeds 0.1 dpa, the formation of large clusters containing more than 15 vacancies, although limited to less than 1% in population, cannot be excluded. Furthermore, the estimated vacancy accumulation trend is consistent with available computational results, and unveils that the vacancy clustering is more pronounced in molybdenum than in tungsten et the early stage evolution.