First-Principles Study of Helium Trapping in Carbide Precipitates (Cr₂₃C₆) in Ferritic-Martensitic Steels

Experiments observed preferential He bubble formation in carbide precipitates M₂₃C₆ during low-temperature He irradiation in ferritic-martensitic steels. However, the process and mechanism of He trapping in M₂₃C₆ present a challenge to measure. Using density functional theory, we have systematically investigated He distribution, migration, and accumulation in Cr₂₃C₆. The formation energies of interstitial and substitutional He in Cr₂₃C₆ are 3.50 and 3.16 eV, respectively, remarkably lower than those in Fe matrix. The higher solubility of He in Cr₂₃C₆ makes it an He-trapping center in martensitic steels. On the other hand, the migration barrier of interstitial He in Cr₂₃C₆ is 2.58 eV, about 2.52 eV higher than that in bulk Fe. Furthermore, we only find a very weak attraction potency for substitutional-interstitial He pair, 0.25 eV, and even no binding trend for interstitial-interstitial or substitutional-substitutional He pairs, which suggests that it is more difficult for He atoms to move and less powerful driving force to accumulate in Cr₂₃C₆ than those in Fe matrix. Our results indicate that the trapping effect results from a lower charge density zone in Cr₂₃C₆, and predict that the small and dense Cr₂₃C₆ particles may hinder bubble growth at the initial stage, which can improve the resistance to irradiation void swelling.