MX precipitate behavior in an irradiated advanced Fe-9Cr steel: Helium effects on phase stability

Precipitates are main microstructural features to provide high temperature creep strength and radiation resistance in structural materials for fusion energy systems. However, the mechanisms of precipitate stability under irradiation in candidate structural materials for fusion first-wall and blanket components are poorly understood. In particular, the dual effects of helium transmutation and irradiation-induced damage on precipitate evolution have not been systematically studied in candidate materials, the leading of which are Fe-9Cr reduced activation ferritic/martensitic (RAFM) alloys. To fill this knowledge gap, a fundamental understanding of the single and combined interactions of helium (0-25 appm He/dpa), temperature (300-600C), and atomic displacements (15-100 dpa) on the behavior of MX (M=metal, X=C and/or N) precipitates in an advanced Fe-9Cr RAFM alloy were studied through the use of dual ion irradiation experiments. It was found that helium suppressed the diffusion-mediated mechanisms of precipitate stability (i.e. radiation-enhanced growth) at elevated temperatures and intermediate damage levels but had no effect on precipitate dissolution in the high dose conditions (>50 dpa). A precipitate stability model was used to rationalize the impacts of helium on ballistic dissolution and radiation-enhanced diffusion which are key contributors to overall precipitate stability. This is the second paper in a series of three to provide a systematic evaluation of MX precipitate behavior in RAFM steels under various fusion-relevant ion irradiation conditions.