  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.
