| Abstract Scope |
The structural materials for a fusion reactor blanket and first wall must withstand high operating temperatures (>600°C), creep fatigue, and lifetime irradiation dose (100 – 200 dpa). Reduced activation ferritic martensitic (RAFM) steels are leading material candidates but have limited thermal creep strength above ~550°C. Castable nanostructured alloys (CNAs) represent a novel class of RAFM steel designed to improve creep strength and radiation tolerance by optimizing high number densities of nanoscale precipitates, MX (M=Ta, Ti, V; X=C, N) and M23C6 (M=Cr, Fe). MX precipitates are thought to enhance creep properties due to their coarsening resistance and location in the matrix and on boundaries. Contrarily, M23C6 precipitates exhibit mixed effects due to their larger size (~100 nm), susceptibility to coarsening, and location only on boundaries. This study investigated two model CNAs with similar grain structure: a low-carbon CNA containing a high number density (~2×1021 m-3) of MX precipitates and a high-carbon CNA containing M23C6 precipitates and MX precipitates (~0.9×1021 m-3). Both alloys exhibit similar tensile properties yet differ significantly in creep properties. Thermal creep testing was performed at 575-650°C and 65-150 MPa applied stress. While it did not always have the lower minimum creep rate, the high-carbon CNA consistently demonstrated longer creep life compared to the low-carbon CNA, despite lower MX precipitate number density. Synchrotron X-ray diffraction and scanning/transmission electron microscopy (S/TEM) investigation revealed that precipitate stability and pinning of grain and subgrain boundaries accounted for the variations in creep behavior, highlighting the importance of precipitate microstructure in determining creep performance. |