Abstract
The goal of the Advanced Materials and Manufacturing Technologies (AMMT) program is to accelerate the deployment of new materials and manufacturing technologies into advanced nuclear-related systems. Although 316H stainless steel fabricated by laser powder bed fusion (LPBF) has already been identified as an alloy that could have a significant effect on various reactor technologies, many other materials and manufacturing techniques are being evaluated. Nickel-based alloys typically offer higher-temperature capabilities compared with advanced stainless steels, and previous reports looked at three Ni-based alloy categories: low-Co alloys with a potential use close to the reactor core; high-temperature, high-strength alloys; and molten salt–compatible alloys. Due to the increasing interest in the low Co 625 alloy for nuclear applications, the alloy was compared at the 91 (ORNL) with 617, 230, two high-temperature, high-strength solution-strengthened alloys. Hot cracking could not be suppressed for alloy 230, and it was shown that these cracks, which were elongated along the build direction (BD), had a drastic effect on the ductility of alloy 230 at room temperature when specimens were machined perpendicular to the BD. Cracking was not as significant for alloy 617, but still led to significant variation in ductility at room temperature along the build direction. On the contrary, LPBF printing of crack-free alloy 625 was achieved using similar printing parameters, and the alloy looks like a very promising candidate for various reactor technologies. The fabrication by additive manufacturing (AM) of the strengthened 282 alloy was also performed, by LPBF at ORNL and by laser powder directed energy deposition (LP-DED) and gas metal arc directed energy deposition (GMA-DED) at the Idaho National Laboratory (INL). Optimization of the printing parameters resulted in materials with low-defect density, but the heat treatment needs to be further optimized to achieve full recrystallization and control the grain size. Characterization performed at INL on the LP-DED 282 alloy confirmed the presence of nano size precipitates in the alloy as well as carbides at grain boundaries. The fabrication of larger builds by LPBF for mechanical testing led to significant local variations in flaws density and computed tomography (XCT) scans of the alloy before and after creep testing at 750°C were carried out to establish correlation between the flaws’ volume fraction, creep ductility, and creep lifetime. While the low creep and creep-fatigue behaviors of the LPBF 282 alloy compared to wrought 282 could be attributed to the presence of flaws, the impact of other microstructural features such as the grain size and the presence of brittle carbides at grain boundaries need to be further analyzed. Finally, single track experiments were performed at ORNL on the two alloys previously identified as good molten salt–resistant, Ni-based candidates: Hastelloy N and 244. Various laser parameters were considered, and cracking was not observed for either of the two alloys. Molten salt exposure at INL confirmed the compatibility of these two alloys in the wrought form with Chloride and Fluoride salts, and the excellent creep strength of wrought 244 makes the alloy a great candidate for molten salt components operating at T>700°C.
