Assessing Deformation Mechanisms in Irradiated Superalloy 718 using Ultra-Miniature Specimens

Ni-based superalloys are a primary candidate alloy class for current and advanced reactor applications because of their intrinsic resistance to creep, adequate corrosion resistance and the ability to tailor the microstructure for high strength. These high strength Ni-based alloys gain their strength primarily through solid solution strengthening from Mo and Nb additions and/or secondary precipitating phases in the lattice. The mechanical behavior of Ni-based alloys traditionally was attributed to conventional face centered cubic (fcc) deformation mechanisms: dislocation plasticity, localized dislocation channeling upon irradiation, and dislocation twinning in the presence of precipitates. Recent NSUF-supported work on unirradiated and 1 dpa neutron irradiated Ni-base alloy 625 revealed surprising room temperature deformation-induced fcc-to-hcp and fcc-to-bcc martensitic transformations previously unreported in Ni superalloys. Superalloy 718 has similar bulk Ni-Cr-Fe composition to alloy 625 with less Mo content and typically contains a high density of Nb-rich γ″ precipitates for high temperature strength. Preliminary tensile results suggest a similar martensitic transformation in superalloy 718 irradiated near 300 °C but not at 600 °C. Previous research on steels shows that the critical transformation stresses for ε-hcp and α’-bcc martensite variants are sensitive to irradiation-induced cavity (i.e., bubbles and void) configurations. We propose to investigate differences in deformation mechanisms between 300 °C and 600 °C irradiated wrought superalloy 718 specimens and understand the microstructural reasons for these differences. Our hypothesis is that martensitic-mediated deformation is active at room temperature after irradiation at 300 °C because a combination of irradiation-induced cavities, dislocation loops, Mo in solid solution, and/or γ" dissolution, all of which act to pin dislocations. The proposing team seeks use, through the Nuclear Science User Facilities, of the LAMDA facility at ORNL for fabrication and room temperature tensile straining of ultra-miniature SS-T specimens with corresponding scanning/transmission electron microscopy (STEM). Microstructural refinement such as nanoscale precipitation or irradiation reduces the extent of the specimen size effect through a larger number density of obstacles, enabling progressively smaller volumes to achieve representative properties. Thus, we propose to use a smaller geometry, colloquially SS-Tiny (SS-T), to produce additional tensile specimens from the already tested SS-J2 tensile heads to capture representative results, reduce the radiological hazards associated with handling tensile specimens, and enable detailed analysis in the Low Activation Materials Development and Analysis (LAMDA) laboratory. In-situ SEM tensile testing of ultra-miniature specimens at room temperature, with High angular Resolution EBSD (HR-EBSD), will enable us to understand how grains, phases, elastic stresses, and geometrically necessary dislocation densities evolve during deformation. Subsequent FIB lift-outs from the gauge of the ultra-miniature specimens will enable us to link the martensite and other deformation features with specific irradiation-induced defects. Thus, the outcome will be a multi-length scale comprehension linking deformation and irradiated induced defects in superalloy 718. In total, the proposed experiments will require an estimation of about 8 days for in-situ straining, 2 days to produce TEM lamella, and 4 days to characterize the irradiated and deformed microstructure.