The challenges for structural materials for high temperature advanced reactors are irradiation effects, corrosion, and elevated-temperature strength in the temperature range of 500-700°C where high temperature helium embrittlement limits component lifetime. Ni-based superalloys are a primary candidate alloy class for 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 and/or secondary precipitating phases in the lattice, such as the intermetallic phases δ, γʹ or γʹʹ or a carbide phase. The poor machinability and extensive work hardening of Ni-based superalloys makes additive or advanced manufacturing (AM) an attractive option for producing geometrically complex components with distinct microstructures while reducing the overall cost and shortening the supply chain. The absorption of transmutation produced helium at grain boundaries becomes a key factor in the propagation of cracks, possibly leading to subcritical crack growth by cavity coalescence. The dislocation-lattice and precipitate-lattice interfaces can act as benign locations in the microstructure to trap helium, reducing the detrimental accumulation of helium at the grain boundaries. In candidate Ni-based superalloys, helium trapping at interfaces will play a large role in the embrittlement response and thus the long-term service life of in-core components. We propose to investigate the effect of precipitate-lattice coherency on helium trapping using in-situ dual ion irradiation and annealing within a transmission electron microscope at high temperatures from 500-700°C. We will investigate the hypothesis that the larger free volume associated with incoherent precipitate-lattice will act as a greater sink for helium compared to coherent precipitates, and thus reduce cavity formation on grain boundaries at high temperature. The samples in this work were manufactured in the MDF using the laser powder bed fusion (LBPF) Concept Laser X-Line AM system and heat treated to produce compositionally identical Ni3Nb precipitates as either the incoherent orthorhombic δ phase or the coherent body centered tetragonal γ″ phase. The proposing team seeks use, through the Nuclear Science User Facilities, of the Michigan Ion Beam Laboratory (MIBL) for in-situ TEM ion irradiation with characterization and the Low Activation Materials Development and Analysis (LAMDA) facility at ORNL for liftout preparation and detailed post irradiation electron microscopy. The outcomes of this work will provide quantitative analysis of helium-induced cavity formation on grain boundaries and precipitates as a function of temperature. Completion of the proposed study will provide several outcomes: the effect of precipitate coherency on helium trapping and the relative partitioning of helium among precipitates and grain boundaries. This knowledge is critical to understanding how the precipitate microstructure of an alloy can be tailored to mitigate high temperature helium embrittlement for advanced nuclear technologies.

The Office of Nuclear Energy (NE) mission is to advance nuclear energy science and technology to meet U.S. energy, environmental, and economic needs. This RTE focuses on the generation of data related to cavity formation and helium buildup at precipitates and grain boundaries at temperatures relevant to high temperature advanced reactor concepts. The use of in-situ ion irradiations for scientific understanding fits the spirit of the Nuclear Energy Enabling Technologies (NEET) Crosscutting Technology Development (CTD) and the Reactor Concepts Research, Development, and Demonstration (RC RD&D) to advance the state of nuclear technology and hasten the development of a robust pipeline for screening proposed advanced reactor materials. Investigation of the processes influencing high temperature helium embrittlement of an additively manufactured nickel-based superalloy also fits within the interests of the RC RD&D’s program activities to address technical and cost issues associated with the advancement of high temperature reactors. Furthermore, the use of AM fits with the mission of the Advanced Methods for Manufacturing (AMM) crosscut program to accelerate innovations that reduce the cost and schedule of constructing new nuclear plants and make fabrication of nuclear power plant components faster, cheaper, and more reliable.