Increasing demand for energy and reduction of CO2 emissions has revived interest in nuclear energy. Designing materials for radiation environments necessitates fundamental understanding of how radiation-induced defects alter mechanical properties. Ion beams create radiation damage efficiently without material activation, but their limited penetration depth requires small-scale testing. However, strength measurements of nano-scale irradiated specimens have not been previously performed. Here we show that yield strengths approaching macroscopic values are measured from irradiated ~400 nm diameter copper specimens. Quantitative in situ nano-compression testing in a transmission electron microscope reveals that the strength of larger samples is controlled by dislocation-irradiation defect interactions, yielding size-independent strengths. Below ~400 nm, size-dependent strength results from dislocation source limitation. This transition length-scale should be universal, but depend on material and irradiation conditions. We conclude that for irradiated copper, and presumably related materials, nano-scale in situ testing can determine bulk-like yield strengths and simultaneously identify deformation mechanisms.

Large, freestanding membranes with remarkably high elastic modulus (>10 GPa) have been fabricated through the self-assembly of ligand-stabilized inorganic nanocrystals, even though these nanocrystals are connected only by soft organic ligands (e.g., dodecanethiol or DNA) that are not cross-linked or entangled. Recent developments in the synthesis of polymer-grafted nanocrystals have greatly expanded the library of accessible superlattice architectures, which allows superlattice mechanical behavior to be linked to specific structural features. Here, colloidal self-assembly is used to organize polystyrene-grafted Au nanocrystals at a fluid interface to form ordered solids with sub-10-nm periodic features. Thin-film buckling and nanoindentation are used to evaluate the mechanical behavior of polymer-grafted nanocrystal superlattices while exploring the role of polymer structural conformation, nanocrystal packing, and superlattice dimensions. Superlattices containing 3-20 vol % Au are found to have an elastic modulus of ∼6-19 GPa, and hardness of ∼120-170 MPa. We find that rapidly self-assembled superlattices have the highest elastic modulus, despite containing significant structural defects. Polymer extension, interdigitation, and grafting density are determined to be critical parameters that govern superlattice elastic and plastic deformation.elasticity | buckling | nanocomposite | thin film | nanoindentation N anocrystal superlattices are ordered arrays of ligand-stabilized colloidal nanocrystals with unique thermal (1, 2), optical (3, 4), and electronic (5, 6) properties due to the nanoscale dimensions and periodic spacing of the inorganic crystals. Nanocrystal superlattices also exhibit superior mechanical performance: superlattice elastic modulus has been shown to rival that of lightweight structural composites (>10 GPa), and superlattice membranes are capable of withstanding repeated indents to large displacements (7)(8)(9). This is all the more remarkable because mechanical cohesion in the superlattices is attributed to van der Waals interactions between ligands on neighboring nanocrystals (10, 11), which are weak enough that the ligands are liquid at room temperature when not attached to nanocrystals. Superlattice strength and stiffness can be further elevated to values that are unprecedented for polymer nanocomposites by cross-linking the organic ligands that coat the nanocrystals (12). The unusual combination of physical properties in nanocrystal superlattices presents intriguing opportunities to use these materials as mechanically actuated optoelectronic sensors (13,14), lightweight solar sails (15), and ultrathin barriers and coatings (16,17), but warrants the development of a thorough understanding of the mechanical behavior of nanocrystal superlattices. In particular, the roles of nanocrystal packing geometry, ligand structural conformation, and ligand-ligand and ligand-nanocrystal interactions must be clarified to design multifunctional, self-assembled polymer nanocomposites with improved mechanical ...

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Joining of dissimilar metals using high energy-density beams such as lasers and electron beams offer several advantages including precision, narrow fusion zones, and narrow heat affected zones (HAZ) that consequently result in reduced part distortion when compared to traditional joining processes. When high energy-density beams are combined with the design freedom offered by additive manufacturing (AM), or a layer-by-layer part fabrication process, it becomes possible to manufacture complex multi-material parts with improved joint characteristics resulting from controlled process parameters. Complex multi-material parts can be achieved that have tremendous impact on applications ranging from nuclear power plant components to repair applications. This research explores the feasibility of joining Inconel 718 with 316L Stainless Steel, and vice versa, by utilizing electron beam melting (EBM) additive manufacturing, a class of powder bed fusion technology. The use of this process can help avoid the use of filler materials, provides an evacuated processing environment resulting in limited contamination of oxides and nitrides, and can provide a high quality metallurgical joint while minimizing the thermal damage to surrounding material. Multi-material components were fabricated and the joint interfaces were characterized. Assessments of the interfaces revealed minimized thermal effects from the process and finer weld joints.