Small specimen testing techniques have a long history in nuclear material research due to the limitations posed by nuclear facilities. The limited space in reactors and the fact that the samples are ...oftentimes radioactive in addition to the increasing need to obtain mechanical properties from ion beam irradiated samples require small specimen mechanical testing. With the application of modern focused ion beam sample preparation techniques and the enhancement of nanoindentation instruments, the size scale has been moved to even smaller scales and new geometries. Micrometer and even nanometer size samples are feasible, but raise the question of comparability to large scale properties for engineering applications. In this review, we summarize available small scale materials testing techniques and potential shortcomings based on examples from the literature, as well as introduce novel experimental approaches conducted using microcompression testing, microbend bar testing, and nanoindentation at ambient and nonambient conditions.
Exposure of austenitic stainless steels to liquid lead–bismuth eutectic with low concentration of dissolved oxygen typically results in selective leaching of highly-soluble alloying elements and ...ferritization of the dissolution-affected zone. In this work, focused ion beam, transmission electron backscatter diffraction and scanning transmission electron microscopy were utilized to elucidate early-stage aspects of the dissolution corrosion process of cold-worked austenitic stainless steels exposed to static, oxygen-poor liquid lead–bismuth eutectic at 450°C for 1000h. It was found that deformation-induced twin boundaries in the cold-worked steel bulk provide paths of accelerated penetration of the liquid metal into the steel bulk.
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The results from a novel, micro-tensile testing technique, employing a micro-electro-mechanical system, operated in a push-to-pull configuration, to study the effects of radiation damage on Inconel ...X-750, are presented. Non-irradiated material, along with material irradiated to 67 dpa and 81 dpa at two irradiation temperatures, 120–280 °C and 300–330 °C, is investigated. This testing approach implements a safe, lift-out, extraction method that enables the evaluation of material tensile behavior in specific locations of radioactive components with complex geometries outside of costly hot cell protective environments. Regional cold working and grinding manufacturing processes that go undetected in bulk component testing can be evaluated in parallel with radiation damage effects. Non-irradiated specimens on the order of 1 µm × 1 µm × 2.5 µm taken from center regions of the material unaffected by processing produced yield strengths of 938 MPa and 1043 MPa, in good agreement with the bulk yield strength of Inconel X-750: 972–1070 MPa. Mechanical strengths of material irradiated to 67 dpa decreased by ~75 MPa for material irradiated at 300–330 °C and ~176 MPa for material irradiated at 120–280 °C, compared to the non-irradiated material. However, material irradiated to 81 dpa has practically identical mechanical strengths at the two irradiation temperatures, and these strengths are ~100 MPa greater than those of non-irradiated material. Average ductility of the material decreases more quickly when irradiated at 300–330 °C, from an initial value of ~15% to ~5% after 67 dpa, whereas the ductility of the material irradiated at 120–280 °C remains close to the initial value at 67 dpa and decreases to ~2% after 81 dpa.
We discuss the challenge of selecting materials for nuclear applications and outline the need for comprehensive databases to assist scientists and engineers in choosing materials that meet ...interdependent physical, chemical, and nuclear criteria. In conventional engineering, chemical and physical properties and the electronic structure of materials are typically the primary considerations; nuclear applications must also consider the nuclear physics characteristics of a material. Development of databases that correlate physical, chemical, and nuclear properties would accelerate and facilitate innovations in nuclear design.
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Increasing demand for energy and reduction of carbon dioxide emissions has revived interest in nuclear energy. Designing materials for radiation environments necessitates a 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 nanoscale 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 nanocompression 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 depends on material and irradiation conditions. We conclude that for irradiated copper, and presumably related materials, nanoscale in situ testing can determine bulk-like yield strengths and simultaneously identify deformation mechanisms.
Small scale mechanical testing has many beneficial features including the ability to sample the properties of a single crystal over a wide temperature range. This manuscript presents the results of ...high temperature, in-situ scanning electron microscope microcompression testing of a single crystal of UO2. Observations indicated that the UO2 microcompression specimens deformed in a ductile fashion at temperatures of 350 °C and above. The slip traces on the micro-compression specimens after deformation appeared on the {100}1/2 system at the temperatures and orientation tested. Lastly, the Peierls stress of UO2 is calculated using the microcompression data.
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This work features the synthesis of nanoscale cavities using a He ion beam implantation method. A multitechnique (AFM, TEM, nanoindentation) post synthesis approach allows for the characterization of ...a 200 nm thin implantation region containing nanocavities. It was found that at lower doses, the cavities arrange themselves in a superlattice and are close to equilibrium pressure while at higher doses the self-organization of the nanoscale cavities is lost, forming cavities that change in nature resulting in a degradation of the mechanical properties. Close correlation between theory and experimental observation is achieved considering for the observed phenomena.
Nanoindentation is a widespread and useful method for evaluating mechanical properties at the sub-micron length scale. However, the indentation size effect remains a major obstacle to obtain ...meaningful macroscopic mechanical properties from small volume testing. This work systematically addresses the Indentation Size Effect (ISE) phenomenon as a function of temperature and defect density in an austenitic Fe-Cr-Ni alloy (800H) to establish the baseline for nanoindentation testing of ion-irradiated alloys in environmental conditions. The 800H steel sample was irradiated with 70 MeV Fe9+ at 450 °C to the total dose of 20.68 dpa. All samples were tested up to 300 °C in order to quantify the effect of temperature on the indentation size effect. It was found that in all cases, the ISE is less pronounced at high temperatures due to the increase of the plastic zone size. For the same grain orientation, the ISE is less pronounced in the irradiated 800H at all temperatures.
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Focused ion beam machining revolutionized the way samples are studied in materials science. The introduction of this tool enabled researchers to break into new frontiers of research. While this ...method focused on the nanometer length scale, the mesoscale manufacturing of samples has not seen such advances. It remains a challenge to manufacture sample geometries too large for focused ion beam milling yet too small for conventional machining. Femtosecond laser ablation opens this length scale of sample space and allows the fabrication of numerous useful geometries. This paper outlines a state-of-the-art femtosecond laser machining system that can be used for rapid, micro- and mesoscale sample preparation. To illustrate the utility of this system, stress–strain data are presented for single-crystal Cu micropillars and three microscale tensile test specimens prepared from physical-vapor-deposited Cu and Ni foils.
•Micro-pillar testing carried out on CuCrZr with and without irradiation defects.•Intrinsic and extrinsic size effects obvious only in unirradiated CuCrZr.•Size-independent results obtained from ...smaller pillars following irradiation.•DBH and BKS models predict hardening using microstructural length-scales.
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The results of small-scale mechanical tests are convoluted by the so-called size effect, whereby materials appear stronger when the scale of the test is reduced to the order of microns or less. The dimensional range over which this occurs has been shown to be linked to a change in sample microstructure, such as the addition of defects induced by irradiation. To investigate this response, a CuCrZr alloy was subjected to proton irradiation and mechanically tested using micro compression of pillars with a range in size. It was found that irradiation defects dominate over the extrinsic size effect and the sensitivity to differences in precipitate microstructure was also somewhat reduced, suggesting that size-independent results could be obtained from much smaller test volumes in irradiated material compared to their non-irradiated counterparts. Finally, comparison was made between the increase in yield strength predicted by models and the experimentally measured values to establish the key parameters driving the strengthening behaviour.
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