Irradiation-induced NanoCluster Evolution

Oxide dispersion strengthened steel (ODS) and commercial ferritic-martensitic (F-M) alloys are widely accepted candidate structural materials for designing advanced nuclear reactors. Nanoclusters embedded in the steel matrix are key microstructural features of both alloy types. Irradiation from nuclear fusion and fission affects the morphology of these nanoparticles, altering the performance of the alloys and potentially decreasing their usable lifetime. Thus, it is important to understand the effect of irradiation on these nanoparticles in order to predict long-term nuclear reactor performance. It was found that the evolution of nanoclusters in each material is different depending on the experimental irradiation parameters. The Nelson-Hudson-Mazey (NHM) model has been refined based on previous experimental work, and has been shown to be an effective model to simulate irradiation-induced nanocluster evolution in ODS and F-M steels. In this work, an NHM simulation tool was developed for nanoHUB, with a simplified user interface that enables rapid prediction of the effect of irradiation on the size of nanoclusters in a variety of Fe-based steels

Enhanced radiation damage tolerance of amorphous interphase and grain boundary complexions in Cu-Ta

Amorphous interfacial complexions are particularly resistant to radiation damage and have been primarily studied in alloys with good glass-forming ability, yet recent reports suggest that these features can form even in immiscible alloys such as Cu-Ta under irradiation. In this study, the mechanisms of damage production and annihilation due to primary knock-on atom collisions are investigated for amorphous interphase and grain boundaries in a Cu-Ta alloy using atomistic simulations. Amorphous complexions, in particular amorphous interphase complexions that separate Cu and Ta grains, result in less residual defect damage than their ordered counterparts. Stemming from the nanophase chemical separation in this alloy, the amorphous complexions exhibit a highly heterogeneous distribution of atomic excess volume, as compared to a good glass former like Cu-Zr. Complexion thickness, a tunable structural descriptor, plays a vital role in damage resistance. Thicker interfacial films are more damage-tolerant because they alter the defect production rate due to differences in intrinsic displacement threshold energies during the collision cascade. Overall, the findings of this work highlight the importance of interfacial engineering in enhancing the properties of materials operating in radiation-prone environments and the promise of amorphous complexions as particularly radiation damage-tolerant microstructural features

The Comparison of Microstructure and Nanocluster Evolution in Proton and Neutron Irradiated Fe–9%Cr ODS Steel to 3 DPA at 500 °C

A model Fe-9%Cr oxide dispersion strengthened (ODS) steel was irradiated with protons or neutrons to a dose of 3 displacements per atom (dpa) at a temperature of 500 °C, enabling a direct comparison of ion to neutron irradiation effects at otherwise fixed irradiation conditions. The irradiated microstructures were characterized using transmission electron microscopy and atom probe tomography including cluster analysis. Both proton and neutron irradiations produced a comparable void and dislocation loop microstructure. However, the irradiation response of the Tie-Ye-O oxide nanoclusters varied. Oxides remained stable under proton irradiation, but exhibited dissolution and an increase in Y:Ti composition ratio under neutron irradiation. Both proton and neutron irradiation also induced varying extents of Si, Ni, and Mn clustering at existing oxide nanoclusters. Protons are able to reproduce the void and loop microstructure of neutron irradiation carried out to the same dose and temperature. However, since nanocluster evolution is controlled by both diffusion and ballistic impacts, protons are rendered unable to reproduce the nanocluster evolution of neutron irradiation at the same dose and temperature

Nanoindentation Investigation of Chloride-Induced Stress Corrosion Crack Propagation in an Austenitic Stainless Steel Weld

Transgranular chloride-induced stress corrosion cracking (TGCISCC) is a mounting concern for the safety and longevity of arc welds on austenitic stainless steel (AuSS) nuclear waste storage canisters. Recent studies have shown the key role of crystallography in the susceptibility and propagation of TGCISCC in SS weldments. Given that crystallography underlies mechanical heterogeneities, the mechanical-crystallographic relationship during TGCISCC growth must be understood. In this study, welded SS 304L coupons are loaded in four-point bend fixtures and then boiled in magnesium chloride to initiate TGCISCC. Nanoindentation mapping is paired with scanning electron microscopy (SEM) electron backscatter diffraction (EBSD) to understand the correlation between grain orientation, grain boundaries, and hardening from TGCISCC propagation. The nanoindentation hardness of individual grains is found to not be a controlling factor for TGCISCC propagation. However, intragranular hardness is generally highest immediately around the crack due to localized strain hardening at the crack tip. This work shows that nanoindentation techniques can be useful in understanding CISCC behaviors when paired with electron microscopy

Microstructure Impacts on Mechanical Properties in a High Temperature Austenitic Stainless Steel

Austenitic and super-austenitic stainless steels are a critical component of the spectrum of high temperature materials. With respect to power generation, alloys such as Super 304H and NF709 span a gap of capability between ferritic and martensitic high chromium steels and nickel-based alloys in boiler tube applications for both conventionally fired boilers and heat-recovery steam generators (HRSG). This research explores a wrought version of a cast austenitic stainless steel, CF8C-Plus or HG10MNN, which offers promise in creep strength at relatively low cost. Various manufacturing techniques have been employed to explore the impact of wrought processing on nano-scale microstructure and ultimately performance, especially in high temperature creep. Transmission electron microscopy has been used to quantify and characterize the creep-strengthening particles examining the relationship between traditional melting and extrusion as compared to powder metallurgy

Method for Evaluating Irradiation Effects on Flow Stress in Fe-9%Cr ODS Using TEM In Situ Cantilevers

Transmission electron microscopic (TEM) in situ mechanical testing has become a widely utilized tool for simultaneously measuring mechanical properties and understanding fundamental deformation mechanisms in irradiated and nuclear materials. Although tensile and compression specimen geometries are among the most common, opportunities remain for investigating alternative geometries that could provide unique insights into the plasticity of irradiated materials. This work demonstrates a new TEM in situ cantilever beam configuration. Cantilevers are produced from as-received and proton-irradiated (1 dpa, 500°C) Fe-9%Cr oxide dispersion-strengthened steel. Flow stress is measured using a TEM in situ depth-sensing mechanical testing holder. A 200-MPa increase in flow stress is measured due to irradiation. Size effects arise when the intrinsic (i.e., microstructural) size approaches the extrinsic (i.e., external dimensions) size and can be described using a power law relationship as a function of the material microstructure and cantilever dimensions

TEM in Situ Micropillar Compression Tests of Ion Irradiated Oxide Dispersion Strengthened Alloy

The growing role of charged particle irradiation in the evaluation of nuclear reactor candidate materials requires the development of novel methods to assess mechanical properties in near-surface irradiation damage layers just a few micrometers thick. In situtransmission electron microscopic (TEM) mechanical testing is one such promising method. In this work, microcompression pillars are fabricated from a Fe2+ ion irradiated bulk specimen of a model Fe-9%Cr oxide dispersion strengthened (ODS) alloy. Yield strengths measured directly from TEM in situ compression tests are within expected values, and are consistent with predictions based on the irradiated microstructure. Measured elastic modulus values, once adjusted for the amount of deformation and deflection in the base material, are also within the expected range. A pillar size effect is only observed in samples with minimum dimension ≤100 nm due to the low inter-obstacle spacing in the as received and irradiated material. TEM in situ micropillar compression tests hold great promise for quantitatively determining mechanical properties of shallow ion-irradiated layers

Method for Fabricating Depth-Specific TEM In Situ Tensile Bars

The growing use of ion irradiation to assess degradation of nuclear materials has created a need to develop novel methods to probe the mechanical response of shallow ion-irradiated layers. Transmission electron microscopy (TEM) in situ mechanical testing can isolate the ion-irradiated layer from its unirradiated substrate. However, there is a lack of established procedures for preparing TEM in situ mechanical testing specimens from bulk materials requiring depth-specific examination, e.g., target dose on the ion irradiation damage profile. This study demonstrates a new method for extracting depth-specific TEM in situ tensile bars from a bulk specimen of Fe-5 wt.%Mo. Measured yield stress, ultimate tensile stress, Young’s modulus, and elongation are consistent with those properties obtained from similarly sized Fe and Mo single-crystal nanowires. Results are discussed in the context of the specimen size effect

TEM \u3cem\u3ein situ\u3c/em\u3e Cube-Corner Indentation Analysis Using ViBe Motion Detection Algorithm

Transmission electron microscopic (TEM) in situ mechanical testing is a promising method for understanding plasticity in shallow ion irradiated layers and other volume-limited materials. One of the simplest TEM in situ experiments is cube-corner indentation of a lamella, but the subsequent analysis and interpretation of the experiment is challenging, especially in engineering materials with complex microstructures. In this work, we: (a) develop MicroViBE, a motion detection and background subtraction-based post-processing approach, and (b) demonstrate the ability of MicroViBe, in combination with post-mortem TEM imaging, to carry out an unbiased qualitative interpretation of TEM indentation videos. We focus this work around a Fe-9%Cr oxide dispersion strengthened (ODS) alloy, irradiated with Fe2+ ions to 3 dpa at 500 °C. MicroViBe identifies changes in Laue contrast that are induced by the indentation; these changes accumulate throughout the mechanical loading to generate a “heatmap” of features in the original TEM video that change the most during the loading. Dislocation loops with b = ½ \u3c111\u3e identified by post-mortem scanning TEM (STEM) imaging correspond to hotspots on the heatmap, whereas positions of dislocation loops with b = \u3c100\u3e do not correspond to hotspots. Further, MicroViBe enables consistent, objective quantitative approximation of the b = ½ \u3c111\u3e dislocation loop number density