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Irradiation-induced ductility loss is a major concern facing structural steels in next-generation nuclear reactors. Currently, the mechanisms for this are unclear but crucial to address for the design of reactor components. Here, the deformation characteristics around nanoindents in Fe and Fe10Cr irradiated with Fe ions to $\sim$1 displacement-per-atom at 313 K are non-destructively studied. Deformation localisation in the irradiated materials is evident from the increased pile-up height and slip step formation, measured by atomic force microscopy. From 3D X-ray Laue diffraction, measurements of lattice rotation and strain fields near the indent site show a large confinement, over 85%, of plasticity in the irradiated material. We find that despite causing increased irradiation hardening, Cr content has little effect on the irradiation-induced changes in pile-up topography and deformation fields. The results demonstrate that varying Cr content in steels has limited impact on retaining strain hardening capacity and reducing irradiation-induced embrittlement., Comment: 23 pages, 9 figures (1 graphical abstract, 4 figures in main text, 4 figures in supplementary file)

Severe plastic deformation changes the microstructure and properties of steels, which may be favourable for their use in structural components of nuclear reactors. In this study, high-pressure torsion (HPT) was used to refine the grain structure of Eurofer-97, a ferritic/ martensitic steel. Electron microscopy and X-ray diffraction were used to characterise the microstructural changes. Following HPT, the average grain size reduced by a factor of $\sim$ 30, with a marked increase in high-angle grain boundaries. Dislocation density also increased by more than one order of magnitude. The thermal stability of the deformed material was investigated via in-situ annealing during synchrotron X-ray diffraction. This revealed substantial recovery between 450 K - 800 K. Irradiation with 20 MeV Fe-ions to $\sim$ 0.1 dpa caused a 20% reduction in dislocation density compared to the as-deformed material. However, HPT deformation prior to irradiation did not have a significant effect in mitigating the irradiation-induced reductions in thermal diffusivity and surface acoustic wave velocity of the material. These results provide a multi-faceted understanding of the changes in ferritic/martensitic steels due to severe plastic deformation, and how these changes can be used to alter material properties., Comment: 59 pages, 19 figures

Ferritic/martensitic steels will be used as structural components in next generation nuclear reactors. Their successful operation relies on an understanding of irradiation-induced defect behaviour in the material. In this study, Fe and FeCr alloys (3-12%Cr) were irradiated with 20 MeV Fe-ions at 313 K to doses ranging between 0.00008 dpa to 6.0 dpa. This dose range covers six orders of magnitude, spanning low, transition and high dose regimes. Lattice strain and hardness in the irradiated material were characterised with micro-beam Laue X-ray diffraction and nanoindentation, respectively. Irradiation hardening was observed even at very low doses (0.00008 dpa) and showed a monotonic increase with dose up to 6.0 dpa. Lattice strain measurements of samples at 0.0008 dpa allow the calculation of equivalent Frenkel pair densities and corrections to the Norgett-Robinson-Torrens (NRT) model for Fe and FeCr alloys at low dose. NRT efficiency for FeCr is 0.2, which agrees with literature values for high irradiation energy. Lattice strain increases up to 0.8 dpa and then decreases when the damage dose is further increased. The strains measured in this study are lower and peak at a larger dose than predicted by atomistic simulations. This difference can be explained by taking temperature and impurities into account., Comment: 49 pages, 9 figures, 3 tables

Ion-irradiated FeCr alloys are useful for understanding and predicting neutron-damage in the structural steels of future nuclear reactors. Previous studies have largely focused on the structure of irradiation-induced defects, probed by transmission electron microscopy (TEM), as well as changes in mechanical properties. Across these studies, a wide range of irradiation conditions has been employed on samples with different processing histories, which complicates the analysis of the relationship between defect structures and material properties. Furthermore, key properties, such as irradiation-induced changes in thermal transport and lattice strain, are little explored. Here we present a systematic study of Fe3Cr, Fe5Cr and Fe10Cr binary alloys implanted with 20 MeV Fe$^{3+}$ ions to nominal doses of 0.01 dpa and 0.1 dpa at room temperature. Nanoindentation, transient grating spectroscopy (TGS) and X-ray micro-beam Laue diffraction were used to study the changes in hardness, thermal diffusivity and strain in the material as a function of damage and Cr content. Our results suggest that Cr leads to an increased retention of irradiation-induced defects, causing substantial changes in hardness and lattice strain. However, thermal diffusivity varies little with increasing damage and instead degrades significantly with increasing Cr content in the material. We find significant lattice strains even in samples exposed to a nominal displacement damage of 0.01 dpa. The defect density predicted from the lattice strain measurements is significantly higher than that observed in previous TEM studies, suggesting that TEM may not fully capture the irradiation-induced defect population.

Ion implantation is widely used as a surrogate for neutron irradiation in the investigation of radiation damage on the properties of materials. Due to the small depth of damage, micromechanical methods must be used to extract material properties. In this work, nanoindentation has been applied to ion irradiated silicon carbide to extract radiation-induced hardening. Residual stress is evaluated using HR-EBSD, AFM swelling measurements, and a novel microcantilever relaxation technique coupled with finite element modelling. Large compressive residual stresses of several GPa are found in the irradiated material, which contribute to the significant hardening observed in nanoindentation measurements. The origin of these residual stresses and the associated hardening is the unirradiated substrate which constrains radiation swelling. Comparisons with other materials susceptible to irradiation swelling show that this effect should not be neglected in studying the effects of ion irradiation damage on mechanical properties. This constraint may also be influencing fundamental radiation defects. This has significant implications for the suitability of ion implantation as a surrogate for neutron irradiations. These results demonstrate the significance of swelling-induced residual stresses in nuclear reactor components, and the impact on structural integrity of reactor components., Comment: 23 pages, 7 figures

Raman spectroscopy has been used to identify defective bonding in neon and silicon ion irradiated single crystals of 6H-SiC. Observable differences exist in the C-C bonding region corresponding to different defect structures for neon and silicon ion implantations. Raman spectra of ion irradiated SiC show less tensile strain than neutron irradiations, explained by a residual compressive stress caused by the swelling constrained by the undamaged substrate. Evidence of oxidation during high temperature ion implantation is observed as C-O and Si-O Raman signals. Annealing irradiated SiC while acquiring Raman spectra shows rapid recovery of Si-C bonding, but not a complete recovery of the unirradiated structure. Annealing irradiated SiC causes surface oxidation where unirradiated SiC does not oxidise. Comparisons are made to the apparent radiation resistance of diamond and silicon which have similar crystal structures, but are monatomic, leading to the suggestion that chemical defects are responsible for increased radiation damage in SiC., Comment: 24 pages, 8 figures

The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, models of lithospheric flexure reveal differences in lithospheric strength that are difficult to reconcile based on such flow laws. Recent rheological data from low-temperature deformation experiments on olivine suggest that this discrepancy may be largely explained by strain hardening. Details of the mechanical data, specifically the effects of temperature-independent back stresses stored in the samples, indicate that strain hardening in olivine occurs primarily due to long-range elastic interactions between dislocations. These interpretations provided the basis for a new flow law that incorporates hardening by development of back stress. Here, we test this hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of < 4.3 GPa or in a nanoindenter at applied contact stresses of > 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10$^{14}$ m$^{-2}$ and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned along slip bands and interacting with dislocations of other types that act as obstacles. The stress heterogeneities and accumulations of dislocations along their slip planes are consistent with strain hardening resulting from long-range back-stresses acting between dislocations. These results corroborate the mechanical data in supporting the form of the new flow law for low-temperature plasticity and provide new microstructural criteria for identifying the operation of this deformation mechanism in natural samples.