Irradiation-induced Strain Research Articles

Synchrotron-based x-ray diffraction analysis of energetic ion-induced strain in GaAs and 4H-SiC

Strain engineering using ion beams is a current topic of research interest in semiconductor materials. Synchrotron-based high-resolution x-ray diffraction has been utilized for strain-depth analysis in GaAs irradiated with 300 keV Ar and 4H-SiC and GaAs irradiated with 100 MeV Ag ions. The direct displacement-related defect formation, anticipated from the elastic energy loss of Ar ions, can well explain the irradiation-induced strain depth profiles. The maximum strain in GaAs is evaluated to be 0.88% after Ar irradiation. The unique energy loss depth profile of 100 MeV Ag (swift heavy ions; SHIs) and resistance of pristine 4H-SiC and GaAs to form amorphous/highly disordered ion tracks by ionization energy loss of monatomic ions allow us to examine strain buildup due to the concentrated displacement damage by the elastic energy loss near the end of ion range (∼12 μm). Interestingly, for the case of SHIs, the strain-depth evolution requires consideration of recovery by ionization energy loss component in addition to the elastic displacement damage. For GaAs, strain builds up throughout the ion range, and the maximum strain increases and then saturates at 0.37% above an ion fluence of 3×1013 Ag/cm2. For 4H-SiC, the maximum strain reaches 4.6% and then starts to recover for fluences above 1×1013 Ag/cm2. Finally, the contribution of irradiation defects and the purely mechanical contribution to the total strain have been considered to understand the response of different compounds to ion irradiation.

Irradiation-induced strain localization and strain burst suppression investigated by microcompression and concurrent acoustic emission experiments

Irradiation-induced strain localization and strain burst suppression investigated by microcompression and concurrent acoustic emission experiments

Detecting irradiation-induced strain localisation on the microstructural level by means of high-resolution digital image correlation

Materials subjected to irradiation damage often undergo local microstructural changes that can affect their expected performance. To investigate such changes, this work proposes a novel approach to detect strain localisation caused by irradiation-induced damage in nuclear materials on the microstructural level, considering a statistically relevant number of grains. This approach determines local strains using high-resolution digital image correlation (HRDIC) and compares them with the underlying material microstructure. Sets of images captured before and after irradiation are compared to generate full-field displacement maps that can then be differentiated to yield high-resolution strain maps. These strain maps can subsequently be used to understand the effects of irradiation-induced dimensional change and cracking on the microscale. Here, the methodology and challenges involved in combining scanning electron microscopy (SEM) with HRDIC to generate strain maps associated with radiation-induced damage are presented. Furthermore, this work demonstrates the capabilities of this methodology by analysing three different materials subjected to proton irradiation: a zircaloy-4 (Zry-4) metal irradiated to 1 & 2 dpa, and two ceramics based on MAX phase compounds, i.e., the Nb4AlC3 ternary compound and a novel (Ta,Ti)3AlC2 solid solution, both irradiated to -0.1 dpa. These results demonstrated that all materials show measurable expansion, and the very high strains seen in the MAX phase ceramics can be easily attributed to their microstructure. Grain-to-grain variability was observed in Zry-4 with a macroscopic expansion along the rolling direction that increased with irradiation damage dose, the Nb4AlC3 ceramic showed significant expansion within individual grains, leading to intergranular cracking, while the less phase-pure (Ta,Ti)3AlC2 ceramic exhibited very high strains at phase boundaries, with limited expansion in the binary carbide phases. This ability to measure irradiation-induced dimensional changes at the microstructural scale is important for designing microstructures that are structurally resilient during irradiation.

Coupled irradiation-thermal-mechanical analysis of the solid-state core in a heat pipe cooled reactor

The solid-state core of a heat pipe cooled reactor operates at high temperatures over 1000 K with thermal and irradiation-induced expansion during burnup. The expansion changes the gap thickness between the solid components and the material properties, and may even cause the gap closure, which then significantly influences the thermal and mechanical characteristics of the reactor core. This study developed an irradiation behavior model for HPRTRAN, a heat pipe reactor system analysis code, to introduce the irradiation effects such as swelling and creep. The megawatt heat pipe reactor MegaPower was chosen as an application case. The coupled irradiation-thermal-mechanical model was developed to simulate the irradiation effects on the heat transfer and stresses of the whole reactor core. The results show that the irradiation deformation effect is significant, with the irradiation-induced strains up to 2.82% for fuel and 0.30% for monolith at the end of the reactor lifetime. The peak temperatures during the lifetime are 1027.3K for the fuel and 956.2K for monolith. The gap closure enhances the heat transfer but caused high stresses exceeding the yield strength in the monolith.

Irradiation hardening and ductility loss of Eurofer97 steel variants after neutron irradiation to ITER-TBM relevant conditions

Ten Eurofer97 steel variants, produced by non-standard fabrication-processing routes and modified alloying chemistries, were studied by neutron irradiations in the high flux isotope reactor. The irradiations were performed to ITER-TBM relevant conditions of ∼255–350 °C, 2.94–3.24 dpa. We quantified the irradiation-induced degradation of the steels using mechanical property tests. All the steels suffered from irradiation hardening, where a significant increase in Vickers microhardness and yield stress (σYS) occurred, accompanied with severe loss of tensile elongation. The extent of hardening was material dependent. For Tirr = 300±30 °C, most steels showed σYS increase in the range of ∼30% to as high as ∼66%, except for a low temperature tempered steel with σYS increase below 15%. Despite large losses in elongation, most failures were ductile. Significant post-necking ductility was retained with reduction in area (RA) between 65–75%, but <50% for low temperature tempered steels. The ultimate tensile stress to yield stress (σUTS/σYS) ratios decreased significantly after irradiation, highlighting irradiation-induced strain hardening capacity reduction. No major effect of irradiation on the plastic instability stress (σPIS) and true fracture stress of the steels was observed. By comparing the tensile stresses in true stress units and with literature, the results suggest that RAFM steel designing should target materials with a large separation between σPIS and σYS, to ensure the materials can maintain large work hardening and uniform deformation capability after irradiation. The tensile data of the steels additionally revealed a compelling evidence of an inverse trend between the change in RA and increase in σYS of the neutron irradiated Eurofer97 type steels.

High-energy 120 MeV Au9+ ion beam-induced modifications and evaluation of craters in surface morphology of SnO2 and TiO2 nanocomposite thin films

The electronic excitation and modification in the properties of the target material occur due to the kinetic energy loss of incident ions mainly through the inelastic collision with atomic electrons in the process of electronic energy stopping. Nanocomposite tin oxide (SnO2) and titanium dioxide (TiO2) thin film were grown on silicon and ITO substrates using RF magnetron-sputtering process. These thin films were irradiated by 120 MeV Au9+ Swift Heavy Ion (SHI) beam with varying fluence from 5 × 1011 ions cm−2 to 2 × 1013 ions cm−2 to induce modifications in thin films by dense electronic excitation. Surface morphology and depth profile of craters (approx. 85 nm) have been characterized by multimode atomic force microscopy (AFM). Grain mapping of AFM images provided the variation in grain size in the range of 72–279 nm and RMS roughness increased (4.01–23.6 nm) with increasing the ion fluence. Scanning Electron Microscopy (SEM) was also carried out to confirm the formation of craters in the irradiated film. X-ray diffraction (XRD) technique has been used for the structural analysis of virgin and irradiated thin films. XRD patterns established the formation of film in mixed (rutile and gamma) phase. Further, the crystallite size was evaluated using the Debye–Scherrer and Williamson–Hall method. It was found that crystallite size lie in the range of 36–54 nm which increased for the lower fluence and then decreased for higher fluence. The ion irradiation-induced strain has been explained by Thermal spike and Coulomb explosion model. UV–Vis study showed that the optical band gap reduced from 3.87 to 3.72 eV with increasing the ion fluence which can be ascribed to the formation of intermediate states and free radicals between the valance and band conduction band. Rutherford Backscattering Spectrometry (RBS) ensured that the film thickness decreased from 210 to 183 nm with increasing the ion fluence. The simulation of the RBS spectra provides the elemental composition of Sn (0.37), Ti (0.65) and O (0.85) in SnO2 and TiO2 nanocomposite thin films.

Evolution of irradiation-induced strain in an equiatomic NiFe alloy

The evolution of irradiation-induced atomic strain in an equiatomic NiFe concentrated solid-solution alloy is investigated using both atomistic simulations and x-ray diffraction analysis. The irradiations are performed using 1.5MeV Ni ions to fluences ranging from 1×1013 to 1×1014cm−2. The irradiation simulations are carried out by overlapping 5keV Ni recoils cascades up to a total of 300 recoils. An increase of volumetric strain is observed at low dose, which is associated with production of point defects and small clusters. Relaxation of strain occurs at higher doses, when large defect clusters, e.g., dislocation loops, dominate.

Mechanical analysis of surface-coated zircaloy cladding

A structural model for stress distributions of coated Zircaloy subjected to realistic incore pressure difference, thermal expansion, irradiation-induced axial growth, and creep has been developed in this study. In normal operation, the structural integrity of coating layers is anticipated to be significantly challenged with increasing burnup. Strain mismatch between the zircaloy and the coated layer, due to their different irradiation-induced axial growth, and creep deformation are found to be the most dominant causes of stress. This study suggests that the compatibility of the high temperature irradiation-induced strains (axial growth and creep) between zircaloy and the coating layer and the capability to undergo plastic strain should be taken as key metrics, along with the traditional focus on chemical protectiveness.

Mechanical response of UO2 single crystals submitted to low-energy ion irradiation

{111}- and {100}-oriented UO2 single crystals were irradiated with 500-keV Ce3+ ions in the 1014–9 × 1014 cm−2 fluence range. The irradiation-induced strain was monitored using high-resolution X-ray diffraction. A mechanical modelling dedicated to thin irradiated layers was applied to account for the reaction of the unirradiated part of the crystals. The elastic strain, which is confined along the surface normal of the samples, increases with ion fluence until it is dramatically relieved. This behaviour is observed for both orientations. While the measured elastic strain depends on the crystallographic direction, the strain due to irradiation defects only is found to be equal for both directions, with a maximum value of ∼0.5%. Strain relaxation takes place at the damage peak, but the in-plane lattice parameter of the irradiated layer remains unchanged and equal to that of the pristine material. Meanwhile, the strain at the damaged/pristine interface continues to increase.

Tensile behavior of EUROFER ODS steel after neutron irradiation up to 16.3 dpa between 250 and 450 °C

During the development of structural material for future fusion reactors, a 50kg heat of reduced-activation ferritic-martensitic 9%CrWVTa steel with nanoscaled Y2O3-particles, EUROFER97 ODS HIP, was produced using powder metallurgy fabrication technology. This first batch of EUROFER97 ODS HIP and, for comparison, the steel EUROFER97 were prepared for a post-irradiation tensile test program. During neutron irradiation in the HFR (High Flux Reactor, The Netherlands), an accumulated dose of up to 16.3dpa was reached for 771 days at full power, with the irradiation temperature ranging between 250 and 450°C. During the post-examinations, all specimens showed the highest tensile strength at lower irradiation temperatures between 250 and 350°C. However, ODS-alloy and steel were found to clearly differ in the mechanical behavior, which could be documented by fully instrumented tensile tests. In the un-irradiated state, tensile strength of the ODS-alloy already was increased considerably by about 60% compared to the steel. Strengthening was further increased by another 20% after neutron irradiation, but with a much better ductility than observed in the steel. The typical irradiation-induced strain localization of EUROFER97 or RAFM steels could not be observed in the EUROFER97 ODS HIP alloy.

Response of cubic zirconia irradiated with 4-MeV Au ions at high temperature: An X-ray diffraction study

Yttria-stabilized cubic zirconia single-crystals have been irradiated with 4-MeV Au2+ ions at fluences ranging from 1012cm−2 to 5×1015cm−2 and at three temperatures (room temperature, 500°C and 800°C). Evaluation of the irradiation-induced strain has been performed by the X-ray diffraction technique. It is found that, whatever the irradiation temperature, the elastic-strain build-up exhibits two steps. An increase of the (tensile) strain-level is observed in the first step. A drastic strain relaxation occurs at a transition fluence, which defines the beginning of the second step. Increasing the irradiation temperature induces a decrease of the strain level and a shift of the transition fluence towards low fluence. Both effects may be explained by an enhanced defect-clustering rate which occurs already at 500°C.

The influence of cold-work level on the irradiation creep and swelling of AISI 316 stainless steel irradiated as pressurized tubes in the EBR-II fast reactor

Pressurized tubes of AISI 316 stainless steel irradiated in the P-1 experiment in the EBR-II fast reactor have been measured to determine the dependence of irradiation-induced strains resulting from plastic deformation, irradiation creep, void swelling and precipitation. It is shown that the Soderberg relation predicting no axial creep strains in biaxially-loaded tubes is correct for both plastic and creep strains. Swelling strains are shown to be isotropically distributed both for stress-free and stress-affected swelling, while precipitation strains are somewhat anisotropic in their distribution. When corrected for stress-enhancement of swelling, the derived irradiation creep strains appear to be identical for both annealed and 20% cold-worked specimens, and also for tubes strained by rise to power increases in pressure. For relatively small creep strains it is often difficult to separate the creep and non-creep components of deformation.

Structural, electrical transport, magnetization, and 1∕f noise studies in 200MeV Ag ion irradiated La0.7Ce0.3MnO3 thin films

The effect of 200MeV Ag ion irradiation on structural, electrical transport, magnetization, and low-frequency conduction noise properties of electron-doped La0.7Ce0.3MnO3 thin films have been investigated. The as-grown thin films show c-axis epitaxial structure along with a small amount of unreacted CeO2 phase. After the irradiation, at the lowest fluence both the magnetization and metal-insulator transition temperature increase. Further increase in fluence reduces the metal-insulator transition temperature and leads to larger resistivity; however, the unreacted phase of CeO2 disappears in the x-ray diffraction pattern. On the other hand, the normalized electrical noise is greatly enhanced even at the lowest nonzero fluence. Surprisingly the conducting noise in the irradiated samples is much higher in the metallic state than in the semiconducting one. The observed modifications in structural, electrical, magnetic, and noise properties of 200MeV Ag ion irradiated La0.7Ce0.3MnO3 thin films have been explained on the basis of effects of the presence of swift heavy-ion irradiation-induced strain and defects.

Modelling of dimensional changes in irradiated nucleargraphites

Neutron irradiation of the polygranular graphites used in reactormoderators results in an initial bulk shrinkage at low neutron fluencefollowed by expansion at higher neutron fluence. Recent work carried out toinvestigate the effects of simultaneous radiolytic oxidation on thedimensional-change behaviour of the Gilsocarbon graphites used in the UKadvanced gas-cooled reactors (AGRs) has led to the development of a newdimensional change model for all irradiation-induced strains. Although inthe future high-temperature graphite-moderated generating systems underdevelopment, concurrent oxidation is excluded, oxidation may still occur toa small degree. This paper presents an approach to modelling dimensionalchange applicable to most situations. For AGR moderator graphite, the modelpresented is able to reproduce semi-quantitatively the effects ofirradiation and radiolytic oxidation on dimensional changes. Further workis required to refine the modelling approach to particular situations, butit is envisaged that the model can be applied to future high-temperaturereactor systems.

Damage and strain in pseudomorphic vs relaxed GexSi1−x layers on Si(100) generated by Si ion irradiation

We compare both the strain and damage that 100 keV Si irradiation at room temperature introduces in pseudomorphic and relaxed GexSi1−x films grown on Si(100) substrates. The ion range is such that the Si/GexSi1−x interface is not significantly damaged. The amount of damage produced in pseudomorphic and relaxed GexSi1−x layers of similar x for irradiation doses up to 2.5 × 1014 Si/cm2 is the same, which proves that a pre-existing uniform strain does not noticeably affect the irradiation-induced damage. However, the irradiation-induced strain does depend on the pre-existing strain of the samples. Possible interpretations are discussed.

Thermally induced strain recovery in irradiated and unirradiated zirconium alloy stress-relaxation specimens

Thermally induced strain recovery has been measured in specimens which were stress relaxed at 570 K either in a reactor or in an autoclave. Strain recovery occurs in both unirradiated and irradiated specimens. Strain recovery in unirradiated specimens is attributed to unpinning of anelastically bowed dislocations. Strain recovery in irradiated specimens occurs in two stages; a rapid stage attributed to unpinning bowed dislocations and a slow stage attributed to annealing of irradiation defects. Assuming that mechanisms proposed for creep are applicable during stress-relaxation the complete recovery of irradiation-induced strains in annealed specimens and partial recovery in specimens with some cold-work agrees with expectations from models based on stress-induced alignment of dislocation loops and irradiation damage. Recovered strains in this experiment were small as were strains measured by others on irradiated specimens with larger deformation strains, thus annealing to reduce strains in reactor structures may not be useful.

Viscoelastic response of graphitic materials in irradiation environments

A numerical method is presented for the stress analysis of reactor graphitic components under conditions of plane stress, plane strain, generalized plane strain or axisymmetry. In the analysis, both the creep deformation and the irradiation-induced strains are the major concern. The mechanical response of the material is assumed to be linear viscoelastic and transversely isotropic. The constitutive relations are formulated in hereditary integral form, and the dependence of the material properties on temperature and neutron flux are considered. To illustrate the method, a numerical example on the analysis of a fuel element is presented.