Mechanical-Properties and Microstructure of 600-Mev Proton-Irradiated Copper Single-Crystals

Low dose (10(-4) - 10(-2) dpa) irradiation of high-purity (99.99%) copper single crystals were performed in the PIREX facility with 600 MeV protons at room temperature. The irradiation hardening effects are investigated by measuring the yield stress or critical resolved shear stress changes and the irradiation induced defect microstructure has been observed under transmission electron microscopy. The variations of the yield stress and defect density as a function of the irradiation fluence are comparable to the existing data from neutron irradiations at similar fluence.

Modelling oriented investigations of primary radiation damage in ultra high purity Fe and FeCr alloys

Anna Prokhodtseva

In the quest of the structural materials for the future fusion reactor, it has been shown that ferritic/martensitic (F/M) steels are very promising candidates, with a good radiation resistance in terms of damage accumulation in the microstructure relative to for example austenitic steels. However, our ability to predict their response to irradiation in such harsh conditions is still not satisfactory. In this view, there is a critical need for information on the primary damage occuring in this materials class, as input to the multiscale modelling of the irradiation effects, including the impact of He and H. Despite numerous studies in the last 50 years, there is still lack of knowledge in many areas of the irradiation response of the microstructure of ferritic steels. This is due to the complexity of these materials, for they contain numerous alloying elements, grains boundaries and precipitates. The strategy nowadays to identify basic mechanisms of primary damage is to investigate so-called model alloys of those, which allows parametric studies in simplified structures. In this work ultra high purity Fe and Fe(Cr) model alloys were investigated in a transmission electron microscope (TEM) under ion irradiation in an attempt to better understand the fundamental mechanisms of radiation damage starting from the lowest doses, and the dependence on Cr content. Attention is also paid to the effects of He and H. Radiation induced dislocation loops and cavities were quantified. This study provides data for validation of the modelling efforts. Single, dual and triple beam ion irradiations were performed in JANNuS facility located on two sites, in Orsay and in Saclay in France. In Orsay, electron transparent thin foils of UHP Fe, Fe -5, -10 and -14Cr were irradiated in situ in TEM as a single beam experiment with 500 keV Fe+ ions and as a dual beam experiment with 500 keV Fe+ and 10 keV He+ ion beams, to a dose of 1 dpa with and without 1000 appm/dpa He at room and liquid nitrogen temperatures in order to observe the very first defects, desirably before their thermal evolution. Effects of dose rate on the produced damage were assessed in UHP Fe. The impact of He and Cr on defect accumulation was scrutinized in terms of the number density, size and Burgers vector of the visible defects. Special care was taken in the analysis of the TEM micrographs, for which new techniques were developed, with a particular one to determine the Burgers vector of a dense dispersion of fine nanometric defects. Emphasis was put on the identification of the loops Burgers vector, which is considered to be either a0〈100〉 or 1/2 a0〈111〉 in ferritic materials. 2 In order to study the effect of the free surfaces of the electron transparent thin specimens on the irradiation induced microstructure, the results obtained for TEM thin foils were compared to bulk samples that were irradiated ex situ in Saclay in single beam experiments with 24 MeV Fe8+ ions, and dual beam ones with 24 MeV Fe8+ to 1 dpa and 2 MeV He+ ions 1000 appm/dpa He at RT. To study synergistic effects of He and H a triple beam bulk irradiation with 24 MeV Fe8+, 2 MeV He+ and 0.6 MeV H+ ions, to 1 dpa, 1000 appm/dpa He and 4000 appm/dpa H at room temperature was preformed. From the bulk specimens thin lamellae were extracted by focused ion beam, which allowed the TEM observation of the damage through the whole implanted range of about 3.5 μm. In this way, the effect of irradiation with and without He and synergistic effects of simultaneous He and H implantation on the produced defects were assessed. In addition, radiation induced hardening was assessed by nano-indentation and related to the radiation induced microstructure. The analysis shows that in thin foils of UHP Fe the defect population is dominated by a0〈100〉 defects, while in presence of He it is dominated by 1/2 a0〈111〉 loops after irradiation to 1 dpa. It indicates that helium stabilizes mobile 1/2 a0〈111〉 loops, which in thin foils otherwise escape to 2 the free surfaces. Irradiation to the lowest dose of 0.05 dpa resulted in the production of either increased ratio of 1/2 a0〈111〉 loops, compared to the highest dose, or in the entire population of 2 1/2 a0〈111〉 loops in all materials in single and dual beam. In bulk UHP Fe and Fr(Cr) samples 2 after single and dual beam irradiation mainly 1/2 a0〈111〉 loops and few a0〈100〉 loops were 2 observed, but of smaller size than in thin foils, emphasizing the effects of free surfaces on the type of produced loops. It is thus inferred that 1/2 a0〈111〉 loops dominate the early loop 2 2 population and visible a0〈100〉 loops observed in UHP Fe and Fe(Cr) thin foils stem from addition and/or absorption reactions between 1/2 a0〈111〉 loops. It follows that these reactions 2 are reduced by the presence of He, which may eventually impede the formation of a0〈100〉 loops, leaving a loop population dominated by 1/2 a0〈111〉 loops. The same type of reactions 2 are valid for Fe(Cr) alloys, but the reduction in mobility of 1/2 a0〈111〉 loops by Cr in the single 2 beam case and the synergistic effects of He and Cr after dual beam irradiation result in a mixed population of 1/2 a0〈111〉 and a0〈100〉 type loops. This confirms the idea that the formation of 2 visible a0〈100〉 loops is promoted by the presence of free surfaces. Irradiation of UHP Fe thin foil at liquid nitrogen temperature confirmed the assumption of initial 1/2 a0〈111〉 loops. These then escape to the free surfaces after warming to RT, the more so 2 in the thinnest regions of the sample leaving a loop population dominated by a0〈100〉 loops. In single beam irradiated UHP Fe no dependence of the total density of defects or their Burgers vector on the rate of irradiation was observed. However, in the presence of He, only 1/2 a0〈111〉 2 loops formed under higher dose/implantation rates, while at the lowest dose rate results were similar to the single beam. This observation is attributed to the local increase of the effective He/defect ratio in the case of the high dose rate due to the increased temporal overlap of displacement cascades. Regarding mechanical properties, it appears that hardness of the as received materials increases with increasing Cr content. Following irradiation there is a significant hardening, which is not monotonous with Cr content, and which increases for all materials when irradiated together with He, and the more so when irradiated simultaneously with He and H. Hardening relates well to the observed microstructure and is the largest for Fe-5Cr.

EPFL2013

The relationship between the irradiation induced damage and the mechanical properties of single crystal Ni

Zhongwen Yao

Irradiation is known to lead to a degradation of the mechanical properties of materials. This is particularly crucial in the case of materials that will be used in the future thermonuclear fusion reactor, where extremely high irradiation doses are expected. In the quest of a better understanding between the irradiation induced defects and the mechanical properties Ni single crystal specimens have been irradiated with 590 MeV protons to doses ranging from 10-3 dpa to 0.3 dpa, at room temperature, 250°C and 350°C. The irradiation induced microstructure has been characterized by transmission electron microscopy, and the mechanical properties have been assessed by mechanical testing. Molecular dynamics (MD) simulations have been conducted in Ni and Cu in order to understand the defect formation and accumulation following a displacement cascade. The following conclusions are established. Following irradiation at room temperature, 43% to 55% of the irradiation induced defects in Ni consist in stacking fault tetrahedra (SFTs), 31%∼41% in loops and about 10 % in unidentified black dots. In the case of Ni irradiated at 250 °C, 44%∼53% of the irradiation induced defects consists in SFTs, 35%∼51% in loops, and less than 5% in black dots. In the case of Ni irradiated at 350 °C, 50% of the irradiation induced defects consists in voids. The remaining 50% include ∼ 14% SFTs and 36% loops. Moreover, it appears that these ratios are independent from the irradiation dose, contrary to what was found in the literature. These results allow clarifying a long standing issue, namely that in fact in Ni there is no transition in the ratio between SFTs and loops with increasing dose. In addition, it appears that in fact the irradiation induced defect density is similar to the one found in other irradiated fcc metals for RT irradiation. The data of the irradiation at 250°C are in agreement with previous published results on a neutron irradiation at 230 °C. It appears that the size of SFTs is independent of the irradiation dose, similar to what is found in irradiated Cu. It depends, however, on the irradiation temperature. MD simulations have been performed in order to understand the influence of the interatomic potential's parameters on the formation of defects following displacement cascades. Starting with a known defect configuration, namely the SFT, 4 different potentials are tested. Results of the formation of an SFT from a triangular platelet of vacancies show that i) the platelet of 6 vacancies did not collapse to SFT regardless of the annealing conditions, but formed a void above 800K, for all selected potentials; ii) the platelet of 15 vacancies collapsed into an SFT when simulated with Farkas-I and Farkas-II potentials, which have low stacking fault energies; iii) the platelet of 66 vacancies easily collapsed to an SFT at 500K for all applied potentials, even with a high stacking fault energy (SFE) of 300 mJ·m-2. The common neighbor analysis and Wigner-Seitz defects analysis are used for studying the resulting structures after a displacement cascade, and results show that the largest number of stacking faults is surprisingly obtained with the potential giving the highest stacking fault energy (300 mJ·m-2). An SFT-like structure appears close to the core of cascades and also an isolated interstitial loop are found with Cleri-Rosato potential. It appears that the defects size, configuration and density relates more to the displacement threshold energy than to the stacking fault energy. It appears that a significant irradiation hardening occurs starting at the lowest dose of 10-3 dpa, at room temperature and 250°C. With increasing dose, the yield shear stress increases. The dose dependence of this increase varies from an irradiation temperature of room temperature to 250°C. In fact the irradiation hardening shows a significant temperature dependence. The thermal activation energy was calculated by measuring the activation volume, which is obtained from relaxation tests. In unirradiated Ni, an activation energy of about 0.3 eV can be extrapolated at RT. In case of irradiated Ni, the unsuccessful fitting with a linear relationship between the thermal energy and temperature suggests that multiple deformation mechanisms are operating simultaneously or in turn over the considered temperature range of –196 °C to 423 °C. The dispersion barrier hardening model is used to interpret the irradiation induced hardening in irradiated Ni. According to the equation, Δτ = αμb(Nd)1/2, with α, the obstacle strength. It appears that α is 0.12 for the room temperature irradiation induced hardening, similar to what was previously found for irradiated Cu, and is 0.22 for the hardening following irradiation at 250°C, which is much larger than previous values. This difference in α is suggested to be related to the presence of voids at high temperatures. It is suggested that voids are stronger obstacles than SFTs or dislocation loops. In situ TEM observations were made in irradiated Cu, in order to assess the strength of irradiation induced defects as obstacles to gliding dislocations. It appeared that the strength of obstacles is approximately 100 MPa. Considering that 90% of the damage consists in SFTs, it is suggested that the obstacles presenting a resistance of 100 MPa are SFTs. In particular, the pulling out of a string from the interaction between what was assumed to be an SFT and a moving dislocation could be recorded for the first time. This reaction implies that the gliding dislocation is strongly pinned by SFTs. TEM observation of the microstructure of the irradiated Ni following plastic deformation shows that defect-free channel formation is the deformation mechanism at the beginning of yielding. The channeling explains the observed softening. As deformation proceeds, a dislocation cell structure is initiated. It takes different forms depending on whether the sample is irradiated or not. In the irradiated case, cell formation initiates from mobile dislocation in the defect free channels. This implies that the eventual cell configuration will retain part of the dimensionality of the channels, for the least their width of 100 nm for the cell size, because of the geometrical constraint. In the unirradiated case the dislocation cell size is larger. The higher flow stress observed in the irradiated case is thus related to the dislocation cell size.

EPFL2005

Differences in the microstructure of the F82H ferritic/martensitic steel after proton and neutron irradiation

The microstructure of the F82H steel that is a candidate for the future fusion reactor first wall is investigated. Protons as well as fission neutrons are used to simulate the irradiation effects of the fusion environment. The differences in the He production rates between the two types of irradiation may lead to differences in the irradiation damage of the F82H microstructure. This paper presents a transmission electron microscopy (TEM) study for three different irradiation conditions: (i) in the PIREX facility with 590 MeV protons, (ii) with fission neutrons at the research reactor in Petten and (iii) with 590 MeV protons in PIREX followed by fission neutrons at the reactor in Studsvik. The latter experiment allows us to vary the He production rate for a given irradiation type. Total doses ranged from 0.3 to 10 dpa and temperatures ranged from room temperature to 310 degreesC. (C) 2000 Elsevier Science B.V. All rights reserved.