Dislocation defect interaction in irradiated Cu

Robin Schäublin, Philippe Spätig, Zhongwen Yao
2005
Article

Résumé

Pure Cu single crystals irradiated at room temperature to low doses with 590MeV protons have been deformed in situ in a transmission electron microscope in order to identify the basic mechanisms at the origin of hardening. Cu irradiated to 10(-4) dpa shows at room temperature a yield shear stress of 13.7 MPa to be compared to the 8.8 MPa of the unirradiated Cu. Irradiation induced damage consists at 90% of 2 mu stacking fault tetrahedra, the remaining being dislocation loops and unidentified defects. In-situ deformation reveals that dislocation-defect interaction can take several forms. Usually, dislocations pinned by defects bow out under the applied stress and escape without leaving any visible defect. From the escape angles obtained at 183 K, an average critical stress of 100 MPa is deduced. In some cases, the pinning of dislocations leads to debris that are about 20 nm long, which formation could be recorded during the in situ experiment. (c) 2005 Elsevier B.V. All rights reserved.

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https://infoscience.epfl.ch/record/120183?ln=fr

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EPFL2009

Chargement

On the relationship between the microstructure and the mechanical properties of an ODS ferritic/martensitic steel

Amuthan Ramar

In the quest of materials for the first wall of the future fusion reactor, it has been shown that oxide dispersion strengthened (ODS) ferritic / martensitic (F/M) steels appear to be promising candidates. The inherent good mechanical properties supported by a good thermal conductivity, swelling resistance and low radiation damage accumulation of the base material, such as EUROFER97, are further enhanced by the presence of a fine dispersion of nanometric oxide particles. They would allow in principle for a higher operating temperature of the fusion reactor, above 550°C, which improves its thermal efficiency. In effect, their strength remains higher than the base material at high temperatures. There are however limitations to their application in the reactor, which relate to the intimate link between the microstructure and the mechanical properties. 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The base of the model is the dispersion barrier strengthening model, which is given by Δσsparticle or defects = Mαμb(Nd)1/2 for the hardening due small defects or oxide particles and is given by Δσd-d = Mαμb(ρ)1/2 for the hardening due to dislocations. In this approximation, the total hardening is then given by Δσtotal = {(Δσd-d)2 + (Δσparticle or defects)2}1/2 . The model is used to explain the hardening due to oxides, dislocations and irradiation induced defects. It appears that dispersed oxides is dominating hardness up to 800°C, while following a heat treatment at 1000°C it is the dislocations that then dominate the hardness. Deformation mechanisms were investigated using differential strain rate experiments. The activation parameters, i.e. the activation volume and activation energy, were calculated. It appears that there is a decrease in the activation volume from room temperature to 400°C, which is less than 3%. 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The peculiar response of the yield stress to irradiation for this class of material was rationalized using the total hardening model we developed. At a dose of 2 dpa, irradiation induced defects are higher in number density than the dispersed yttria particles, which although they are weaker than the yttria particles they can explain the significant hardening observed in the material. Conversely, the shallow impact of irradiation on ODS F/M steels at low doses, relative to base F/M steels, is explained by the fact that the amount of dispersoid is found to be higher than the density of irradiation induced defects.

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Chargement

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

Chargement

On the relationship between the microstructure and the mechanical properties of an ODS ferritic/martensitic steel

Amuthan Ramar

EPFL2008

EPFL2009

EPFL2013