R. Schäublin

Irradiation-induced stacking fault tetrahedra in fcc metals

Irradiation induces the formation of stacking fault tetrahedra (SFTs) in a number of fcc metals, such as stainless steel and pure copper. In order to understand the role of the materials parameters on this formation, pure Cu, Ni, Pd and Al, having a respective stacking fault energy of 45, 125, 180 and 166 mJ m−2, have been irradiated with high energy protons to a dose of about 10−2 dpa at room temperature. The irradiation-induced microstructure has been investigated using transmission electron microscopy. All irradiated metals but Al present SFTs. The proportion of perfect, truncated and grouped SFTs has been determined. The SFT energy as a function of size has been calculated using elasticity of the continuum, with respect to the energy of a number of other possible defect configurations. It appears that the key parameters are the stacking fault energy and the shear modulus. Their implication on the formation and stability of the SFTs is discussed.

Helium effects on displacement cascades in α-iron

Molecular dynamics simulations have been performed to investigate the effects of helium on the displacement cascades in α-iron. Besides conventional analysis tools, a new graphical representation of the data based on ternary plots has been introduced. Results show that the production of defects and their subsequent clustering appear to be greatly influenced by the presence of helium. Calculations reveal that the location of helium atoms, substitutional or interstitial, plays a major role. Compared to pure iron, interstitial helium atoms increase the amount of Frenkel pairs generated during the cascades. Conversely, substitutional helium atoms tend to decrease this production. However, in both cases, it is observed that helium atoms stabilize larger self-interstitial clusters, due to a strong binding energy. These simulations show that helium atoms trap self-interstitial clusters and would thus slow down their subsequent migration. Some helium–vacancy clusters are generated in the core of the displacement cascades but also grow at the periphery of self-interstitial clusters. It is shown that results greatly depend on the irradiation temperature.

Tensile properties and microstructure of 590 MeV proton-irradiated pure Fe and a Fe-Cr alloy

Polycrystalline pure Fe and Fe-12Cr alloy have been proton-irradiated to doses of 10(-3)-3 x 10(-1) dpa at room temperature and 523 K. Samples were mechanically tested in tension and transmission electron microscopy (TEM) was performed on the as-irradiated material as well as on the irradiated and deformed material. At room temperature, pure Fe presents an initial yield point, that increases with the dose, followed by a yield region, indicative of a localized deformation mode. The Fe-12Cr alloy shows the same behavior, but the irradiation amplifies the effects previously mentioned. No yield point or yield region appears after irradiation at 523 K. TEM observations of the irradiated material showed a high density of small defect clusters which increases with the dose. Defect-free channels are observed in Fe-12Cr irradiated and deformed at room temperature. Results of the present investigation are compared with neutron irradiation of similar materials

The effects of irradiation and testing temperature on tensile behaviour of stainless steels

Abstract Irradiated 304 and 316 stainless steel samples were investigated. The steels were neutron irradiated to 1.5 and 7.5 dpa at 550 K. Similar types of steels were irradiated at 550 K with 590 MeV protons in the PIREX facility at PSI, Switzerland. The doses reached in this case were 0.15 and 0.3 dpa. The stress–strain relationships at different temperatures were measured. The irradiation and deformation microstructures were investigated using transmission electron microscopy (TEM). Two modes of deformation were found, twinning and channelling, depending on the testing temperature. These results are discussed in terms of deformation mechanisms at different temperatures correlated with radiation hardening. Finally, possible correlations between deformation modes and previous irradiation assisted stress corrosion cracking (IASCC) studies are discussed.

Evolution of the mechanical properties of the F82H ferritic/martensitic steel after 590 MeV proton irradiation

Tensile properties have been investigated on the low activation F82H ferritic/martensitic steels for both unirradiated and irradiated material. The irradiations have been carried out with protons of medium energy (590 MeV) at the PIREX facility and doses from 0.2 up to 1.75 dpa have been reached for temperature ranging between 310 and 673 K. The irradiation induced hardening has been systematically measured at room temperature for the different irradiation conditions (dose and irradiation temperature). The tensile tests have been supplemented with stress relaxation experiments in order to determine the activation volume of the rate controlling mechanism for dislocation motion. The effects of irradiation on the tensile properties and activation volumes are presented.

State of a pressurized helium bubble in iron

We study the state of a nanometric helium bubble in bcc-iron as a function of temperature and He content using atomistic calculations. It appears that up to moderate temperatures the Fe lattice can confine He to solid state, in good agreement with known solid-liquid transition diagram of pure He. However, He in the bubble forms an amorphous phase, while an fcc structure is expected at the same temperature and He density. In addition, the He bubble forms a polyhedron whose morphology depends on either the surface energy or the elastic-plastic properties of Fe at either low or high pressure, respectively. Indeed, at high He contents the bubble surface breaks down at the mechanical stability limit of the Fe crystal, leading to a pressure decrease in the bubble.

The microstructure and tensile properties of pure Ni single crystal irradiated with high energy protons

The microstructure and tensile behavior of pure single crystalline Ni irradiated with 590 MeV protons are investigated. First results have been obtained for a damage level of 0.13 dpa at room temperature and 0.002 dpa at 523 K. The irradiation induced defect microstructure observed under transmission electron microscopy consists of loops and stacking fault tetrahedra in a lower density. Tensile tests were performed at room temperature and show observable radiation induced hardening already at 0.002 dpa. Dislocation channels and cells were observed and the transition from the initial channel formation to the development of the deformation cells is investigated. First results are presented here.

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°C.

Influence of the stress field due to pressurized nanometric He bubbles on the mobility of an edge dislocation in iron

Voids and He bubbles are strong obstacles to dislocation, which induce hardening and loss of ductility. In Fe, molecular dynamics simulation is used to investigate the basic mechanisms of the interaction between a moving edge dislocation and a void or He bubble, as a function of its He content, temperature, interatomic potentials and interaction geometry. Different interatomic potentials for Fe–Fe and Fe–He interactions are used. It appears that temperature eases the dislocation release, due to the increased mobility of the screw segments appearing on the dislocation line upon bowing from the void or He bubble. The mobility includes the cross-slipping of these segments, which leads to the formation of a jog. It appears that the He bubble induces an inhomogeneous stress field in its surroundings, which strongly influences the dislocation passage depending on the geometry of the interaction.

-Loop characterization in α-Fe: comparison between experiments and modeling

Ferritic/martensitic steels considered as first-wall candidate materials for fusion reactors experience significant radiation hardening at temperatures below similar to400 degreesC. Experimental evidence suggests that the observed defects are large interstitial dislocation loops with Burgers vector b = and b = (1)/(2) . In this work, the atomic character of and (1)/(2) loops is investigated with molecular dynamics simulations and the atomic configurations are used to calculate the defect image contrast through direct simulation of TEM images. The simulated images are subsequently compared with actual TEM micrographs of irradiated ferritic materials and are in very good qualitative agreement, providing strong indication that the observed b = loops are interstitial in nature