[en] Oxide dispersion strengthened (ODS) ferritic-martensitic steels are being considered for high temperature, high fluence nuclear applications, like fuel pin cladding in Fast Breeder Reactors. ODS alloys offer improved out of pile strength characteristics at temperature above 550 deg.C and ferritic-martensitic matrix is highly swelling resistant. A clad in an ODS ferritic steel, call DY (Fe-13Cr-1,5Mo+TiO₂+Y₂O₃) has been irradiated in the experimental reactor Phenix. Under irradiation oxide dissolution occurs. Microstructural observations indicated that oxide evolution is correlated with the dose and consist in four phenomena: the interfaces of oxide particles with the matrix become irregular, the uniform distribution of the finest oxide (< 20 nm) disappear, the modification of oxide composition, and a halo of fine oxides appear around the larger oxides. The use of such a material requires a study of oxide stability under irradiation, since the oxide particles provide the desired mechanical properties. The study is based on two types of alloys, the DY and four ferritic steels Fe-9Cr-1Mo reinforced by Al₂O₃, Y₂O₃, MgO or MgAl₂O₄. These materials were irradiated with charged particles in order to gain a better understanding of the mechanisms of dissolution. Irradiation with 1 MeV Helium does not induce any modification, neither in the chemical modification of the particles nor in their spatial and size distribution. Since most of the energy of helium ions is lost by inelastic interaction, this result proves that this kind of interaction does not induce oxide dissolution. Irradiation with 1 MeV or 1.2 MeV electrons leads to a significant dissolution with a radius decrease proportional to the dose. These experiments prove that oxide dissolution can be induced by Frenkel pairs alone, provided that metallic atoms are displaced. The comparison between irradiation with ions (displacements cascades) and electrons (Frenkel pairs only) shows the importance of free point defects in the dissolution phenomena. For all the irradiations (ions or electrons) the spinel MgAl₂O₄ seems more resistant than Y₂O₃ to dissolution, and MgO and Al₂O₃ are even less resistant. This is the order of stability under irradiation of bulk oxides. (author)

[en] Full text of publication follows: The austenitic stainless steels are largely employed for the industrial components which operate at temperatures ranging between 300 deg. C and 600 deg. C. In the Sodium Fast Reactors (SFR), these steels and in particular the steels stabilized with titanium are employed for pipes of the secondary circuit and the exchangers. The whole of these components, and more particularly those in Ti-stabilized steel (AISI 321), have been the subject of an attention supported for more than ten years. The non destructive examinations carried out on these pipes showed indications on the heat affected zones (HAZ) of some circular welds. The first metallographic expertises carried out on the welds confirm the presence of real defects, intergranular cracks which appear on the intern surface at the weld toes and parallel with the fusion line. These defects were attributed to the stress relief cracking. The expertises made it possible to highlight a certain number of significant parameters for this mechanism. Temperature. No case of cracking was observed on the circuits working at 350 deg. C, whereas some cases were revealed on the circuits at 475 deg. C, the percentage of affected welds increasing for the circuit at 550 deg. C. The presence of a defect at the weld root. Each cracked weld shows that intergranular crack was propagated from a shrinkage or a defect of rolling. Initial work hardening in weld root generally observed on the cracked welds. Local loadings, and in particular the residual stresses of welding. Geometrical discontinuities or difference in materials also constitute factors worsening with respect to the mechanism. Microstructural examinations carried out by transmission electron microscopy on different welds affected or not by the stress relief cracking mechanism made it possible to establish a grid of risk of appearance of this type of cracking based on four microstructural features: Dislocations density close to the interface with the melted zone, The morphology of this dislocation network with or not presence of sub-grain boundaries, and of cells of dislocations, The secondary precipitation of titanium carbide finely dispersed in the grains, near the interface with the melted zone, The gradient of microstructure of the base metal towards the HAZ/melted zone interface. These results lead to change material and to use a 316LN stainless steel. These criteria were then used in order to evaluate the risk of cracking on other components having also the pipes welded out of steel 321. (authors)

[en] Aimed at better understanding of the mechanism of oxide particles dissolution in dispersion-strengthened ferritic-martensitic steels the methods of electron microscopy and scanning energy dispersive spectrometry are applied to study variation of microstructure and strengthening phase chemical composition in steels DY (Fe-13 Cr-1.5 Mo + TiO₂ + 0.5 Y₂O₃), EM10 + Y₂O₃ and EM10 + MgO irradiated under various conditions. Neutron irradiation is performed up to 81 dpa maximal dose at temperatures of 400-580 deg C in Phenix reactor. The irradiation with 1 and 1.2 MeV electrons is carried out in an electron microscope column at 300-550 deg C, and 1 MeV helium ion irradiation is performed in an accelerator at 400 deg C. It is stated that helium ion irradiation has no effect on steel structure and oxide chemical composition as the most part of ion energy is lost in inelastic interactions. The irradiation with 1.2 MeV electrons is shown to promote the dissolution of yttrium oxides as this taken place radii of particles decrease with a dose growth, suggesting that point defects play an important role in dissolution processes. In-core irradiation of dispersion-strengthened steels in the form of fuel pin cans results in the following changes: a disappearance of uniform distribution of fine (< 20 nm) particles; the formation of haloes of fine particles around the larger ones; the occurrence of irregular oxide-matrix interfaces; variations in oxide chemical composition

No abstract available

[en] This paper aims to simulate the behavior of some alloys in high temperature fatigue-relaxation (creep), for a long hold time (one month for each cycle) when laboratory experiments are difficult to realize. The first part presents an estimation of the internal intergranular stresses. The second part deals with the recovery occurring during the hold time. (A.L.B.)

[en] In this work, Al_xGa_1_−_xN (x = 0; 0.1; 0.3; 0.5; 0.65; 0.7; 0.8; 1) wurtzite epilayers, grown on c-plane sapphire substrates, have been irradiated with Swift Heavy Ions at GANIL facility. Modifications induced by irradiation are characterized with in-situ optical absorption spectroscopy at 15 K. Spectra of these irradiated alloys exhibit optical absorption band formation, related to new energy levels in their bandgaps, whose positions only depend on the composition of the layer. However, these absorption bands are not observed in the Al_xGa_1_−_xN with Al molar fraction less than 0.3, likely because the energy level of the corresponding defect is located above the conduction band. Moreover, using different irradiation conditions, a coupled effect between nuclear collisions and electronic excitations for these color center creation have been investigated. A synergy between these two phenomena has been shown and appears to be independent of the composition of the alloy.

[en] Plastic deformation in irradiated zirconium alloys occurs in a very heterogeneous manner at the grain scale by the clearing up of radiation-induced prismatic loops by gliding dislocations and the channeling of these dislocations inside narrow bands of the grain. A statistical TEM investigation of this mechanism has been performed on neutron irradiated recrystallized Zr alloys tested at 350 deg. C. Due to the strong anisotropy of plastic deformation, different loading conditions have been investigated. It is shown that for transverse tensile tests and closed end burst tests, only basal channels are observed, whereas for axial tensile tests, prismatic and pyramidal channels are observed. This phenomenon can be understood in terms of texture and interactions between dislocations and irradiation induced loops. From Schmid factor calculations, we have also been able to prove that irradiation leads to a higher increase of critical resolved shear stresses for prismatic and pyramidal slip systems than for basal slip system

No abstract available

[en] This paper details in-situ studies of modifications induced by swift heavy ion irradiation in α-Al2O3. This complex behaviour is intermediary between the behaviour of amorphizable and non-amorphizable materials, respectively. A unique combination of irradiation experiments was performed at the IRRSUD beam line of the GANIL facility, with three different characterisation techniques: in-situ UV–Vis absorption, in-situ grazing incidence X-Ray diffraction and ex-situ transmission electron microscopy. This allows a complete study of point defects, and by depth profile of structural and microstructural modifications created on the trajectory of the incident ion. The α-Al2O3 crystals have been irradiated by 92 MeV Xenon and 74 MeV Krypton ions, the irradiation conditions have been chosen rather similar with an energy range where the ratio between electronic and nuclear stopping power changes dramatically as function of depth penetration. The main contribution of electronic excitation, above the threshold for track formation, is present beneath the surface to finally get almost only elastic collisions at the end of the projected range. Amorphization kinetics by the overlapping of multiple ion tracks is observed. In the crystalline matrix, long range strains, unit-cell swelling, local microstrain, domain size decrease, disordering of oxygen sublattice as well as colour centre formation are found. This study highlights the relationship between ion energy losses into a material and its response. While amorphization requires electronic stopping values above a certain threshold, point defects are predominantly induced by elastic collisions, while some structural modifications of the crystalline matrix, such as unit-cell swelling, are due to contribution of both electronic and nuclear processes.

[en] Structural modifications of aluminium oxide induced by swift heavy ion irradiation are investigated. (0 0 0 1)-Al₂O₃ single crystals have been irradiated at room temperature along the c-direction by 92 MeV ¹²⁹Xe²³⁺ with fluences up to 1.2 × 10¹⁵ ions/cm². High resolution and in-plane X-Ray diffraction is mainly used to characterize samples with various sensitivities to the surface and to the irradiated depth. As function of fluence the mechanisms of structural modifications are investigated. Different evolutions of the c-parameter (parallel to ion beam) and the a-parameter (perpendicular to ion beam), affected by irradiation in the MeV range, are observed. 2D maps around (0 0 0 6)-reflection indicate a clear broadening on 2θ axis with the formation of new contributions which are due to tensile strain induced by irradiation. In the damage profile, by combining with RBS/C and TEM results, we suggest that the c-parameter is larger near the surface and decreases gradually with depth. In addition, the damage profile evidences an increase of damaged depth as function of the fluence which occurs before saturation linked to the formation at the surface of an amorphous layer.