Fragilisation par l'hydrogène

vignette|Fissure d'un acier trempé provoquée par de l'hydrogène, observée au microscopie électronique à balayage. La fragilisation par l'hydrogène est un phénomène de fissuration de certains métaux au contact de l'hydrogène. Le rôle de l'hydrogène dissous dans un métal est connu depuis la fin du , mais encore incomplètement compris. Histoire scientifique et technique Dès 1983, Airey et Van Rooyen démontrent qu'une présence d'hydrogène dissous dans le milieu environnant d'un métal peut aggraver le risque de corrosion, notamment de corrosion sous contrainte. Depuis, les effets de l’hydrogène sur les métaux continuent à être étudiés par de nombreux laboratoires et chercheurs, car il s'agit d'un enjeu industriel, économique et de sécurité important, par exemple dans le domaine du transport et du stockage de l'hydrogène et dans celui de l'industrie nucléaire. On s'intéresse aussi aux effets des produits et sous-produits de la corrosion dispersés par l'eau dans l'installation .

Creep Properties and Related Microstructural Changes of a Lamellar Titanium Aluminide Alloy before and after Helium-Implantation

Per Magnusson

The creep properties of a cast gamma-alpha-2 lamellar titanium aluminide of type Ti-46Al-2W-0.5Si (at. %) containing beta-particles was investigated using both helium-implanted and non-implanted material. Creep tests were performed in vacuum under constant stress using small miniaturized dog-bone shaped samples with gauge dimensions of 10 × 2 × 0.2 mm3. Creep temperatures ranged from 700 °C to 1000 °C and applied stresses ranged from 75 MPa to 400 MPa. Helium implantation was carried out with 0-24 MeV helium ions homogeneously implanting the miniaturized samples from 6.3 to 1333 appm He, at temperatures of 150 °C and 630 to 1000 °C. Non-implanted samples showed very good creep resistance up to temperatures of roughly 850 °C; at temperatures above 900 °C a decrease in creep resistance was observed. For temperatures up to 900 °C the activation energy of creep was determined to 405 kJ/mol and the stress exponent of 7.2. Size and geometry effects of miniaturized samples were observed in form of large scatter in creep strain rates and lower strains at fracture. Electron microscopy showed that the initial microstructure was unstable and that the alloy had a tendency to decompose the alpha-2 phase and form beta-gamma structures, loss of initial microstructure was accelerated with increasing temperature. The creep deformation mechanism was determined to be a combination of stress-induced phase change and dislocation creep. A gradual change in fracture behavior was observed, at lower temperatures fracture was brittle with little necking, at the highest temperature the fracture was completely ductile. Material crept and implanted at 700 and 800 °C exhibited embrittlement at helium contents above 10 appm, characterized by a strong loss of ductility and creep lifetime. Samples crept and implanted at 900 °C experienced embrittlement for all tested helium contents. Investigations with transmission electron microscopy revealed a substantial clustering and growth of helium bubbles at interfaces in the alpha-2 phase. An average bubble size for embrittlement between 5.3 and 6.7 nm was determined. The behavior and evolution of helium bubbles were studied with post implantation annealing experiments. Samples were implanted at 150, 630, 800 and 1000 °C and subsequently annealed at temperatures from 600 to 900 °C. Material implanted at 150 °C showed no bubbles before annealing, only after post-implantation annealing was widespread clustering observed. Material implanted at temperatures of 630 °C or higher, exhibited bubbles before post-implantation annealing. Depending on the implantation temperature, two different branches were observed in the size and distribution of helium bubbles even after post-implantation annealing. Samples implanted at 150 or 630 °C and then annealed at higher temperatures had high bubble density with small bubbles found in the entire microstructure. Samples implanted at 800 or 1000 °C had low bubble densities and the bubbles were only observed at alpha-2/beta and alpha-2/gamma interfaces, with sizes larger than the average bubble radius for embrittlement. Based on the post-implantation annealing experiments, a temperature limit for the onset of helium embrittlement between 630 and 700 °C is predicted. The helium embrittlement, the loss of initial microstructure and decreased creep resistance at temperatures above 900 °C suggests a limited applicability of lamellar TiAl in advanced nuclear reactor environments.

EPFL2012

Embrittlement induced by the synergistic effects of radiation damage and helium in structural steels

Kun Wang

Ferritic/martensitic steels are candidate materials for fusion reactor structural components, liquid metal containers of spallation neutron sources, and accelerator driven systems, with good radiation resistance and thermo-mechanical properties. However, embrittlement resulting from the combined effects of radiation induced displacement damage (measured in dpa = displacement per atom) and transmutation products, especially helium gas, is one of the key issues. Four different steels were selected for mechanical and microstructural studies to understand the mechanisms of embrittlement induced by the combined effects of displacement damage and helium after irradiation in SINQ, the Swiss spallation source. The irradiations conditions were in the range: 10.7 ¿ 20.4 dpa with 850-1750 appm He at 160-300 °C. The evolution of the mechanical properties after irradiation was investigated by tensile and hardness tests. Radiation-induced defect clusters and helium bubbles were quantified by transmission electron microscopy (TEM). Emphasis was put on the deformation mechanisms under the different observed fracture mode, (ductile, quasi-cleavage and intergranular), whose occurrence depends on the irradiation conditions (dpa, He content and irradiation temperature). The tensile stress-strain curves and the scanning electron microscopy images of fracture surfaces showed distinct fracture mechanisms under different irradiation and test conditions. The tensile tests showed a yielding stress increase and loss of ductility of irradiated specimens. Hardness was measured on the specimens before tensile testing. The hardness results demonstrated an increasing trend with irradiation dose and helium content. TEM observations were done for all irradiated fractured specimens. Small defect clusters were observed in the 12.3 dpa specimen, but large defect clusters with loop-shape were very few. In the specimens of 17.2, 17.7 and 20.4 dpa, many large dislocation loops were detected besides small clusters. In addition, helium bubbles were observed in all specimens. The average size of defect clusters increased from 4.2 nm to 11.8 nm with dose increasing from 12.3 dpa to 20.4 dpa, whereas the number density did not change significantly. Meanwhile, the average size of visible helium bubbles increased from 1.03 nm to 1.93 nm. The microstructures in deformed area of irradiated specimens were observed and defect free channels with {110} and {112} slip planes were found in some specimens, indicating plastic flow localization. The average width of the channels is about 100 nm. Regarding the brittle samples, the TEM-lamella were extracted directly below intergranular fracture surfaces or cleavage surfaces by focused ion beam. Strikingly, deformation twinning was observed as the main feature in three irradiated specimens at high dose. Only twins with {112} planes were observed in all of these samples. The average thickness of twins is about 34 nm. Twins started from a fracture surface became gradually thinner with distance away from the fracture surface and stopped in the matrix finally. Features such as twin-precipitates interaction, twin-grain boundary and/or lath boundary interaction were observed. Twinning bands were seen to be arrested by grain boundaries or large precipitates, but could penetrate martensitic lath boundaries. Unlike the case of defect free channels, inside twins small defect-clusters, dislocation loops and dense small helium bubbles were observed.

EPFL2015