New insights into microstructure of irradiated beryllium based on experiments and computer simulations

The microstructural response of beryllium after neutron irradiation at various temperatures (643–923 K) was systematically studied using analytical transmission electron microscope that together with outcomes from advanced atomistic modelling provides new insights in the mechanisms of microstructural changes in this material. The most prominent feature of microstructural modification is the formation of gas bubbles, which is revealed at all studied irradiation temperatures. Except for the lowest irradiation temperature, gas bubbles have the shape of thin hexagonal prisms with average height and diameter increasing with temperature. A high number density of small bubbles is observed within grains, while significantly larger bubbles are formed along high-angle grain boundaries (GB). Denuded zones (DZ) nearly free from bubbles are found along both high- and low-angle grain boundaries. Precipitations of secondary phases (mainly intermetallic Al-Fe-Be) were observed inside grains, along dislocation lines and at GBs. EDX analysis has revealed homogeneous segregation of chromium and iron along GBs. The observed features are discussed with respect to the available atomistic modelling results. In particular, we present a plausible reasoning for the abundant formation of gas bubbles on intermetallic precipitates, observation of various thickness of zones denuded in gas bubbles and precipitates, and their relation to the atomic scale diffusion mechanisms of solute-vacancy clusters

On the Influence of Nb/Ti Ratio on Environmentally-Assisted Crack Growth in High-Strength Nickel-Based Superalloys

The effect of Nb/Ti ratio on environmentally-assisted crack growth of three prototype Ni-based superalloys is studied. For these alloys, the yield strength is unaltered with increasing Nb/Ti ratio due to an increase in grain size. This situation has allowed the rationalization of the factors influencing damage tolerance at 700 °C. Primary intergranular cracks have been investigated using energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope and the analysis of electron back-scatter diffraction patterns. Any possible detrimental effect of Nb on the observed crack tip damage due to Nb-rich oxide formation is not observed. Instead, evidence is presented to indicate that the tertiary γ′-precipitates are dissolving ahead of the crack consistent with the formation of oxides such as alumina and rutile. Our results have implications for alloy design efforts; at any given strength level, both more and less damage-tolerant variants of these alloys can be designed

Anomalies of critical state in fracturing geophysical objects

International audienceNon-linear time-sequences of fracture-related events were studied in drifting sea-ice and fracturing rock. A reversible drop of the b-value was detected prior to the large-scale sea-ice cover fragmentation, when the time sequence of impact interactions between ice-fields was fully decorrelated. A similar loss of the temporal invariance of the fracture process was revealed in the time sequence of microfracture events detected in a loaded rock sample. These temporal gaps in the continuous critical state of the considered self-organizing, open systems were attributed to the property of hierarchicity inherent in the geophysical objects. A combination of scaling and hierarchic features in the behavior of fracturing solids manifests itself in the heterogeneity of the temporal pattern of fracture process

Behaviour of P, Si, Ni impurities and Cr in self ion irradiated Fe–Cr alloys – Comparison to neutron irradiation

International audienceThis paper presents an atom probe tomography study of phase transformation and solute segregation in Fe-Cr alloys of low purity under self-ion irradiation. Fe-9%Cr and Fe-12%Cr were irradiated at 100 °C, 300 °C and 420 °C at a dose of 0.5 dpa. Homogeneously distributed clusters enriched in Cr, P, Si and Ni are shown to form at 300 °C and 420 °C but not at 100 °C. Study of the evolution of the segregation intensities of Cr, Si and P in the clusters with temperature under ion irradiation indicates that they form by a radiation induced mechanism. No a 0 clusters were observed whatever the irradiation temperature whereas they were observed in the same alloys after neutron irradiation at 300 °C at 0.6 dpa. Comparison of the solute cluster composition after ion irradiation and neutron irradiation, suggests that P atoms could play an important role in the appearance of the solute clusters by stabilizing point defect clusters that could later be enriched in Ni, Si and Cr

Intra granular precipitation and grain boundary segregation under neutron irradiation in a low purity Fe–Cr based alloy

International audienceA nanoscale description of intra- and inter-granular segregation and precipitation in a Fe–12 at.%Cr model alloy of low purity after neutron irradiation at 300 °C up to 0.6 dpa, has been performed owing to Atom Probe Tomography (APT). Two different populations of clusters have been observed inside the grains: Cr-enriched and NiSiPCr-enriched clusters. As expected with a process of enhanced precipitation, Cr-enriched clusters are homogeneously distributed inside grains. The NiSiPCr-enriched clusters, which are probably formed by radiation induced segregations, are independent of the Cr-enriched clusters. Investigation of a low angle grain boundary has revealed Si, Cr and P segregation on the dislocation network