[en] Cavity evolutions in a normalized and tempered martensitic steel (TMS) and two nanostructured ferritic alloys (NFA) under Fe³⁺ and He⁺ dual ion beam irradiations (DII) at 500 °C and 650 °C were characterized. The irradiation conditions encompass a wide range of displacement per atom damage (dpa), He and He/dpa. The 500 °C DII produced damage and He levels of ≈10–47 dpa and ≈400–2000 appm, respectively. Transmission electron microscopy (TEM) showed that DII of a 8Cr TMS, at 500 °C to up to 60 dpa and 2100 appm He, produced a moderate density of non-uniformly distributed cavities with bimodal sizes ranging from ≈1 nm He bubbles to ≈20 nm faceted voids, and swelling ≈0.44%. In contrast, the same irradiation conditions produced only small ≈1.3 nm diameter bubbles and swelling of ≈0.05% in the NFA MA957. Similar bubble distributions were observed in MA957 and a developmental NFA DII at 650 °C up to ≈80 dpa and ≈3900 appm He. These results demonstrate the outstanding He management capability of the oxide nano-features in the NFA. The various data trends are shown as a function of dpa, He, He/dpa and He^*dpa

[en] Microstructural changes in a Fe–15 at.%Cr model alloy neutron irradiated to 1.82 dpa at 290 °C were characterized by atom probe tomography. Homogenously distributed α′ precipitates as well as fewer clusters containing Si, P, Ni, and Cr, were observed in the matrix. Grain boundary analyses before and after irradiation revealed segregation of Cr, with W-shape concentration profiles developing in the vicinity of grain boundary carbide and nitride particles. After irradiation, impurities such as C, Si and P were segregated to the grain boundaries. Zones depleted of α′ clusters, and Si were found at the interfaces of carbide and nitride precipitates and along grain boundaries in the vicinity of these precipitates

[en] Graphical abstract: -- A series of model Fe–Cr alloys containing 3–18 at.% Cr was neutron irradiated at a nominal temperature of 563 K to 1.82 dpa. Solute distributions were analyzed by atom probe tomography, which revealed α′ precipitation for alloys containing more than 9 at.% Cr. Both the Cr concentration dependence of α′ precipitation and the measured matrix compositions are in agreement with the recently published Fe–Cr phase diagrams. An irradiation-accelerated precipitation process is strongly suggested

[en] The microstructural and chemical changes in a Fe–6 at.%Cr binary model alloy neutron irradiated to 1.82 dpa at 290 °C were investigated using atom probe tomography. After irradiation, Si and Cr are found segregated to dislocation loops, which were analyzed in terms of number density, size, and habit plane. Grain boundary chemistry was quantitatively compared between the as-received and the neutron irradiated alloys. The results are discussed in the context of equilibrium segregation, radiation-enhanced diffusion, and/or radiation induced segregation

[en] Highlights: • Neutron irradiation accelerates Cr-rich α′ precipitation in Fe-Cr alloys. • α′ precipitate Cr composition measured by APT depends on precipitate size. • For small α′ sizes, APT technique artifacts affect composition measurements. • α′ precipitates reach a thermodynamically stable composition at high temperatures. • Cascade ballistic mixing likely lowers α′ precipitate compositions at lower temperatures.

[en] Density functional theory calculations have been performed to study the structural, electronic and elastic properties, and formation energies, of pyrochlore Y₂Ti₂O₇ and orthorhombic Y₂TiO₅. These oxides are among the hardening features in nano-dispersion-strengthened ferritic alloys that are promising for future advanced fission/fusion energy applications. Calculations employ pseudo-potentials generated within the projector-augmented wave (PAW) method and the generalized gradient approximation (GGA) as well as the local density approximation (LDA) to the exchange-correlation functional. The calculated equilibrium structural parameters agree well with experiment for both oxides. The PAW-GGA yields the better bulk modulus of Y₂Ti₂O₇, and hence is used for all the elastic calculations. A complete set of elastic parameters for both oxides are reported. Analysis of electron localization functions, charge densities, and densities of states suggest that the oxide bonds are highly ionic, but with some covalent character. In both cases the Ti-O bonds exhibit higher covalency than the Y-O bonds.

[en] Precipitation of α′ in FeCr alloys under neutron irradiation is thermodynamically driven while being accelerated by radiation-enhanced diffusion. However, similar alloys under ion irradiation at high dose rates (> 10^{− 4} dpa/s) fail to exhibit α′ precipitation. Here, the microstructure of an Fe18Cr alloy under ion or neutron-irradiation at 300 °C at dose rates from ~ 10^{− 7} to 10^{− 4} dpa/s was analyzed by atom probe tomography. The steady-state composition content of the clusters depends on the ion irradiation dose and dose rate, confirming the contribution of ballistic mixing in diluting the Cr concentration in non-equilibrium α′ precipitates.

[en] We have investigated magnetic properties for reactor pressure vessel model alloys with variable Cu contents, subjected to neutron irradiation up to a fluence of $9\times {10}^{19}$ n cm${}^{-2}$. Unlike a monotonic increase of microhardness with neutron fluence, the major-loop coercivity decreases at a higher fluence and the decrease becomes larger for the alloy containing a higher amount of Cu. The measurements of first-order reversal curves (FORCs) for the high-Cu alloy show that the position of the FORC distribution peak shifts toward a lower coercivity just after neutron irradiation, followed by a slight increase, associated with the broadening along both the coercivity and interaction field axes. The results can be explained by the enhancement of magnetic inhomogeneity in a matrix due to Cu precipitation and an increasing magnetostatic interaction between local magnetic regions with different coercivity. The magnetic method using FORCs can be a possible technique which provides in-depth information on microstructural changes due to neutron irradiation, which is not obtained by measurements of a conventional major hysteresis loop.

[en] Nano-scale Y₂Ti₂O₇ and Y₂TiO₅ oxides are the major features that provide high strength and irradiation tolerance in nano-structured ferritic alloys. Here, we employ density functional theory to study helium trapping in Y₂TiO₅. The results suggest that helium is more deeply trapped in Y₂TiO₅ compared to Y₂Ti₂O₇. Helium occupies open channels in Y₂TiO₅, where it weakly chemically interacts with neighboring oxygen anions, and results in less volume expansion compared to Y₂Ti₂O₇, reducing strains in the iron matrix. The corresponding helium mobility in these channels is very high. While its ultimate fate is to form oxide/matrix interface bubbles, transient deep trapping of helium in oxides plays a major role in the ability of NFA to manage helium distribution.

[en] Pyrochlore Y₂Ti₂O₇ is a primary precipitate phase in nano-structured ferritic alloys (NFAs) for fission and fusion energy applications. We report a theoretical study for assessing the relative stability of trapping helium in Y₂Ti₂O₇ versus in matrix iron. Various defect structures and the associated energies are examined and compared. Results reveal that helium can be deeply trapped in Y₂Ti₂O₇ and that the corresponding self-interaction is essentially repulsive. Transmutant helium in NFAs prefers to occupy individual octa-interstitial sites in Y₂Ti₂O₇, before forming small clusters in Y₂Ti₂O₇. Helium partitioning in NFAs depends on the number and dispersion of Y₂Ti₂O₇; and thus initially, bubble formation and growth in iron matrix can be largely suppressed. Charge transfer occurs from helium to neighboring oxygen anions, but not to neighboring metal cations, suggesting a general effectiveness of trapping helium in oxides. Reasons for the ultimate fate of helium to form small nm-scale interface bubbles are also discussed