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By exploiting Monte Carlo methodology and molecular dynamics, we computationally simulate the spinodal decomposition of iron–chromium binary alloys and analyze the relationship between the increase of yield strength induced by the phase separation phenomenon, and statistical parameters of the atomistic configuration. We successfully model the experimentally-discovered proportional relationship between the hardness and the variation parameter (or V ), and also found that the adequacy of the parameter V as an empirical indicator of hardening is limited, because it does not properly capture short-range atomistic configurations that influence the hardening. We suggest that the short-range-order parameter has more potential to become universal descriptor of the phenomenon. [ABSTRACT FROM AUTHOR]

We have investigated a post-annealing effect on microscopic structural changes of kink boundaries (KBs), which have been initially introduced by a hot-extrusion of an Mg-Zn-Y alloy containing long period stacking/order (LPSO) phase, based on scanning transmission electron microscopy (STEM) and atom-probe tomography (APT). STEM observations show that most of the KBs appear to be segmented into several submicron-scale, each of which is composed of arrays of extended basal (a -) dislocations that are geometrically necessary to account for the LPSO crystalline rotations. Atomic-scale STEM/APT observations reveal significant reconstructions of these extended dislocation-core structures, which occur at the solute-enriched face-centered cubic layers in the LPSO structures and forms local hexagonal close-packed Mg region with distinct solute depletions. These recovery-like features indeed enhance the thermal stability of the KBs, and the corresponding driving force can be reasonably described according to disclinations, whose elastic energy is defined by crystal rotations and the relevant dipole distances. • Microscopic structures of kink boundaries in unique long-period structured Mg-Zn-Y alloys have been investigated. • Microscopic segmentations of kink boundaries have been promoted during the post-annealing of the hot-extruded specimen. • Dislocation-core structures at kink boundaries have been significantly reconstructed with distinct solute redistributions. • Driving force to promote the observed recovery features can be reasonably described according to disclinations. [ABSTRACT FROM AUTHOR]

We investigated the fluence dependence of irradiation-induced solute cluster, dislocation loop, and very small defect to reveal the hardening mechanism in surveillance test specimens from a reactor pressure vessel steel with low-Cu content (0.04 wt%) using atom probe tomography (APT), weak-beam scanning transmission electron microscopy (WB-STEM), and positron annihilation spectroscopy. A high number density (>10 23  m −3 ) of solute clusters mainly composed of Ni, Mn, and Si atoms were found in highly neutron irradiated specimens (∼10 24 neutrons m −2 ( E  > 1 MeV)) by APT. These solute clusters were one of the main sources of hardening as reported previously. On the other hand, it was also revealed that dislocation loops were formed with a number density of ∼10 22  m −3 in the high-fluence specimens by WB-STEM. The estimated hardening due to dislocation loops was more than half of the actual hardening, showing that dislocation loops are also main source of irradiation hardening at high neutron fluence with the solid experimental evidences. Regarding specimens subjected to a low neutron fluence (∼10 23 neutrons m −2 ), very small defects, not detected by either WB-STEM or APT, were formed by positron annihilation spectroscopy. This result suggested that, at a low neutron fluence, the defects were the initial hardening source and they may grow the dislocation loops observed by WB-STEM at high fluence range. [ABSTRACT FROM AUTHOR]

To clarify the irradiation hardening behavior of neutron-irradiated tungsten(W), the formation behavior of rhenium(Re)-enriched cluster and W-Re precipitation with displacement damage in fission reactors environment was investigated by the atom probe tomography (APT) technique, and the results obtained by APT and microstructural observation by transmission electron microscopy (TEM) were compared. Neutron-irradiated pure W and W-10Re alloys were used for the analysis. These samples were irradiated to 0.96 dpa at 538 °C in a fast experimental reactor Joyo, and irradiated to 0.90 dpa at 500 °C in a mixed spectrum reactor HFIR. The chemical compositions of the matrix, Re clusters, and precipitates were determined by the AP analysis. The Joyo-irradiated samples contained lower levels of transmuted Re and Os, while the HFIR-irradiated samples contained higher levels of Re and Os. The Re-enriched clusters demonstrated characteristic spatial distribution related to defect cluster distribution in each irradiated specimen. [ABSTRACT FROM AUTHOR]

Abstract The composition evolution of γ′ nanoparticles as a function of aging duration in the Ti-doped CoFeCrNi high entropy alloy was studied by atom probe tomography (APT) and first-principle calculations. The APT results indicate that the evolution behaviors of Co, Fe, and Cr in the Ni 3 Ti-type γ′ nanoparticles are significantly different. Co saturates to a stable concentration in the γ′ nanoparticles rapidly in the initial stage of the precipitation, whereas Fe and Cr atoms are incorporated into the γ′ nanoparticles during the coarsening. The APT results are supported by the first-principle calculations from the perspective of the electronic state. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

[Display omitted] • The addition of Re and Cr suppressed the formation of vacancy clusters in W after high-temperature irradiation, while no effect was observed for W-5 at.% Ta and W-2.5 at.% Mo alloys. • Fine Cr-rich and Re-rich precipitates were observed in W-0.3 at.% Cr and W-5 at.% Re alloys, respectively, where the distributions of Mo and Ta in W-2.5 at.% Mo and W-5 at.% Ta alloys were uniform even after irradiation. • Irradiation hardening was observed for all materials but that of W-5 at.% Re alloy was significantly smaller than the hardening of W, W-2.5 at.% Mo and W-5 at.% Ta alloys. • The stability of mixed dumbbells plays an important role in the vacancy cluster formation, precipitation and irradiation hardening of W alloys. Irradiation responses of binary W alloys were investigated systematically from the perspective of the binding energy of an alloying element with a W self-interstitial atom (W-SIA). Plates of W, W-0.3 at.% Cr, W-5 at.% Re, W-2.5 at.% Mo and W-5 at.% Ta alloys were irradiated at 1073 K with 6.4 MeV Fe ions to 0.26 dpa at the damage peak, where the binding energy of alloying element with W-SIA is in order of Cr > Re > Mo > Ta. The formation of vacancy-type defects (vacancies and vacancy clusters) was studied by using positron lifetime measurement. The precipitation of alloying elements was studied by using atom probe tomography (APT) and the hardness changes in the irradiated volumes were measured by the nanoindentation technique. The formation of vacancy-type defects was strongly suppressed by the addition of Cr and Re, while Ta and Mo had no noticeable suppression effect. The APT measurements showed fine Cr- and Re-rich precipitates in W-0.3 at.% Cr and W-5 at.% Re alloys, respectively, where the density of precipitates in the latter was clearly lower than that in the former. The distributions of Mo and Ta were uniform even after irradiation. Irradiation hardening was observed for all materials but that of W-5 at.% Re alloy was significantly smaller than the hardening of W, W-2.5 at.% Mo and W-5 at.% Ta alloys. These observations suggest that the irradiation hardening of W, W-2.5 at.% Mo, and W-5 at.% Ta alloys were mainly caused by vacancy-type defects. It was concluded that an alloying element with moderate binding energy with a W-SIA effectively suppresses vacancy formation without significantly enhanced precipitation and consequently mitigates irradiation hardening. [ABSTRACT FROM AUTHOR]

• The role of Si content on irradiation embrittlement was clarified. • Solute atom clusters of highly neutron-irradiated Japanese pressurized water reactor surveillance test specimens were analyzed by atom probe tomography. • The cluster radius and number density decreased and increased, respectively, with increasing Si content, resulting in a constant volume fraction of SCs. • Increase in Si reduces the degree of irradiation embrittlement. This is consistent with the results of our previous study based on Bayesian statistical analysis. Solute clusters (SCs) formed in pressurized water reactor surveillance test specimens neutron-irradiated to a fluence of 1 × 1020 n/cm2 were analyzed via atom probe tomography to understand the effect of silicon on solute clustering and irradiation embrittlement of reactor pressure vessel steels. In high-Cu bearing materials, Cu atoms were aggregated at the center of cluster surrounded by the Mn, Ni, and Si atoms like a core-shell structure. In low-Cu bearing materials, Mn, Ni, and Si atoms formed cluster and these solutes were not comprised core-shell structure in SCs. While the number of Cu atoms in clusters was decreased with decreasing nominal Cu content, the number of Si atoms had clearly increased. The cluster radius (r) and number density (N d) decreased and increased with increasing nominal Si content, respectively. The shift in the reference temperature for nil-ductility transition (ΔRT NDT) showed a good correlation with the square root of volume fraction (V f) multiplied by ( V f × r). The negative relation between the nominal Si content and ΔRT NDT indicated that increasing of nominal Si content reduces the degree of embrittlement. [ABSTRACT FROM AUTHOR]

Bimetallic plates of CuCrZr/316LN are important heat sink components in the ITER first wall. Their in-reactor performance was investigated with 6.4 MeV Fe3+ ion irradiation (500 °C/10.5 dpa), as an analogue for neutron damage. Detailed characterizations showed that the CuCrZr/316LN joints have maintained good structural integrity after irradiation. The as-irradiated interface consisted of a nanocrystalline interlayer, typically of <1 μm in width, mixed of CuCrZr and 316LN grains measuring 20–250 nm in size. Black-spots and loop/loop rafts dominated the damage microstructure on 316LN and CuCrZr sides of the interface, respectively. A series of 30 min long post-irradiation annealing experiments saw the onset of interlayer evolution in CuCrZr/316LN joints at 400 °C by grain boundary motion and Ostwald ripening, and further acceleration at ≥600 °C. Structural collapse of the interlayer is expected at extended time scales. Finally, a brief comparison between heavy-ion and neutron irradiation damage (Yi et al. 2021) in CuCrZr/316LN joints is made, with regard to defect production and interfacial transport behaviour of chemical elements. • High quality FIB lift-outs of CuCrZr/316LN were prepared by tailored geometry between Ga+ ions and the interface. • The CuCrZr/316LN joints maintained structural integrity after heavy-ion irradiation at 500 °C, up to 10.5 dpa. • Black-spots and loop/loop rafts dominated the damage microstructure on both sides of the joint. • The as-irradiated interface consisted of a stable nanocrystalline interlayer, mixed of CuCrZr and 316LN grains • The interlayer structure evolved during post-irradiation annealing (≥ 400 °C). [ABSTRACT FROM AUTHOR]

• The grain-boundary P segregation for A533B RPV steels irradiated to high fluences in PWRs or a MTRs was analyzed using AES. • An increase in irradiation-induced grain-boundary P segregation was confirmed by a simulation based on a rate theory model as well as AES analysis. • No flux effect on grain-boundary P segregation was confirmed for A533B RPV steels with bulk P contents equivalent to that in RPV steels employed in Japanese nuclear power plants. • Intergranular embrittlement is not likely to occur for RPV steels with a bulk P content, which is higher than that of most U.S. A533B steels. Reactor pressure vessel (RPV) steels for pressurized water reactors (PWRs) with bulk P contents ranging from 0.007 to 0.012wt.% were subjected to neutron irradiation at fluences ranging from 0.3 to 1.2 × 1020 n/cm2 (E > 1 MeV) in PWRs or a materials testing reactor (MTR). Grain-boundary P segregation, which was analyzed using Auger electron spectroscopy (AES) on intergranular facets, increased with increasing neutron fluence. A rate theory model based on four diffusion-reaction equations for substitutional P atoms, octahedral interstitial P atoms, vacancies, and self-interstitial atoms was also used to simulate the increase in grain-boundary P segregation for RPV steels with a bulk P content up to 0.020wt.%, using parameters optimized by the present AES data. The increase in grain-boundary P segregation in RPV steel with a bulk P content of 0.015wt.%, which is the maximum P concentration in RPV steels used in Japanese nuclear power plants intended for restart, was estimated to be less than 0.1 in monolayer coverage at 1 × 1020 n/cm2 (E > 1 MeV). A comparison of the PWR data with the MTR data, including that from the literature, showed that neutron flux had no effect upon grain-boundary P segregation for A533B steels. The relationships of the ductile-brittle transition temperature (DBTT) shifts to grain-boundary P segregation and to yield strength were also discussed. A linear relationship between the yield strength and the DBTT shift with a slope of 0.63 was obtained for RPV steels with a bulk P content up to 0.026wt.%, which is higher than that of most U.S. A533B steels. It is concluded that the intergranular embrittlement is unlikely to occur for RPV steels irradiated in PWRs. [ABSTRACT FROM AUTHOR]

Spinodal decomposition in thermally aged Fe–Cr alloys leads to significant hardening, which is the direct cause of the so-called 475 ∘ C-embrittlement. To illustrate how spinodal decomposition induces hardening by atomistic interactions, we conducted a series of numerical simulations as well as reference experiments. The numerical results indicated that the hardness scales linearly with the short-range order (SRO) parameter, while the experimental result reproduced this relationship within statistical error. Both seemingly suggest that neighboring Cr–Cr atomic pairs essentially cause hardening, because SRO is by definition uniquely dependent on the appearance probability of such pairs. A further numerical investigation supported this notion, as it suggests that the dominant cause of hardening is the pinning effect of dislocations passing over such Cr–Cr pairs. Moreover, this finding has a practical merit, that is, the SRO parameter can serve as a good index of hardening of steels, which is critical for evaluating their lifetime. [ABSTRACT FROM AUTHOR]

• The mechanism of surface activated bonding (SAB) at low temperature is clarified. • Atomic diffusion in the bonding process is a key concept to make tough interfaces. • The diffusion would be assisted by the defects introduced in the activation process. • There is a trade-off relationship between bonding strength and electronic properties. Bonding mechanism at room temperature (RT) in GaAs/Si heterointerfaces fabricated by surface-activated bonding (SAB) is examined using cross-sectional scanning transmission electron microscopy combined with low-temperature focused ion beam and time-of-flight secondary ion mass spectrometry. In the bonding process at RT, atomic intermixing at the interfaces, presumably due to the transient enhanced diffusion assisted by the point defects introduced in the surface activation process, is confirmed. The defect-assisted atomic diffusion at the interfaces, as well as the formation of atomically clean and activated surfaces, would be the key concept of SAB, by which we can create tough heterointerfaces at RT. Meanwhile, the defects on the activated surfaces would degrade the interface resistance. The degraded properties can be recovered by an appropriate annealing after the SAB processes, although the atomistic structure around the heterointerfaces would be modified during the annealing. By controlling SAB and subsequent annealing conditions, we can obtain low-resistance heterointerfaces via the optimization of the trade-off relationship between the chemical bonding strength and the electronic properties, determined by the activated surfaces before bonding. [ABSTRACT FROM AUTHOR]

Ultra-high purity (> 99.9999 wt%) chemical vapour deposited tungsten (CVD W) samples were neutron irradiated in the BR2 reactor (Belgium) at T irr < 210 °C to ~ 0.15 dpa, followed by isochronal annealing at 500, 800 and 1100 °C. Defect characterization showed that dislocation loops dominated the as-irradiated damage microstructure and were mostly ≤5 nm. Void formation was observed after post-irradiation annealing at 1100 °C. The mechanical and thermal properties of CVD-W were evaluated based on tensile tests, Vickers hardness and temperature wave analysis. Fractography study suggested that a transition from intergranular fracture to cleavage fracture took place in the material after neutron irradiation. Hardening was found ~ 23% after irradiation. Subsequent annealing below 800 °C saw further increase in hardness, featuring a maximum value of about Hv = 487. Softening occurred at 1100 °C. Thermal diffusivity dropped by ~ 65% after irradiation and ~ 40% of this degradation recovered at 1100 °C. • CVD-W coatings were neutron irradiated to 0.15 dpa at T irr < 210°C, followed by isochronal annealing. • Small loops dominated the damage microstructure. Voids were observed after post-irradiation annealing at 1100 °C. • Irradiation hardening was found to be ~ 23%. Further increase was found during post-irradiation annealing. • A transition of intergranular fracture to cleavage fracture took place in CVD-W after neutron irradiation. • Thermal diffusivity degraded by ~ 65% in CVD-W after neutron irradiation. [ABSTRACT FROM AUTHOR]