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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]

[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]

Abstract: Irradiation-induced Ti-rich precipitates in V–Ti and V–4Cr–4Ti alloys are studied by TEM and positron annihilation methods (positron lifetime, and coincidence Doppler broadening (CDB)). The characteristics of small defect clusters formed in V alloys containing Ti at irradiation temperatures below 300°C have not been identified by TEM techniques. Strong interaction between vacancy and Ti solute atoms for irradiated V alloys containing Ti at irradiation temperatures from 220 to 350°C are observed by positron lifetime measurement. The vacancy-multi Ti solute complexes in V-alloys containing Ti are definitely identified by using CDB measurement. It is suggested that ultra-fine Ti-rich precipitates or Ti segregation at periphery of dislocation loops are formed in V alloys containing Ti at irradiation temperatures below 300°C. [Copyright &y& Elsevier]

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]

• Explosion-welded CuCrZr/316LN joints were neutron irradiated to 0.3 dpa at T irr < 190 °C. Good structural integrity has been maintained. • Black-spot contrasts dominated the damage microstructure on both sides of the CuCrZr/316LN interface. They were all under 4 nm in average size and achieved number densities on the order of 1022 m−3. • Radiation-enhanced diffusion might have favoured the dissolution of interlayers in pristine CuCrZr/316LN joints, and have accelerated the interfacial diffusion of key elements by at least two orders of magnitude. • The joint interface experienced an increase of Vickers hardness by ~ 21% after neutron irradiation. Heat treatments of 500 °C/1 h resulted in moderate softening, while 800 °C/1 h gave rise to a complete elimination of irradiation-induced hardening, respectively. Explosion-welded CuCrZr/316LN joints, which serve as key joining components in enhanced-heat-flux type first wall panels of ITER, have been neutron irradiated in the BR2 reactor (Belgium) at <190 °C, up to 0.3 dpa. The joints have maintained good structural integrity after irradiation and were free of void swelling. Radiation defects featuring black-spot contrasts were produced on both sides of the CuCrZr/316LN interface. They were on the order of 1022 m−3 in number density and all less than 4 nm in average size. Radiation-enhanced diffusion might have favoured the dissolution of interlayers which were characteristic of pristine CuCrZr/316LN joints, and accelerated the diffusion of key elements (e.g. Cu, Fe) across the interface. The joint interface experienced an increase of Vickers hardness by ~ 21% (∆H v = 48) after neutron irradiation. Heat treatments of 500 °C/1 h resulted in moderate softening of the interface, while 800 °C/1 h gave rise to a complete elimination of irradiation-induced hardening effect, respectively. [ABSTRACT FROM AUTHOR]