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In this work, three NiCoCr-based medium-entropy alloys (MEAs), i.e., NiCoCr, NiCoCrW 0.1 and NiCoCrTi 0.1 , were irradiated with helium (He) ions at 400 ℃ and 700 ℃ to study the radiation hardening and He-induced cavity growth behavior. The typical quaternary FeCoNiCr alloy was irradiated similarly and evaluated for comparison. Using transmission electron microscopy and nanoindentation measurements, He-induced microstructural evolution and mechanical changes were systematically investigated. Here we show that mitigated radiation hardening at 400 ℃ and cavity growth behavior at 700 ℃ were exhibited in the NiCoCr-based MEAs, in contrast to significant radiation hardening and He cavity growth of FeCoNiCr. Among the studied alloys, NiCoCrW 0.1 displayed the best radiation hardening resistance, while NiCoCrTi 0.1 presented the strongest tolerance in terms of He cavity growth. The underlying reasons were carefully analyzed based on the observed microstructures. Our results demonstrate the great potential of NiCoCr-based MEAs to function as critical nuclear structural components at relatively low and high temperatures. [ABSTRACT FROM AUTHOR]

• NiCo and NiFe were irradiated by 275 keV He+ ions at 673 K, 773 K and 873 K. • NiFe exhibits better helium cavity growth resistance than NiCo at 673 K, whereas the trend reverses at 773 K and 873 K. • The growth of dislocation loops tends to be delayed in NiFe with the increasing irradiation temperature. Irradiation temperature and the species of elemental constituents are two pivotal factors in determining the helium (He) radiation behaviors. However, their synergistic effect on the He-induced defect evolution in single-phase concentrated solid-solution alloys (SP-CSSAs) has rarely been studied. In this work, the evolution of He cavities and dislocation structural damages in two equiatomic SP-CSSAs, i.e., NiCo and NiFe, were investigated at 673 K, 773 K and 873 K with 275 keV He+ ions. He cavity formation behaviors were diverse at different temperature regimes. NiFe was found to have a stronger suppression effect on He cavity growth at 673 K. However, with the increasing temperature, a more pronounced increment of He cavity size was exhibited in NiFe. Unlike the evolution of He cavities, the growth of dislocation loops tends to be delayed in NiFe over the studied irradiation temperatures. The underlying mechanisms for the temperature-dependent defect evolution in NiCo and NiFe are discussed. [ABSTRACT FROM AUTHOR]

A series of NiCoCrSi x (x = 0, 0.1, 0.2, 0.3) medium-entropy alloys were synthesized to investigate the effect of Si addition on the microstructures, mechanical properties and helium irradiation resistance of the NiCoCr-based alloys. The microstructures and the phase evolution of these alloys were examined using electron microscopy as well as the existing models. The results show that the alloys exhibit a single face-centered-cubic structure when the Si content is in the range of 0–6.25 at.%, whereas phase decomposition occurs as the Si content further increases (∼9 at.%). Comparing with NiCoCr, NiCoCrSi 0.2 exhibited improved strength without sacrificing the ductility. The yield strength and ultimate tensile strength increased from 438 to 599 MPa and 921 to 1032 MPa, respectively, whilst the ductility kept as high as ∼50%. Our analysis indicates that the increased strength is mainly ascribed to the grain boundary strengthening. The large ductility is primarily due to the low stacking fault energy and shear modulus of the NiCoCrSi 0.2 alloy, which makes mechanical twinning in small grains accessible and provides a steady strain-hardening rate in a wide strain regime. Besides, the enhanced compositional complexity and local lattice distortion induced by the introduction of Si can improve the helium irradiation resistance of NiCoCr, resulting in reduced average bubble size. • Microstructure,mechanical properties and helium irradiation resistance of NiCoCrSi x MEAs were investigated. • Enhanced strength was obtained in NiCoCrSi 0.2 without the sacrifice of ductility. • Comparing with NiCoCr, NiCoCrSi 0.2 exhibits improved helium irradiation resistance. [ABSTRACT FROM AUTHOR]

For the structure materials applied in advanced nuclear energy system, helium bubble formation is always a big concern which will severely degrade the performance of materials around or above the half-melting temperature regime (∼0.5 T m). To explore the He bubble formation resistance in the FeCoNiCr alloy, that is a novel face-centered cubic (fcc) high-entropy alloy (HEA) showing excellent radiation damage tolerance, we conducted a series of 2 MeV He ions irradiation experiments on them at three different temperatures (0.46, 0.51 and 0.57 T m). For reference purpose, a model fcc metallic system of pure Ni was irradiated simultaneously. Through transmission electron microscopy (TEM), He bubble formation in the irradiated samples was systematically investigated. The results show that in any designated temperature, He bubbles have a smaller size, higher number density, and denser distribution in the HEA when comparing to that of pure Ni. The volume fraction of He bubbles is also less in the HEA, suggesting a suppressed bubble evolution. For the underlying mechanism of the He bubble formation resistance of HEA, we suggest that the featured energy barriers for point defects migration in the HEA will promote the recombination of defects and somewhat reduce the vacancy concentration during irradiation. Such unique effect could suppress the He diffusion through vacancy mechanism, it will finally influence the evolution of He bubbles in the HEA. •. FeCoNiCr high-entropy alloy weas irradiated by 2 MeV He+ in the half-melting temperature regime. •. He bubbles have a smaller size, higher number density, and denser distribution, in high-entropy alloy. •. The helium diffusion through vacancy mechanism was suppressed in high-entropy alloy. [ABSTRACT FROM AUTHOR]