The dramatically increasing population and developing global economy urgently calls for more reliable, environmentally sustainable, and cost-effective energy. Nuclear fusion energy is expected to be the best option in the future due to its intrinsically safe and environmentally sound features, and limitless energy source. Due to the extraordinarily limited solubility, the abundant helium (He) atoms generated during fusion reaction will precipitate and form He cavities in alloys, which could seriously impair the performance of structural materials and detrimental to the lifetime of reactors. Also, dislocation structures generated due to displacement cascade from the high-energetic He atoms can lead to radiation-induced hardening and substantial reduction in the uniform elongation of structural materials. Therefore, developing high-performance structural materials can be one of the main technical barriers to realizing the future success of proposed fusion energy reactors. Recently, a novel class of materials called single-phase multicomponent alloys, including high-entropy alloys (HEAs), have been of substantial interest owing to their attractive properties. Previous studies have demonstrated that the face-centered-cubic-type (FCC-type) multicomponent alloys exhibit remarkable radiation resistance due to the suppressed accumulation of radiation-induced damage. In such a background, this thesis is concerned with understanding the He behavior in FCC-type multicomponent alloys and designing high-performance FCC-type multicomponent alloys for future reactors. The thesis could be divided into the following three parts:

In the first part, the helium cavity evolution in FCC-type multicomponent alloys was investigated. Pure Ni and five equiatomic alloys, i.e., NiCo, NiFe, NiCoCe, NiCoFeCr and NiCoFeCrMn were irradiated He⁺ ions at 673 ~ 973 K. The results revealed that the He cavity formation behavior of FCC-type multicomponent alloys with different elemental constituents types and numbers were diverse at different regimes. For the NiCo and NiFe alloys with the same numbers but different types of elemental constituents, NiFe displays a smaller average He cavity size at 673 K due to the smaller gap between vacancy and interstitial migration energies. However, the vacancy mobility tends to be enhanced via the oversized Fe atoms in NiFe with the increasing temperature, leading to enhanced vacancy clustering and He accumulation. This behavior will overwhelm the recombination effect, resulting in a higher He cavity growth rate in NiFe comparing with NiCo. On the other hand, the increasing elemental constituents numbers can effectively suppress the He cavity growth at low temperature (below half-melting temperature). This can be attributed to the suppressed mobility and enhanced recombination of point defects due to the increasing chemical complexity and lattice distortion. Nevertheless, chemically biased vacancy diffusion still overwhelms the recombination effect and dominates the He cavity growth at high temperatures. The results of Fe/Cr depletion around the He cavities indicated a preferential vacancy migration toward He cavities via oversized atoms, which results in enhanced He cavity growth with increasing elemental constituent numbers.

Following the above part, we studied the evolution of He-induced dislocation structures in FCC-type multicomponent alloys. At 673 K, faulted 1/3 <111> dislocation loops prevail in the materials. The growth of faulted loops in FCC-type multicomponent alloys can be dominated by stacking fault energy (SFE) and lattice distortion. In most cases, the effect of severe lattice distortion will overwhelm the SFE effect and significantly hinder the dislocation loop growth in FCC-type multicomponent alloys, especially in materials with more elemental constituents like NiCoFeCr and NiCoFeCrMn. With similar lattice distortion, lower SFE tends to enhance the growth of faulted dislocation loops. As the temperature increases, the dislocation loops in the studied materials were thermally unstable and dissociated into long dislocation segments. 

Thirdly, NiCoCr was selected to be a based alloy to developed novel alloys with enhanced performance considering its excellent combination properties. Si was added into NiCoCr to investigate the microstructure, mechanical properties and He irradiation resistance. The single FCC structure can be maintained in the NiCoCrSi_0.2 alloy, while an amount of Cr-rich and CrSi-rich phases precipitate out in the NiCoCrSi_0.3 alloy. Comparing with NiCoCr, increments of yield strength and ultimate tensile strength by ~ 37 % and ~ 12 %, respectively, but without a sacrifice of the ductility can be achieved in the NiCoCrSi_0.2 alloy. The TEM study revealed the formation of mechanical twinning during deformation in the NiCoCrSi_0.2 alloy, which is the same as NiCoCr but activated at an earlier deformation stage. Therefore, nanotwinning occurred over an extended strain range, promoting the work hardening behavior and in turn postponing the necking instability of the NiCoCrSi_0.2 alloy. Moreover, the He cavity growth was found to be delayed in the NiCoCrSi_0.2 alloy after being irradiated with He ions at 673 K, attributing to the enhanced compositional complexity and atomic-level local lattice distortion by the introduction of Si. Hence, we demonstrated that the Si-doped NiCoCr alloy shows great potential to be utilized in future reactors.

We believe that our present findings not only are important for the understanding of the synergistic effect of compositional complexity and irradiation temperature on the He-induced defect evolution in FCC-type multicomponent alloys, but also shed light on the future design of structural materials for fusion reactor systems.