Development and performance investigation of low activation medium entropy alloy FeCr₂VW_x via arc melting for fusion applications

High-entropy alloys (HEAs) are a novel class of metal alloys composed of five or more metal elements, with each primary element's concentration ranging from 5% to 35%. Due to their outstanding mechanical properties, creep resistance, high oxidation resistance, and irradiation durability, HEAs are considered ideal candidate materials for structural components in nuclear reactors. This study aims to design a medium-entropy alloy (MEAs) consisting of high-melting-point, low-activation alloy elements as a potential nuclear structural material. Alloy samples were fabricated using arc melting. The microstructure, phase characterization, mechanical properties, and oxidation resistance of the arc-melted low-activation medium-entropy alloys were investigated. Additionally, annealing heat treatment was employed to optimize the mechanical properties of the samples.

Firstly, thermodynamic calculations were employed to guide alloy design. Feasibility analysis was then performed on newly designed low-activation medium-entropy alloys produced by arc melting. These materials were manufactured using arc melting, and their microstructure and mechanical properties were studied. The results indicated that a developed MEA FeCr₂VW_0.1 composition displayed a similar microstructure to FeCr₂V. The prepared FeCr₂VW_0.1 showed improved mechanical properties compared to FeCr₂V. First-principles calculations based on density functional theory (DFT) and theoretical strength calculations confirmed that the inclusion of W in MEAs based on FeCr₂V significantly enhanced Solid State Strengthening (SSS) and Precipitate Strengthening (PS) of the investigated alloy, in line with experimental results.

Building upon the feasibility manufacturing study, the microstructure of arc-melted low-activation medium-entropy alloys with varying W contents was characterized, and their mechanical properties were tested at room temperature and high temperatures. The studied FeCr₂VW_x (x = 0, 0.1, 0.3) medium-entropy alloys exhibited a dual-phase microstructure composed of body-centered cubic (BCC) phases. With further W addition, the microstructure of the FeCr₂VW_0.5 sample featured additional W-rich phases. Compression tests conducted at both room temperature and high temperature revealed an improvement in the compression performance of MEA with the introduction of W. These compression properties are comparable to existing low-activation refractory high-entropy alloys. The excellent high-temperature performance is attributed to the enhanced precipitation and solid solution strengthening effects post W addition.

After confirming the excellent mechanical properties of the novel low-activation medium-entropy alloys, their oxidation resistance was studied. Oxidation kinetics studies demonstrated that different FeCr₂VW_x (x = 0, 0.1, 0.3, 0.5) medium-entropy alloys followed a parabolic kinetic trend at 650°C in air. Compared to the traditional nuclear material P91 steel, FeCr₂VW_x MEAs exhibited superior oxidation resistance. Additionally, the addition of W further enhanced their oxidation resistance. Finally, the mechanical properties of the fabricated structures were optimized through post-annealing treatment. Solid solution annealing treatment at 1200°C refined the microstructure of the low-activation medium-entropy alloy samples. Samples subjected to post-annealing treatment at 1200°C displayed a more uniform elemental distributions, larger grain sizes, and an appropriate combination of precipitates, resulting in improved mechanical properties.

Based on the outcomes of this study, it can be concluded that the designed low-activation medium-entropy alloys are viable. They exhibit mechanical properties comparable to existing low-activation refractory high-entropy alloys and superior oxidation resistance compared to traditional nuclear materials. Furthermore, subsequent annealing treatment effectively enhanced the comprehensive mechanical properties of the alloys. The outstanding resistance of MEA to high temperatures, along with its high strength and resistance to oxidation, positions it as a promising candidate for applications in the nuclear energy sector, critical component manufacturing in aerospace, and the marine chemical industry. These results in the study provide valuable insights for the future development and microstructure engineering of novel MEAs, as well as optimizing the mechanical properties of MEAs.