Kooknoh Yoon

Nuclear fusion is an enticing alternative to current sources of energy, with multilayered Rare-Earth Barium Copper Oxide (REBCO) coated conductors deemed pivotal in the race toward fully realized, commercially viable, and magnetic confinement fusion reactors. In this study, we simulated the ion spectrum expected to evolve from REBCO's nickel-based Hastelloy C-276 substrate and copper stabilizer in an affordable robust compact-like reactor. We then emulated this gas production through helium implantation to investigate changes in materials and superconducting properties. Our results revealed that the substrate and stabilizer are capable of producing protons energetic enough to recoil throughout the tape thickness in appreciable doses, and alphas energetic enough to deposit 7.54 × 1014 ions/cm2 or 50.1 helium appm in the superconducting layer over a 30-year reactor lifetime. The superconducting layer of SuperPower® tapes exhibited at least double the swelling rate of the other major layers, and both SuperPower and Fujikura Ltd. tapes displayed microstructural changes in the REBCO layer not observed in isotropic metals. For the estimated lifetime fluence, the Fujikura tapes showed a ∼1 K reduction in critical temperature and a 32% degradation in critical current for compact reactor-relevant conditions (16 T, 20 K). Nuclear transmutation, low-temperature solder implantations, gas-ion evolution, the influence of gas production on vortex dynamics, and other related considerations are also discussed.

Refractory high‐entropy alloys (RHEAs) are considered promising candidate materials for next‐generation nuclear reactors due to their superior mechanical strength, irradiation resistance, and thermal stability at high temperatures. However, the significant positive heat of mixing between refractory alloying elements and Cu, commonly used in cooling systems, poses challenges in forming composite structures. This study addresses the issue using a liquid metal dealloying (LMD) process. A precursor alloy (WTaVTi) with a directional dendrite‐interdendrite structure is fabricated and reacted with molten Cu at 1200 °C for 96 h. This approach produced a RHEA‐Cu composite with a stable interface between RHEA (W_31.5Ta_30.9V_21.4Ti_14.3) and Cu, featuring a spontaneously formed W‐rich interlayer that enhances interfacial bonding. The composite showed excellent irradiation resistance, with 30% less swelling under α‐ion irradiation than pure W. It also exhibited low thermal conductivity at room temperature, but reached ≈120 W m⁻¹·K⁻¹ at ≈650 °C, surpassing pure W. This temperature‐dependent rise in κ, with a positive gradient of +0.075 W m⁻¹·K⁻², is attributed to decreasing diffuse mismatch at elevated temperatures. The large‐scale reaction and stable microstructure achieved through LMD process highlight its industrial potential. This work offers a strategy for developing high‐performance materials by combining RHEA's radiation resistance with Cu's thermal conductivity for extreme environments.

Metals often lose their ductility at cryogenic temperatures owing to the decreased mobility of dislocations. TRansformation-induced plasticity (TRIP), a toughening mechanism at room temperature, can increase damage susceptibility at low temperatures, as the resultant martensite phases can become more brittle than the parent phases. Herein, we develop a high-entropy alloy (HEA) with an improved low-temperature impact-damage tolerance through a sequential plasticity mechanism. We design a trip-assisted dual-phase HEA (TADP HEA) and investigate the effects of Al addition on its mechanical properties upon deformation at different temperatures, depending on stacking fault energy (SFE). Our analysis shows that a senary (Cr20Mn6Fe34Co34Ni6)98Al2 HEA exhibits superior mechanical properties, including a 641 MPa yield strength (σy), 964 MPa ultimate tensile strength (σUTS), and 40% uniform elongation (ɛUTS) at ambient temperature (25 °C), and a 1 GPa σy, 1.5 GPa σUTS, and 36% ɛUTS at −100 °C. Notably, despite the presence of hexagonal-close packed martensite, the HEA exhibits a higher Charpy impact energy (406 J) than Cantor HEA (344 J) at −100 °C. We attribute this improvement to the sequential deformation mechanism of mechanical twinning and martensitic transformation in the HEA at −100 °C, which results in sustainable steady strain-hardening during deformation. We suggest that optimizing the sequential deformation mechanism by manipulating SFE in multi-component alloys can be an effective route for improving the damage tolerance of metals at cryogenic temperatures.

The formation of a single phase is an important requirement for high-entropy ceramics (HECs) because precipitation of unwanted phases generally degrades their functional properties. This paper provides a useful guideline for the single-phase formation of HECs. First, metal elements constituting HECs can be divided into two groups: elements that have a parent phase as a stable phase and elements that have a phase with the same stoichiometry as the parent phase but a different crystal structure. Second, even when the latter elements are added in an HEC, we can stabilize the parent phase if stabilizing energy by configurational entropy is larger than the difference in formation energy due to their stable phase, which can be quantitatively calculated through first-principles calculation. Interestingly, based on these guidelines, (CrMnFeCoNi)Si HE silicide with a single B20 structure was sequentially developed from mono-silicide. In particular, the HEC with maximized configurational entropy was searched in our HEC system by adding NiSi to (CrMnFeCo)Si, which is stable in B31 and B20 structures. This study offers a chance to increase the structural and compositional complexity in HECs, enabling the expansion of the single-phase region in HECs.

Herein, we carefully investigate the effect of nitrogen doping in the equiatomic CoCrFeMnNi high-entropy alloy (HEA) on the microstructure evolution and mechanical properties. After homogenization (1100 °C for 20 h), cold-rolling (reduction ratio of 60%) and subsequent annealing (800 °C for 1 h), a unique complex heterogeneous microstructure consisting of fine recrystallized grains, large non-recrystallized grains, and nanoscale Cr2N precipitates, were obtained in nitrogen-doped (0.3 wt.%) CoCrFeMnNi HEA. The yield strength and ultimate tensile strength can be significantly improved in nitrogen-doped (0.3 wt.%) CoCrFeMnNi HEA with a complex heterogeneous microstructure, which shows more than two times higher than those compared to CoCrFeMnNi HEA under the identical process condition. It is achieved by the simultaneous operation of various strengthening mechanisms from the complex heterogeneous microstructure. Although it still has not solved the problem of ductility reduction, as the strength increases because the microstructure optimization is not yet complete, it is expected that precise control of the unique complex heterogeneous structure in nitrogen-doped CoCrFeMnNi HEA can open a new era in overcoming the strength–ductility trade-off, one of the oldest dilemmas of structural materials.

Quantitative and well-targeted design of modern alloys is extremely challenging due to their immense compositional space. When considering only 50 elements for compositional blending the number of possible alloys is practically infinite, as is the associated unexplored property realm. In this paper, we present a simple property-targeted quantitative design approach for atomic-level complexity in complex concentrated and high-entropy alloys, based on quantum-mechanically derived atomic-level pressure approximation. It allows identification of the best suited element mix for high solid-solution strengthening using the simple electronegativity difference among the constituent elements. This approach can be used for designing alloys with customized properties, such as a simple binary NiV solid solution whose yield strength exceeds that of the Cantor high-entropy alloy by nearly a factor of two. This study provides general design rules that enable effective utilization of atomic level information to reduce the immense degrees of freedom in compositional space without sacrificing physics-related plausibility.