High-entropy carbides (HECs) are a new class of materials with properties that are promising for applications in extreme environments, involving high temperature, corrosion, and high ion-flux. In HECs, multiple principal cations form solid solutions, similar to medium/high-entropy alloys (M/HEA). However, mixing of atoms can be non-ideal, resulting in chemical short-range order (CSRO). CSRO has been already reported in M/HEAs, cation-disordered oxides, and high-entropy oxides and in many cases, it was found to have significant impact on materials properties. CSRO in covalently-bonded high-entropy ceramics has not been observed so far, and its potential impact on materials properties is unknown. In contrast to M/HEAs, in HECs only one of the sublattices forms a solid solution, and therefore it is unclear whether the concept of CSRO extends to HECs. Here, we report the observation of CSRO in multiple HECs using a combination of atomistic simulations and scanning transmission electron microscopy. We find that CSRO in HECs can be controlled by both selection of chemical elements and heat treatment, and it significantly improves radiation resistance, although it is not the only factor. Our findings expand the understanding of CSRO to HECs and provide a pathway for design of new materials for extreme environments.

Durability and longevity are among the key factors for applications of structural metallic materials. Since it is unrealistic to prevent stress-induced damage, self-healing could provide an alternative pathway to extend the material lifetime. Self-healing has been explored before in metals reinforced with shape-memory alloy (SMA). However, the poor interfacial bonding between embedded SMA and metallic matrices resulted in limited strength recovery. Here, we discovered that solid-solid phase transformation within the stress-induced amorphous shear bands in Al-Sm systems provides a promising strategy for strength recovery through self-healing. Based on a combination of experimental characterization and atomistic simulations, we demonstrated that amorphous shear bands in the Al-Sm system could transform back to crystalline structure during annealing at temperatures as low as 150 degrees C. This self-healing mechanism, driven by the amorphous-to-crystalline phase transformation allows the recovered materials to restore mechanical properties to a similar level as those of the undeformed samples. Our findings provide a potential strategy for the design of easy-self-healing materials with recyclable mechanical properties.

The formation of deuterium supersaturated surface layers (DSSL) in tungsten (W) under D irradiation is a critical phenomenon influencing defect evolution and fuel retention in plasma-facing materials (PFMs). However, the role of ubiquitous C/O impurities in DSSL development remains insufficiently elucidated. This study systematically investigated the impact of C/O impurities and D irradiation conditions on DSSL by employing two distinct D ion beam systems: a standard Kaufman source (K system, with inherent C/O), and a magnetically filtered highpurity source (T system). X-ray photoelectron spectroscopy confirmed significantly lower C/O co-implantation from the T system. Elastic recoil detection analysis revealed that DSSL formed in the K system were narrow (similar to 27 nm), whereas those in the T system were substantially broader (>110 nm). This broadening in the lowimpurity T system is attributed to unimpeded D diffusion, while C/O impurities in the K system inhibit D diffusion, confining the DSSL. Sequential irradiation experiments-K system pre-irradiation followed by T system irradiation-confirmed that pre-existing C/O impurities effectively narrow the DSSL, challenging the conventional view that DSSL formation is solely D-driven. While C/O impurities restrict DSSL broadening, this effect diminishes with increasing D flux and fluence. In the absence of significant C/O interference, DSSL concentration and total D retention were found to be more strongly dependent on D fluence than flux. This work presents the crucial role of C/O impurities in dictating DSSL morphology and provides benchmark data on intrinsic DSSL behavior, offering vital insights for understanding deuterium-material interactions in fusion environments.

Tungsten aluminum boride (WAlB) has been proposed as a promising candidate for integrated plasma-facing and shielding materials in compact tokamaks due to its excellent neutron and gamma-ray shielding properties, as well as its resistance to radiation-induced damage. However, the hydrogen isotope erosion and retention properties remain unclear. This study assesses and compares the erosion and retention properties of WAlB, molybdenum aluminum boride (MoAlB), and tungsten (W) using a combination of experiments and first-principle calculations. The WAlB sample, which contained impurity phases of W-Al and W-B, was annealed at 600 degrees C for 2 h prior to irradiation. This treatment increased the WAlB phase to over 80 %, making it the primary focus of this investigation. Both WAlB and W were subjected to deuterium (D) ion irradiation with fluences of 7.20 x 1023-1.71 x 1024 D/m2 at temperatures of 452-598 K. Results show that WAlB undergoes preferential sputtering under D ions irradiation with a sputtering yield slightly higher than that of W but lower than MoAlB. D retention in the near-surface region of WAlB is only half of that in pure W but greater than that in MoAlB. Notably, no significant blistering or plastic deformation appeared on the WAlB surface, whereas a substantial amount of D-induced blisters formed on the W surface. These findings imply that while WAlB may not match W in D retention resistance, it demonstrates superior resistance to surface blister formation. Optimistically, this work suggests that future advancements in composition and structural design could enhance WAlB's resistance to D retention and sputtering, boosting its potential for application in compact tokamak reactors.

Amorphous shear bands that can form in crystalline materials during high strain-rate deformation are typically precursors to fracture. Since shear bands in crystalline material typically do not persist over a wide range of strain rates, the effect of strain rate on solid-solid phase transformation within shear bands has rarely been discussed. Recent findings reveal that in some materials, shear bands can instead enable plasticity at low strain rates. However, it remains unknown how the mechanics of such shear bands and the resulting mechanical properties depend on the strain rate. Here, the strain-rate dependence of shear bands is investigated in two different compositions of Al-Sm system, where shear bands lead to fracture in one of the compositions and plasticity in the other. We discovered that strain rate directly affects the shear-band density and shear-band behavior, which in turn alters the mechanical properties. In contrast to low strain rates, high strain rates lead to the formation of immature partially amorphous shear bands which then transition into fully amorphous shear bands. The underlying reasons and the mechanisms underlying shear-band formation have been elucidated using a combination of experiments and molecular dynamics simulations across a large range of strain rates.

Radiation and corrosion can be coupled to each other in non-trivial ways and such coupling is of critical importance for the performance of materials in extreme environments. However, it has been rarely studied in ceramics and therefore it is not well understood to what extent these two phenomena are coupled and by what mechanisms. Here, we discover that radiation-induced chemical changes at grain boundaries of ceramics can have a significant (and positive) impact on the corrosion resistance of these materials. Specifically, we demonstrate using a combination of experimental and simulation studies that segregation of C to grain boundaries of silicon carbide leads to improved corrosion resistance. Our results imply that tunning of stoichiometry at grain boundaries either through the sample preparation process or via radiation-induced segregation can provide an effective method for suppressing surface corrosion.

Ultraviolet (UV) radiation and corrosion can be coupled in non-trivial ways and such coupling is of critical importance for the performance of materials in extreme environments. However, the confluence of these phenomena and their collective impact remains underexplored. This study bridges this knowledge gap by presenting a thorough investigation of Zr alloys exposed to corrosive high-temperature water under in-situ UV irradiation. We found that initial UV exposure significantly accelerates corrosion and induces distinct changes in the microstructure of ZrO2 oxides. Over time, we observed the formation of Fe3O4 particles on the ZrO2 surface due to photo-dissolution of zirconia and photo-deposition of iron oxide magnetite. The emergence of Fe3O4 particles could enhance the corrosion resistance of the Zr alloy substrate. Furthermore, our results indicate that UV irradiation modulates the density and distribution of nanopores within the oxide layer by influencing oxygen speciation and the dynamics of oxide growth. These insights advance our understanding of the complex interplay between UV irradiation and high-temperature water corrosion.

Nano-porosity development in thermally grown oxides plays a significant role in oxidation kinetics as nanopores may provide pathways for oxidizing species. As an example, Zr alloys are the most commonly used cladding materials in pressurized and boiling water nuclear reactors and are known to develop significant oxide nanoporosity. Corrosion results in the formation of an oxide layer containing nanopores while still being protective. It is still under debate whether the nanopores can provide fast diffusion paths for the oxidizing and hydriding species, and possibly accelerate corrosion. In the current study, we precisely quantify the nanoporosity in different regions of the oxide layer using a machine-learning-based method reported in a previous publication. In addition, the entire oxide layer depth and each pore within the layer are imaged, and a TEMbased 3D tomographic reconstruction is obtained. We further investigate the interconnectivity of the pores and the shortest path through the pores as functions of oxide depth, exposure time, and corrosion rate. Results demonstrate that interconnectivity is highest in the proximity of the W/O interface and gradually decreases towards the M/O interface. Specifically, higher temperatures, longer exposure times, and higher corrosion rates are correlated to increased interconnectivity among pores. This work provides essential evidence that pores in the near water/oxide interface region can provide paths for oxidizing species.

The oxidation behavior of alumina-forming austenitic (AFA) stainless steels exposed to 600 degrees C supercritical water (SCW) was studied via several high-resolution characterization techniques. The addition of 1.5-3.5 wt% aluminum (Al) significantly affected the microstructure of AFA alloys compared with Al-free alloys, with the formation and re-distribution of precipitates, including Laves, B2-NiAl, and delta-ferrite. After exposure to SCW, a typical duplex oxide scale was formed on the surface, and the oxide thickness decreased with increasing Al content, which was ascribed to the formation of a protective inner oxide layer. The threshold Al content was determined to be 3.5 wt%, as only the AFA alloy with a 3.5 wt% Al addition formed a distinct alumina scale and exhibited the lowest weight gain. The relationship between the Al content, precipitates, and mechanism of oxide film formation was revealed. In the AFA alloys, the volume fraction of B2-NiAl increased with increasing Al content, resulting in a more continuous alumina film formation. The addition of Al promoted the spheroidization of the needle-like Laves phase, leading to the formation of a protective Si-containing oxide film. Additionally, the higher content of Al addition also favored the formation of the delta-ferrite phase, which formed a slightly thicker but uneven alumina film after exposure to SCW.

Understanding the effects of Scandium (Sc) on the hydrogen absorption and desorption capabilities and the underlying mechanisms within Ti-Sc alloys is crucial for their applications as hydrogen storage materials. In this study, we explore the influence of Sc on deuterium (D) behavior in Titanium (Ti) films through a combination of experimental characterization and theoretical simulations. The Sc-doped Ti films with different Sc ratio were prepared using magnetron sputtering. Our findings indicate that Sc doping raises the D absorption temperature from 350 degrees C to 500 degrees C, and the Sc-doped Ti deuteride films exhibit higher apparent activation energies and temperatures for D desorption. Results from both experiments and DFT calculations indicate that these phenomena are attributed to the atomic size effect, in which the larger Sc atoms reduce the Ti-H spacing, thereby strengthening the interaction between the adjacent Ti atoms and H atoms. The enhanced interaction presents a greater challenge for H atom diffusion within alpha-Ti and complicates the dissociation of TiH2. This study provides a vital theoretical and experimental basis for understanding the effects of Sc doping on the hydrogen absorption and desorption properties in titanium alloys.

Accident-tolerant fuels (ATF) have been extensively studied to reduce the production rate and total amount of heat and hydrogen generated from high-temperature steam oxidation during severe accidents. Chromium-coated zirconium alloy cladding has become one of the most promising candidates for ATF because of its excellent corrosion and oxidation resistance. This paper provides a comprehensive overview of the research progress on oxidation behaviors and degradation mechanisms in chromium-coated zirconium alloy cladding under high-temperature conditions. Potential techniques to strengthen the oxidation resistance are highlighted and compared. Finally, challenges and opportunities for various future directions are addressed.

Tungsten (W), as a plasma-facing material, is subjected to high fluence plasma, resulting in both radiation damage and deuterium (D) retention. This study explores the blistering behavior and D supersaturated layers in both pre-damaged and undamaged W samples. To achieve a uniform damage profile similar to neutron irradiation, carbon (C) ions with multiple energies and angles were employed. The samples were further irradiated with low-energy D ions under two fluences at temperatures from ~ 380 to 640 K. Using various experimental techniques, we found that pre-damaged W samples can effectively suppress the formation of blistering and lower the threshold temperature at which the surface remains free of blistering. We also observed the prominent D supersaturated peaks within the depth of 20 nm in both pre-damaged and undamaged W. Furthermore, a close correlation between blistering and the D supersaturation was investigated. The results suggest that neutron irradiation in the fusion reactors could increase the critical D concentration for blistering, while a greater depth of damage facilitated the inward diffusion of D, thereby reducing the D concentration gradient and alleviating surface blistering.

Radiation-induced segregation (RIS) is one of the most dramatic changes that can occur at grain boundaries (GBs) during irradiation. In ceramics, RIS has been rarely studied and the underlying mechanisms are not well understood. Here, we used a combination of experiments and simulations to demonstrate RIS in TiB2. Specifically, we found that radiation causes a significant B depletion and a modest Ti enrichment in the GB regions. We demonstrate that B depletion is a result of the formation of BI-VTi complex and the migration of B vacancies to the GB, where the BI represents B interstitial and VTi represents the Ti vacancy. The Ti enrichment is driven by the differences between the diffusivities of Ti interstitials and vacancies. The distinct RIS mechanisms found in TiB2 shed new light on the relation between the complex energy landscape and defect evolution in ceramics.

Future nuclear reactors and advanced power generators require materials with good stability and damage tolerance under harsh conditions, including high temperatures and high-dose radiation. Ti3SiC2 MAX phase has good physical properties and mechanical strength. It can remain crystalline under serious microstructure damage due to the nanolaminate structure. In this study, the effects of helium in irradiated Ti3SiC2 at up to 1100 degrees C were investigated by microstructural and chemical composition analysis. The concentrated helium can grow into large bubbles without significant confinement or capture by the nano-laminated layers. A new hexagonal to fcc phase transformation mechanism, driven mainly by the evolution of the helium bubbles accompanied by Si diffusion and depletion, is found and investigated. Si interstitials are forced to move out from the peak helium region by the helium evolution and segregate at the outermost surface, forming a thin Si-O layer, at 1100 degrees C. The formation of the fcc phase is the result of chemical compositional changes and local compressive stress contributed by He bubbles.

For high temperature applications Mo base alloy requirements include both superior structural performance and environmental resistance. To address these requirements alloys in the Mo-Si-B system and refractory multi-principal element alloys (RMPEA) are being developed that exhibit a promising potential, but also have some remaining challenges to improve ductility, lower density and enhance environmental resistance. In the Mo-Si-B system microstructures with a Mo solid solution (Moss) Mo3Si and Mo5SiB2 (T2) phases have been the focus of attention. However, the Si solubility in the Moss phase diminishes the ductility and toughness. In order to address this issue a new design based upon Moss, Mo2B and T2 phases lowers the Si solubility in the Moss to improve ductility while the T2 phase maintains the oxidation performance. Selected additions of Al and Ti enable a density reduction to below 8 g/cm3. The RMPEA designs for Mo-rich alloys provide for excellent structural performance, but the complex oxidation products provide no protection. In this case a new coating design has been introduced that provides the required environmental resistance.

Refractory multi-principal-element alloys (RMPEAs) exhibit high specific strength at elevated temperatures ( T ). However, current RMPEAs lack a balance of room-temperature (RT) ductility, high- T strength, and high- T creep resistance. Using density-functional theory methods, we scanned composition space using four criteria: (1) formation energies for operational stability: -150≤E_f ≤ +70 meV per atom; (2) higher strength found via interstitial electron density with Young’s moduli E > 250 GPa; (3) inverse Pugh ratio for ductility: G / B < 0.57; and (4) high melting points: T m > 2500 °C. Using rapid bulk alloy synthesis and characterization, we validated theory and down-selected promising alloy compositions and discovered Mo 72.3 W 12.8 Ta 10.0 Ti 2.5 Zr 2.5 having well-balanced RT and high- T mechanical properties. This alloy has comparable high- T compressive strength to well-known MoNbTaW but is more ductile and more creep resistant. It is also superior to a commercial Mo-based refractory alloy and a nickel-based superalloy (Haynes-282) with improved high- T tensile strength and creep resistance.

Refractory multiple-principal-element alloys (RMPEAs) are promising structural materials to enable increased power efficiency in high-temperature oxidation environments, but the oxidation behavior and microstructures of the oxides, especially at the beginning of the oxidation, have received limited attention. The oxidation mechanism in an equimolar W-Mo-Ta-Nb-V was investigated at 1300 degrees C and compared with the equimolar W-Mo-Ta-Nb alloy without V. The oxide scale on WMoTaNbV after 1 min exposure is shown to be composed of a degradation layer at the interface of the alloy/oxide, an initial oxide transition layer, followed by the main phase aggregate oxide layer, and an outermost oxide layer. At the early stage of oxidation, the absorption of oxygen by the RMPEA substrate forms a solid solution. Vanadium accelerates the initial degradation process as it forms VO. The initial oxidation-induced degradation of RMPEA follows a sequence governed by the free energy change accompanied by local element segregation. Liquid V-Mo oxides aggregate in the intermediate oxide layer. The inward growth of the oxide scale is controlled by the local composition changes, the orientation of the substrate, the crystal structure, and physical properties such as melting points of the oxides.

A Mo-Si-B based coating has been applied on a refractory multi-principal element alloy (RMPEA) using a twostep coating strategy and has endured more than 750 h of thermal cycling oxidation exposure between room temperature to 1300 & DEG;C with a minimal weight change. The formation of the Mo5SiB2 diffusion barrier in the coating prevents the inward diffusion of the coating components. Then, boron is captured by the RMPEA, resulting in the formation of a solid solution X5SiB2 (X = W, Mo, Ta, Nb, V) layer and an RMPEA-B boride layer underneath it that acts as the second diffusion barrier.

Traditionally, the formation of amorphous shear bands in crystalline materials has been undesirable, because shear bands can nucleate voids and act as precursors to fracture. They also form as a final stage of accumulated damage. Only recently were shear bands found to form in undefected crystals, where they serve as the primary driver of plasticity without nucleating voids. Here we have discovered trends in materials properties that determine when amorphous shear bands will form and whether they will drive plasticity or lead to fracture. We have identified the materials systems that exhibit shear-band deformation, and by varying the composition, we were able to switch from ductile to brittle behaviour. Our findings are based on a combination of experimental characterization and atomistic simulations, and they provide a potential strategy for increasing the toughness of nominally brittle materials. Amorphous shear bands in crystalline materials are found to increase the toughness of brittle materials, in contrast to their traditional role as precursors to fracture. Criteria for this toughening have been identified.

We have investigated the radiation tolerance of Cr-B binaries and Cr-Al-B ternary MAB phases using a combination of ab initio calculations and experiments. One key discovery is that Cr3AlB4 has excellent resistance to radiation-induced amorphization, and therefore it is a promising material for applications in extreme environments. We also demonstrate that both the type of B network and the presence of Al layer in the structure have important implications for defect kinetics. We find that the order of the tolerance to radiation-induced amorphization is, from high to low, CrB, Cr3AlB4, Cr3B4, Cr4AlB6, and Cr2AlB2 at 150 °C and 0.5 dpa, and Cr3AlB4, CrB, Cr3B4, Cr4AlB6, and Cr2AlB2 at 300 °C and 1.0 dpa. The results are explained in terms of defect properties determined from ab initio calculations.