The interactions between gaseous hydrogen and metallic compounds are of significant basic scientific interest as well as important in various practical applications, including metal hydride formation, energy storage systems, and catalysis. Typically, the initial hydrogenation process involves surface hydrogen dissociation on the native oxide layers followed by atomic hydrogen penetration through those layers into the underlying metallic compounds. Recent research has demonstrated that common impurities (e.g., CO, CO2, O2) in the hydrogen stream inhibit the initial hydrogenation process but do not prevent it completely. Even at relatively high impurity concentrations, localized hydride "patches" may form, which in some instances are sufficiently large to disrupt the oxide overlayer. Consequently, during the hydrogenation process hydrogen may interact with two distinct surface types: the native oxide of the metallic compound and the hydride patches formed during the initial hydrogenation stage. This study investigates the impact of gaseous impurities on hydrogen-hydride surface interactions through both experimental and theoretical approaches. Density Functional Theory (DFT) calculations were employed to evaluate the molecular adsorption energies and interaction energies between hydrogen and impurities on the hydride surface. Application of the Langmuir model, incorporating calculated adsorption energies at various pressures and temperatures, indicates complete impurity coverage of the hydride surface, even at very low impurity concentrations. This suggests that, when the hydrogen gas phase contains such impurities, the essential H2 dissociation step is likely to be inhibited on the regular GdH2 surface. Furthermore, if dissociation occurs (at surface defects or on the coexisting oxide surface near the hydride), the penetration of the hydridic moiety (H-delta) through the hydride is generally suppressed due to its capture by impurities, forming new adsorbed species such as H2CO, HCO2, and OH. The results of these calculations were experimentally compared. The influence of the above impurities on hydrogenation kinetics was studied using gravimetric analysis combined with X-ray diffraction (XRD) measurements. The results corroborate with the theoretical findings, demonstrating that pure hydrogen yields the highest formation rate, while the presence of low impurity concentrations in the hydrogen stream suppresses and limits the progress of the hydrogenation process on the dihydride surface. The elucidated hydrogenation mechanisms of the impurity effect on the gadolinium hydride development provide valuable insights for the deployment of novel hydrogen storage procedures.

The present research combines Transmission Electron Microscopy and planar impact testing to investigate the geometry, concentration, and individual strengths of Ni3Al precipitates in Ni-Al and Ni-Al-Fe alloys. The study focuses on homogenized and aged samples of a binary (88.5% Ni, 11.5% Al) and a ternary (79.3% Ni, 10.2% Al, 10.5% Fe) systems. A detailed analysis of experimental results reveals significant difference between structures of the Ni3Al precipitates in these alloys. In the binary Ni-Al alloy, precipitates are predominantly small and dense spheres with an average diameter of 4 nm. Conversely, the ternary Ni-Al-Fe alloy exhibits larger and loose spherical precipitates, averaging 50 nm in diameter. A key finding of this study is the superior strength of smaller precipitates, which are approximately 40% stronger than their larger counterparts. The latter are identified as regions of Ni-based solid solution densely packed with minuscule, 2 nm-diameter, islands of the ordered Ni3Al phase. These insights contribute to a deeper understanding of the microstructural factors influencing the mechanical properties of Ni-Al based alloys.

Aluminum alloys, mainly from the 5xxx and 6xxx families, are used extensively in research reactors due to their suitable properties. However, alloys common in nuclear applications are not yet suitable for Additive Manufacturing (AM). The alloy AlSi10Mg has good engineering properties and is the most used and investigated AM Al-alloy. In this work, stress relieved (SR) samples of AM AlSi10Mg have been compared to samples that underwent a subsequent heat treatment at 500°C (SHT), producing two distinctly different microstructures. The SHT samples contain larger grains with a cleaner matrix and coarser silicon particles compared to the SR samples. Samples with the two microstructures were irradiated using silicon ions up to damage of ∼20 dpa at a depth of about 1 µm. To compare the response of both conditions to irradiation, properties were measured using nano-indentation and transient grating spectroscopy (TGS). Microstructure was characterized using TEM. Results show that the SHT condition is much more sensitive to irradiation, compared to SR. Irradiation caused thermal diffusivity to decrease by ∼5% in the SR condition and by more than 50% in the SHT condition. Hardness increased by 12% and 24% in the SR and SHT conditions, respectively. Also, the void number density was significantly larger in the SHT condition. No change was observed in young's modulus. The lower sensitivity of the SR condition is attributed to a higher sink density present in the initial microstructure, resulting in overall less irradiation damage retained in the material after irradiation. The explanation offered for the major drop in thermal diffusivity observed only in the SHT condition is the large size of the silicon particles, which increases their instability during irradiation, causing them to partly dissolve and release atomic silicon into the matrix around them. These silicon atoms are effective electron and phonon scatter centers. The findings of this work show that the pre-irradiated microstructure of AM AlSi10Mg has a crucial role in the material's performance in nuclear reactor core conditions. Careful thought and care need to be implemented to the heat treatment applied to an as-built part.

High and medium entropy alloys (HEAs and MEAs) exhibit superior resistance to irradiation damage: there is thus great incentive to study uranium-based HEAs and MEAs. These materials could potentially be suitable candidates for use as nuclear fuels where severe irradiation regimes prevail. In the present study, the ability to predict single phase regions in the quaternary system, Mo–Nb–U–Zr, using thermodynamic calculations was demonstrated and proven experimentally. Similarly to other known bcc HEAs systems, the bcc phase in the present MEA system was shown to exhibit high yield strength but also brittleness. It was also shown that a single phase in the MEA system, Mo–Nb–U–Zr, could be obtained only for compositions containing no more than 10 at

High Entropy Alloys (HEA's) are highly promising compounds, mainly from mechanical aspects. In addition, their magnetic properties are potentially beneficial, primarily as soft magnets that might be magnetically controllable by changing their atomic and phase concentration. However, as they incorporate a relatively large number of transition metals, the nature of their magnetic order might be very complicated. To check the magnetic properties of these compounds stringently, we measured several single-phased (B2) AlxCoCrFeNi compounds. Our results point to a complex magnetic ordering evidenced in two different magnetic transitions for most compounds measured. In addition, increasing the Al concentration decreases the magnetic interaction strength. We claim that this decrease is due to a subsequent drop in the magnetic atoms (Fe, Co, Ni, Cr) content in the samples, initiating an effective increase in the interatomic distance between them that weakens the magnetic exchange interaction.

Uranium is traditionally stabilized in its ductile gamma (BCC) phase by the addition of elements such as Mo, Nb or Zr (Mo is used in metallic fuel for reactor applications). Due to the resemblance of uranium alloys to ferrous alloys, an attempt was made to synthesize single phase U-based high entropy alloys (HEAs) in the U-Mo-Nb-Zr system, following empirical rules related to enthalpies of mixing and atomic radius differences. Microstructure, phase and elemental compositional were characterized, and mechanical properties were measured. This research showed that despite the expectation that a single gamma (BCC) phase would be formed, these alloys usually presented two-phase structures; a U-rich gamma phase and a Mo-Nb rich BCC phase or a Laves phase. Thermodynamic calculations were successful in predicting the content of phases in the alloys but were not in full agreement with the experimental results. Small punch tests (SPT) showed that most of the studied samples were hard and brittle, which could be attributed to the presence of the Laves phases or alternatively, could be correlated with the multi-component gamma (BCC) phase, since other BCC HEAs also tend to be hard and brittle. (C) 2021 Elsevier B.V. All rights reserved.

The interactions of hydrogen with metallic surfaces play a major role in many important practical processes such as metal hydride formation, energy storage systems, and catalysis. Generally, the interactions involve hydrogen dissociation, producing atomic hydrogen that penetrates the metallic compound via its native oxide. In the present study, the effect of common gas phase impurities such as CO, CO2, and O-2 on the hydriding process of gadolinium was investigated experimentally and theoretically using DFT calculations. The experimental part shows that the presence of gas-phase impurities does not block the initial hydrogen ingress through the oxide overlayer and even at relatively high impurity concentrations (5%), the hydrogen reacts with the gadolinium metal beneath the oxide, and small hydride blisters appear on the surface. However, the presence of even a relatively small concentration (0.5%) of each impurity suppresses the massive attack of hydrogen and the formation of the large hydride blisters (growth centers). The DFT calculations reveal that this behavior can be explained by the buildup of new moieties such as center dot H2 center dot CO, center dot OH, center dot HCO2, or center dot HCO that strongly adsorb to the surface, accumulate with increasing exposure, and thereby suppress the hydrogen ingress flux and the development of massive hydride growth centers.

The Al0.5CoCrFeNi alloy has been shown to possess a high potential for applicability by virtue of its me-chanical properties. Its duplex nature proved to be stable after heating cycles to near melting and after aging treatments. In the as-cast state, this alloy contains similar to 90 vol% of dendrite core (DC) regions having FCC structure, while the remainder consists of inter-dendritic (ID) regions with B2/BCC mixture, characterized by its typical 'Chinese letter' morphology. At 600 degrees C, ordered FCC (L12) nano-precipitations have been shown to form uniformly within the DC regions. These precipitates can potentially increase the DC hardness due to precipitation hardening during aging treatments. This article investigates the phase evolution and changes in the L12 phase morphology during a variety of aging treatments at 600 degrees C. The effect of aging on the alloy's hardness was also evaluated. It was found that during aging, the L12 phase changed its mor-phology from individual particles to a 'tweed-like' morphology, which caused a sharp increase in hardness (600 degrees C/17d: 388.5 +/- 11.4HV) compared to a non-aged sample (600 degrees C/q: 218.7 +/- 5.4HV). With further aging, the L12 symmetry evolved into orthorhombic symmetry before finally transiting into a lamellar B2 morphology at the over-aged state, associated with a decrease in hardness. The increase in the DC hardness during aging seems to cause a delay in the formation of a tetragonal (Sigma) Cr-Fe rich phase, which tends to precipitate along the DC/ID boundaries in this alloy. In the ID region, the phase evolution starts with an HCP precipitation that remains stable as long as the B2/BCC mixture is stable, but once the BCC phase is transformed to FCC, the HCP particles become extinct. The experimental results were compared with the predicted phase content using the TCHEA4 thermodynamic database, and the differences are discussed.(c) 2022 Elsevier B.V. All rights reserved.

High-entropy alloys (HEAs) have been extensively investigated primarily because of their wide range of properties (mechanical, thermal, corrosion, etc.) that enable their application in countless applications. One of the most promising families is the AlxCoCrFeNi based HEA, which tends to form simple phases and exhibit simple phase evolution with temperature. In this system, the complexity of the phase evolution tends to increase with decreasing temperatures, which complicates the collection and analysis of experimental thermodynamic data. Therefore, most of the work done in this system regarding phase transitions has been at high temperatures (T > 1000 degrees C). Al0.5CoCrFeNi has promising mechanical properties due to its duplex nature. It has an FCC dendrite core (DC) region at low temperatures, which occupies about 90 vol%: the reminder is a B2/BCC mixture inter-dendritic (ID) region. Each region has a different chemical composition, which leads to different phase evolution at intermediate temperatures (400 <= T <= 900 degrees C). The phase composition and evolution were studied using high-sensitivity calorimetry followed by electron microscopy and XRD characterization. The DC region exhibits a simpler phase evolution with ordered FCC (L1(2)) nano-precipitation at 508 degrees C that transitions to B2 precipitation at 778 degrees C. The L1(2) -> B2 transition is associated with a large exothermic event caused by the release of 'symmetry-breaking' strain and significant shrinkage. The ID region exhibits a more complicated phase evolution that starts with precipitation of a Co-Cr rich HCP phase at 626 degrees C and continues with the precipitation of Cr-Fe rich sigma (sigma) phase along the boundaries between the regions at 699 degrees C, followed by the transition of the BCC precipitation into FCC at 733 degrees C. All these transitions in the ID region are associated with non-linear expansion. The experimental findings were compared with the thermodynamic evolution made using the ThermoCalc software and two thermodynamic databases. (C) 2021 Elsevier B.V. All rights reserved.

Al0.5CoCrFeNi contains a mixture of the ductile fcc (A1) phase and the hard ordered bcc (B2) phase. In this paper, the microstructure, composition, precipitation kinetics, and thermodynamic calculation of the equilibrium state of the Al0.5CoCrFeNi alloy were studied. It was found that the morphology of the B2 phase (rich Al-Ni) is temperature-dependent; up to 1000 degrees C the dominant morphology is needle/plate-like whereas above this temperature, equiaxed morphology takes over until the needle morphology disappears completely at above 1200 degrees C. It was confirmed experimentally, and by thermodynamic calculation with good agreement between the two, that the phase compositions are temperature dependent. Nevertheless, there is still disagreement between the thermodynamic model and the experimental results for the A1 to B2 phase ratios and compositions at similar to 1200 degrees C and above. The inter-diffusion activation energy (Q = 187 +/- 20 kJ/mol) for the B2 phase growth was found to be lower than the activation energy reported for the AlCoCrFeNi alloy. This result is attributed to the different lattice distortion parameters of the two alloys. It is further suggested that the morphology transformation of the B2 phase from needle-like to equiaxed with increase in temperature, occurs because of increased density of dislocations at the B2/A1 interface.

Aluminum alloys are widely used as fuel cladding materials for research reactors. Neutron irradiation damage is a major concern in the nuclear industry, and it is of great interest to study irradiation damage by ions, which can cause similar damage to materials. In this study, Al 6063 alloy was irradiated with Si ions at 333 K up to 90 dpa to mimic the effects of both elemental Si, a transmutation product of thermal neutrons, and Frenkel pair formation, an effect of fast neutrons. The irradiation damage was investigated by transmission electron microscopy and the nanoindentation technique. The irradiated sample's net dislocation density was about 1.2·1014 m−2 in the vicinity of the voids and 2·1014 m−2 in the vicinity of the Si peak. The implanted Si peak concentration was ~ 6 at% at a depth of ~ 1100 nm. The formation of voids with an average size of 8 nm, and a peak number density of ~1022 m−3 was identified, and the void annihilation time dependence was found to be proportional to (1t)0.25. The amorphous to crystalline structure transformation temperature of the intermetallic Al8Fe2Si occurs at 723 K and follows the Johnson-Mell-Avrami diffusion control model. The hardness and estimated ultimate tensile strength of the irradiated surface layer, obtained by the nanoindentation measurements, increased by ~65% in comparison to the unirradiated sample. A similar increase in yield stress and ultimate tensile strength published values of neutron irradiation damage, occurs at much higher dpa (~260 dpa). This difference is suggested to be related to the high dpa rate and higher at%Si/dpa ratio in the present ion irradiation experiments.

The two phases of the AlxCoCrFeNi multi-component system, A2 (Im3m, bcc) and B2 (Pm3m, primitive) do not appear as single phases in the entire range studied to date (0 < x < 3). To measure the thermochemical properties of alloys and improve the accuracy of thermodynamic models, single phases need to be synthesized and characterized. Toward this end, we first study the mixture of A2 and B2 phases in the Al2.75CoCrFeNi multi-component alloy. After determining the composition of each phase, we attempted to synthesis each individual phase. The “B2” alloy is rich in Al, Ni, and Co and consists of a single primitive structure with a nonuniform composition. The “A2” alloy is a Cr- and Fe-rich alloy of compositional uniformity on the microscale, but that phase separates on the nanoscale into Al–Ni–Co-rich, nanometer-sized B2 precipitates in an A2 matrix rich in Cr and Fe. This result reveals the profound impact of Al on the stabilization of the B2 phase in the AlxCoCrFeNi system. Although the B2 alloy is adequate for thermochemical study, further efforts are required to synthesize an A2 alloy. In this case, the Al content should be less than 10 at.% and the Ni content should be about 1 at.%. These low contents of Al and Ni should prevent the formation of stable “Al–Ni-like” B2 ordered intermetallics.

Al0.5CoCrFeNi is a promising alloy in the AlxCoCrFeNi series, since it contains a mixture of the ductile fcc phase (A1) and the hard and ordered bcc phase (B2). The combination of the two forms a "metallic composite" with mechanical properties that can be modified by different thermo-mechanical treatments. In this study, it was shown that deformation prior to heat treatment can alter the B2 phase morphology from "plate-like" to equiaxed morphology. The mechanical properties of Al0.5CoCrFeNi were determined by utilizing compression experiments in conjunction with non-linear finite element analysis. The results showed that in all cases, pre-heat treatment deformation led to a reduction in the effective yield stress. This behavior correlates with the changes in the B2 morphology resulting from the pre-heat treatment deformation. Moreover, the effective yield stress decreased with increasing heat treatment temperature for both pre-deformed and undeformed samples.

The nearly infinite compositional design space of high entropy alloys (HEAs) presents many opportunities to improve performance in extreme environments, particularly for nuclear reactors. The ability of some HEAs to resist high amounts of radiation damage, while well documented, has not yet been fully exploited. We studied the irradiation effect of different ions (Si and Ni) on the microstructure and mechanical properties of the Al0.5CoCrFeNi alloy at its equilibrium state, and of the individual response of each phase to irradiation. The results show a stronger effect of Si ions and differences in response of the ductile FCC (A1) and brittle ordered BCC (B2) phases towards irradiation. This finding highlights the need to further probe unexplored compositional spaces in HEAs, as further optimization is likely to yield further compositional and microstructural stability with practical applications.

AlxCoCrFeNi high-entropy alloys have received significant attention recently because of their promising mechanical and corrosion-resistance properties. These alloys tend to form a mixture of fcc and bcc phases, where the latter has an important role in material hardening. In many cases, the bcc phase is a mixture of disordered bcc (A2), which is an Fe- and Cr-rich phase, and ordered bcc (primitive cubic, B2), which is an Al- , Ni- , and Co-rich phase. Although phase diagrams above x = 2 are somewhat consistent, they unfortunately contain no valuable data about the mole fraction and phase composition. Moreover, Alx > 2CoCrFeNi alloys suffer from a lack of systematic experimental investigation into the kinetics of the phase transformation. To clarify these points, the present study investigates the phase relations and precipitation kinetics of A2 from the B2 matrix in Al2.75CoCrFeNi. The results show that the compositions of the A2 and B2 phases are temperature-dependent and that, with increasing temperature in the B2 phase, the Al content decreases while the Cr content increases, which correlates with thermodynamic calculations. In addition, the equilibrium composition and phase content lead to a reduced lattice distortion parameter compared with that of the nominal alloy. Concerning the kinetics of phase transformation, the results suggest that, to precipitate in the solid state, the A2 phase within the B2 matrix must overcome the internal stresses that are due to the different lattice parameters of the two phases. Furthermore, the diffusion activation energy is estimated and its implications are discussed from the perspective of sluggish diffusion in multicomponent systems. Finally, the coefficients of thermal expansion of Al2.75CoCrFeNi alloy and of Al–Ni–Co- and Cr–Fe-rich alloys (both alloys containing Al, Co, Cr, Fe, Ni) were measured and are discussed in relation to phase transformation.

The interactions of water vapor with rare earth oxide surfaces play a major role in many important practical processes, such as heterogeneous catalysis and corrosion phenomena, especially for "real life" metals (metals with thin oxide overlays). Generally, these interactions take place by two different routes, a pure ionic dissociation, producing only hydroxyls, and a redox reaction, producing atomic hydrogen and an oxidic oxygen (or hydroxyl). In the presence of oxygen, however, the redox route is eliminated and only the pure ionic dissociation prevails. In the present study, the effect of oxygen on the dissociation of water vapor over GdO1.5 was investigated experimentally and theoretically using density functional theory (DFT) calculations. The DFT calculations revealed that water vapor will follow the redox route only on nonstoichiometric, oxygen-deficient surfaces by producing a hydroxyl and a reduced H-delta moiety that relaxes into an oxygen vacancy site. This moiety may diffuse into the oxide-metal interface (for oxide-coated metals) to form hydrides or it may associate on the oxide surface into H-2. In the presence of oxygen, the formation of H-delta is prevented and therefore the formation of hydrides on oxide-coated metals is not expected. The experimental results are in agreement with the DFT analysis for the reaction of gadolinium (coated by its native oxide) with humidity to form gadolinium hydride islands on the surface. However, in the presence of oxygen, the extent of this reaction was very limited.

Spontaneous infiltration of molten metals in to ceramic skeletons, in the course MMCs' production, is related to improved wetting of the ceramic by metals. TiC is considered a "metal-like" carbide and is supposed to be wetted well by metals through metallic bonding mechanism. Nevertheless, TiC/Cu exhibit an unusual behavior since spontaneous infiltration of molten Cu takes place, while TiC is partially wetted by Cu (theta= 90 degrees).In this work we studied the relation between wetting and spontaneous infiltration in the TaC/Au, Al and Cu systems. TaC is also considered a "metal-like" carbide and indeed no chemical interaction was observed at the interfaces of the studied systems.Sessile drop experiments showed almost perfect wetting in the three system but spontaneous infiltration occurred only in the first two (e.g. TaC/Au or Al). Thermodynamic calculation shows the difference between the systems which also has its' influence on the mechanical properties of the MMCs'. Further calculation clarifies the difference between TaC/Cu and TiC/Cu infiltration behavior, but is unable to explain the wetting results differences.Correlation between wetting and spontaneous infiltration in some cases is not straight forward and more studies and calculations on the atomistic level should be done in order to clarify this matter.

The effects of MgO as a sintering additive, sintering duration, and post-heat treatment on mechanical properties and microstructure of spark plasma-sintered aluminum powders were investigated. The sinterability of aluminum with or without MgO was found to be sensitive to the aluminum average particle size, meaning the amount of native oxide within the raw aluminum powders. The fracture mode changes gradually from a brittle mode (after short SPS), through a mixed brittle-ductile fracture mode (after long SPS), ending with the pure ductile form (short SPS followed by heat treatment). Maxima flexural strength and elongation were found in samples with particles size of about 44 μm and the addition of 2 wt.% MgO after short SPS process followed by an additional heat treatment. The addition of MgO may contribute to perforation of the aluminum native oxide and enhance aluminum diffusion during the heat treatment.

β-U could be metastably retained at room temperature when alloyed with various elements such as gallium. Since iron is a common impurity, we investigated two different kinetic processes which occurs simultaneously; one is the \( \beta \to \alpha \) transformation and the other is the precipitation of the U6Fe phase from the iron dilute U(Ga)–Fe alloy. Furthermore, we determined the solubility of iron and gallium solubility in quenched uranium from 800 to 720 °C (γ and β phase regions, respectively). It was shown that the \( \beta \to \alpha \) phase transformation is sluggish and has a character of massive transformation. The retention from the β phase region does not fully stabilize β phase, and α phase is also formed during quenching, while the γ-quenched sample remains metastable at room temperature, and only high-temperature heat treatment transforms β to α. The precipitates found in the system are U6Fe and U2Ga3, no ternary intermetallics were detected, and the morphology of the precipitates does not depend on the heat treatment course. The gallium and iron contents found in the γ- and β-quenched samples are X Ga = 0.72 wt% X Fe = 0.18 wt% and X Ga = 0.42 wt% X Fe = 0.06 wt%, respectively. The different gallium contents in the γ and β explain the differences in the \( \beta \to \alpha \) phase transformation kinetics. Quenching U–Ga alloys (above the solubility limit of gallium) from the γ region stabilizes β-U better than quenching from the β region.

The morphology of U6Fe precipitates were investigated in the different allotropic states of uranium matrix. The morphology of γ-U is equiaxed and the characteristic size is ~65 μm. The excess dissolved iron in β-U precipitates discontinuously through a eutectoid reaction during cooling at the grain boundaries, while α-phase precipitates have a “needle-like” morphology (in many cases, aspect ratio ≫10) with a well-defined direction in the uranium matrix. This morphology may compensate for the large density differences of ~9 % between α-U and U6Fe (~19 and ~17.4 g/cm3, respectively) and the fact that precipitation takes place at relatively low temperatures. It was shown that U6Fe precipitates do not introduce new nucleation sites in β and α phases of uranium.