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V-Cr-Ti alloys with reduced Ti contents are expected to be good candidates as recyclable structural materials for fusion reactors. Herein, He irradiation was applied to V-4Cr-xTi (x = 0 to 4) alloys to investigate the additional effect of Ti and gas interstitial impurities on their microstructural evolution and irradiation hardening. Following He irradiation of the specimens at 700 °C with a maximum damage depth of 0.5 dpa, transmission electron microscopy and nanoindentation hardness tests were conducted. The microstructural evolution of the titanium-oxycarbonitride (Ti(CON)) precipitates formed during He ion irradiation was observed. The behavior of irradiation hardening is explained by the formation process of Ti(CON) precipitates and the magnitude of irradiation hardening depends on the amount of gas interstitial impurities that contribute to precipitate formation.

Using transmission electron microscopy, the features of the grain and defect structure of the V–4Ti–4Cr alloy after 16 acts of biaxial and triaxial pressing at room temperature are studied. The microstructure of the alloy after these treatments is a structural composite with the elements of fine-crystalline and submicrocrystalline states. The defect structure is characterized by a crystal lattice curvature from 10 to 30°/μm. It is established that biaxial pressing contributes to a more intensive grinding of the crystallites and a formation of an anisotropic microstructure. The dislocation-disclination mechanism and localization in the direction of maximum shear stresses are thought to be the main mechanisms of structural transformation under conditions of high-strength state, formed as a consequence of dispersion strengthening by the second phase particles. The role of local internal stresses and their gradients in the processes of crystal lattice fragmentation under conditions of severe plastic deformations is discussed. [ABSTRACT FROM AUTHOR]

The oxidation stability and phase formation sequence for pure aluminum APK and Al–2.3%V alloy heated in air at rates of up to 100°C/min were analyzed by thermogravimetry with differential scanning calorimetry and X-ray diffraction using synchrotron radiation. It was established that an increase in the heating rate from 10 to 100°C/min does not significantly change the thermal stability of the modified Al powder. The presence of Al3V and Al10V intermetallic compounds, as well as a small amount of γ-Al2O3, in the structure of the alloy should favor consolidation of metal particles and reduce the porosity of the resulting product during selective laser melting (SLM). [ABSTRACT FROM AUTHOR]

Vanadium alloys are an attractive candidate material for advanced fusion reactors’ structural components. Some leading vanadium alloys, such as V−(4−5)Cr−(4−5)Ti alloy, exhibit several important advantages, including excellent strength at elevated temperatures, high resistance to neutron irradiation damage, inherently low long-term activation, as well as good fabricability and weldability. However, the corrosion and embrittlement via oxygen pickup during the high-temperature oxidation process of vanadium alloys remains a key issue, restricting their operation conditions and long service life. In a high-pressure oxygen environment, the main oxidation product V2O5, with a low melting point of ～680 ℃, is formed on the vanadium alloy surface, which cannot offer reliable protection to mitigate further oxidation over 650 ℃. However, despite being exposed to a very low-pressure oxygen environment, it is still unlikely for vanadium alloys to form an effective oxidation film to retard the oxygen absorption at temperatures over 450 ℃, mainly due to the high solubility of oxygen in vanadium. When the oxygen concentration reaches 0.2% in the matrix of V−4Cr−4Ti alloy, it can cause severe oxygen embrittlement, possibly due to oxygen accumulation and formation of fine oxidation precipitates at the grain boundaries and the adjacent matrix. Therefore, it is significantly important to enhance the high-temperature oxidation-resistant performance of the vanadium alloy to broaden the operation conditions. In this work, this research progress on the high-temperature oxidation resistance of vanadium alloys is systematically reviewed. In summary, three main methods for enhancing the oxidation–corrosion resistance of vanadium alloys at elevated temperatures are elaborated, i.e., oxidation-resistant element addition, diffusion coating, and overlay coating. Additionally, the characteristics and existing problems of these methods and the responding examples are also analyzed and discussed in detail. In the first two methods, it is impossible to completely isolate the alloy substrate from the service environment; thus, the typical oxidation product V2O5 is easily formed in the high-pressure oxygen environment, leading to severe oxidation corrosion and embrittlement, especially at elevated temperatures. Expectedly, the dense overlay coating presents a greater potential application mainly because of the thorough protection from the service environment. Finally, the development trend in the modification and technical requirements of the advanced overlay coatings on high-performance oxidation resistance are prospected in this paper as per the practical application demands for vanadium alloys, aiming to provide a beneficial reference for further research.

The effect of annealing temperature on the grain structure and microhardness values of the V–4Ti–4Cr alloy after thermomechanical treatment with rolling has been investigated. The specificity of the change in the texture of the V–4Ti–4Cr alloy with an increase in the annealing temperature was studied by x-ray diffraction analysis. Based on the EBSD analysis data, changes in the grain structure after annealing at different temperatures were revealed. The temperature intervals for the implementation of the main processes of relaxation and recrystallization of the alloy under study were revealed. The microband (layered) structural state formed after the rolling stage is characterized by a rolling texture of the {100}<110> type which is stable up to a temperature of 700°C at which partial recovery begins with relaxation of the most defective areas. At 800°C, primary recrystallization processes are activated, which at 900°C cover the entire volume of the material. An equilibrium structural state with no rolling texture is formed after annealing at 1000°C. Secondary recrystallization processes are activated at 1300°C. With an increase in the annealing temperature from 700 to 900°C, the microhardness decreases from 2.3 to 1.75 GPa. No nonequilibrium solid solutions of interstitial atoms were found in the entire annealing temperature range. [ABSTRACT FROM AUTHOR]

In the current work, an analysis of the effects of Y on the radiation hardening and microstructure response of a V-4Cr-4Ti alloy has been conducted after 30 keV D ion irradiation at room temperature using transmission electron microscopy (TEM) and nanoindentation. The results show that the formation of large Y2O3 and small Y2V2O7 nanoparticles was confirmed, indicating that the addition of Y reduces the amount of dissolved oxygen. The addition of Y has been shown to affect the radiation-induced dislocation loops, radiation hardening, and Ti-rich segregation of the V-4Cr-4Ti alloy. With the addition of Y, the mean size of the radiation-induced dislocation loop decreased, which may result from the strong sink strength of the nanoparticle/matrix interface, interactions between Y atoms and SIA clusters, and the strong binding energy of vacancy–oxygen pairs. Some particles with core–shell structures were observed after ion irradiation, where Ti-rich segregations at the nanoparticle/matrix interface were confirmed. These results indicate that Y might promote abnormal segregation. Possible causes for this include the lower interface energy at the particle/matrix interface and the interaction between oxygen and solute atoms. [ABSTRACT FROM AUTHOR]

The flash-electropolishing of focused ion beam samples for V-4Cr-4Ti alloys is established, and the microstructures of high-purity V-4Cr-4Ti alloys after He ion irradiation are examined by transmission electron microscopy from room temperature to 700 °C. The correlation between irradiation hardening behavior and microstructural changes is clarified. During room temperature irradiation, defect clusters are formed at shallow positions in the specimens and no He bubbles are observed at the damage peak position. In contrast, 500 and 700 °C, TiCON precipitates are predominantly formed and He bubbles and voids were formed at the damage peak position. The results of nanoindentation tests and a comparison of irradiation hardening by irradiation damage indicate that the obstacle barrier strength factorαof TiCON is 0.45 while that of the irradiation defect clusters irradiated at room temperature is 0.10. Irradiation damage in the He ion range extends toward the interior of the specimens with increasing irradiation temperature. [ABSTRACT FROM AUTHOR]

V-based alloy membranes have huge potential for hydrogen separation and purification due to their higher hydrogen permeability, but suffer from hydrogen embrittlement. To address this matter, Pd and Fe have been chosen as alloying additions in V-based membranes to inhibit hydrogen embrittlement and maintain high hydrogen permeation performance based on the lattice-matching strategy. The phase structure, composition, microstructure and hydrogen permeation properties of V-Pd-Fe ternary alloy are investigated systematically, and the V-Fe and V-Pd binary alloys are presented for comparison. The results show that the prepared V-Pd-Fe ternary can match almost perfectly with pure V in lattice constants. Attributing to the lattice-matching strategy, the V-Pd-Fe ternary alloy exhibits more excellent hydrogen permeable properties than that of V-Fe and V-Pd binary alloys. The reduction of hydrogen embrittlement is ascribed to the lower hydrogen solubility that induced by the lattice distortion and electronegativity of Fe and Pd. Meanwhile, the manipulated lattice constant that parallel to pure V maintains high hydrogen permeation performance. Additionally, the designed ternary V 87.8 Pd 8 Fe 4.2 alloy exhibits exceptional long-term hydrogen permeation stability at 573 and 623 K, and hydrogen flux gradually decreases at 673 and 723 K is observed due to the interdiffusion between Pd film and V 87.8 Pd 8 Fe 4.2 substrate and defects in the Pd coating. This study offers new scenario and method for optimized hydrogen permeable properties and restrains hydrogen embrittlement in the alloy membrane. • Constructing the lattice structure of V-Pd-Fe alloy by modulating Pd and Fe composition is almost identical to that of pure V. • The ternary V-Pd-Fe alloy exhibits excellent hydrogen permeability proprieties compared to alloys of binary V-Pd and V-Fe. • The mechanism for H solubility reduction and long-term H permeation deterioration in V-Pd-Fe alloys are studied. [ABSTRACT FROM AUTHOR]

V-Ti-based refractory high entropy alloys (RHEAs), e.g., VTiTa and VTiTaNb, have been found with outstanding mechanical properties and irradiation resistance, granting them application potential in nuclear engineering. Nonetheless, similar to conventional V-Ti-based alloys such as V-4Ti-4Cr, these alloys are sensitive to C/N/O interstitial impurities. Under irradiation, such impurities may promote the formation of precipitates which may affect the defect evolution, especially at the appearance of helium. Previous studies have observed different helium behavior in the above alloys, including the heterogeneous distribution of helium bubbles in VTiTa, but the origin was not clear. In this work, detailed microstructural characterization is performed to analyze the irradiation induced precipitates and their impact on helium bubble formation in these alloys. Under helium ion irradiation at 700°C, the TiN precipitates with the NaCl-type structure are formed in all the three alloys. Nonetheless, the interfaces between precipitates and matrix in VTiTa and VTiTaNb are semi-coherent while those in V-4Ti-4Cr are coherent, due to the various lattice parameters of the alloys. Moreover, severe lattice distortion is also found inside the precipitates in the two RHEAs. Consequently, significant heterogeneous nucleation of He bubbles is observed in these two RHEAs but not in V-4Ti-4Cr. Among the three alloys, the growth of precipitates is the fastest in VTiTa, causing the formation of bubble clusters. Furthermore, the growth direction as well as the shape of precipitates affects the shape of bubbles, leading to the {001} cuboids shape in V-4Ti-4Cr and the truncated {110} dodecahedra shape in V-Ti-Ta and V-Ti-Ta-Nb. [ABSTRACT FROM AUTHOR]

The irradiation behavior of vanadium alloys was examined to evaluate the effect of temperature history during reactor operation. Irradiation experiments were performed in a fast reactor, Joyo, with an irradiation-temperature-controlled irradiation rig, MARICO-II. In-core irradiation rigs that control the irradiation temperature and are not influenced by reactor power were used. Irradiation conditions with or without the MARICO-II rig were performed. Significant differences in irradiation hardening and swelling behavior in undersized vanadium binary alloys were visible between temperature-controlled and conventional rigs. The temperature history affects the microstructural evolution, especially the nucleation and growth of voids during startup and shutdown procedures. No difference was observed in the vanadium alloys with oversized solute atom because the vacancy migration was suppressed by tight binding between oversized solute atoms and vacancies during irradiation. V–4Cr–4Ti–(Si,Al,Y) alloys showed significant irradiation hardening at high-temperature irradiation even though the contribution of dislocation and precipitates for irradiation hardening was insufficient to satisfy the measured irradiation hardening. The significant irradiation hardening in the V–4Cr–4Ti–(Si,Al,Y) alloys may be caused by transmission-electron-microscope-invisible clusters including minor elements (Si, Al, Y) that were formed at a high temperature during irradiation.

Intermetallic compounds often have a low radiation tolerance due to the low recombination rate of radiation-induced defects. In the present work, we designed a novel multicomponent vanadium-based alloy (MVA), V34Ti25Cr10Ni30Pd1, containing a micron-scale TiNi matrix phase with VCr nanoprecipitates (NPs) and a micron-scale VCr matrix phase with TiNi NPs. The MVA was irradiated with 6 MeV Ti3+ ions with a radiation dose of 5×1015ions/cm2 at room temperature. Results indicated that the micron-scale intermetallic TiNi matrix along with VCr NPs inside both became amorphous, while the micron-scale VCr matrix including numerous intermetallic TiNi NPs both exhibited a high structural stability after ion irradiation. The intermetallic TiNi matrix becomes amorphous due to the accumulation of radiation-induced defects, and the intermetallic TiNi NPs in VCr matrix have a high stability of crystallographic structure due to high-density interfaces between NPs and matrix. These results indicate that the phase stability of TiNi intermetallic compound is increased after nanocrystallization. Besides, the discrepancy of two matrix phases including precipitates after ion irradiation, and the underlying mechanisms are discussed in detail in this work. This work gives a guidance for designing new vanadium alloys and intermetallic compounds with enhanced structural stability under ion irradiation.

In the current work, an analysis of the effects of Y on the radiation hardening and microstructure response of a V-4Cr-4Ti alloy has been conducted after 30 keV D ion irradiation at room temperature using transmission electron microscopy (TEM) and nanoindentation. The results show that the formation of large Y2O3 and small Y2V2O7 nanoparticles was confirmed, indicating that the addition of Y reduces the amount of dissolved oxygen. The addition of Y has been shown to affect the radiation-induced dislocation loops, radiation hardening, and Ti-rich segregation of the V-4Cr-4Ti alloy. With the addition of Y, the mean size of the radiation-induced dislocation loop decreased, which may result from the strong sink strength of the nanoparticle/matrix interface, interactions between Y atoms and SIA clusters, and the strong binding energy of vacancy–oxygen pairs. Some particles with core–shell structures were observed after ion irradiation, where Ti-rich segregations at the nanoparticle/matrix interface were confirmed. These results indicate that Y might promote abnormal segregation. Possible causes for this include the lower interface energy at the particle/matrix interface and the interaction between oxygen and solute atoms.

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Activation analysis of impurities was performed on a test specimen of the reference low-activation vanadium alloy NIFS-HEAT-2 (V-4Cr-4Ti) which has been irradiated in Japan Materials Testing Reactor (JMTR) ~16 years before. Since the radioactive nuclides from V, Cr and Ti are produced with short half-lives, intense peaks of 60Co (T1/2 = 5.27 years) were dominant in the measured gamma-ray energy spectra and peaks of 94Nb (2.03 × 104 years) were also observed. The concentrations were evaluated to be 0.41 wppm and 0.71 wppm respectively from comparison with results of radioactivity calculation using the FISPACT-2005 code. In the analysis, detection of 108mAg (418 years), which would have a considerable influence on the material recycling after the usage in a fusion reactor, was intense motivation. Although the peaks of 108mAg has not been detected, the concentration of Ag was estimated to be

An on-line instrumented material irradiation rig (MARICO-II) was used for investigating temperature history effect for void swelling behavior in V-Fe binary alloys irradiated at a fast rector, Joyo. Neutron irradiation were performed at 477C with 7.5 dpa in the temperature control capsule (TC capsule) and at 482C maximum temperature with 20.4 dpa in the conventional capsule with monitoring temperature. In the TC capsule, voids was formed and distributed homogeneously, on the other hand a lot of tiny void formation or bimodal void distribution were observed in the conventional capsule, The void swelling rate of V-Fe alloys irradiated in the TC capsule was larger than that in the conventional capsule and it is caused by the imbalance of point defect flux to sink due to void nucleation at the low temperature transient regime in start-up and shutdown period of each cycle in neutron irradiation, that is temperature history effect for swelling behavior of V-Fe binary alloys during neutron irradiation.

• Ne ion irradiation produced significant hardening than Fe ion irradiation in V alloy. • The significant hardening produced by Ne ion irradiation contributed to the formation of more dislocation loops rather than bubbles with limited sizes. • The presence of inert gas atoms had a thermal stabilization effect on irradiation hardening with a required critical atom concentration. Vanadium (V) alloys are attractive candidate materials for tritium breeding modules (TBM) of blankets in fusion reactors. However, irradiation-induced hardening and embrittlement give rise to significant challenges for the application of V alloys in advanced nuclear reactors, particularly in the presence of inert gas atoms. In this study, V-5Cr-5Ti alloys were irradiated by 122 MeV Ne ions (at doses of 0.1 and 0.3 dpa) and 352 MeV Fe ions (at doses of 0.06, 0.20 and 0.35 dpa) at a temperature around 100 °C, and subsequently annealed at the temperature ranging from 212 to 500 °C. During the annealing process, the hardening produced by Fe ion irradiation started to recover at a temperature higher than 212 °C and was eliminated at a temperature higher than 375 °C. For the sample with Ne ions irradiation, the irradiation hardening of the sample with 0.3 dpa can be sustained at a temperature up to 450 oC, but an evident reduction of hardening in the sample with 0.1 dpa was observed after annealing at 212 oC. This suggested that the existence of inert gas atoms had a thermal stabilization effect on irradiation hardening, but it required a critical atom concentration. The different annealing behaviors of irradiation hardening at different doses under Ne ions irradiation were considered to be from the coupling effect of bubble growth and the interaction between interstitial clusters and Ne atoms. [ABSTRACT FROM AUTHOR]

V–4Cr–xTi (x = 0 to 4) alloys were used to investigate the additional effect of Cr, Ti and interstitial impurities on the microstructural evolution in He-irradiated V–Cr–Ti alloys to minimize radioactivity after fusion neutron irradiation. Transmission electron microscopy and atom probe tomography were carried out to the He-irradiated specimens at 500 °C with 0.5 dpa at peak damage. A flash electro-polishing method for the FIB-extracted specimen was established for the ion-irradiated vanadium alloys. The microstructural evolution of the irradiation-induced titanium-oxycarbonitride, Ti(CON) precipitates was observed and was influenced by the effect of Ti addition on the Ti(CON) precipitation. Apparent Ti(CON) precipitates formed in V-4Cr-xTi with 2% addition of Ti. In the V-4Cr-1Ti alloy, a high density Ti enriched cluster was formed. The origin of the irradiation hardening increase resulted from the size distribution of Ti(CON) precipitation from the dispersed barrier-hardening theory.

Microstructure and irradiation hardening of V-4Cr-4Ti alloys with different yttrium (Y) contents were studied by self-ion irradiation at 550 °C via TEM and nano-indentation test technology. The peak damage of the V-4Cr-4Ti-xY alloy (x = 0, 0.1, 0.5, and 1, wt.%) irradiated by 2.5 MeV self-ion (V2+) is 8 dpa. Dense dislocation loops were observed in all vanadium alloy samples after irradiation. With the increase of Y content, both average size and number density of dislocation loops using g = near the pole [001] decreased, while the irradiation hardness increment first decreased and then increased. In order to better reduce the irradiation hardening, it is considered that the addition of 0.1 wt.% Y in V-4Cr-4Ti alloy is reasonable.

The paper studies the effect of the annealing temperature on microstructural transformation and micro-hardness variation in an internally oxidized vanadium alloy V—Cr—ZrO2 deformed by high-pressure torsion in Bridgman anvils. It is shown that the development of large plastic strains and subsequent annealing of the particle-reinforced V—Cr—ZrO2 alloy lead to the formation of a fine-grained structure (with about 1 µm grain size) with a high density of high-angle boundaries pinned by ZrO2-based nanoparticles. Such high-defect states are characterized by a more than twofold increase in microhardness with the major contribution of grain boundary hardening. The conducted research has revealed the main relaxation features of V—Cr—ZrO2 alloy deformed by high-pressure torsion at room temperature. The heat treatment of the studied material at 800°C is shown to activate recovery and polygonization. Primary recrystallization is observed upon temperature increase to S00°C. A further increase in temperature in the interval 950–1050°C intensifies collective recrystallization, due to which the fraction of equi-axed grains increases significantly. Secondary recrystallization is activated at 1200°C and, as a result, individual grains grow in size. These processes are accompanied by a decrease in the V—Cr—ZrO2 alloy microhardness from 3500 to 2000 MPa. Dispersion and substructural hardening are analyzed, and their contribution to the strength is studied. It is shown that the high thermal stability of the nanostructural and fine-grained states is ensured by the high density of uniformly distributed ZrO2-based nanoparticles (of size 3–10 nm) that pin the high-angle grain boundaries. [ABSTRACT FROM AUTHOR]

TEM observation and tensile test were examined for vanadium alloys irradiated in a temperature control rig in JMTR at 290°C with damage level ranged from 0.003 to 0.06dpa. With the increase of the neutron dose, irradiation hardening could be observed in all the vanadium alloys except for the V–5Nb alloy. In the case of pure vanadium, the relationship between irradiation hardening and neutron dose was described as Δσ ∝ (ϕt)0.35-0.53. For V–5Cr alloy and V–4Cr–4Ti–0.1Si alloy, the dose dependence on irradiation hardening increase was shown as Δσ ∝ (ϕt)0.8 and Δσ ∝ (ϕt)0.8-1.0, respectively. From the TEM observation, the hardening source of radiation-induced defects was mainly determined to be dislocation loops for pure vanadium, loops with voids for V–5Cr and, loops and {100} precipitates for V–4Cr–4Ti–0.1Si and V–3Fe–4Ti–0.1Si alloys. From the strain rate dependence of 8% stress for V–4Cr–4Ti–0.1Si alloys tested at RT, the strain rate sensitivity, m=1/σ*(dσ/dln(dε/dt)) shows positive. Therefore, the dynamic interaction between interstitial impurities and dislocation is not strong in V–4Cr–4Ti alloys in the temperature range from RT to 290°C. A discrepancy of deformation mode of irradiated V–4Cr–4Ti–0.1Si alloys with 0.068dpa could be seen when the charpy impact test indicated the brittle behavior and the tensile test indicated the ductile behavior at room temperature. It can be explained by the difference of strain rate for the value of yield stress between tensile test and charpy test and the critical fracture stress.

A set of V–(4–8)Cr–(0–4)Ti alloys was fabricated to survey an optimum composition to reduce the radioactivity of V–Cr–Ti alloys. These alloys were subjected to nano-indenter tests before and after 2-MeV He-ion irradiation at 500 °C and 700 °C with 0.5 dpa at peak damage to investigate the effect of Cr and Ti addition and gas impurities for irradiation hardening behavior in V–Cr–Ti alloys. Cr and Ti addition to V–Cr–Ti alloys for solid–solution hardening remains small in the unirradiated V–(4–8)Cr–(0–4)Ti alloys. Irradiation hardening occurred for all V–Cr–Ti alloys. The V–4Cr–1Ti alloy shows the highest irradiation hardening among all V–Cr–Ti alloys and the gas impurity was enhanced to increase the irradiation hardening. These results may arise from the formation of Ti(CON) precipitate that was produced by He-ion irradiation. Irradiation hardening of V–Cr–1Ti did not depend significantly on Cr addition. Consequently, for irradiation hardening and void-swelling suppression, the optimum composition of V–Cr–Ti alloys for structural materials of fusion reactor engineering is proposed to be a highly purified V–(6–8)Cr–2Ti alloy.

In order to improve the properties of high vanadium wear‐resistant alloy and obtain the optimal process parameters, the effect of heat treatment on the microstructure and properties of high vanadium wear‐resistant alloy was studied by means of optical microscope, the scanning electron microscope, X‐ray diffraction, energy dispersive spectrometer, Rockwell and Vickers hardness tester. The results show that the microstructure of as‐cast high vanadium wear‐resistant alloy was composed of matrix and eutectic structure. The matrix was mainly composed of martensite and retained austenite, and the eutectics mainly included M7C3 and VC type carbide. After quenching, the matrix structure of the alloy remains martensite and retained austenite. M23C6 type carbide was precipitated while quenching at 1100 °C. The type of eutectic structure has no obvious change after tempering. Tempering has great influence on the amount of retained austenite in the matrix. The hardness of the alloy is the highest (about 69.1 HRC) when the quenching temperature and tempering temperature are 1000 °C and 550 °C respectively. [ABSTRACT FROM AUTHOR]

A comparative investigation of the influence of thermomechanical treatment modes on the structural-phase state and mechanical properties of a V-Cr-Zr-Ta low-activation vanadium alloy is performed. It is demonstrated that the use of thermomechanical treatment improves the level of short-term strength at room and elevated temperatures. The alloy is characterized by high ductility in a wide range of temperatures irrespective of the regimes of thermomechanical treatment. It is found out that the V-Cr-Zr-Ta alloy possesses higher thermal microstructure stability and mechanical properties compared to those of the V-Ti-Cr alloys. [ABSTRACT FROM AUTHOR]

The methods of optical and electron microscopy are used to study the structure of specimens of a three-layer material based on vanadium alloy V - 4% Ti - 4%Cr and ferritic steel 08Kh17T after radial forging conducted by simulated regimes of joint deformation. The mechanical properties of the three-layer material are studied in tensile tests of two-layer "steel/vanadium alloy" specimens. [ABSTRACT FROM AUTHOR]

A priority in atomic power today is to develop a new material for fuel-rod casings in fast-neutron reactors. A radiation- and corrosion-resistant three-layer composite based on vanadium alloy and stainless steel has been developed. This composite potentially meets the operational requirements on fuel-rod casings in very challenging operating conditions (high temperatures, radiation, and aggressive media). The performance of this material depends on the quality of the joint between the three layers, which is determined by the preliminary deformation and heat treatment. In the present work, the influence of tempering on the chemical composition, structure, and strength of the joint between the vanadium alloy and steel in the sandwich obtained by hot pressing a three-layer pipe blank at 1100°C is studied. The components of the pipe are 20Kh13 (Russian standard) steel for the external layers and V-4Ti-4Cr vanadium alloy in the core. The structure and chemical composition at the interfaces is investigated by optical and electronic microscopy, with X-ray spectral analysis. The strength of the steel-alloy bond is assessed in compressive tests of an annular three-layer sample with a cut; acoustic-emission measurements are employed. Pressing is found to form a transition zone of thickness 10-15 μm between the vanadium alloy and the steel, which is characterized by diffusional interaction and has a variable chemical composition. This zone consists of a series of solid solutions, without the deposition of brittle phases, and consequently the junction between the layers is strong. No pores, peeling, or defect are observed at the steel-alloy junction. However, in compressive tests of semiannular three-layer samples with a cut after hot pressing, a crack is formed in the steel layer at the tip of the cut. Annealing at 800°C improves the transition zone by increasing the thickness corresponding to diffusional interaction. Consequently, in mechanical tests, the sandwich behaves as a monolithic material, without cracking or peeling between the steel and the vanadium alloy. [ABSTRACT FROM AUTHOR]

Vanadium alloys are promising structural materials for fuel cladding tubes for fast-neutron reactors. However, high solubility of oxygen and nitrogen in vanadium alloys at operating temperatures of 700 °C limits their application. In this work, we present a novel composite structure consisting of vanadium alloy V-4Ti-4Cr (provides high long-term strength of the material) and stainless steel Fe-0.2C-13Cr (as a corrosion resistant protective layer). It is produced by co-extrusion of these materials forming a three-layered tube. Finite element simulations were utilised to explore the influence of the various co-extrusion parameters on manufacturability of multi-layered tubes. Experimental verification of the numerical modelling was performed using co-extrusion with the process parameters suggested by the numerical simulations. Scanning electron microscopy and microhardness measurements revealed a defect-free diffusion layer at the interfaces between both materials indicating a good quality bonding for these co-extrusion conditions. [ABSTRACT FROM AUTHOR]

We have systematically investigated the disorder dependence electron phonon scattering rate in three dimensional disordered V 82 Pd 18 − x Fe x alloys. A minimum in temperature dependence resistivity curve has been observed at low temperature T = T m . In the temperature range 5 K ≤ T ≤ T m the resistivity correction follows ρ o 5 / 2 T 1 / 2 law. The dephasing scattering time has been calculated from analysis of magnetoresistivity by weak localization theory. The electron dephasing time is dominated by electron–phonon scattering and follows anomalous temperature ( T ) and disorder ( ρ 0 ) dependence behaviour like τ e - ph − 1 ∝ T 2 / ρ 0 , where ρ 0 is the impurity resistivity. The magnitude of the saturated dephasing scattering time ( τ 0 ) at zero temperature decreases with increasing disorder of the samples. Such anomalous behaviour of dephasing scattering rate is still unresolved. [ABSTRACT FROM AUTHOR]

As one of the most promising candidate structural materials for fusion reactors, the irradiation damage behavior of vanadium alloys has been a research focus. Helium ions with 100 keV were adopted to irradiate V-4Cr-4Ti and China low activity martensitic (CLAM) steels at room temperature. Combined with selected area electron diffraction (SAED), X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS), the microstructure especially the precipitate evolution before and after He+ irradiation has been inves¬tigated by advanced transmission electron microscopy (TEM). The results show that the precipitate present in the vanadium matrix is TiC with FCC structure and they grew along a specific crystal orientation. The length is about 200-1000 nm. The irradiation of helium ion at room temperature induced lots of fine helium bubbles, ran¬ging from 2 to 8 nm in diameter. Distinctly, large bubbles aggregated at the interface between the precipitate and the matrix. The attice parameters did not change significantly before and after irradiation at the present conditions. Besides, compared with CLAM steel, V-4Cr-4Ti alloy demonstrated better performance of irradia-tion damage resistance. [ABSTRACT FROM AUTHOR]

A layered hybrid material is prepared by high-pressure torsion of a three-layer (vanadium alloy-zirconium alloy-vanadium alloy) billet. A mixed nanostructure with a highly uniform distribution of zirconium and vanadium alloy regions forms under the chosen severe plastic deformation conditions in the middle layer of the material. The microhardness of this layer increases by a factor of about 2.5 as compared to the initial state. [ABSTRACT FROM AUTHOR]

Vanadium-based alloys are considered to be one of the most promising hydrogen separation membranes due to their high hydrogen permeability. In this study, we investigate the dissolution and diffusion behaviors of hydrogen in vanadium-based binary alloys, V 15 M (where M = Al, Ti, Cr, Fe, Ni and Nb) alloys, using first-principles method based on density functional theory. The dissolution of hydrogen in V 15 M alloys is affected by both the elastic and electronic properties, but the elastic effect is the main factor. The H solution energies in the alloys follow the sequence: V Ti < V Nb < V Al < V Cr < V Ni < V Fe, and a smaller atom size increase the H solution energy. Therefore, the addition of alloying elements with smaller atomic sizes can reduce the solubility of hydrogen in vanadium and inhibit hydrogen embrittlement. For hydrogen diffusion, alloying elements Al, Ti and Nb can be good candidates because they have a higher diffusion coefficient. The V Ti alloy has the highest hydrogen permeability, but will have serious hydrogen embrittlement due to the increased H solubility. [ABSTRACT FROM AUTHOR]

The alloying effects of iron and aluminum on the formation and decomposition temperatures of vanadium hydride, β phase, have been investigated by a series of in - situ X-ray diffraction measurements in hydrogen atmosphere. It is found that the addition of iron increases the formation temperature of β phase, while the addition of aluminum decreases the formation temperature drastically. According to the first principal calculations, the addition of iron, chromium or nickel increases the hydrogen dissolution energy when hydrogen atom is inserted to the nearest neighbor site of alloying element. These results is considered to correspond to the increase in the plateau pressure of α–β phase transition by the addition of these alloying elements. The alloying effect on the change in the total energy upon hydrogen insertion into T-site is larger than O-site. As a result, the energy difference between T-site model and O-site model becomes small by the addition of these alloying elements, which will lead to increase the formation temperature of β phase. Aluminum in vanadium shows strong repulsive interaction with hydrogen so that hydrogen cannot occupy the nearest neighbor sites of aluminum energetically. Owing to this blocking effects, the formation temperature of β phase may decreases drastically by the addition of aluminum into vanadium. [ABSTRACT FROM AUTHOR]

The present work deals with synthesis, characterisation and elevated temperature mechanical property evaluation of V–4Cr–4Ti and oxide (yttria = 0.3, 0.6 and 0.9 at%) dispersion strengthened V–4Cr–4Ti alloy processed by mechanical alloying and field-assisted sintering, under optimal conditions. Microstructural parameters of both powder and sintered samples were deduced by X-ray diffraction (XRD) and further confirmed with high resolution transmission electron microscopy. Powder diffraction and electron microscopy study show that ball milling of starting elemental powders (V–4Cr–4Ti) with and without yttria addition has resulted in single phase α–V (V–4Cr–4Ti) alloy. Wherein, XRD and electron microscopy images of sintered samples have revealed phase separation (viz., Cr–V and Ti–V) and domain size reduction, with yttria addition. The reasons behind phase separation and domain size reduction with yttria addition during sintering are extensively discussed. Microhardness and high temperature compression tests were done on sintered samples. Yttria addition (0.3 and 0.6 at.%) increases the elevated temperature compressive strength and strain hardening exponent of α–V alloys. High temperature compression test of 0.9 at% yttria dispersed α–V alloy reveals a glassy behaviour. [ABSTRACT FROM AUTHOR]

A comparative study of the influence of the concentration of oxygen introduced during chemical-heat treatment and the temperature of final annealing on the mechanical properties and fracture features of low-activation vanadium alloys of the V–Me (Cr,W)–Zr system at different temperatures was carried out. It was found that the Zr-containing vanadium alloys studied in this work demonstrate an increase in the values of the yield strength with an increase in the concentration of introduced oxygen, while the maximum level of strength properties is achieved near the stoichiometric ratio С O /С Zr = 2. In all cases, an acceptable level of ductility of the studied alloys is observed. The study of the thermal stability of the structural-phase state and mechanical properties of alloys was carried out depending on the chemical-heat treatment regimes. It was found that the activation of the processes of coagulation and phase transformation of ZrO 2 particles occurs at a temperature of 1200°С (0.67 T melt). It is shown that an increase in the strength values at elevated temperatures of tension is provided by dispersion strengthening according to the Orowan mechanism, for the effective implementation of which it is sufficient to transform only a part of the ZrO 2 particles into the nanosized state. The influence of the oxygen concentration on the fracture features of the studied alloys at different tensile temperatures is revealed. It was found that by effectively fixing the grain boundaries, fine ZrO 2 particles contribute to the embrittlement of V–Cr–W–Zr alloy even at high tensile temperatures. [ABSTRACT FROM AUTHOR]

A cluster formula of [M − V]M was formed for vanadium alloys based on a cluster-plus-glue-atom model for BCC solid solutions, where the clusters [M − V] were centered by one solute M and surrounded by fourteen solvent atoms V, and M could also be served as a glue atom to link clusters. The [M − V]M formula with M = VCrTi, an equal-molar combination of V, Cr and Ti, corresponded to the typical V-4Cr-4Ti alloy (wt.%). Based on this formula, a series of new alloys with Ta and Zr substitution for V and Ti respectively in M, were designed and molded into ϕ3 mm rods by copper-mold suction-cast method. These alloys were solid-solutioned at 1273 K for 2 h followed by water-quenching. For Zr-added alloys, the second phase VZr was prone to be precipitated, that made alloys much brittle and worse corrosion-resistant in Cl solution. While Ta-alloyed alloys exhibited a single BCC structure, the Vickers hardness HV of alloys were enhanced obviously. Among them, the Ta-added alloy with M = TaCrTi (VTaCrTi wt.%) displayed both higher microhardness and better corrosion-resistance in Cl solution. [ABSTRACT FROM AUTHOR]

Abstracts First-principles calculations are performed to derive promising alloying elements which inhibit inter-diffusion between vanadium and palladium-coating layers in vanadium-based hydrogen separation membranes. Paying attention to the inhibition of inter-diffusion by the grain boundary segregation of impurities, several physical features of various solute elements, grain boundary segregation, grain boundary embrittlement and solute–solute binding tendencies, are evaluated to construct a large database. Several alloying elements and their combinations similar to or even better than a previously reported yttrium are suggested for future experimental works by comprehensively analyzing these physical features. [ABSTRACT FROM AUTHOR]

• The single crystal elastic constants of V44 were determined at several temperatures • V44's elastic response was isotropic up to 550°C, becoming anisotropic at 700°C • V44 elastic constants were calculated by DFT with short-range order effects • DFT results showed the importance of lattice constant and alloying elements Cr/Ti Vanadium base alloys represent potentially promising candidate structural materials for use in nuclear fusion reactors due to vanadium's low activity, high thermal strength, and good swelling resistance. In this work, the mechanical properties of the current frontrunner vanadium base alloy, V-4Cr-4Ti, have been interrogated using in-situ high energy X-ray diffraction (XRD) tensile testing at varying temperatures. The single crystal elastic constants of the samples were determined from the in-situ XRD data and used to evaluate results from density functional theory calculations. Polycrystalline elastic properties and Zener anisotropy were calculated from the single crystal elastic constants produced, revealing the effect of elevated temperature on the alloy's elastic properties. These results characterise important thermomechanical properties, valuable in mechanical modelling, that will allow further and improved analysis of the structural suitability of V-4Cr-4Ti ahead of alloy adoption in the mainstream. [ABSTRACT FROM AUTHOR]

The hydrogen solubility in a FLiNaK molten salt mixed with Ti powder was investigated. A hydrogen-soluble metal powder mixed with a molten salt can increase the effective hydrogen solubility of the molten salt, which is currently a major disadvantage of molten salts. A significant increase in hydrogen solubility was observed, even with a mass fraction of Ti powder of only 0.1 wt%. The increase of hydrogen solubility was so large that a vanadium alloy would be compatible with the Ti powder/molten salt mixture, unlike typical molten salts that result in an unacceptably large tritium inventory in the vanadium alloy. In addition, contamination of the Ti powder by oxidation suppressed the hydrogen uptake and release capability. Controlling the purity of the molten salt was found to be one of the key issues for the metal powder mixture concept. [ABSTRACT FROM AUTHOR]

Inter-diffusion between vanadium and palladium coating layers in vanadium-based hydrogen separation membranes is investigated by using a computational approach based on first-principles calculations and semi-empirical atomistic simulations, paying attention to the surface stability and the prevention of the degradation of hydrogen permeability. It is found that the governing mechanism of the inter-diffusion is the grain boundary diffusion, and therefore a diffusion barrier based on the grain boundary segregation of impurities can be an efficient way to inhibit the inter-diffusion that causes the degradation. An interesting aspect in previous experimental works that showed a good resistance to the inter-diffusion by an addition of a trace amount of yttrium is discussed from the view point of the grain boundary segregation. An experiment that proves the validity of the present alloy design scheme (inhibition of inter-diffusion using grain boundary segregation) is carried out, and a process to maximize the sustainability of the membrane is also proposed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Addition of 10mol% Cr to V increases the β-hydride T C to >200°C. [•] Addition of 10mol% Ni to V increases the β-hydride T C to >400°C. [•] Addition of 10mol% Al to V decreases the β-hydride T C to <30°C. [•] V90Al10 membrane can be cycled to <30°C under H2 without β-hydride formation. [ABSTRACT FROM AUTHOR]

A comparative study of the effectiveness of carbide and oxide types of strengthening of V–Cr–Zr alloy was carried out by means of a comprehensive certification of structural-phase state parameters and measuring the mechanical properties characteristics. It has been shown that the use of chemical-heat treatment contributes to a significant increase in the thermal stability of the microstructure and mechanical properties of V–Cr–Zr alloy in comparison with carbide strengthening under the conditions of thermomechanical treatment. A controlled increase in the volume fraction of fine particles based on ZrO 2 , along with an increase in the concentration of oxygen in the solid solution, leads to a decrease in the rate of oxides coagulation and an increase in the thermal stability of high disperse heterophase structure. These effects contribute to the retention of high defect structural states with nonzero values of crystal lattice curvature even after high-temperature (0.67 T melt) anneals. The high efficiency of dispersion and substructural strengthening is a consequence of blocking dislocation slip by fine particles stabilized by oxygen in a solid solution. [ABSTRACT FROM AUTHOR]