Vanadium-base Alloys Research Articles

The structure evolution of titanium–vacancy complex in a vanadium-based alloy

Under irradiation conditions, excess point defects interact strongly with the solute atoms in fusion reactor materials. The solutes can bind with point defects and form clusters such as solute–vacancy complexes. The evolution of these complexes at long timescale significantly influences the mechanical properties of the irradiated materials. In this study, we mainly use climbing image nudged elastic band method to investigate the evolution and migration of titanium–vacancy complexes in a vanadium-based alloy. The transition pathways associated with atomic trajectories of complex evolution in bulk and in grain boundary vicinity are calculated. Results show that both basin energies and transition barriers are affected when a complex is in grain boundary vicinity. The evolution is revealed to be driven by multiple titanium–vacancy interactions synergistically. The migration is supposed to be induced by two mechanisms: the vacancy reorientation jumps and complex dissociation–reformation which are both based on titanium–vacancy interactions. This research contributes to the understanding of solute diffusion and early-stage radiation defects in vanadium-based alloys.

Site preference, magnetic and electronic properties of half-metallic Vanadium-based full Heusler alloys

We reported a new atomic site preference occupation for Vanadium-based X2YZ full Heusler alloys. It tells that alloys with less 24 valence electrons form the L21-type structure, while others form the XA-type structure. This fact means that the site preference rule for Vanadium-based alloys depends on the total number of valence electrons instead of the electropositive between X and Y atoms. The magnetic moments of XA-type alloys follow the Slater-Pauling rules, with two forms, Mt = NV − 24 and Mt = NV − 18 corresponding to different origins of the band gaps. XA-type V2YZ (Y = Cr, Mn and Fe; Z = Ga, Ge and As), V2CoGa, V2CoGe and V2NiGa alloys have half-metallic property. L21-type V2YZ (Y = Cu and Zn; Z = Ga, Ge and As) alloys show no spin polarization and no half-metallic property due to the structural symmetry and the non-magnetism of copper and zinc atoms.

Influence of Nb, Ti and Mo on Microstructure and Mechanical Properties of Vanadium Solid Solutions

The present study addresses the microstructural evolution and mechanical properties of three different cast V-5X (X = Nb, Ti and Mo) alloys. All alloy compositions are located in the vanadium solid solution (VSS) phase field in their respective phase diagrams. While the alloys V-5Ti and V-5Nb solidified as single-phase VSS after arc-melting, the alloy V-5Mo additionally shows some amount of molybdenum solid solution (MoSS) in the resulting as-cast microstructure. To improve the mechanical database on vanadium-based alloys, room temperature compression tests were performed and microhardness measurements were carried out by the Vickers indentation method. The lattice parameters were determined by X-ray diffraction analysis and they were calculated using the density functional theory. By carrying out these experiments, the data available for vanadium-based alloys were extended being now available for further optimization of novel V-Si-B high temperature alloys.

Angular dependent interatomic potential for Ti–V system for molecular dynamics simulations

An interatomic potential for the Ti–V binary alloy focusing on the evolution of defects, including ones arising as a result of the irradiation process, was constructed within the Lipnitskii–Saveliev approach, which accurately takes into account three-particle interactions and the sum of all multi-particle interactions of a higher order in the framework of the centrally symmetric approximation. In the new potential, Ti–V interactions were fitted to the density functional theory data on a set of model structures with different coordination numbers, including ones with vacancies. The properties used for fitting are accurately reproduced by the present potentials for both pure elements and alloy systems. The potential was tested on the binding energies between Ti atoms and self-point defects in bcc V, elastic moduli, thermal expansion and melting point of some alloys, and diffusion. We obtained qualitative agreement for these properties with available theoretical and experimental data. Finally, we investigated the evolution of excess vacancies in the V–4 at% Ti alloy at 700 K, which are typical conditions of vanadium-based alloys for fusion applications. We found that no vacancy loop is formed in the alloy in contrast to the pure V, which agrees with the experimental observations. The potential is expected to be especially suitable for irradiation simulations of vanadium-based V–Ti alloys.

Effect of partial Ni substitution in V85Ni15 by Ti on microstructure, mechanical properties and hydrogen permeability of V-based BCC alloy membranes

Vanadium-based alloy membranes with body-centred-cubic (BCC) structure are considered as one of the leading alternatives to Pd-based alloys for hydrogen separation applications due to their lower cost and higher permeability. As permeability and mechanical properties depend on what microstructure can be produced mainly by alloy composition under same processing conditions, the effect of alloy composition on microstructure, mechanical properties and hydrogen permeability has been investigated for the V85Ni15 and V85Ni10Ti5 (at%) alloys prepared by a same process route. All Ni atoms dissolve into the V-matrix to form a single highly supersaturated solid solution with dendritic segregation of Ni-solute atoms in the binary alloy. A part of Ni replacement with 5 at% Ti leads to the formation of small interdendritic phases NiTi and NiTi2 in addition to major phase of V-based solid solution. The mechanical property testing shows that the ultimate strength of the ternary alloy is higher than that of the binary alloy, but the elongation and rollability are lower due to a combination of solid solution hardening and particle strengthening effect. The addition of Ti can greatly increase permeability about 4 times greater than the binary alloy at a permeation testing of 400 °C. But the presence of small amounts of interdendritic compounds provides a barrier to hydrogen migration, resulting in a relatively lower hydrogen diffusion coefficient. In theory, the diffusivity and solubility of hydrogen atom in the presence of alloying element Ti is higher than that in the presence of alloying element Ni in vanadium. This is demonstrated using first principles calculation which further explains the mechanism of hydrogen permeation.

Deuterium permeation through the low-activated V–4Cr–4Ti alloy under plasma irradiation

Abstract Vanadium-based alloys are considered as candidate structural materials for nuclear fusion and fast nuclear reactors. Some properties of vanadium alloys are especially advantageous for nuclear fusion facilities, namely, a fast decay of induced activity, resistance to liquid lithium media and high-temperature strength. At the same time, V-alloys absorb and dissolve hydrogen isotopes in large quantities, and the mobility of dissolved hydrogen is high even at cryogenic temperatures. Thus, vanadium and its alloys, as some other metals of the fifth group (Nb, Ta), are characterized by high hydrogen permeability. This allows their usage as membranes for the diffusive purification of hydrogen isotopes. Under plasma irradiation of an inlet surface of such membranes, suprathermal hydrogen atoms and ions with quite high possibility penetrate through the adsorption barrier and diffusing inside materials bulk can reach an outlet surface. This paper aims to study the plasma-driven permeation (PDP) of deuterium through V–4Cr–4Ti alloy. It was shown that the permeation probability of deuterium under plasma irradiation for V–4Cr–4Ti alloy varies in a range of χ = 0.03 ÷ 0.12 for T = 600 ÷ 800 K, which leaves open the possibility of such alloys usage for hydrogen isotopes purification in the fusion reactor exhaust.

Angular dependent interatomic potential for Ti-V system for molecular dynamics simulations

Abstract An interatomic potential for the Ti-V binary alloy focusing on evolution of defects, including ones arising as a result of the irradiation process, was constructed within the Lipnitskii-Saveliev approach, which accurately takes into account three-particle interactions and the sum of all multi-particle interactions of a higher order in the framework of the centrally symmetric approximation. In the new potential, Ti-V interactions were fitted to the DFT data on set of model structures with different coordination numbers, including ones with vacancies. The properties used for fitting are accurately reproduced by the present potentials for both pure elements and alloy systems. The potential was tested on the binding energies between Ti atoms and self-point defects in bcc V, elastic moduli, thermal expansion and melting point of some alloys, and diffusion. We obtained qualitative agreement for these properties with available theoretical and experimental data. Finally, we investigated evolution of excess vacancies in the V-4 at. \% Ti alloy at 700 K, which are typical conditions of vanadium-based alloys for fusion applications. We found that no vacancy loop is formed in the alloy in contrast to the pure V, which agrees with the experimental observations. The potential is expected to be especially suitable for irradiation simulations of vanadium based V-Ti alloys.

The interstitial emission mechanism in a vanadium-based alloy

Under nuclear fusion environments, displacement cascades in the potential first protective wall material vanadium (V) and its alloys lead to a large number of point defects. As sink of point defects, grain boundaries are observed to significantly affect the radiation resistance of structured metals. By using potential energy surface searching tools, the interaction between the interstitial-loaded ∑3<110>{111} symmetric tilt grain boundary (STGB) and the point defects in V-based alloy is investigated. For the self-interstitial atoms (SIA)-loaded STGB, the vacancy located within certain distances from the STGB can be effectively annihilated within several nanoseconds via the interstitial emission (IE) mechanism. If an alloy doping interstitial as a Cr atom substitute the SIA, or the vacancy is stabilized by alloy solutes as Ti atoms forming a solute-vacancy complex near the STGB, IE works more effectively with lower activation energy barriers and less thermal activation time. The results indicate that the STGB-induced IE mechanism raises the radiation resistance of the V-based alloy. The results contribute to the micro-structure and constituent optimizations in designing an excellent V-based first protective wall material.

Effect of precipitates on high-temperature strength and irradiation behavior of vanadium-based alloys

The formation of precipitates and their effect on mechanical properties and irradiation damage in V–4Cr–4Ti alloys have been investigated using transmission electron microscope, high-temperature tensile tests and high-voltage electron microscope (HVEM). V–4Cr–4Ti alloys were aged for 20–40 h at 873 K, and extremely fine precipitates were produced during the aging process, which coarsened with aging time. The results of high-temperature (773, 873 and 973 K) tensile tests showed an increase in strength with increasing aging time (i.e., increasing size of precipitates) while a decrease in the uniform elongation. There was evidence that presence of precipitates strengthened V–4Cr–4Ti alloys. In situ HVEM observations showed that the precipitates restricted the growth rate of dislocation loops induced during electron irradiation at 773 K. The interactions between precipitates and irradiation-induced dislocation loops were discussed. The presence of particles alleviated the increase in irradiation hardening up to an irradiation dose of 3.42 dpa (displacement per atom). The precipitates shrank in size during electron irradiation, which was due to the dissolution of constituent atoms of the precipitates into the alloy matrix.

Effects of alloying elements (Al, Mn, Ru) on desorption plateau pressures of vanadium hydrides: An experimental and first-principles study

Vanadium-based alloys are promising for the reversible and compact storage of renewable hydrogen. To improve the hydrogen desorption plateau pressure of vanadium hydrides, V–3A binary alloys of V97Al3, V97Mn3, and V97Ru3 were prepared by vacuum arc melting. Hydrogen absorption and desorption properties of the newly prepared V–3A alloys were studied, and compared to vanadium hydrides, by pressure-composition-temperature measurements and first-principles calculations. Among the studied materials, V–3Ru showed the highest hydrogen desorption plateau pressures between T = 373 and 433 K. During hydrogen loading, V–3Ru also reached the highest hydrogen to metal ratio between the alloys. Meanwhile, V–3Ru was also the alloy with the highest hardness. Findings of hydrogen absorption and desorption measurements were supported with DFT calculations of hydride formation energies. The calculated DOS, ELF and atomic charges were used to study the effects of alloying on the electronic properties of hydrides. The bonding interactions in vanadium dihydrides were influenced the most through Al alloying, whereas V–3Mn and V–3Ru hydrides showed comparable electronic properties.

Influence of Processing Techniques on the Surface Microstructure of V85Ni15 Membrane Alloy

Vanadium-based alloys with a BCC structure are considered to be an alternative to palladium alloys for hydrogen purification. Since the performance of membranes is influenced by not only their bulk characteristics but also their surface condition, we have investigated the effect of different surface preparation techniques (abrasive grinding and polishing, electrolytic polishing, and ion etching) on the surface microstructure of V85Ni15 membrane alloy using X-ray photoelectron spectroscopy. After electrolytic polishing and ion etching, we observed an increase in the percentages of vanadium and nickel metals in the surface layer of the alloy in comparison with its state after abrasive polishing. Such changes are expected to be favorable for an increase in the rate of hydrogen dissociation and recombination on the surface of the membrane material, eventually improving hydrogen transport efficiency.

Effect of Deuterium Plasma Irradiation on V–Ga Alloys

Indentation tests are used to investigate the effect of high-power nanosecond deuterium ion pulses (~100 keV, ~1010 W/cm2) and dense deuterium plasma (~100 eV, ~107 W/cm2) generated by a Plasma Focus machine on the mechanical properties of vanadium-based alloys such as V–5Ti–5Cr, V–5Ga–5Cr, V–5Ga, and V–5Ga–0.1Ce. It is established that the V–Ga–Cr alloys have a higher resistance to embrittlement during radiation-thermal exposure than that of V–Ti–Cr alloys. X-ray diffraction analysis shows that the structure of solid solution is retained in all investigated alloys after pulsed ion and plasma irradiation. No signs of solid solution decomposition and the precipitation of second phases are found. It is established that pulsed ion-plasma irradiation suppresses the rolling texture that is typical of the alloys in the initial state and decreases the lattice parameter of the alloys, which in the case of the V–Ga alloy can be explained by the escape of gallium from the recast surface layer. Doping with a rare earth element (cerium) increases the radiation resistance of the V–Ga alloy, which results in the stability of the mechanical properties and the lattice parameter after irradiation.

First-principles study of hydrogen behavior in vanadium-based binary alloy membranes for hydrogen separation

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, V15M (where M = Al, Ti, Cr, Fe, Ni and Nb) alloys, using first-principles method based on density functional theory. The dissolution of hydrogen in V15M 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: VTi < VNb < VAl < VCr < VNi < VFe, 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 VTi alloy has the highest hydrogen permeability, but will have serious hydrogen embrittlement due to the increased H solubility.

Selective laser melting additive manufacturing of advanced nuclear materials V-6Cr-6Ti

Selective laser melting (SLM) additive manufacturing technology has been a prevailing method to fabricate components with complex physical geometry or novel structural design. However, original nuclear powder material such as vanadium-based alloy appropriate for SLM processing has yet not been obtained commercially, which significantly restricts the development of nuclear component manufacturing. In this study, near-sphere, uniform and fine V-6Cr-6Ti pre-alloy powder which met the performance demands for SLM processing was successfully obtained by high-energy ball milling. Subsequently, the V-6Cr-6Ti part was fabricated by SLM consolidation of as-prepared pre-alloy powder with double-region orthometric scanning strategy, forming strong texture feature within molten pool. The compression test was performed, showing that the maximum compression stress reached 1078MPa and the accumulated strain was about 0.32.

Ti-rich precipitate evolution in vanadium-based alloys during annealing above 400 °C

We have assessed the plate-like TiO precipitate evolution in V-4Ti and V-4Ti-4Cr alloys during isochronal annealing above 400 °C, by combining Vickers hardness, positron lifetime and coincidence Doppler broadening measurements. Our results reveal the formation of additional TiO precipitates in both alloys at temperatures of 450–600 °C in both alloys. The implanted positrons become trapped at the nm-thick TiO/matrix interface, and act as effective probes of the concomitant annealing of vacancies taking place inside the TiO precipitates above 550 °C in V-4Ti alloy. The presence of Cr in the ternary alloy not only retards the recovery of dislocations, but also enhances the oxygen diffusivity and therefore decreases the vacancy content in the TiO precipitates. These results will impact the expected alloy stability and capacity to bind light elements in the operational temperature window of these alloys for fusion reaction applications.

On the interdiffusion in multilayered silicide coatings for the vanadium-based alloy V-4Cr-4Ti

To provide protection against corrosion at high temperatures, silicide diffusion coatings were developed for the V-4Cr-4Ti alloy, which can be used as the fuel cladding in next-generation sodium-cooled fast breeder reactors. The multilayered coatings were prepared by halide-activated pack cementation using MgF2 as the transport agent and pure silicon (high activity) as the master alloy. Coated pure vanadium and coated V-4Cr-4Ti alloy were studied and compared as substrates. In both cases, the growth of the silicide layers (V3Si, V5Si3, V6Si5 and VSi2) was controlled exclusively by solid-state diffusion, and the growth kinetics followed a parabolic law. Wagner's analysis was adopted to calculate the integrated diffusion coefficients for all silicides. The estimated values of the integrated diffusion coefficients range from approximately 10−9 to 10−13 cm2 s−1. Then, a diffusion-based numerical approach was used to evaluate the growth and consumption of the layers when the coated substrates were exposed at critical temperatures. The estimated lifetimes of the upper VSi2 layer were 400 h and 280 h for pure vanadium and the V-4Cr-4Ti alloy, respectively. The result from the numeric simulation was in good agreement with the layer thicknesses measured after aging the coated samples at 1150 °C under vacuum.

Thermodynamic and experimental study of corrosion behavior of vanadium-based alloy in liquid sodium-potassium coolant

A preliminary assessment of oxygen effect on vanadium solubility in Na–K melt eutectic composition has been carried out using mathematical framework of the subregular solution model and equations of coordination-cluster model. The effect of oxygen on the solubility of vanadium in the Na–K alloy can be considered as the result of short-range ordering in liquid metal solution. The negative deviations from the ideality for dilute oxygen solutions in Na–K solvent is one reason that explains the quantitative differences between Na and Na–K coolants, when we need to estimate the threshold oxygen concentration for the formation of ternary oxide NaVO2 on the surface of the solid vanadium in liquid sodium and in Na–K alloy. Isothermal capsule experiments qualitatively confirmed the results of calculations of vanadium solubility in Na0.32K0.68 alloy.

High temperature compression strength and oxidation of a V-9Si-13B alloy

Vanadium-based alloys are attractive relative to nickel base superalloys for high temperature applications due to significantly reduced density and increased melting temperatures. In this exploratory study, the compressive strength and oxidation behavior of a powder metallurgically processed V-Si-B multiphase alloy is examined. Results are compared to CMSX-4 and other existing vanadium alloys. Compressive yield strength is comparable to the nickel base superalloy at 1000°C but the mass gain during isothermal oxidation at 600°C is higher, implying the need for a protective coating for sustained high temperature exposure.

The Accumulation and Annealing of Radiation-Induced Defects and the Effect of Hydrogen on the Physicomechanical Properties of the V−4Ti−4Cr and V−10Ti−5Cr Vanadium-Based Alloys under Low-Temperature (at 77 K) Neutron Irradiation

The processes of the accumulation and annealing of radiation-induced defects that occur under low-temperature (at 77 K) irradiation (with an energy E > 0.1 MeV) of V−4Ti−4Cr and V−10Ti−5Cr bcc alloys both nonmodified and modified with hydrogen isotopes in a concentration of 200 ppm, as well as the effect of these processes on the physicomechanical properties of these alloys, have been studied. It has been found that the saturation of these alloys with hydrogen leads to slight changes in their strength and ductility characteristics. The irradiation of the alloys at the temperature of 77 K results in a substantial increase in their yield stress and ultimate strength, as well as a decrease in their ductility. In the course of the postradiation annealing of the alloys at a temperature of 130 K, the stage related to the migration of interstitial atoms is observed. At temperatures of 290–320 K, the recovery stage occurs due to the formation of vacancy clusters. The stage that occurs at a temperature of 470 K can be attributed to the formation of impurity-vacancy clusters. Possible mechanisms of the radiation-induced strengthening of the alloys during irradiation and subsequent annealing have been discussed.

Characterization of dislocation loops in hydrogen-ion irradiated vanadium

Vanadium alloys are considered as the candidate materials for structure application in fusion reactors because of their low radiation-induced activation, high resistance to radiation damage, high thermal conduction, and low thermal expansion coefficient. Before these materials, which will be exposed to high-flux hydrogen and helium isotopes, may be safely used in fusion device much more data based on irradiation damage are required. The study of dislocation loops in vanadium is designed to indicate the mechanism of void growing under irradiation. The mechanism is that different types of dislocation loops have different bias which represent their abilities to absorb point defects. It is possible to explain the irradiation swelling performance in the material with the bias of loops. The thin disks samples used in this experiment are made of pure vanadium and vanadium alloy (V-4Cr-4Ti) by twin-jet electro-polishing. Electrolyte of H2SO4-CH3OH (1 : 6 by volume) at -20 ℃ is used in a current of 80～120 mA. To get a clear view of dislocation loops, the SRIM code is used to simulate the implantation of hydrogen ions into vanadium. The ion irradiation is carried out to a dose of 51016H+/cm2, at an energy of 30 keV. Microstructure observations are performed on a Tecnai G2 F20 (transmission electron microscope, TEM) at an accelerating voltage of 200 kV. The Burger's vectors and nature of the dislocation loops formed in pure vanadium by hydrogen implantation are confirmed by TEM. This experiment has focused on as many as 76 dislocation loops, lots of images are taken under different diffraction conditions from the same areas of interest. Results show that most of the dislocation loops have a Burger's vectors of 1/2111 (90%), and a few of 110. No loops with b= 100 loops can be found in this study. The nature of dislocation loops is determined by the inside-outside method. The number of the dislocation loops that can make sure of their nature is 29, and all of them are conformed to be interstitial type, their habit planes are from {110} to {112}. No vacancy type loops are found. The density and average size of dislocation loops in vanadium and vanadium alloy are also analysed. Compared with the pure vanadium, the loops in vanadium alloy of V-4Cr-Ti are formed in a smaller size and higher number density. As a future work the difference of the loops nature between pure vanadium and vanadium-based alloys should be investigated to illustrate their behaviour of irradiation swelling.