Investigation on Retention and Release Behaviors of Hydrogen and Helium in Vanadium Alloy

Vanadium alloy is proposed as an attractive candidate for first wall and blanket structural material of fusion reactors. The retention and release behaviors of hydrogen and helium in vanadium alloy may be an important issue. In the present work, 1.7 keV deuterium and 5 keV helium ions are respectively implanted into V-4Cr-4Ti and V-4Ti at room temperature. The retention and release of deuterium and helium are measured with thermal desorption spectroscopy (TDS). When the helium ion fluence is larger than 3 × 1017 He/cm2, the retained helium saturates with a value of approximately 2.5 ×1017 He/cm2. However, when the ion fluence is 1×1019 D/cm2, the hydrogen saturation in vanadium alloy does not take place. Experimental results indicates that hydrogen and helium retention in vanadium alloy may lead to serious problems and special attention should be paid when it is applied to fusion reactors.

The Synergetic Effects of Hydrogen and Oxygen on the Strength and Ductility of Vanadium Alloys

A V4Ti alloy and several V4Cr4Ti alloys with different oxygen contents were studiedon their tensile properties with the effect of hydrogen concentrations.The ductility of the alloysshowed a successive decrease In a varied rate with an increased hydrogen concentration,while theultimate tensile strength remained unchanged or even decreased for the high oxygen content alloy

Characterization for Fusion Candidate Vanadium Alloys

This paper summarizes recent achievements in the characterization ofcandidate vanadium alloys obtained for fusion in the framework of the Japan-China Core UniversityProgram. National Institute for Fusion Science (NIFS) has a program of fabricating high-purityV-4Cr-4Ti alloys. The resulting products (NIFS-HEAT-1,2), were characterized by various researchgroups in the world including Chinese partners. South Western Institute of Physics (SWIP) fabricateda new V-4Cr-4Ti alloy (SWIP-Heat), and carried out a comparative evaluation of hydrogenembrittlement of NIFS-HEATs and SWIP-Heat. The tensile test of hydrogen-doped alloys showed that theNIFS-HEAT maintained the ductility to relatively high hydrogen levels. The comparison of the datawith those of previous studies suggested that the reduced oxygen level in the NIFS-HEATs should beresponsible for the increased resistance to hydrogen embrittlement. Based on the chemical analysisdata of NIFS-HEATs and SWIP-Heats, neutron-induced activation was analyzed in Institute of PlasmaPhysics (IPP-CAS) as a function of cooling time after the use in the fusion first wall. The resultsshowed that the low level of Co dominates the activity up to 50 years followed by a domination of Nbor Nb and Al in the respective alloys. It was suggested that reduction of Co and Nb, both of whichare thought to have been introduced via cross-contamination into the alloys from the molds usedshould be crucial for reducing further the activation.

Electron beam braze-welding of vanadium alloy to stainless steel with electroplated Cu/Ag coatings

Cracks may easily occur in the fusion weld between vanadium alloys and stainless steel due to the brittle intermetallics and welding stress. The high vacuum electron beam braze-welding has been successfully used to join vanadium alloy（V-5Cr-STi） to stainless steel （HR-2） with electroplated Cu and Ag coating. To investigate the effects of electroplated coating on the weldability, the joint appearaace, the microstrueture and the mechanical properties of the joints have been thoroughly analyzed. The results show that the joint surface configuration was good and root reinforcement was full and smooth. A reaction zone （RZ） was gained on the interface between the V-5 Cr-5 Ti alloy and HR-2 stainless steel base metals. The width of reaction zone at the top of the joint was up to O. 65 mm, wider than that in the bottom of the joint （ 0.46 mm）. The reaction zone consisted of considerably smaller dendritic structures with an average grain size of less than 10μm. Element Ag and Cu almost enriched the interface between V-SCr-5Ti alloy substrate and RZ, serving as a physical barrier which decreases or avoids the formation of intermetallics. The maximum tensile strength of vanadium alloy^stainless steel dissimilar alloy joint was more than 300 MPa. The joint was defects free.

Dissimilar-metal bonding of a carbide-dispersion strengthened vanadium alloy for V/Li fusion blanket application

Carbide-dispersion strengthened(CDS)vanadium alloy is a kind of advanced structural material for V/Li fusion blanket.The welding technology for the alloy is very important for its application in fusion reactors.In the present study,preliminary investigation on dissimilar-metal bonding by hot isostatic pressing(HIP)was carried out for the CDS V alloy by joining with the conventional V-4Cr-4Ti alloy.Microstructural characterization for the bonding interfaces and mechanical tests for bonding property evaluation were analyzed for the HIP joint.The results show that after the HIP,the grain size of the V-4Cr-4Ti base metal(BM)obviously increases,while the grain size near the interface is much smaller than that in the V-4Cr-4Ti BM.This is the reason why the hardness near the interface is larger than that of the V-4Cr-4Ti BM.By shear test evaluation,the HIP joint possesses good bonding property with sound strength and ductility.Because there is no phase transformation during HIP,no post-bond heat treatment(PBHT)is necessary for the dissimilar-metal joint.

The Effects of Al,Cr on the Oxidation Behavior of a V-4Ti Alloy

Oxidation experiment was performed in air at elevated temperature for three kinds of vanadium alloys. The features of the oxides and the role of the alloying elements were analyzed. All specimens exhibited a parabolic kinetic behavior of weight gain with the exposure time. The alloys can't be used in air at the temperature over 700℃, below which, V4Ti3Al showed a much lower oxidation rate than the V4Cr4Ti alloy. It was found that Al in the alloy was segregated to the specimen surface in the process, and formed into Al2O3 on the surface, hence decreasing the formation of V2O5. The oxides on the surface were nucleated in a small number density and grew to a large size, giving more protection to the matrix alloy.

Mechanical properties of vanadium-alloyed austempered ductile iron for crankshaft applications

This study focused on the development of austempered ductile iron(ADI)with desirable combination of mechanical properties for crankshaft applications by the combined effect of vanadium(V)alloying and an optimized heat treatment process.The produced unalloyed GGG60,0.15%V-alloyed GGG60(V-15),and 0.30%V-alloyed GGG60 samples were subjected to austenitizing at 900℃for 1 h and subsequent austempering processes at 250,300,and 350℃for 15,30,60,90,and 180 min.As a result of these austempering processes,different bainitic structures were obtained,which led to the formation of diverse combinations of mechanical properties.The mechanical properties of the austempered samples were tested comprehensively,and the results were correlated with their microstructures and the stability of the retained austenite phases.From the microstructural observations,the V-alloyed samples exhibited a finer microstructure and a more acicular ferrite phase than unalloyed samples.The V addition delayed the coarsening of the acicular ferrite structures and considerably contributed to the improvement of the mechanical properties of GGG60.Moreover,the X-ray diffraction results revealed that the retained austenite volume and the carbon enrichment of austenite phases in ADI samples were remarkably affected by the addition of vanadium.The increase in volume fraction of retained austenite and its carbon content provided favorable ductility and toughness to V-15,as confirmed by the elongation and impact test results.Consequently,the dual-phase ausferrite microstructure of V-15 that was austempered at 300℃for 60 min exhibited high strength with substantial ductility and toughness for crankshaft applications.

Response of structural and magnetic properties of ultra-thin FeCo–V foils to high-energy beam welding processes

Microstructural evolutions and grain-boundary-character distribution during high-energy-beam welding of ultra-thin Fe Co-V foils were studied. Detailed data about the boundaries, coincidence site lattice（CSL） relationships, grain sizes, and microstructural features were acquired from electron-backscatter diffraction（EBSD） maps. Moreover, the evolution of the magnetic properties during high-energy-beam welding was studied using vibrating sample magnetometry（VSM）. The fraction of low-angle boundaries was observed to increase in the fusion zones of both electron- and laser-beam-welded foils. The results showed that the fractions of low-Σ CSL boundaries（particularly twin boundaries, Σ3） in the fusion zones of the welded foils are higher than those in the base metal. Because the strain rates produced during high-energy-beam welding are very high（because of the extremely high cooling rate）, grain deformation by a slip mechanism is limited; therefore, deformation by grain twinning is dominant. VSM analysis showed that the magnetic properties of the welded foils, i.e., their remanence, coercive force, and energy product, changed significantly. The formation of large grains with preferred orientation parallel to the easy axis of magnetization was the main reason for the diminished magnetic properties.

A review of solute-point defect interactions in vanadium and its alloys: first-principles modeling and simulation

Vanadium alloys are the promising first wall and blanket materials for fusion reactors.Large amounts of helium(He)and hydrogen(H)impurities are produced inside the materials along with irradiation defects under neutron irradiation,leading to bubble formation and microstructure changes,which will degrade the thermal and mechanical properties of vanadium alloys.The microstructure changes of materials are influenced by the interactions of point defects with solute atoms.Nowadays,first-principles calculations are intensively performed to elucidate these interactions,clustering,and dissolution,which can provide valuable information for the design of high-performance anti-irradiation materials.This paper reviews the recent findings of the interactions of point defects(vacancies,self-interstitial atoms)with substitutional solutes and interstitial solutes(C,O,N,H,and He)as well as their clusters in vanadium and its alloys from first-principles calculations.

Fusion Material Studies Relating to Safety in Russia in 2002

The paper is a summary of Russian material studies performed in frames of activi-ties aiming at substantiation of safety of the International Thermonuclear Experimental Reactor(ITER) after 2001. Subthreshold sputtering of tungsten by 5 eV deuterons was revealed at temper-atures above 1150℃. Mechanism of globular films formation was further studied. Computationsof tritium permeation into vacuum vessel coolant confirmed the acceptability of vacuum vesselcooling system for removal of the decay heat. The most dangerous accident with high-currentare in toroidal superconducting magnets able to burn out a bore up to 0.6 m in diameter in thecryostat vessel was determined. Radiochemical reprocessing of V-Cr-Ti alloy and its purificationfrom activation products down to a contact dose rate of ～ 10 μSv/h was developed.

Recent progress of vanadium-based alloys for fusion application

Low-activation vanadium alloys,with the reference composition of V-4Cr-4Ti have been considered as one of the most promising candidate materials for structural components such as the blanket in future fusion reactors,thanks to their excellent neutron irradiation resistance,superior high-temperature mechanical properties,and high compatibility with liquid lithium blankets.The self-cooled liquid lithium blanket using structural materials of vanadium alloys is an attractive concept because of the high heat transfer and high tritium breeding capability.After more than 2 decades of research,technological progress has been made in reducing the number of critical issues for application of vanadium alloys to fusion reactors.In this paper,the recent research and development activities of vanadium alloys are summarized,including significant progress achieved on fabrication technology and composition optimization,coating and corrosion,improved understanding of irradiation effects upon microstructure and material properties,retention of hydrogen isotopes,as well as advancements in joining and weld-ing.In particular,the fact that recent products from China,Japan,US and France showed similar properties which meant the fabrication technology has been almost standardized.

Phase Relationship in the Al-rich(Al+0.15at %V)-Mg-Zn System

Isothermal sections at room temperature and 450°C for Al-rich (Al+0.15at%V)-Mg-Zn system have been determined by X-ray diffraction, microscopic observation and DTA methods. Comparing with Al-Mg-Zn system the α-phase region in isothermal sections at room temperature and 450°C for the system with 0.15at%V is slightly narrowed and expanded respectively. The solubility limits of Zn and Mg in α-Al have been found to be 3.3at% (7.6wt%) and 5.5at%(5.0wt%) respectively at room temperature, and 6.0at% (13.4wt%) and 11.0at% (10.0wt%) at 450°C. The τ phase in this system is a ternary intermetallic compound, which undergoes an allotropic transformation at about 427°C.