4Cr 4Ti Alloy Research Articles

Performance of V–4Cr–4Ti alloy exposed to the JFT-2M tokamak environment

A long-term test has been conducted in the JFT-2M tokamak fusion device to determine the effects of environmental exposure on the mechanical and chemical behavior of a V–4Cr–4Ti alloy. Test specimens of the alloy were exposed in the outward lower divertor chamber of JFT-2M in a region away from direct contact with the plasma and were preheated to 300°C just prior to and during selected plasma discharges. During their nine-month residence time in JFT-2M, the specimens experienced approximately 200 lower-single-null divertor shots at 300°C, during which high energy particle fluxes to the preheated test specimens were significant, and approximately 2010 upper-single-null divertor shots and non-divertor shots at room temperature, for which high energy particle fluxes to and expected particle retention in the test specimens were very low. Data from post-exposure tests have indicated that the performance of the V–4Cr–4Ti alloy would not be significantly affected by environmental exposure to gaseous species at partial pressures typical for tokamak operation. Deuterium retention in the exposed alloy was also low (<2 ppm). Absorption of interstitials by the alloy was limited to the very near surface, and neither the strength nor the Charpy impact properties of the alloy appeared to be significantly changed from the exposure to the JFT-2M tokamak environment.

Solute interactions in pure vanadium and V–4Cr–4Ti alloy

Room temperature electrical resistivity and microhardness measurements were performed on cold-worked pure vanadium, solution annealed and cold-worked V–4Cr–4Ti alloy and fusion weld material over the isochronal annealing temperature range from 200°C to 1200°C. The results suggest that Ti solutes in the vanadium alloy interact with interstitial O, C and N elements at temperatures of 200°C and higher. This interaction lowers the effective diffusivity of the interstitials in the alloys, which shifts the interaction of interstitials with dislocations to higher temperatures as compared to the cold-worked vanadium. Over temperature ranges associated with recovery and recrystallization, general trends showed that the hardness decreased and resistivity first decreased but then increased in each material. Microstructural changes caused by precipitation and by reduction in dislocation density during recrystallization may account for most of the differences observed in the measured properties of each material.

Synergistic effects of hydrogen and helium on microstructural evolution in vanadium alloys by triple ion beam irradiation

In fusion materials, irradiation of 14 MeV neutrons produces He and H atoms at a high generation rate. The objective of this study is to clarify synergistic effects of He and H on microstructural evolution in pure vanadium and two vanadium alloys including candidate ternary alloy V–5Cr–5Ti. The specimens are irradiated with 12 MeV Ni3+ ions at 873 K with simultaneous implantation of 1 MeV He+ and 350 keV H+ ions. Helium and hydrogen implantation ratios are independently controlled at two different He/dpa and H/dpa rates. Dual beam irradiation with heavy ions and He ions or H ions, and single beam irradiation experiments are also performed. Triple beam irradiation strongly enhances growth of cavities and swelling in pure vanadium compared with those under dual beam irradiation with He and single beam irradiation, whereas simultaneous implantation of H without He does not affect cavity growth and swelling. In V–5Cr–5Ti alloy, no cavities are detected without implantation of He. However, Ni, He and H triple beam irradiation is found to enhance swelling. These results are discussed in terms of dislocation and cavity evolution in the irradiated vanadium alloys.

Effect of strain rate on the tensile properties of unirradiated and irradiated V–4Cr–4Ti

Tensile tests were carried out on an annealed, unirradiated V–4Cr–4Ti alloy from RT to 850°C at strain rates ranging from 10−1 to 10−5 s−1. Below 300°C, where interstitial solutes are relatively immobile, deformation is homogeneous, and the strain rate sensitivity (SRS) of the yield and flow stress is positive. Between 300°C and 700°C, the formation of solute atmospheres at locked dislocations results in dynamic strain-aging (DSA), deformation becomes heterogeneous, and the SRS of the flow stress is negative; in this regime the lower yield stress is independent of strain rate. Above 700°C, substitutional solutes are also mobile, DSA declines, and the material enters a power law creep regime in which the SRS becomes positive again. Following neutron irradiation to 0.5 dpa at temperatures ⩽400°C, severe flow localization occurs due to the high number density of 〈110〉 and 〈111〉 loops. However, above 400°C, strain hardening capacity returns but without the Lüders extension. At 500°C, after several percent plastic deformation, DSA occurs as interstitial solutes are released from the defect structure.

A physically based constitutive model for a V–4Cr–4Ti alloy

A constitutive model for low-to-intermediate temperatures, strains, and strain rates is developed for the program heat of V–4Cr–4Ti. The basic form of the model is derived from more general dislocation-based models of yield stress and strain hardening. The physically based forms are fit to a database derived from tensile tests carried out over a wide range of temperatures and strain rates. Yield and post-yield strain-hardening contributions to the flow stress are additive. The yield stress has both thermally activated and athermal components. The former is described by a two-mechanism activated dislocation slip model, with contributions that appear to arise from both lattice friction (at lower temperatures) and dislocation pinning by interstitial impurities (at higher temperatures). The yield stress data can be correlated using a strain rate-compensated temperature. The model uses a temperature-weighted average of the two mechanisms. Post-yield strain hardening was found to be approximately athermal. Strain hardening is fit to a two-component modified Voce-type saturating flow stress model. The constitutive model is also used to determine the flow stability limits as estimates of uniform tensile strains. The relatively compact, but mechanism-based, semi-empirical model has a number of both fundamental and practical advantages that are briefly outlined.

On the mechanisms and mechanics of fracture toughness of a V–4Cr–4Ti alloy

Effects of irradiation and crossrolling on fracture toughness–temperature curves of the program heat of the V–4Cr–4Ti were evaluated. The alloy undergoes normal stress controlled cleavage fracture at low temperatures and ductile tearing fracture at higher temperature. It appears that crossrolling lowers the cleavage transition temperature. A critical stress–critical-stressed area cleavage criterion is used to model toughness–temperature curves in the cleavage transition, including the effect of specimen size. Irradiation induced increases in the transition temperature are accurately predicted by an equivalent yield stress model. Ductile tearing toughness is also degraded by irradiation, correlating with reduced uniform strains measured in tensile tests. Large cleavage transition temperature shifts and ductile toughness decreases may limit the application of V–4Cr–4Ti alloys for service in low-to-intermediate temperature irradiation environments.

Tensile and impact properties of V–4Cr–4Ti alloy heats 832665 and 832864

Two large heats of V–4Cr–4Ti alloy were produced in the US in the past few years. The first, 832665, was a 500 kg heat procured by the US Department of Energy for basic fusion structural materials research. The second, 832864, was a 1300 kg heat procured by General Atomics for the DIII-D radiative divertor upgrade. Both heats were produced by Oremet-Wah Chang (previously Teledyne Wah Chang of Albany). Tensile properties up to 800°C and Charpy V-notch impact properties down to liquid nitrogen temperature were measured for both heats. The product forms tested for both heats were rolled sheets annealed at 1000°C for 1 h in vacuum. Testing results show the behavior of the two heats to be similar and the reduction of strengths with temperature to be insignificant up to at least 750°C. Ductility of both materials is good in the test temperature range. Impact properties for both heats are excellent – no brittle failures at temperatures above −150°C. Compared to the data for previous smaller laboratory heats of 15–50 kg, the results show that scale-up of vanadium alloy ingot production to sizes useful for reactor blanket design can be successfully achieved as long as reasonable process control is implemented (H. Tsai, et al., Fusion Materials Semiannual Progress Report for Period Ending 30th June 1998, DOE/ER-0313/24, p. 3; H. Tsai, et al., Fusion Materials Semiannual Progress Report for Period Ending 31st December 1998, DOE/ER-0313/25, p. 3).

Effect of low temperature irradiation on the mechanical properties of ternary V–Cr–Ti alloys as determined by tensile tests and shear punch tests

Tensile tests and shear punch tests were performed on a variety of vanadium alloys that were irradiated in the Advanced Test Reactor (ATR) at temperatures between 200°C and 300°C to doses between 3 and 5 dpa. Tests were performed at room temperature and the irradiation temperature. The results of both the tensile tests and the shear punch tests show that following low temperature irradiation, the yield strength (YS) increased by a factor of 3–4 while the ultimate strength increased by a factor of approximately 3. Uniform elongation (UE) and tensile reduction in area show that the ductility diminishes following irradiation. The correlation between uniaxial ultimate strength and effective shear maximum strength was in excellent agreement with previous studies on other materials. Using the room temperature test data, the correlation between uniaxial YS and effective shear YS was in excellent agreement with previous studies on other materials.

NIFS program for large ingot production of a V–Cr–Ti alloy

The National Institute for Fusion Science (NIFS), in collaboration with Japanese industry, has initiated a research program for vanadium alloys, which will be highlighted by the production of a large V–4Cr–4Ti ingot. In this program, the technology and underlying science associated with vanadium alloy fabrication will be enhanced. Effort has been focused on the control of interstitial impurities such as C, O and N. Purification of the present commercial metal vanadium was carried out by improvement of the processes. Medium size (30 kg), high purity V–4Cr–4Ti ingots have been produced, designated as NIFS-HEAT-1, and a large size (200 kg) ingot is planned. In this study, the technology for fabricating large V–Cr–Ti ingots with ∼50 ppm C, ∼180 ppm O and ∼100 ppm N has been demonstrated.

Mechanical behavior and microstructural evolution of vanadium alloys irradiated in ATR-A1

An irradiation experiment has been done in the ATR-A1 to investigate irradiation behavior of vanadium alloys in the low temperature regime from 200°C to 300°C with damage levels of 3 to 4 dpa. In creep measurements, creep tubes of V–3Fe–4Ti–0.1Si with inner pressures up to 165 MPa did not rupture during irradiation. The effective strain rate of creep was below 0.2% dpa−1 and it showed the same tendency as V–4Cr–4Ti alloys. In Charpy impact tests, all specimens of V–4Cr–4Ti–0.1Si and V–3Fe–4Ti–0.1Si showed brittle behavior at room temperature and the DBTT increased to 60–150°C. The fracture surface showed cleavage. Tensile tests conducted both at room temperature and at the irradiation temperature showed significant irradiation hardening and brittle responses. TEM showed that high densities of tiny defect clusters were formed in V–Cr–Ti and V–Fe–Ti alloys. Precipitates could not be seen in specimens irradiated below 300°C, however, fine defect clusters are considered to be the origin of brittle behavior in V–Cr–Ti alloys irradiated at low temperatures.

Experimental test of structural materials activation in the IFMIF neutron spectrum

Activation cross sections contained in a new evaluated library, the intermediate energy activation file (IEAF-99), were tested in an integral experiment with a neutron spectrum very closely resembling the spectrum of the projected international fusion materials irradiation facility (IFMIF) by irradiating samples of SS-316 steel, F82H steel, pure vanadium, and V–4Ti–4Cr alloy. γ-Spectrometry of the samples yielded induced activities of more than 40 product nuclides. By comparison with calculations using IEAF-99 and the ALARA inventory code, calculation-over-experiment ratios were obtained, the majority of which is within a factor of 2 from unity.

Transition metal alloys: elastic properties and Peierls–Nabarro stresses

As an adjunct to alloying transition metals, mainly those in the second long row of the Periodic Table, with each other, published elastic properties have been analyzed and the Peierls–Nabarro (P–N) stresses computed for edge and screw dislocations. For binary alloys of Ti–Nb–Mo–Re, average tensile and shear elastic moduli, and Poisson's ratio were computed from the anisotropic elastic constants, c ij , which are also used to compute the energy factors for dislocations. From the latter, P–N stresses were computed for dislocations on the {110} and {112} planes. Computed values are compared with experimentally derived values for Nb and Mo. On the basis of measured values, the elastic constants are estimated for a series of Nb–Cr–Ti alloys of current interest.

Internal friction and Young’s modulus of V–Ti–Cr alloy before, during and after proton irradiation

Using a composite oscillator, the Young’s modulus and the internal friction of annealed, plastically bent and bent–straightened V–4Ti–4Cr alloy specimens have been measured, in a wide amplitude range, before, during and after proton irradiation. The proton energy and flux were 8 MeV and 10 12 p cm −2 s −1. The in situ dependencies on the proton beam current and temperature have been obtained. Softening and overheating (which looks like softening–strenghtening of the alloy) effects, which arise due to radiation damage, have been revealed. The influence of beam heating and defect accumulation–annihilation on the acoustic properties of the annealed and pre-strained samples, during and after proton irradiation, are briefly discussed.

Slip line formation during tensile straining experiments at ambient temperature in a Nb–Cr–Ti alloy

Tensile deformation experiments were conducted on a high fracture toughness experimental alloy 50Nb–13Cr–37Ti (at.%) at ambient temperature. Stress–strain curves were measured for large grained specimens in the laboratory and within the scanning electron microscope. Strain gages were used to measure macroscopic strain and stereoimaging was used to measure microscopic strains within several grains that formed slip lines. Proportional limit was about 850 MPa and strain hardening coefficient was about 0.05. Creep was encountered at stresses just above the proportional limit. One of the deformed specimens was subsequently examined by transmission electron microscopy. The stress–strain characteristics measured are compared to Nb–Al–Ti alloys.

Procedure development of laser welding of V–4Cr–4Ti alloy

V–4Cr–4Ti alloy is selected as the structure material for the first wall/blanket in a fusion power reactor. A systematic study was conducted to develop a laser welding procedure for fabrication of vanadium alloy for the first wall/blanket systems. A 1.6 kW pulsed Nd:YAG laser with fiber optic beam delivery was used to carry out the bead-on-plate welding on 4 mm thick V–4Cr–4Ti plates. The process parameters, such as laser schedule power settings, beam travel speed, and welding atmosphere control, and their effects on weld quality, such as weld depth, porosity, and oxygen uptake were studied. Results from metallurgical characterization of the welds are presented. An innovative laser welding procedure has been developed to obtain deep penetration, defect-free, and oxygen contamination-free welds.

Vanadium-base alloys for fusion first-wall/blanket applications

Vanadium alloys have been identified as a leading candidate material for fusion first-wall/blanket applications. Certain vanadium alloys exhibit favorable safety and environmental characteristics, good fabricability, high temperature and heat load capability, good compatibility with liquid metal coolants and resistance to irradiation damage. The current focus is on vanadium alloys with (3–9 wt%) Cr and (3–10 wt%) Ti with a V–4Cr–4Ti alloy as the reference composition. Substantial progress has been made in the development of vanadium alloys for the fusion first wall/blanket applications including production and welding, characterization of baseline properties, corrosion/compatibility, and effects of irradiation on the properties. This paper presents an overview of the development of vanadium alloys for fusion applications and a summary of key issues requiring further research.

Reduction of impurity levels of vanadium and its alloys for fusion application

Reduction of interstitial impurity (C, N and O) levels is essential for application of vanadium alloys to low activation structural materials of fusion reactors. This paper summarizes the effort of reducing the impurity levels of vanadium and vanadium alloys at the National Institute for Fusion Science (NIFS) Japan, program of fabricating high purity V–4Cr–4Ti alloy ingots.

電池材料 熱処理によるＴｉ‐Ｖ‐Ｃｒ系合金の組織変化とプロチウム吸蔵特性

Ti-V-Cr BCC solid solutions are known to absorb a high amount of protium. But, relations of their microstructures with their protium absorption-desorption properties are not systematically studied. The purpose of this study is to clarify the effects of V content and heat-treatment on the protium absorption-desorption properties of Ti-(0-35)at%V-Cr alloys.Addition of V to Ti-Cr alloys was effective in forming a BCC phase. However, with decreasing V content, a Laves phase was formed and an effective protium capacity decreased.Anealling at 1573 K was effective to flatten the plateau region and enlarge the effective protium capacity. A Ti-10V-55.4Cr alloy, which contained a laves phase in the as-cast state, became a BCC single-phase after annealing over 1573K. Even though the alloy contains only 10 at%V, this heat-treated alloy showed an effective protium capacity of 2.5 mass%H.

New V-based alloys with high protium absorption and desorption capacity

Abstract The hydrogen absorption properties of V–Zr–Ti–M(M=Fe, Mn, Ni) alloys were examined in order to develop the alloys with high hydrogen capacity. It was found that the best composition among the studied alloys is the V–Zr–Ti–Ni system. A suitable amount of the vanadium in the V–Zr–Ti–Ni system was studied in correlation with its hydrogen absorption properties, and turned out to be around 75 at.%V. Substitution with Zr improved the hydrogen absorption properties in forming the grain boundary network phases of C14 Laves phase. Heat treating the alloys (homogenize and quench) drastically improves the plateau region of PCT curves. Zr addition also improves the properties for V–Zr–Ti–Cr alloys.

Oxygen in Ti-(Cr, Mn)-Si icosahedral phases and approximants

Abstract Nearly single-phase icosahedral quasicrystals can be obtained in Ti-Cr and Ti-Mn alloys by rapid quenching, when the amounts of silicon and oxygen are carefully controlled. The role of phase stabilization by these elements, however, is unknown. Here, the essential role of both oxygen and silicon for the formation of phases with local icosahedral symmetry is demonstrated in these alloys. Rapidly quenched and as-cast samples of Ti-Cr-Si and Ti-Mn-Si alloys made without oxygen form primarily the β solid solution phase. In contrast as-cast alloys of Ti-Cr-Si containing more than 10 at.% oxygen and Ti-Mn-Si alloys made with more than 6at.% oxygen contain significant quantities of the 1/1 crystal approximant. Correspondingly, as-quenched alloys of these compositions form the icosahedral phase. Annealing studies of samples containing the 1/1 phase demonstrate that the oxygen has a thermodynamic rather than a kinetic role in phase stabilization. It is likely that the oxygen atoms bind with Ti atoms, form...