4Cr 4Ti Alloy Research Articles

The degradation of the corrosion resistance of sputter-deposited chromium–titanium alloys by nanoscale heterogeneity

The effect of heat treatment, inducing the formation of nanocrystalline two phase alloys on the corrosion resistance of sputter-deposited Cr–Ti alloys containing 22, 48, 60, and 72 at.% titanium has been studied in 6 M HCl. The as-deposited homogeneous Cr–Ti alloys are spontaneously passive in 6 M HCl, except Cr–72Ti alloy, although both chromium and titanium metals undergo active dissolution in this environment. The air-formed surface films on the alloys containing sufficient amounts of both chromium and titanium are highly protective, being responsible for spontaneous passivation. The heat treatment of the alloys leading to the formation of hcp titanium or bcc chromium with a grain size of 20–25 nm as well as Cr 2Ti is detrimental for the high corrosion resistance of these alloys. However, when Cr 2Ti and bcc solid solution of titanium containing ∼14 at.% chromium are formed, the high corrosion resistance of the as-deposited alloys is sustained. The high corrosion resistance of the sputter-deposited Cr–Ti alloys is largely reduced by the formation of less corrosion-resistant hcp titanium or bcc chromium phases, but is maintained when only the phases containing certain amounts of both elements are formed by heat treatment.

Fatigue failure analysis of V–4Ti–4Cr alloy

In the present work, the fatigue fracture and failure behavior of a V–4Ti–4Cr has been studied. Static tests were conducted to study the overloading behavior and to select the magnitude of the stress level for the fatigue studies. Fatigue tests were performed using single edge notched (SEN) specimens under tension–tension load control conditions. Fatigue crack propagation (FCP) data such as the crack length, number of cycles, and hysteresis loops were recorded to calculate the crack speed, the energy release rate, and the change in work expended on damage formation and dissipative processes within the material. Parameters characterizing the fatigue fracture resistance of V–4Ti–4Cr alloy, namely the specific energy of damage ( γ ′), and the dissipative coefficient ( β ′), were determined from the fatigue data using the modified crack layer (MCL) theory. Fracture surface examination using scanning electron microscopy (SEM) revealed ductile failure mechanisms under tensile overloading conditions. The fatigue fracture surface of the V–4Ti–4Cr consists of three distinct regions, corresponding to the threshold, stable and unstable crack propagation stages.

Reactions of hydrogen with V–Cr–Ti alloys

The interest in vanadium alloys for use as a first-wall material in fusion reactor applications has led to a study to determine the effects of hydrogen on the mechanical properties of certain V–Cr–Ti alloys. Hydrogen uptake by the alloys is a function of temperature and pressure. In the absence of increases in oxygen concentration, additions of up to 400 wpm hydrogen to V–4Cr–4Ti did not result in any significant embrittlement. However, when hydrogen approached 500 wppm, rapid embrittlement occurred at 325°C that was suggested to result from formation of a hydride phase upon cooling. When oxygen was added as well, either during or prior to hydrogen exposure, synergistic effects depending on grain size occurred that led to significant embrittlement by 35–130 wppm hydrogen. Because of its synergism with hydrogen and ubiquitous presence in most environments, oxygen pick-up remains as one of the major concerns for use of V–Cr–Ti alloys in fusion reactor applications.

Effects of the alloying element Cr on the burning behavior of titanium alloys

The alloys C (Ti–35V–15Cr) and Ti40 (Ti–25V–15Cr–0.2Si) exhibit good burn resistance. However, the influence of the alloying element Cr on the burning behavior is not clear. The effects of the alloying element Cr on the itanium burning behavior are studied by means of the DCSB method. The results reveal that the burn resistance is improved when the Cr content is larger than 10%, especially larger than 15%. However, Binary Ti–Cr alloys do not exhibit burn resistant behavior. Their burning product surface is cracked and porous. The interface between the burning products and the matrix is also cracked if the Cr content is less than 10%, which can not retard oxygen diffusion into the matrix. The interface of Ti–(>10)Cr alloys is enriched with Cr, and Cr-oxides are tenacious, which can retard the oxygen diffusion into the matrix and decrease the burning velocity. The burning products of Ti–Cr alloys are TiO2, TiO and TiCrO3.

Spinodal decomposition existence of the β Ti–Cr binary alloy: computer simulation of the real alloy system and experimental investigations

The spinodal decomposition of β Ti–Cr binary alloys system still questionable, since there are rare experimental data moreover simulation results for the real alloy system have not been found. Transmission electron microscopy (TEM) and quantitative computer simulations results based on Khachaturyan's diffusion equation have been employed to study the microstructure evolution occurring in the β Ti–Cr alloys. Our study results reveal that the metastable β undergoes a phase separation reaction through a spinodal decomposition. The coherent two phase fields show extremely fine plate-like precipitates lying parallel to {1 0 0} plane. Those precipitates are high elastically-induced from the first step of phase separation.

Internal friction and anelastic properties of vanadium and V–Ti–Cr alloys

Using an acoustic technique of composite oscillator, the effect of preliminary deformation on the amplitude dependence (amplitude range from 10 −6 to 3 × 10 −4) of ultrasound damping and the Young's modulus (resonance frequencies) for V and V–(4–10)Ti–(4–10)Cr alloys has been studied. The alloys differed in composition, manufacture technology and also irradiation type (protons, neutrons), environments and fluence were varied. Neutron irradiation (fast reactor, BR-10) of the samples was carried out in argon (irradiation temperature T I=410°C) and liquid lithium ( T I=480°C) environments. It has been found that, for anelastic deformations of the order of 10 −8–10 −6, irradiation may result in most cases in noticeable anelastic softening of alloys characterized by a Young's modulus decrease. The softening of the V–4Ti–4Cr alloy irradiated in Ar appeared to be the most significant. The experimental results of acoustic studies might be explained by changing the mobile dislocation point defect interaction, perhaps additionally influenced by existing internal stresses.

Fabrication of a 1200 kg ingot of V–4Cr–4Ti alloy for the DIII–D radiative divertor program

Vanadium–chromium–titanium alloys are attractive materials for fusion reactors because of their high-temperature capability and their potential for low neutron activation and rapid activation decay. A V–4Cr–4Ti alloy has been selected in the US as the current leading candidate vanadium alloy for future use in fusion reactor structural applications. General Atomics (GA), in conjunction with the Department of Energy's (DOE) DIII–D Program, is carrying out a plan for the utilization of this vanadium alloy in the DIII–D tokamak. The plan will culminate in the fabrication, installation, and operation of a V–4Cr–4Ti alloy structure in the DIII–D Radiative Divertor (RD) upgrade. The deployment of this vanadium alloy will provide a meaningful step in the development and technology acceptance of this advanced material for future fusion power devices. Under a GA contract and material specification, an industrial scale 1200 kg heat (ingot) of a V–4Cr–4Ti alloy has been produced and converted into product forms by Wah Chang of Albany, Oregon (WCA). To assure the proper control of minor and trace impurities which affect the mechanical and activation behavior of this vanadium alloy, selected lots of raw vanadium base metal were processed by aluminothermic reduction of high purity vanadium oxide, and were then electron beam melted into two high purity vanadium ingots. The ingots were then consolidated with high purity Cr and Ti, and double vacuum-arc melted to obtain a 1200 kg V–4Cr–4Ti alloy ingot. Several billets were extruded from the ingot, and were then fabricated into plate, sheet, and rod at WCA. Tubing was subsequently processed from plate material. The chemistry and fabrication procedures for the product forms were specified on the basis of experience and knowledge gained from DOE Fusion Materials Program studies on previous laboratory-scale heats and a large scale ingot (500 kg) of V–4Cr–4Ti alloy produced by WCA for Argonne National Laboratory (ANL). Charpy V-Notch (CVN) impact tests have been conducted on sheet material from the 1200-kg heat. The values are compared to data obtained on previous heats of the alloy.

Metallurgical bonding development of V–4Cr–4Ti alloy for the DIII-D radiative divertor program

General Atomics (GA), in conjunction with the Department of Energy's (DOE) DIII-D Program, is carrying out a plan to utilize a vanadium alloy in the DIII-D tokamak as part of the DIII-D Radiative Divertor (RD) upgrade. The V–4Cr–4Ti alloy has been selected in the U.S. as the leading candidate vanadium alloy for fusion applications. This alloy will be used for the divertor fabrication. Manufacturing development with the V–4Cr–4Ti alloy is a focus of the DIII-D RD Program. The RD structure, part of which will be fabricated from V–4Cr–4Ti alloy, will require many product forms and types of metal/metal bonded joints. Metallurgical bonding methods development on this vanadium alloy is therefore a key area of study by GA. Several solid-state (non-fusion weld) and fusion weld joining methods are being investigated. To date, GA has been successful in producing ductile, high-strength, vacuum leak-tight joints by all of the methods under investigation. The solid-state joining was accomplished in air, i.e., without the need for a vacuum or inert gas environment to prevent interstitial impurity contamination of the V–4Cr–4Ti alloy.

Microstructure of V–4Cr–4Ti alloy after low-temperature irradiation by ions and neutrons

Recent interest in the application of a V–4Cr–4Ti alloy in the ITER prompted an investigation of the effects of low-to-moderate temperature irradiation (<420°C) on the alloy's mechanical properties. Two sets of experiments were conducted. The effects of fast neutron irradiation to ≈4 dpa at 390°C were investigated in the X530 experiment in the EBR-II reactor. Irradiation with single (4.5-MeV Ni 2+) and dual ion beams (350-keV He + simultaneously with 4.5-MeV Ni 2+) complemented this study. TEM observations showed the formation of a high density of point-defect clusters and dislocation loops (<30 nm diameter) distributed uniformly in both types of specimens. Mechanical property testing of neutron irradiated material showed embrittlement of the alloy. The deformed microstructures were examined by TEM to determine the causes of embrittlement and revealed dislocation channels propagating through the undeformed matrix. The channels are the sole slip paths and they cause early onset of necking and loss of work-hardening. Based on a review of the available literature, suggestions are made for further research of slip localization in V–4Cr–4Ti alloys.

Effects of oxygen and oxidation on tensile behavior of V–4Cr–4Ti alloy

Vanadium-base alloys are potential candidates for applications such as the first wall and other structural components of fusion reactors, but a good understanding of the oxidation behavior of the alloys intended for elevated-temperature use is essential. We conducted a systematic study to determine the effects of time and temperature of air exposure on the oxidation behavior and microstructure of V–4Cr–4Ti alloy. Uniaxial tensile tests were conducted at room temperature and at 500°C on preoxidized specimens of the alloy to examine the effects of oxidation time and oxygen migration on maximum engineering stress and uniform and total elongation. The effect of preexposure of the specimens to environments with varying oxygen partial pressures on the tensile properties of the alloy was investigated. Extensive microstructural analyses of the oxygen-exposed/tensile-tested specimens were conducted to evaluate the cracking propensity for the alloy. In addition, tensile-property data for the alloy were correlated with oxygen pressure in the exposure environment, test temperature, and exposure time.

Analysis of V–Cr–Ti alloys in terms of activation of impurities

Vanadium alloys offer many advantages including reduced neutron activation. However, activation of V–Cr–Ti alloys is controlled by impurities. This study analyzes four impurity levels for the alloy V–4Cr–4Ti. The first is an existing commercial heat of the alloy and the others are based on impurity levels attainable by three processes. One process uses a dedicated production plant; another uses a chemical recrystallization process, and the remaining one uses state-of-the-art laboratory purification. It was assumed that the alloys were irradiated as first wall and blanket structures in a fusion reactor operating for four full power years at a wall loading of 5 MW y/m 2. It was found that the two purest alloys and, with special selection, the three purest alloys will meet the criteria for shallow land burial after a cooling time of 50 years. However, none of the alloys were found to meet the criteria for hands-on reprocessing after a cooling time of 100 years.

Influence of thermal treatment on helium trapping at fine-size precipitates in V–4Cr–4Ti

Thermal Helium Desorption Spectrometry (THDS) studies carried out on V–4Cr–4Ti alloy have revealed two types of sinks for helium: (1) helium–vacancy clusters nucleated at interstitially dissolved gas impurities, and (2) pre-existing traps, e.g. fine-size precipitates. The effect of thermal treatment on the precipitates was studied by pre-annealing of the samples before the THDS measurements. The pre-annealing was carried out in high vacuum <10 −7 Pa for 1 h in the temperature interval from 600 to 1700 K. It was observed that helium trapping into the pre-existing traps was dramatically suppressed for annealing at 1200–1300 K. However, a short annealing at 1400 K completely restored the efficiency of the pre-existing traps. Thermally induced modifications of the precipitate–host interface are held responsible for the observed effect.

Irradiation creep of vanadium-base alloys

A study of irradiation creep in vanadium-base alloys is underway with experiments in the Advanced Test Reactor (ATR) and the High Flux Isotope Reactor (HFIR) in US. Test specimens are thin-wall sealed tubes with internal pressure loading. The results from the initial ATR irradiation at low temperature (200–300°C) to a neutron damage level of 4.7 dpa show creep rates ranging from ≈0 to 1.2 × 10 −5/dpa/MPa for a 500-kg heat of V–4Cr–4Ti alloy. These rates were generally lower than reported from a previous experiment in BR-10. Because both the attained neutron damage levels and the creep strains were low in the present study, however, these creep rates should be regarded as only preliminary. Substantially more testing is required before a data base on irradiation creep of vanadium alloys can be developed and used with confidence.

Performance of V–4Cr–4Ti material exposed to the DIII-D tokamak environment

A series of tests is being conducted in the DIII-D tokamak to determine the effects of environmental exposure on a V–4Cr–4Ti vanadium alloy. These tests are part of the effort to build and install a water-cooled vanadium alloy structure in the DIII-D radiative divertor upgrade. Data from the test series indicate that the performance of the V–4Cr–4Ti alloy would not be significantly affected by environmental exposure. Interstitial absorption by the material appears to be limited to the surface, and neither the tensile nor the impact properties of the material appear to be affected by the exposure.

Tensile properties and fracture behaviour of V–Cr–Ti alloys after neutron irradiation at 330°C

Results of irradiation effects in the fast neutron spectrum of the BOR-60 reactor on the mechanical properties and fracture behaviour of vanadium as well as on binary and ternary alloys (including V–4Cr–4Ti and V–5Cr–10Ti) are presented. The irradiation was carried out at 330°C in a 7Li isotope environment to a damage dose of 18 dpa. It is shown that all alloys without exception experienced severe radiation embrittlement. Most specimens failed without discernible traces of ductile strain at room temperature. At a test temperature of 350°C, ductility was observed but the total elongation, as a rule, did not exceed 1%. The fracture was mixed: transgranular brittle cleavage and ductile dimple rupture. As the test temperature was increased from 20°C to 350°C, the brittle fracture fraction decreased. At 600°C only ductile fracture was observed. The behaviour of V–4Cr–4Ti alloys of American and Russian production is analyzed in the 275–530°C temperature range, where a shift in behaviour from brittle to ductile failure is observed.

Development of techniques for welding V–Cr–Ti alloys

Welding vanadium alloys is complicated by interstitial impurity introduction and redistribution at elevated temperatures. Gas tungsten arc (GTA) welding, which will probably be required for the fabrication of large tokamak structures, must be done in a glove box environment. Welds were evaluated by Charpy testing. GTA welds could be made with a ductile to brittle transition temperature (DBTT) of 50°C with a post-weld heat treatment (PWHT) or by using a heated Ti getter system on the glove box to reduce interstitial contamination. Titanium-O,N,C precipitates in the fusion zone were found to transform to a more oxygen-rich phase during a PWHT of 950°C/2 h. Hydrogen was found to promote cleavage cracking following welding in cases where the atmosphere was contaminated. Grain size and microstructure also affected weld embrittlement.

Temperature dependence of the radiation damage microstructure in V–4Cr–4Ti neutron irradiated to low dose

Transmission electron microscopy (TEM) was performed on a V–4Cr–4Ti alloy irradiated to damage levels of 0.1–0.5 displacements per atom (dpa) at 110–505°C. A high density of small faulted dislocation loops were observed at irradiation temperatures below 275°C. These dislocation loops became unfaulted at temperatures above ∼275°C, and a high density of small Ti-rich defect clusters lying on {0 0 1} planes appeared along with the unfaulted loops at temperatures above 300°C. Based on the TEM and tensile measurements, the dislocation barrier strengths of the faulted dislocation loops and {0 0 1} defect clusters are ∼0.4–0.5 and 0.25, respectively. This indicates that both types of defects can be easily sheared by dislocations during deformation. Cleared dislocation channels were observed following tensile deformation in a specimen irradiated at 268°C.

Revision of the tensile database for V–Ti and V–Cr–Ti alloys tested at ANL

The database for the tensile properties of unirradiated and irradiated V–(0–18) wt%Ti and V–(4–15) wt%Cr–(3–15) wt%Ti alloys tested at Argonne National Laboratory (ANL) has been reviewed and, when necessary, revised. A consistent methodology, based on ASTM terminology and standards, has been used to reanalyze the unpublished load vs. displacement curves for 162 unirradiated samples and 91 irradiated samples to determine revised values for yield strength (YS), ultimate tensile strength (UTS), uniform elongation (UE), and total elongation (TE). The revised values for UE and TE are lower by 5 ± 2% (strain) and 4 ± 2% (strain), respectively, than previously reported values, which included displacements due to machine compliance. Revised values for YS and UTS are consistent with previously published values in that they are within the scatter usually associated with these properties.

Research and development on vanadium alloys for fusion applications

The current status of research and development on unirradiated and irradiated V–Cr–Ti alloys intended for fusion reactor structural applications is reviewed, with particular emphasis on the flow and fracture behavior of neutron-irradiated vanadium alloys. Recent progress on fabrication, joining, oxidation behavior, and the development of insulator coatings is also summarized. Pronounced flow localization and loss of strain hardening capacity with uniform elongations <1% generally occurs in vanadium alloys for irradiation temperatures below ∼400°C (0.31 T M). These changes in tensile properties for T irr < 400°C are generally accompanied by large increases in the ductile-to-brittle transition temperature measured under both dynamic and quasi-static loading conditions. The relationships between radiation hardening, flow localization, strain rate, and fracture properties are examined. The irradiated mechanical properties at temperatures between 430°C and 650°C are acceptable for most structural applications. Further work is needed to determine how far the allowable lower and upper operating temperature limits can be expanded beyond the 430–650°C range.

A comparative study of the theory of the O-lattice and the phenomenological theory of martensite crystallography to phase transformations

The approaches of the phenomenological theory of martensite crystallography (PTMC) and the theory of the O-lattice (TOL) to describe the elements of a phase transformation are compared for an O-line transformation strain. A number of simple relationships between the O-lattice treatment and the PTMC have been established. The displacement associated with the long range strain component is shown to be identical in both treatments when the planes defined by the path of the O-lines are correlated to the slip plane of the dislocations. A characteristic feature of the TOL approach is that this displacement is allowed to vary for a fixed habit plane. When the displacement is associated with an interfacial step, it is equivalent to a transformation dislocation, whose Burgers vector is explicitly expressed in terms of a D-lattice vector in the TOL formula. This method is extended to consider the tilt or tent-shaped surface relief effects observed at precipitates in a Ti–Cr alloy. Good agreement is found between the predictions of this approach and the measurements reported in the literature.