4Cr 4Ti Alloy Research Articles

Radiochemical reprocessing of V–Cr–Ti alloy and its feasibility study

An extraction scheme for radiochemical reprocessing of an activated vanadium–chromium–titanium alloy after a fusion reactor decommissioning was developed and checked experimentally. It is based on extraction of V, Cr and Ti freed of activation products from the alloy dissolved in nitric acid. The solution of di-2-ethyl-hexyl-phosphoric acid (D2EHPA) in a hydrocarbon solvent (dodecane) serves as an extractant. It takes 50 extraction steps to recover V, Cr and Ti down to an effective dose rate <12.5 μSv/h, permitting the refabrication of these metals without biological shielding from ionizing radiation. Technical and economic analysis suggests that the reprocessing alternative is more attractive economically than the burial of spent V–Cr–Ti alloy components.

Materials to deliver the promise of fusion power – progress and challenges

High-performance reduced-activation materials are crucial for fulfillment of the promise of fusion to provide safe, economical, and environmentally acceptable energy. Three reduced activation structural materials have emerged as promising candidates, based on 8–9Cr ferritic/martensitic steels, V–Cr–Ti alloys, and SiC/SiC composites. Due to advances in understanding how to control and engineer the nanoscale phase stability required for harsh neutron irradiation environments, these reduced activation materials have unirradiated properties that are superior to commercially available analogs. Perhaps the most important accomplishment to date from fusion materials research is the radiation effects knowledge base. Models of radiation effects and supporting experiments highlight the critical role of helium production on the microstructural stability and lifetime of irradiated materials. The proposed International Fusion Materials Irradiation Facility (IFMIF) would fill a critical need for fusion materials development.

Effects of internal strains on hardness of nanocrystalline Al–Fe–Cr–Ti alloys

The effects of internal strain on the microhardness of nanocrystalline metals were investigated using an Al 93Fe 3Ti 2Cr 2 alloy prepared via mechanical alloying. The results clearly indicate that the hardness of the nanocrystalline Al alloy decreases with decreasing the internal strain at a given grain size, even though dislocation activity within the grain interior is absent in all cases.

Precipitation hardening of Cu–Ti–Cr alloys

Precipitation hardening of Cu-4Ti-1Cd alloy has been studied using hardness measurements and transmission electron microscopy. This alloy exhibited hardness of 238 Hv in solution treated (ST) condition and attained peak hardness of 318 Hv after ageing at 450°C for 40 h. Electrical conductivity of Cu-4Ti-1Cd alloy increased from 5.7 %IACS (International Annealed Copper standard) in ST condition to 8.9 %IACS on ageing at 450°C for 16 h. This alloy exhibited markedly higher yield strength (751 MPa in the peak-aged condition) compared to Cu-4.5Ti alloy but the increase in UTS due to cadmium addition was less significant. The higher yield strength of ternary alloy in peak aged condition is due to the solid solution strengthening of cadmium as well as the presence of β′-Cu4Ti precipitate. On over-ageing the alloy showed a decrease in hardness as a result of formation of equilibrium precipitate, β-Cu3Ti. Optical microscopy reveals single phase with equiaxed grains in solution treated condition. A coherent, metastable phase β′-Cu4Ti is responsible for high strength and hardness in peak aged condition. The over-ageing in this alloy shows the formation of cellular structure at the grain boundaries of the matrix phase.

Hydrogenation properties and structure of Ti–Cr alloy prepared by mechanical grinding

The C14 intermetallic TiCr 1.9 compound was prepared by arc melting, annealed at 1623 K in helium atmosphere for 8 h, and then quenched in water. The X-ray diffraction profiles showed that the crystal structure transformed from C14 to bcc after mechanical grinding for 18 h. The hydrogenation properties of the sample were examined by pressure–composition isotherm measurements. This sample with bcc structure reacted with hydrogen by maintaining the bcc structure. The maximum hydrogen content was found to be about 0.5 H/M at 273 K and 10 MPa, where H/M means the atomic ratio of hydrogen to metal. This hydrogen content was smaller than that of other bcc structure alloys. The sample prepared by MG consists of nanosized grains of polycrystalline TiCr 1.9 and amorphous-like structure regions, judging from the bright field image and the corresponding selected area diffraction pattern. The hydrogen storage capacity is reduced due to the amorphous-like structure.

Compressive behavior of an extruded nanocrystalline Al–Fe–Cr–Ti alloy

The investigation of the compressive behavior of an extruded nanocrystalline Al–Fe–Cr–Ti alloy reveals that the ductility of the alloy is determined by the status of the oxide film at the prior powder particle boundaries. When properly extruded, the alloy exhibits more than −0.45 compressive strain at 25 °C coupled with superior specific strengths up to 400 °C. The enhanced strength is attributed to grain refinement, intermetallic precipitation, and solid solution.

Corrosion Behavior and Surface Characterization of Ti-20Cr Alloy in a Solution Containing Fluoride

The purpose of this study was to investigate the correlation between corrosion resistance and surface composition of an experimental Ti-20 mass% Cr casting alloy in a saline solution containing fluoride. The alloy had a greater resistance to corrosion in a fluoride-containing saline solution than did commercially pure titanium. However, with confirmed dissolution of titanium and chromium, it meant that the fluoride in the saline solution corroded the alloy slightly. X-ray photoelectron spectroscopy analysis revealed that the surface composition of the alloy consisted of titanium and chromium oxides containing hydroxide. The [Ti]/([Ti] + [Cr]) ratio in the surface oxide film decreased when immersed in fluoride-containing saline solution, that is, the surface oxide film became chromium-rich oxide. Therefore, the alloy obtained good corrosion resistance to fluoride due to formation of a chromium-rich oxide film.

Recrystallization and precipitation behavior of low-activation V–Cr–Ti alloys after cold rolling

Recrystallization and precipitation behavior after cold working were investigated for three V–4Cr–4Ti alloys, NIFS-HEAT-1, NIFS-HEAT-2 and US 832665, which contain different levels of impurities. The decrease in hardness and the initiation of recrystallization against test temperature do not depend on the oxygen level. However, the rate of grain growth decreased with the increase in the impurity levels. Bimodal distribution of the precipitates was observed after annealing at 1273 K. The large precipitates were Ti-rich precipitates, and the small ones were composed mainly of Ti, C and O (Ti–C–O precipitates). The impurities were dissolved from Ti–C–O precipitates above 1273 K, and the level of impurities in the matrix increased and resulted in the increase in the hardness. Based on the results, the annealing temperature for V–4Cr–4Ti alloys is recommended not to exceed 1273 K for the purpose of maintaining good mechanical properties such as tensile and impact properties.

Formation of periodic microstructures involving the L1 2 phase in eutectic Al–Ti–Cr alloys

The temperature–concentration parameters defining the existence of the eutectic transformation L⇄L1 2+β at the Al-rich corner of the Al–Ti–Cr phase diagram have been studied. The temperature of this transformation decreases from 1275 to 1250 °C with decreasing titanium content in the alloy, and its temperature interval is at most 10 °C. The univariant eutectic transformation line L⇄L1 2+β was constructed, whose coordinates on the concentration triangle are defined by the equation (in at.%): C Al=95.48–3.45 C Ti + 0.068 ( C Ti) 2. As a result of this transformation, periodic microstructures consisting of lamellae and fibres of the cubic L1 2 and β phases are produced. Transformations in the solid state which occur during cooling lead to the precipitation of intermetallic compounds from the β-solid solution: TiAlCr (structural type C14) or AlCr 2 (structural type C11 b). The position of the boundary at which the change of precipitating phases occurs was determined. The analysis of mechanical properties shows that the transition from single-phase L1 2 aloys to eutectic microstructures is accompanied by enhancement of both strength and plasticity, while retaining a high elastic modulus.

Compositional dependence of phase stability of γ phase formed in vanadium alloys

Alloying effects have been investigated systematically on the stability of the γ phase (VH 2) in various binary V–M and ternary V–Ti–M alloys, where M is a 3d transition metal. It was found that Ti and M in ternary V–Ti–M alloys operate almost independently of each other on the stability of the γ phase. It was also shown that the logarithm of the plateau pressure changes almost linearly with the amount of alloying element, M, in dilute binary V–M alloys. Using this linear relationship, the plateau pressures can be estimated quantitatively even in concentrated V–Ti–Ru and V–Ti–Cr alloys.

Improvement of initial activation of a Ti–Cr alloy by surface modification with potassium (K)

The alkaline treatment of the surface of a Ti–Cr hydrogen storage alloy was made by sticking potassium (K) metal onto the alloy block surface or by milling the alloy powder with K metal blocks instead of the use of KOH solution. This dry surface treatment was found effective to reduce the work function of electrons of the alloy surface and to ease the rate of hydrogen dissociation, resulting in an accelerated activation rate. The K atoms were found distributed in the surface oxide layer; however, X-ray photo spectroscopy (XPS) analyses in the vicinity of Fermi level suggest rather the induction of a metallic feature of the surface modified by K atoms.

Appearance of icosahedral atomic clusters in the formation of theMgCu2-type structure from the bcc structure in Ti-Cr alloys

The ${\mathrm{Cu}}_{2}\mathrm{Mg}$-type structure, called the C15 structure, is one of the covalency-involved structures in alloys and is characterized by a three-dimensional network of the covalent tetrahedra of small majority atoms. In order to understand the formation of covalent bonds in alloys, the crystallographic features of the C15 structure in the $(\mathrm{bcc}\ensuremath{\rightarrow}\mathrm{hcp}+\mathrm{C}15)$ reaction of the Ti-(30- and 40-at. %) Cr alloys have been examined by transmission electron microscopy. It was found that the C15 structure was formed from the metallic bcc structure via the formation of icosahedral atomic clusters. Based on the shape of the icosahedral cluster, the cluster should be produced as a result of the local formation of covalent bonds along one of the $〈110{〉}_{B}$ directions. It can thus be said that, in relation to the formation of the covalency-involved C15 structure from the metallic bcc structures, the local covalent bonds between two Cr atoms are first formed in the bcc matrix, and then that the local bonds are developed into the three-dimensional network of the covalent tetrahedra consisting of four Cr atoms.

Microstructure and hardness of nanostructured Al–Fe–Cr–Ti alloys through mechanical alloying

This study investigates the microstructural and compositional evolution of Al 93Fe 3Ti 2Cr 2 alloys prepared via mechanical alloying (MA) starting from elemental powders. The potential of the MA-processed Al 93Fe 3Ti 2Cr 2 alloy for structural applications at both ambient and elevated temperatures has also been studied. It is found that the nanocrystallization process of the Al alloy proceeds via an initial increase followed by a subsequent decrease in dislocation density to form nano-grains with a dislocation-free interior. The difficulty with which to form supersaturated Al-based solid solutions depends on alloying elements with Fe being the easiest element and Cr the most difficult one to form supersaturated Al-based solid solutions. The hardness of MA-processed Al 93Fe 3Ti 2Cr 2 alloys at the as-milled condition can be twice that of 6061-Al alloys and higher than that of the cold drawn 1045 steel. The enhanced strengthening is attributed to grain refinement, formation of supersaturated solid solutions, and work hardening due to ball milling. After exposure to high temperatures (up to 450 °C) the MA-processed Al 93Fe 3Ti 2Cr 2 alloy still possesses superior room-temperature hardness due to strengthening of intermetallic precipitation and the retention of fcc-Al nano-grains.

Ti–Cr–X protium absorbing alloys with high protium content for fuel-cell

The effects of additional elements to the Ti–Cr alloys on their protium absorption–desorption properties were investigated. Although Ti–Cr alloys with a b.c.c. structure is expected to exhibit a high protium content, it is difficult to obtain single b.c.c. phase quenching from the high temperature. The effects of additional elements such as V, Mo, W, Nb, Ta, Ru, Rh, Re, Os, Ir etc. on stabilizing b.c.c. phase in the alloys were investigated. Then, it was found that the additions of V, Mo, W, Ru, Rh, Re, Os, Ir effectively stabilized b.c.c. phases. But the addition of Nb and Ta preferentially formed Laves phases. Specially, the addition of Mo and Ru is effective to stabilize the b.c.c. phases with high protium content. The Ti–Cr–(1∼2.5%)Mo alloys show almost same capacity of protium as that reported in Ti–Cr–(5–7.5)V alloys. The addition of Ru to Ti–Cr alloys also shows the capacity of 3.0 mass% protium. These V-free Ti–Cr alloys will be also suitable for the future applications.

Wetting behavior of Co based active brazing alloys on SiC and the interfacial reactions

The wetting behavior of Co-based alloys on silicon carbide (SiC) was investigated by the sessile drop method. In vacuum at 1493 K for 10 min, the CoFeNi(Si, B)–(8–15)Cr–(14–21)Ti alloy exhibits good wettability on SiC, but after cooling fracture occurred within the SiC. The interfacial reactions were studied. The newly designed CoNi(Si, B)–Cr–Ti alloy is a promising brazing alloy for SiC.

Suppression of hydrogen absorption to V–4Cr–4Ti alloy by TiO 2/TiC coating

Titanium oxide film was coated on the surface of a V–4Cr–4Ti alloy to reduce hydrogen absorption at low temperature region. Titanium oxide was deposited by rf reactive magnetron sputtering with a titanium target using oxygen as sputter gas. The film consisted of a mixture of TiO 2 and TiC. The content of TiO 2 was approximately 80%. The atomic composition remained the same in the temperature range below 873 K. The film thickness also remained the same in the temperature region below 773 K. At the temperature higher than 973 K, the oxygen concentration decreased due to diffusion into the bulk of V-alloy, and the carbon concentration increased due to diffusion from the bulk of V-alloy. The hydrogen absorption rate of Ti-oxide coated V-alloy decreased with increase of the film thickness. In the case of the film with the thickness of 0.5 μm, the absorption rate was much smaller than that of non-coated V-alloy at the absorption temperature of 573 K.

Hydrogen solubility in V–4Cr–4Ti alloy

A systematic investigation was conducted to provide an accurate determination of the hydrogen solubility in the V–4Cr–4Ti alloy at temperatures in the range 400–600 °C, which is of primary interest for fusion applications. Results have been obtained by the method of hydrogen absorption and desorption into flowing helium with controlled concentrations of hydrogen to provide accurate measurements of the Sieverts constants for the alloy. Results obtained in this investigation indicate that the temperature dependent Sieverts constant for the V–4Cr–4Ti alloy is very similar to that for unalloyed vanadium. The increase in hydrogen solubility produced by the titanium is essentially balanced by the decrease in hydrogen solubility produced by the chromium.

High temperature performance of highly purified V–4Cr–4Ti alloy, NIFS-Heat1

High temperature performance tests including tensile, creep and creep-fatigue tests were conducted using specimens from a large ingot of highly purified V–4Cr–4Ti alloys, NIFS-Heat1. The obtained tensile data of NIFS-Heat1 alloys agree well with previous V–4Cr–4Ti alloys data. The most significant effect for the impurity reduction in NIFS-Heat1 alloys was the higher temperature for the appearance of the dynamic strain hardening compared to previous heats of V–4Cr–4Ti. From uniaxial creep tests, a creep-stress exponent of the NIFS-Heat1 alloy is about 5–8 and the characteristic of creep mechanism is the dislocation creep in pure metals. The apparent activation energies of creep deformation ranged from 180 to 210 kJ/mol at a temperature regime from 750 to 800 °C. No significant degradation due to fatigue behavior was observed during creep-fatigue tests. The creep-fatigue behavior was superior to the load-constant creep behavior in the same stress and temperature conditions at a lower stress condition.

Point defect behavior in electron irradiated V–4Cr–4Ti alloy

To investigate the migration behavior of interstitials and vacancies in vanadium alloy V–4Cr–4Ti, high purity V alloys were irradiated with 2 MeV electrons using a high voltage electron microscope (HVEM) and 28 MeV electrons using a electron linear accelerator. Interstitial type dislocation loops formed quickly in all cases of HVEM irradiation. The saturation density of the dislocation loops was independent of irradiation temperature and defect production rate at temperatures in the range of 323–448 K. However, the logarithm of saturated dislocation loop density was inversely proportional to the irradiation temperature, and dislocation loop density was directly proportional to the square root of the defect production rate above 448 K. The dissociation energy of the interstitial-impurity complex in V–4Cr–4Ti alloy was estimated to be 1.1 eV from the HVEM experiment. It was found from positron lifetime measurements that the vacancies form vacancy clusters at 373 K.

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

As a first step to demonstrate the viability of using vanadium-base alloys for structural applications in fusion devices, six welded upper radiative divertor baffle support brackets made from a V–4Cr–4Ti alloy were installed in the DIII-D tokamak. To determine the effects of exposure to the tokamak environment, lead tests are being conducted on parent metal and weldment specimens. One of the issues to be addressed is whether excessive hydrogen uptake may occur to cause material embrittlement. Some of the lead tests have been completed. Data from samples exposed to up to four DIII-D operating cycles (≈4 years) indicate that the performance of the V–4Cr–4Ti alloy would not be significantly affected by the exposure. Brittle cleavage fractures are noted near the surface of some of the impact specimens, but only at very low (<−150 °C) test temperatures. Above −150 °C, all fractures are ductile.