4Ti Alloy Research Articles

Structure–property relationships during high temperature deformation of Cu–4·5Ti alloy

A Cu–4·5Ti (wt-%) alloy was subjected to hot compression tests at temperatures ranging from 750 to 900°C and strain rates from 100 to 10-3 s-1. A hardness of 180 HV10 was observed after deformation at a low temperature of 750°C, at small as well as large strain rates, whereas the alloy deformed at high temperature (900°C) exhibited high hardness values, i.e. 300 HV10 at a strain rate of 100 s-1 and 280 HV10 at 103 s-1. Furthermore, a reduction in grain size from 110 µm at a strain rate of 103 s-1 to ∼55 µm at 100 s-1 was observed when the alloy was deformed at 900°C. Optical and SEM observations revealed a lamellar structure at 750°C, while equiaxed grains with some lamellar growth were seen in the alloy deformed at 900°C, at both strain rates. The difference in behaviour is attributed to the presence of equilibrium precipitate β-Cu3Ti at 750°C, whereas it is almost absent at 900°C. The high hardness observed at 900°C is due to the formation of fine scale precipitation during air cooling of the deformed alloy.

Effect of hydrogen accumulation on mechanical property and microstructure of V–Cr–Ti alloys

Pure vanadium, V–5Cr, V–5Ti and V–5Cr–5Ti alloys doped with different amounts of hydrogen were tested under tension and observed under TEM in order to clarify the interaction between hydrogen and vanadium. Hydrogenated samples tested under tension showed a typical hydrogen-embrittlement behavior. TEM observation suggests that the embrittlement is caused by hydrogen diffusion at the triple point of grain boundaries, hydride formation and the subsequent generation of dislocations and micro-cracks.

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.

Grain boundary chemistry and heat treatment effects on the ductile-to-brittle transition behavior of vanadium alloys

The ductile-to-brittle transition (DBTT) behavior of vanadium alloys currently being developed for fusion power systems is sensitive to thermo-mechanical processing variables and history. Factors which contribute to this sensitivity are (1) pickup of interstitial impurities such as oxygen, nitrogen and carbon during heat treatments and elevated temperature forming operations, (2) the final grain size achieved, (3) removal of impurities from solid solution due to precipitation reactions, and (4) segregation of impurities to grain boundaries. Previous work on a V–5Cr–5Ti (Heat No. 832394) alloy suggested that sulfur segregation or precipitation during final mill annealing may play a role in determining DBTT behavior. The effect of heat treatment on grain boundary chemistry and Charpy impact behavior was investigated using a production-scale heat of V–4Cr–4Ti (Heat No. 832665). Specimens were examined with Auger electron spectroscopy to characterize grain boundary microchemistry for correlation with Charpy impact test results obtained from one-third size specimens.

Solid state bonding of beryllium to copper and vanadium using transition layers

Beryllium is a material under consideration for divertor surfaces in the international thermonuclear experimental reactor (ITER). Since Be itself is not a suitable structural material for constructing the divertor, it needs to be bonded to other materials with sufficiently high thermal conductivity, which at the same time satisfy structural requirements such as adequate strength and fracture toughness. Bonding of Be to other materials is usually accompanied by (a) mismatches in thermal expansion and/or (b) metallurgical incompatibilities. In an attempt to minimize the thermal expansion mismatch we employed Fe or Ni transition layers for the bonding of Be to a copper alloy by hot isostatic pressing. To alleviate the thermodynamic incompatibility between Be and most other metals, thin Ag foil (130 μm) was used as a reaction barrier. Other experiments involved bonding Be (via an Ag reaction barrier) to a V–5Cr–5Ti (wt%) alloy. An Al–Be transition layer for bonding Be to a copper alloy was also explored. The microstructures of the interfaces were examined by optical and scanning electron microscopy. Shear tests carried out with Cu/Fe/Ag/Be and V/Ag/Be specimens indicated average room temperature shear strengths of 52 and 78 MPa, respectively. Fracture occurred usually at the Ag/Be interfaces, which were therefore the weakest link in the bonded specimens.

Oxidation kinetics and microstructure of V-(4–5) wt% Cr-(4–5) wt% Ti alloys exposed to air at 300–650°C

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 and V5Cr5Ti alloys. All samples were from 1 mm thick cold-rolled sheets, and each was annealed in vacuum at 1050°C for 1 h prior to high-temperature exposure. Different samples from each alloy were heated in ambient air at 500°C for times ranging from 24 to ≈2000 h, and in a thermogravimetric analysis (TGA) apparatus at temperatures ranging from 300 to 650°C. Models describing the oxidation kinetics, oxide type and its thickness, alloy grain size, and depth of oxygen diffusion in the substrate alloy were determined for the two alloys and compared. The results showed that the oxide layers that formed on the surfaces of both alloys in air at 300–650°C are protective, and that the V5Cr5Ti alloy is slightly more oxidation-resistant than the V4Cr4Ti alloy.

Evaluation of flow properties in weldments of vanadium alloys using novel indentation technique

A new approach, known as automated ball indentation testing, was successfully employed to determine the flow properties of gas tungsten arc and electron beam welds of V–4Cr–4Ti and V–5Cr–5Ti alloys. The results showed clear distinction among the properties of the fusion zone, heat affected zone, and base metal in both welds of the two alloys. The two welds of both vanadium alloys in the as welded condition showed strengthening (as well as hardening) of fusion zone and heat affected zone (compared with base metal), with the fusion zone having higher strength than the heat affected zone. However, the gas tungsten arc welds of both alloys, after a post-weld heat treatment of 950°C for 2 h, showed a recovery of properties. These measurements correlated reasonably well with the reported recovery of the Charpy impact properties of the welds after the post-weld heat treatment.

Low-activation properties of novel Cr-based materials for fusion reactors

Due to the low neutron-induced radioactivity of their main constituent, two novel Cr-based structural materials are examined, from the low-activation properties viewpoint, as candidate first-wall materials for fusion reactors. The two alloys are Cr 5Fe 1Y 2O 3, and Cr 44Fe 5Al 0.3Ti 0.5Y 2O 3. Their low-activation properties have been compared with those of some reference materials, in particular the austenitic steel AISI 316L, the Mn-based reduced-activation steel OPTSTAB, a V 5Ti alloy and a ceramic matrix composite (SiC/SiC). Impurities and tramp elements have been included in the calculations. Irradiation conditions envisaged for the SEAFP (Safety and Environmental Assessment of Fusion Power) study have been adopted. Advantages of the proposed materials with respect to steels, either reference alloys or reduced-activation ones, appear evident from the results of the activation analysis. Cr 5Fe 1Y 2O 3, in particular, show excellent low-activation characteristics. This material can be a good choice for further development and characterization of first-wall and blanket structural materials for fusion power reactors.

Effect of oxygen and oxidation on tensile properties of V5Cr5Ti alloy

Oxidation studies were conducted on V5Cr5Ti alloy specimens in an air environment to evaluate the oxygen uptake behavior of the alloy as a function of temperature and exposure time. The oxidation rates calculated from parabolic kinetic measurements of thermogravimetric testing and confirmed by microscopic analyses of cross sections of exposed specimens were 5, 17, and 27 μm per year after exposure at 300, 400, and 500°C, respectively. Uniaxial tensile tests were conducted at room temperature and at 500°C on preoxidized specimens of the alloy to examine the effects of oxidation and oxygen migration on tensile strength and ductility. Microstructural characteristics of several of the tested specimens were determined by electron optics techniques. Correlations were developed between tensile strength and ductility of the oxidized alloy and microstructural characteristics such as oxide thickness, depth of hardened layer, depth of intergranular fracture zone, and transverse crack length.

Fatigue and crack growth behavior of VCrTi alloys

The results of in-vacuum low cycle fatigue tests are presented for unirradiated V5Cr5Ti tested at 25, 250, and 400°C. A pronounced environmental degradation of the fatigue properties is observed in this alloy at 25°C. Fatigue life was reduced by as much as 84% when testing was completed in a rough vacuum. Cyclic stress range data and SEM observations suggest that this reduction is due to a combination of increases in rates of crack initiation and subsequent growth. In high vacuum, the fatigue results also show a trend of increasing cyclic life with increasing temperature between 25 and 400°C. At 250°C, life averages 1.7 times that at 25°C, and at 400°C, life averages 3.2 times that at 25°C. A comparison of low cycle fatigue results for V5Cr5Ti was made to 20% cold-worked 316 stainless steel and several vanadium-base alloys. The results suggest that V5Cr5Ti has better resistance to fatigue than 316-SS in the temperature range of 25 to 400°C. At 400°C, the data also show that V5Cr5Ti out performs Vanstar alloys 7 and 8 over the entire range of strains investigated. Furthermore, the fatigue properties of the V5Cr5Ti alloy compare favorably to V15Cr5Ti (at 25°C) and Vanstar 9 (at 400°C) at strains greater than 1%. At lower strains, the lower fatigue resistance of V5Cr5Ti is attributed to the higher strengths of the V15Cr5Ti and Vanstar 9 alloys.

Effects of neutron irradiation and hydrogen on ductile-brittle transition temperatures of V-Cr-Ti alloys

The effects of neutron irradiation and hydrogen on the ductile-brittle transition temperatures (DBTTs) of unalloyed vanadium and V-Cr-Ti alloys were determined from Charpy-impact tests on 1 3 ASTM-standard-size specimens and from impact tests on 3-mm diameter discs. The tests were conducted on specimens containing < 30 appm hydrogen and 600–1200 appm hydrogen and on specimens after neutron irradiation to 28–46 atom displacements per atom at 420, 520, and 600°C. The DBTTs were minimum (< −220°C) for V-(1–5)Ti alloys and for V-4Cr- 4Ti alloy with < 30 appm hydrogen. The effect of 600–1200 appm hydrogen in the specimens was to raise the DBTTs by 60–100°C. The DBTTs were minimum (< −200°C) for V-(3–5)Ti and V-4Cr-4Ti alloys after neutron irradiation.

Materials analysis of the TITAN-I reversed-field-pinch fusion power core

The operating conditions of a compact, high-neutron-wall-loading fusion reactor severely limit the choices for structural, shield, insulator, and breeder materials. In particular the response of plasma-facing materials to radiation, thermal and pressure stresses, and their compatibility with coolants are of primary concern. Material selection issues are investigated for the compact, high mass-power-density TITAN-I reactor design study. In this paper the major findings regarding material performance are discussed. The retention of mechanical strength at relatively high temperatures, low thermal stresses, and compatibility with liquid lithium make vanadium-base alloys a promising material for structural components. Based on limited data, the thermal creep behaviour of V3TiISi and V15Cr5Ti alloys is approximated using the modified minimum committment method. In addition, the effects of irradiation and helium generation are superimposed on the creep behavior of V3Ti1Si. Coolant compatibility issues are investigated. The liquid lithium compatibility of the two vanadium alloys, V15Cr5Ti and V3Ti1Si, are compared, and the latter was chosen as the primary structural-material candidate for the liquid-lithium-cooled TITAN-I reactor. Electrically insulating materials, capable of operating at high temperatures are necessary throughout the fusion reactor device. Electrical insulator-material issues of concern include irradiation induced swelling and conductivity. Both issues are investigated and operating temperatures for minimum swelling and dielectric breakdown strength are identified for spinel (MgAI2O4).

Microstructure in rapidly solidified AlTi alloys

Rapidly solidified ribbons (50–70 μm thick) of AlTi alloys containing 2, 5 and 10 wt.% Ti have been melt spun under a helium atmosphere and the resulting microstructures characterized using analytical electron microscopy. As a result of the high cooling rates (about 105 K s−1) achieved, the formation of the coarse primary equilibrium intermetallic phase β (b.c.t., DO22 Al3Ti) in this system is suppressed. Ribbons of Al2Ti ald Al5Ti alloys comprise two microstructural zones in sections normal to the ribbon plane. The zone adjacent to the substrate surface is exposed to the largest degree of undercooling and is a single-phase, f.c.c. solid solution (α) of titanium in aluminium, with a grain size of 1–3 μm and a maximum titanium content of 4.0±1.0 wt.% in the Al5Ti alloy. Nearer the free surface, there is a two-phase structure comprising cuboidal particles (0.1–0.3 μm) of an ordered cubic, metastable intermetallic phase β′(L12, a = 0.404 nm) in α-Al solid solution. The metastable phase contains approximately 20 at.% Ti. It forms as primary particles directly from the melt and these particles subsequently provide effective nucleation sites for supersaturated α solid solution. The microstructure in rapidly solidified Al10Ti ribbons is uniform through the ribbon thickness and comprises particles of both the metastable β′ and the equilibrium β phases in an α matrix. Epitaxial growth of the metastable phase is commonly observed to surround particles of the equilibrium phase which are incompletely dissolved during the melt spinning cycle. This rim is effective in subsequent nucleation of several grains of the α solid solution, each sharing an identity orientation relationship with the metastable cubic phase.

Microstructural evolution and yield stress increase for ion-irradiated V-15Cr-5Ti alloy

The effect of 4-MeV 58Ni2+-ion irradiation on the microstructure of the V15Cr5Ti alloy was determined for irradiation temperatures ranging from 550 to 750°C and for damage levels of 50 to 260 dpa. The swelling of the alloy that could be attributed to voids was negligible ( < 0.1%). The principle effect of the irradiation was to induce the formation of disc-shape precipitates in the alloy microstructure. The increase of the yield stress for the alloy resulting from the irradiations was evaluated from the irradiation-produced dislocation density and the number density and size of irradiation-produced precipitates. These evaluations showed that the increase of yield stress for the alloy on ion irradiation had a maximum value at ~ 50 dpa and the increase of yield stress decreased for irradiation temperatures above 600°C.

Quench-Ageing Behaviour of 40Co–38Ni–17Cr–5Ti Alloy

The quench-ageing behaviour of a new Co–Ni–Cr-base high-temperature alloy containing Ti has been studied. The as-quenched structure of the alloy was fcc, and on ageing between 700 and 900° C (973 and 1173 K) the main strengthening phase was observed to be an ordered fcc γ′ phase. On overageing, this phase transforms to a stable hexagonal Ni3Ti η phase. The grain boundaries provide nucleation sites for the cellular precipitation of η phase. TiC and M23C6 carbides were also observed. However, during overageing TiC transforms to M23C6 carbides.