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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

The effects of Ti content and quenching on phase transformations, microstructures, and mechanical properties in uranium-titanium alloys

In this work, the effect of Ti content on phase transformations, microstructures, and mechanical properties of U-Ti alloys are described for alloys containing 0.3 wt.% to 2.0 wt.%Ti. Rapid cooling is required to overcome diffusional decomposition of γ-phase and facilitate diffusionless transformation to supersaturated variants of α-phase. Critical cooling rate increases with increasing Ti content, opposite to the trend observed in U-Mo and U-Nb alloys. This difference occurs because the martensite transformation temperatures in these relatively dilute U-Ti alloys are above the knee of the C-curve for diffusional decomposition, unlike those in the more concentrated U-Mo and U-Nb alloys. In these U-Ti alloys critical cooling rate depends on the amount of undercooling required to reach M s , which increases with increasing Ti content, and the time for diffusional decomposition to occur just above M s , which decreases with increasing Ti content. The net result is that higher cooling rates are required as Ti content increases. Full quenching results in diffusionless transformation of γ-phase to supersaturated variants of α-phase. Very dilute alloys transform via a γ → β → α m sequence of massive transformations. Martensitic γ → α' a transformation begins at ~0.4%Ti, and 100% α' a microstructures are obtained from ~0.65% to ~1.4% Ti. A transition to banded α'b martensite occurs at ~1.5%Ti. Evidence suggests that the α' a to α' b transition may occur when the cubic γ-phase first transforms to tetragonal γ°, which in turn transforms to orthorhombic α' b via the sequence γ → γ° → α' b . Fully quenched alloys exhibit moderate strengths and ductilities, and their supersaturation with Ti makes them amenable to subsequent age hardening. Subcritical quenching typically results in two-phase microstructures with lower ductilities and near-zero Ti-supersaturation, eliminating the possibility of subsequent age hardening.

36 MATERIALS SCIENCE↗
DOE OSTI2022

The effect of Ti content on age hardening and mechanical properties of uranium-titanium alloys

Here, the effect of Ti content on age hardening and the resulting mechanical properties are described for -quenched U-Ti alloys containing 0.3 wt.% to 2.0 wt.%Ti. Age hardening occurs between ~250°C and ~450°C. Overaging occurs at higher temperatures by cellular decomposition. Age hardening kinetics suggest that different mechanisms occur depending on Ti content and initial microstructure. Strengthening in 'a acicular martensites begins by the formation of Ti clusters which evolve into thin U 2 Ti disc shaped precipitates and later mature into continuous U 2 Ti rods beginning at peak hardness. The mechanism of hardening in 'b banded martensite is more elusive, as significant hardening occurs where atomic mobility is lower than that required for precipitate formation, similar to that reported for age hardening in ''b banded martensite in U-6%Nb. The activation energy for aging varies with Ti content and microstructure. In fully martensitic alloys containing 0.75% to 2.0%Ti it is in the vicinity of ~44 kcal/mole (184 kJ/mole). But it is lower in alloys containing less than 0.6%Ti where quenched microstructures are less than fully martensitic. Tensile ductility is high prior to aging, decreases with age hardening, is effectively zero at peak hardness, and remains low in overaged conditions. Attractive combinations of strength and ductility are best obtained in alloys containing 0.6% to 1.0%Ti which have been partially aged to fractional hardening levels no greater than ~0.6. This corresponds to the very early stages of aging, associated with clustering and the earliest stages of U 2 Ti disc formation. Alloys containing 0.45%Ti or less are not as responsive to age hardening. Alloys containing 1.5% and 2.0%Ti can be aged to higher strengths, but extreme quench rate sensitivity prevents them from being effectively heat treated in realistic section thicknesses.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Insights into radiation resistance of titanium alloys from displacement cascade simulations

Radiation damage in beam window materials limits the use of high power proton beams in high energy physics research. The alloy Ti-6Al-4V is presently used as a beam window material but a prospective alternative, Ti-15V-3Cr-3Sn-3Al has been proposed. Since both these alloys contain dual phases at room temperature, we compare the radiation damage in the α and ß-phases of these two materials via primary knock-on atom (PKA) cascade simulations in the 10-40keV energy range. At PKA energies 30 and 40keV, the number of Frenkel pairs in the ballistic stage is higher in the ß-phase of Ti-6Al-4V than that in the ß-phase of Ti-15V-3Cr-3Sn-3Al almost by a factor of 2. The α-phase, of both these alloys, by far outperforms the ß-phases of the two alloys, both in terms of damage during the ballistic stage and in terms of the surviving defects. The average displacement threshold energy (E d ) in the α-phase of both alloys was found to be 66eV while that in the ß-phase of Ti-15-3 was 55 eV and in the ß-phase of Ti-6-4 was 46. Further, while the number of surviving defects is almost equal in both alloys, the vacancy and interstitial clustering mechanisms differ notably, which can impact the degree of radiation hardening and loss of ductility. Our simulations show larger vacancy and interstitial clusters form in Ti-6Al-4V as compared to that in Ti-15-3-3-3 alloy. These results indicate that Ti-15-3-3-3 alloys may be a promising candidate for next generation beam window material with a higher radiation tolerance than the existing Ti-6-4 alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Titanium doped kagome superconductor CsV 3–x Ti x Sb 5 and two distinct phases

The vanadium-based kagome superconductor CsV 3 Sb 5 has attracted tremendous attention due to its unexcepted anomalous Hall effect (AHE), charge density waves (CDWs), nematicity, and a pseudogap pair density wave (PDW) coexisting with unconventional strong-coupling superconductivity. The origins of CDWs, unconventional superconductivity, and their correlation with different electronic states in this kagome system are of great significance, but so far, are still under debate. Chemical doping in the kagome layer provides one of the most direct ways to reveal the intrinsic physics, but remains unexplored. Here, we report, for the first time, the synthesis of Ti-substituted CsV 3 Sb 5 single crystals and its rich phase diagram mapping the evolution of intertwining electronic states. The Ti atoms directly substitute for V in the kagome layers. CsV 3–x Ti x Sb 5 shows two distinct superconductivity phases upon substitution. The Ti slightly-substituted phase displays an unconventional V-shaped superconductivity gap, coexisting with weakening CDW, PDW, AHE, and nematicity. The Ti highly-substituted phase has a U-shaped superconductivity gap concomitant with a short-range rotation symmetry breaking CDW, while long-range CDW, twofold symmetry of in-plane resistivity, AHE, and PDW are absent. Furthermore, we also demonstrate the chemical substitution of V atoms with other elements such as Cr and Nb, showing a different modulation on the superconductivity phases and CDWs. These findings open up a way to synthesise a new family of doped CsV 3 Sb 5 materials, and further represent a new platform for tuning the different correlated electronic states and superconducting pairing in kagome superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Stabilizing alkaline fuel cells with a niobium-doped brookite titanium dioxide catalyst support

Anion-exchange membrane fuel cells represent a promising and scalable approach for hydrogen energy utilization. However, their development is hindered by the weak bonding between metal catalysts and carbon supports, along with challenges in fabricating electronically/ionically conductive electrodes. Here, we report a composite cathode of Nb-doped brookite TiO 2 nanorods that have robust stability when combined with Pt nanoscale catalysts in an alkaline fuel cell. The composite cathode, fabricated without the addition of an ionomer, delivers a power density of 419 mW cm −2 at a current density of 650 mA cm −2 and a voltage retention of 81% at 100 mA cm −2 after 25 h, substantially outperforming a cathode fabricated from commercial Pt/C. Further investigations of the chemical structure, anion exchange capacity, and mass transfer resistance reveal that a solvent residue derived from N-methylpyrrolidone plays an important role in charge transfer and mass transport in the alkaline fuel cell.

alkaline fuel cell↗
DOE OSTI2020

Titanium Niobium Oxide: From Discovery to Application in Fast-Charging Lithium-Ion Batteries

Lithium-ion batteries are essential for portable technology and are now poised to disrupt a century of combustion-based transportation. The electrification revolution could eliminate our reliance on fossil fuels and enable a clean energy future; advanced batteries would facilitate this transition. However, owing to the demanding performance, cost, and safety requirements, it is challenging to translate new materials from laboratory prototypes to industrial-scale products. This Perspective describes that journey for a new lithium-ion battery anode material, TiNb 2 O 7 (TNO). TNO is intended as an alternative to graphite or Li 4 Ti 5 O 12 with better rate and safety characteristics than the former and higher energy density than the latter. The high capacity of TNO stems from the multielectron redox of Nb 5+ to Nb 3+ , its operating voltage window well above the Li + /Li reduction potential prevents lithium dendrite formation, and its open crystal structure leads to high-power performance. Nevertheless, the creation of a practical TNO anode was nonlinear and nontrivial. Its history is built on 30 years of fundamental science that preceded its application as a battery anode, and its battery development included a nearly 30-year gap. The insights and lessons contained in this Perspective, many of them acquired firsthand, serve two purposes: (i) to unite the disparate studies of TiNb 2 O 7 into a coherent modern understanding relevant to its application as a battery material and (ii) to highlight briefly some of the challenges faced when scaling up a new material that affect TiNb 2 O 7 as well as new electrode candidates more generally.

25 ENERGY STORAGE↗