Mobile Vacancies Research Articles

Effects of transition metal carbide dispersoids on helium bubble formation in dispersion-strengthened tungsten

The formation of helium bubbles and subsequent property degradation poses a significant challenge to tungsten as a plasma-facing material in future long-pulse plasma-burning fusion reactors. In this study, we investigated helium bubble formation in dispersion-strengthened tungsten doped with transition metal carbides, including TaC, ZrC, and TiC. Of the three dispersoids, TaC exhibited the highest resistance to helium bubble formation, possibly due to the low vacancy mobility in the Group VB metal carbide and oxide phases. Under identical irradiation conditions, large helium bubbles formed at grain boundaries in tungsten, while no bubbles were observed at the interfaces between the carbide dispersoid and tungsten matrix. Moreover, our results showed the interfaces could suppress helium bubble formation in the nearby tungsten matrix, suggesting that the interfaces are more effective in trapping helium as tiny clusters. Our research provided new insights into optimizing the microstructure of dispersion-strengthened tungsten alloys to enhance their performance.

Effects of Carbon on Void Nucleation in Self-Ion–Irradiated Pure Iron

Impurities such as carbon atoms play a significant role in the void swelling of irradiated metals. The phenomenon is important to both materials designs in which impurities are intentionally introduced and accelerator-based ion irradiation testing in which impurities are introduced unintentionally as contaminants. Here, we report rate theory simulations of void nucleation in pure Fe, which are irradiated by 5-MeV Fe ions, as one typical irradiation condition used in nuclear material testing. Based on kinetics obtained previously from ab initio calculations, Multiphysics Object-Oriented Simulation Environment (MOOSE)–based numerical solvers were used to simulate defect distributions and void nucleation. Vacancy-carbon interactions increase the effective migration energies of carbon and decrease the diffusivity prefactors. The vacancy mobility reduction decreases both interstitial flux and vacancy flux. However, the vacancy flux reduction is more significant than that of interstitials, leading to reduced void nucleation in bulk. On the other hand, reduced vacancy flux toward the surface leads to local vacancy pileups, leading to locally enhanced void nucleation. These two combined effects make the void nucleation profile deviate from the displacements per atom (dpa) peak, and void swelling peaks shift to the near-surface region. The transition from deep swelling to near-surface swelling is plotted as a function of dpa rate, carbon concentration, and temperature. The study shows that the swelling peak shifting caused by the carbon effect can be avoided by either reducing dpa rates or increasing irradiation temperatures. The study is important to understand swelling behaviors and to optimize irradiation parameters for accelerator-based swelling testing.

Peculiarities of the Behavior of Point Defects under the Optoplastic Effect in Copper

The authors previously discovered a new optoplastic effect and observed it under the action of a nanosecond UV laser pulse irradiation of subcritical intensity. In this paper it is shown that under this effect no micropores arise in the subsurface layer of metal. This proves the statement that swelling of metal under laser impact of moderate (subcritical) intensity occurs due to interstitial atoms migrating to the surface and not due to melting with formation of bubbles. At a abrupt cooling (for ~20 μs) interstitial atoms migrate to the surface by the Schottky mechanism due to abnormal mass transfer and the less mobile vacancies have no time to coagulate with formation of micropores in the time of the process.

Peculiarities of the Behavior of Point Defects under the Optoplastic Effect in Copper

The authors previously discovered a new optoplastic effect and observed it under the action of a nanosecond UV laser pulse irradiation of subcritical intensity. In this paper it is shown that under this effect no microporesarise in the subsurface layer of metal. This proves the statement that swelling of metal under laser impactof moderate (subcritical) intensity occurs due to interstitial atoms migrating to the surface and not due to melting with formation of bubbles. At a abrupt cooling (for ~20 μs) interstitial atoms migrate to the surfaceby the Schottky mechanism due to abnormal mass transfer and the less mobile vacancies have no time tocoagulate with formation of micropores in the time of the process.

Research Trends on the Influence of Oxygen Vacancies in Post BaTiO3 (BT) Ceramics for Next-Generation MLCCs

In line with the trend towards electrification in mobility, there is a demand for the development of next-generation Multilayer Ceramic Capacitors(MLCCs) with superior properties compared to those using the conventional BaTiO<sub>3</sub> (BT) ceramics. For this, various high-performing ferroelectric ceramics have been proposed as post-BT materials, and numerous studies have been conducted on the role of oxygen vacancies within these materials. It has been confirmed that oxygen vacancies in the ceramic material have a significant impact on various properties such as oxygen ionic conduction, IR degradation, microstructure, aging degradation, and hardening effect, and by controlling the concentration and mobility of oxygen vacancies, it is possible to adjust these properties. We hope that research on the role of oxygen vacancies in various high-performing ferroelectric ceramics will be utilized as a foundation of knowledge for the development of next-generation MLCCs in the future.

Sterically Suppressed Phase Segregation in 3D Hollow Mixed-Halide Wide Band Gap Perovskites.

Band gap tuning in mixed-halide perovskites enables efficient multijunction solar cells and LEDs. However, these wide band gap perovskites, which contain a mixture of iodide and bromide ions, are known to phase segregate under illumination, introducing voltage losses that limit stability. Previous studies have employed inorganic perovskites, halide alloys, and grain/interface passivation to minimize halide segregation, yet photostability can be further advanced. By focusing on the role of halide vacancies in anion migration, one expects to be able to erect local barriers to ion migration. To achieve this, we employ a 3D "hollow" perovskite structure, wherein a molecule that is otherwise too large for the perovskite lattice is incorporated. The amount of hollowing agent, ethane-1,2-diammonium dihydroiodide (EDA), varies the density of the hollow sites. Photoluminescence measurements reveal that 1% EDA in the perovskite bulk can stabilize a 40% bromine mixed-halide perovskite at 1 sun illumination intensity. These, along with capacitance-frequency measurements, suggest that hollow sites limit the mobility of the halide vacancies.

Effect of the free surface on near-surface void swelling in self-ion irradiated single crystal pure iron considering the carbon effect

The influence of ion-incident free surfaces on void swelling in near-surface regions was systematically studied using self-ion irradiation of single crystal pure iron. The key interest was to evaluate the effect of free surfaces not only on the formation of void-denuded zones but also on the swelling in the region immediately adjacent to the void-denuded zone. The irradiation matrix included beam energies varying from 1.0 to 5.0 MeV, peak displacements-per-atom levels of 50 and 100 dpa, and irradiation temperatures at 425 °C (at 6 × 10−3 peak dpa/sec, 475 °C (at 1.2 × 10−3 peak dpa/sec) and 525 °C (at 6 × 10−3 peak dpa/sec). The observed denuded depth Δx obtained from transmission electron microscopy was modified to incorporate the sputtering effect. The derived activation energy governing the denuding process is Ex4=1.65±0.03eV, which is significantly higher than the vacancy migration energy EVm known for Fe to be 0.67 eV. This difference is speculated to be due to the strong effect of relatively small amounts of dissolved carbon at 103 appm, which reduces the effective vacancy mobility and thereby increases the effective migration energy. Based on the effective vacancy migration energy extrapolated from a previous first-principle calculation for Fe containing various carbon levels, rate theory calculation was used to model the surface influence and the results support the speculation of carbon influence. For the region immediately adjacent to the void-denuded zone, swelling is locally suppressed, rising to reach the bulk swelling at a depth increment essentially equal to the denuded width. This suppression effect causes shifting of swelling vs. local dpa curves obtained from different peak dpa irradiations. Such shifting is proposed in the present study as a method to define the boundary of the surface-induced suppressed swelling zone beyond which the swelling can be considered to be fully free of surface influence.

Metal Particle Composite Hardening in Ba0.85Ca0.15Ti0.90Zr0.10O3 Piezoceramics

AbstractHard‐type piezoceramics are key materials in high‐power transducers and transformers. Acceptor doping is the most widely used piezoelectric hardening approach, but the mobility of oxygen vacancies at large electric fields or at high temperatures inevitably leads to the deterioration of hardening performance. The present study proposes a new hardening method associated with intragranular metal particles for achieving strong pinning of ferroelectric domain walls. Highly effective piezoelectric hardening via intragranular Ag particles in Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramic is realized, where the mechanical quality factor Qm and the coercive field Ec increase by 170% and 53%, respectively. The Ba0.85Ca0.15Ti0.90Zr0.10O3/0.10Ag sample features a larger high‐power mechanical quality factor than the pure Ba0.85Ca0.15Ti0.90Zr0.10O3. Moreover, the piezoelectric properties (d31 and k31) of the Ba0.85Ca0.15Ti0.90Zr0.10O3/0.10Ag sample show exceptional stability with the increase in vibration velocity. This composite approach of introducing metal particles can be considered as a generic hardening method and can be extended to other ferroelectric systems.

Defect properties of a body-centered cubic equiatomic TiVZrTa high-entropy alloy from atomistic simulations

TiVZrTa high-entropy alloys (HEAs) have been experimentally proven to exhibit excellent irradiation tolerance. In this work, defect energies and evolution were studied to reveal the underlying mechanisms of the excellent irradiation tolerance in TiVZrTa HEA via molecular statics calculations and molecular dynamics simulations. The atomic size mismatch of TiVZrTa is ∼6%, suggesting a larger lattice distortion compared to most face-centered cubic and body-centered cubic M/HEAs. Compared to pure Ta and V, smaller vacancy formation and migration energies with large energy spreads lead to higher equilibrium vacancy concentration and faster vacancy diffusion via low-energy migration paths. Vacancies in TiVZrTa have weaker abilities to form large vacancy clusters and prefer to form small clusters, indicating excellent resistance to radiation swelling. The formation energies of different types of dumbbells in TiVZrTa show significant differences and have large energy spreads. The binding abilities of interstitials in TiVZrTa are weaker compared to that in pure Ta and V. In TiVZrTa, fast vacancy diffusion and slow interstitial diffusion result in closer mobilities of vacancies and interstitials, significantly promoting point defect recombination. We further studied the effects of short-range ordered structures (SROs) on defect diffusion and evolution. SROs in TiVZrTa can effectively lead to higher fractions of defect recombination and fewer surviving defects. Our findings provide a comprehensive understanding of the underlying mechanisms of the high irradiation tolerance in body-centered cubic HEAs with large lattice distortion and suggest SROs are beneficial microstructures for enhancing irradiation tolerance.

Investigation of vacancy trapping by solutes during quenching in aluminum alloys

In aluminum alloys, excess vacancies often form pairs or clusters with solute atoms. This is commonly referred to as vacancy trapping by solutes. Such trapping results in a change in the number of mobile or free vacancies, which in turn affects solute diffusion and precipitate formation during heat treatment. In the current work, such trapping is investigated indirectly in the Al-Sn binary model alloy via studying the dependence of Frank loop formation on the availability of free vacancies. Current experimental results show that the addition of 100 at. ppm. Sn to aluminum leads to a significant reduction in the number density of quenched-in Frank loops. Modeling of mono- and divacancy trapping by solutes during quenching demonstrates the influences of the vacancy-solute binding energy, solute concentration, and temperature on the trapping process. The results reveal that the binding between vacancies and solutes significantly reduces the concentration of free vacancies after quenching in Al-Sn.

Oxygen stoichiometry and isotope exchange of oxides Ba0.5Sr0.5Co0.8Fe0.2O3−δ doped with Ta, Nb, Mo or W

This study focuses on the oxygen exchange kinetics and the oxygen diffusivity of the Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxides doped with high-charged cations M = Ta, Nb, Mo or W and factors governing the oxygen vacancies mobility and surface exchange activity. The dependence of oxygen content in Nb-doped oxide has been investigated as the function of oxygen pressure and temperature within the ranges of − 4.5 ≤ log(Po2/P0o2) ≤ − 0.7 and 600–900 °C by means of the quasi-equilibrium oxygen release method. Based on «T − Po2 – δ» diagrams the modelling of point defect equilibrium has been performed. The composition and morphology of the surface of all the oxides have been studied by the X-ray photoelectron spectroscopy (XPS) and the scanning electron microscopy (SEM), respectively. The kinetics of interaction of gas phase oxygen with the oxides has been explored by the isotope exchange method with gas phase equilibration (IE-GPE) in the temperature range of 600–800 °C and the oxygen pressure at 6.7 mbar. All the oxides have been divided into two groups with respect to the type of rate-determining step of oxygen surface exchange process, while the strong dependence of oxygen vacancy mobility on dopant concentration has been found. The obtained experimental data and proposed models of point defect equilibrium allowed to reveal the factors governing oxygen diffusion and oxygen surface exchange mechanism in series of oxides.

Cyclic deformation mechanism and fracture behaviour of 316L stainless steel under thermomechanical fatigue loading

Thermomechanical fatigue (TMF) behaviour of 316L is investigated based on the internal stress and microstructural characterization at various cyclic stages. Results reveal that the cyclic stress response shows initial hardening followed by cyclic softening at a high strain amplitude, while presenting continuous cyclic hardening as the strain amplitude decreases. Among these, Out-of-phase TMF always shows cyclic softening irrespective of strain amplitude. It is important to note that the hardening of back stress and friction stress contribute to the initial and subsequent cyclic hardening. In contrast, the subsequent cyclic softening is mainly dominated by the reduced friction stress. It is observed that the proliferation of dislocations at grain boundaries and the enhanced dislocation interactions within the grains are primarily responsible for the hardening of back stress and friction stress, respectively. Specifically, the cyclic softening of friction stress is attributed to the annihilated dislocations as a result of cross-slip. It is also found from the hysteresis curves that the intensity of dynamic strain ageing is related to strain amplitude, loading history and phase angle, which correlates to the occurrence of cross-slip, the diffusion rate of the solute atom, the density of point defect and the mobility of vacancies. Finally, the effect of loading conditions on fracture behaviour is demonstrated from the point view of crack sources, secondary cracks, creep cavities and oxidation damage.

Transitions from soft to hard piezoelectricity of (Ba, Ca)(Ti, Zr)O3 ceramics via precipitation hardening

Hard piezoelectrics are essential for high-power applications, the market share of which has increased significantly over the past few years. Acceptor-doping hardening, which relies on oxygen vacancies, has been proven to be a good practice to improve mechanical quality factors. However, the high mobility of oxygen vacancies restricts the use of acceptor-doping hardening to low driving fields and temperatures. Here, we extend the design of hard-type piezoceramics via precipitation hardening and demonstrate its large potential in (Ba,Ca)(Ti,Zr)O3. A soft-to-hard transition has been realized in (Ba,Ca)(Ti,Zr)O3 ceramics with an introduction of precipitates, where the mechanical quality factor and coercive field increase by over 180% and 120%, respectively. Through synchrotron x-ray diffraction and Rayleigh analysis, it is revealed that the hardening effect is attributed to the inhibition of domain wall motion by the intragranular CaTiO3 precipitates. This precipitation-hardening approach offers great potential for the design of hard piezoceramics.

Atomistic simulations of enhanced irradiation resistance and defect properties in body-centered cubic W-V-Cr and W-Ta-V alloys

Compared to face-centered cubic (fcc) high-entropy alloys (HEAs), body-centered cubic (bcc) medium/high-entropy alloys (M/HEAs) generally possess larger lattice distortion and exhibit distinct defect properties. In this work, molecular dynamics (MD) simulations of collision cascades and molecular statics (MS) calculations of defect properties were performed to investigate the irradiation resistance mechanism revealed from the point defect behaviors in W-V-Cr and W-Ta-V alloys. Low vacancy formation and migration energies with large energy spreads suggest the higher vacancy concentration and fast diffusions near the thermal spike. Slow self-interstitial atom (SIA) diffusions and fast vacancy diffusions lead to the closer mobilities of vacancies and SIAs, which significantly promotes the encountering probability and enhances the defect recombination in the cooling process of the thermal spike. Furthermore, formation and binding energies of W- and Ta-containing dumbbells are significantly larger than that of V- and Cr-related dumbbells, leading to higher occurrence probabilities and surviving fractions of V and Cr SIAs. Weak binding abilities of V and Cr SIAs can effectively suppress the clustering behaviors and make the surviving SIA clusters smaller. Our findings suggest that the irradiation resistance mechanisms in bcc M/HEAs are different from that in fcc M/HEAs, providing a deeper understanding of the radiation resistance mechanism in bcc M/HEAs with large lattice distortion.

Lithium ion transport in water-containing Li(SCN) over a wide compositional range: From water doping to hydration

This work explores the Li+ cation transport mechanisms in the system Li(SCN) · x H2O, complementary to our previous studies on crystal structures and hydration properties of lithium thiocyanate Li(SCN). Anhydrous Li(SCN) is a Schottky defective material with lithium vacancies VLi' being the dominant mobile defect. Two water incorporation mechanisms are identified: (i) At very low concentrations H2O acts as a dopant and forms H2OSCN• defects, constituting a substitutive incorporation mechanism. Doping with water not only increases the concentration of VLi' in Li(SCN), but also the vacancy mobility, making H2O-doping a special case. A complete quantitative defect chemical model is derived, including enthalpies and entropies of defect formation, association, and migration. (ii) Higher water activities induce chemical and structural changes in Li(SCN) · x H2O, i.e. hydration by reactive incorporation occurs. Single-phases as well as composites are investigated for their Li+ ion transport mechanisms. In case of x ≤ 1, defect associates determine the conductivity resulting in exceptionally high activation enthalpies. Further hydration to 1 < x < 2 creates composites of solid mono- and dihydrates, in which the low-melting diyhdrate can form percolating pathways and increase the conductivity to values as high as 7 × 10−4 S/cm at room temperature. Further increase of x leads to complete liquefaction. The entire range of x in the Li(SCN) – H2O system was studied regarding ion transport properties, identifying the individual mechanisms and temperature dependences. The results of this work deepen the understanding of hydration effects on solid ion conductors in solid state ionics, and help to elucidate transport phenomena in salt – solvent systems.

Synergistic effect of strain and oxygen vacancy on the ferroelectric properties of hafnium oxide-based ferroelectric films

In this paper, a multiphase coexistence phase field model considering mobile oxygen vacancy is established to investigate the synergistic effect of strain and oxygen vacancy on the ferroelectric properties of hafnium oxide-based ferroelectric films. Uniform strain introduced by flat substrate and heterogeneous strain caused by miscut substrate are considered. Our study demonstrates that whether the HfO2 film is under flat substrate-induced strain or miscut substrate-induced strain, the regulation of oxygen vacancy concentration on the ferroelectric phase transition and domain switching of the film is double-sided. When the oxygen vacancy concentration is 1 × 1020 cm−3, the film under 1.2 % uniform strain reaches a maximum remanent polarization of 0.36 C/m2. The optimal concentration interval of oxygen vacancy can be widened by −1.2 % flat substrate-induced uniform strain or 6° and 8° miscut substrate-induced heterogeneous strain. These results embodied the synergistic relationship between oxygen vacancy and strain in regulating the ferroelectric properties of hafnium-oxide based films, which offers guidance for us to understand the phase transition behavior of hafnium-based ferroelectric films and to optimize the electric properties of thin films in device applications.

Monovacancy-hydrogen interaction in pure aluminum: Experimental and ab-initio theoretical positron annihilation study

We report here on hydrogen-vacancy interactions in high purity aluminum by employing positron annihilation spectroscopy (PAS) analysis of hydrogen-loaded samples, aiming to study the mobility of vacancies. The samples were heat treated at 893 K in an atmosphere consisting of a mixture of H2 and Ar gas and, thus, loaded with hydrogen. The samples were then quenched to ice water and subsequently measured in-situ at different temperatures. In parallel we performed ab-initio density functional theory (DFT) calculations of lifetimes of positrons trapped in vacancies associated with 1–8 H atoms. Our experimental results suggest in comparison with the ab-initio calculations that complexes of vacancies with one hydrogen atom (V-H pairs) were formed in Al samples annealed in a mixture of H2 and Ar gas. Furthermore, hydrogen absorbed in aluminum immobilizes vacancies, i.e. the recovery of vacancies is delayed from 220 K up to around 280 K. At that temperature, V-H complexes start to dissociate, and hydrogen atoms previously bound to vacancies are released. In contrast, for Al samples not loaded with hydrogen isolated monovacancies become mobile around 220 K. In both cases mobile vacancies start to form vacancy clusters. From our experimental data we determined that the formation energy of monovacancies in Al is 0.62 ± 0.01 eV. This value is in very good agreement with 0.63 eV obtained by our ab-initio DFT calculations.

On the role of grain size variation on pitting corrosion and passive film behavior of Alloy 600H

AbstractIn this work, the effect of grain size variation on the pitting corrosion as well as the passivation behavior of the Alloy 600H has been critically assessed by various electrochemical techniques. The grain size modifications have been achieved by a series of thermal and thermo‐mechanical processing. The potentiodynamic polarization study has revealed that the coarse‐grained specimens exhibit higher corrosion and pitting resistance than the fine‐grained ones. This has been attributed to the greater extent of random high‐angle grain boundary (RHAGB) surface area in the fine‐grained specimens, which acted as favorable locations for pit initiation. The electrochemical impedance spectroscopy (EIS) result has indicated the formation of more imperfect (nonstoichiometric) passive layers in fine‐grained specimens due to the predominant presence of high energy and defective RHAGBs surface. Moreover, the Mott–Schottky analysis has revealed that the fine‐grained specimens generate higher defect densities at the interfaces, which leads to passive film instability due to increased mobility of cation interstitial and cation vacancies at the interfaces.

The interactions between dislocations and displacement cascades in FeCoCrNi concentrated solid-solution alloy and pure Ni

Concentrated solid-solution alloys (CSAs) have been proposed as structural materials for advanced nuclear reactors due to their great irradiation tolerance. We studied and compared the interaction processes of displacement cascades and edge dislocations in FeCoCrNi CSA and pure Ni by molecular dynamics to explore the influence of dislocations on the irradiation resistances of CSAs. The edge dislocations preferentially absorb interstitials in both materials, however, the differences between the numbers of absorbed interstitials and vacancies in FeCoCrNi are smaller than in Ni, which is attributed to the smaller difference of mobilities of interstitials and vacancies in FeCoCrNi. When the centers of displacement cascades coincide with edge dislocations, the numbers of absorbed interstitials and vacancies in edge dislocations of FeCoCrNi are essentially the same, which is independent with PKA energies. The amounts of survived interstitials and vacancies in the matrix of FeCoCrNi are closer than that in pure Ni, which would avoid the formation of numerous large vacancy clusters induced by excessive vacancies. Furthermore, the edge dislocations are enriched with Ni, Co and depleted with Fe, Cr after interactions with displacement cascades, which is consistent with experimental results. The molecular dynamics simulation results of current study reveal that the edge dislocations in CSAs can probably serve as weak biased sinks for the annihilation of irradiation-induced defects and enhancing the irradiation resistances of CSAs.

Significant difference of lanthanide fission products diffusion in Cr and α-Fe: An atomic-level study

By using first-principles calculations, diffusion of lanthanides (Ln) in bcc Cr and bcc Fe was studied to identify the microscopic mechanisms occurring in barrier materials that mitigate fuel cladding chemical interaction (FCCI) between the U-Pu-Zr and ferritic/martensitic steel systems. The properties of Ln-vacancy complexes were investigated by an oversized solute atoms (OSA) model. Results showed that strong lanthanides-vacancy binding, steep migration barriers and low vacancy concentration could lead to poor vacancy mobility in the vicinity of Ln in Cr. The Ln diffusion coefficients can be 4–8 orders of magnitude lower in Cr than those in Fe, confirming remarkable mitigating effects of Cr on Ln diffusion. Charge transfer and localization in the Cr matrix are analyzed to be responsible for the steep migration barriers. Owning to their high valence bond energies, V, Mo, and W are argued to have potentially similar or better performance in mitigating the diffusion of Ln.