Vacancy Diffusion Research Articles

Structure of PtRu/Ru(0 0 0 1) and AgPd/Pd(1 1 1) surface alloys: A kinetic Monte Carlo study

Bimetallic surfaces allow tailoring their catalytic activity by modifying their composition and/or structure. However, under operating conditions, catalytically active bimetallic structures are often not stable and change their morphology which might reduce their functionality. Still, catalytically active structures do not necessarily need to be thermodynamically stable and might also be kinetically stabilized. Here we report kinetic Monte Carlo simulations based on density functional theory calculation to address the meta-stability of surface alloy systems. As structural changes can typically only occur via vacancy diffusion in the surface, we first determine the vacancy diffusion barrier as a function of their bimetallic environment. By determining the temporal evolution of the bimetallic surface alloys as a function of temperature, we analyze the factors underlying the stability and structure of the bimetallic surface alloys.

A numerical study on the influence of grain boundary oxides on dwell fatigue crack growth of a nickel-based superalloy

• A computational multi-physical modeling approach to oxide-assisted intergranular dwell crack growth of γ′ strengthened nickel-based superalloys is presented. The modeling framework explicitly couples mass transport of chemical species involved in oxide formation to the evolution of mechanical fields ahead of a crack tip. The viscoplastic response accounts for the influence of the γ′ dispersion. • Detailed Quantitative study of the interaction between the dynamic stress-assisted oxide formation ahead of the crack tip and the resulting near crack-tip stress fields was implemented. • A method for extracting crack growth rates based on failure of the oxide wedge ahead of a crack tip is presented • The predicted rates are shown to be in good agreement with available experimental data without any direct information or calibration of model parameters to crack growth rate data in advance. • Established model shows capability of reveal the effect of particle dispersion of nickel-based superalloy on the dwell fatigue crack growth rate. A theoretical treatment on the oxide-controlled dwell fatigue crack growth of a γ’ strengthened nickel-based superalloys is presented. In particular, this study investigates the influence of an externally applied load and variations in the γ’ dispersion on the grain boundary oxide growth kinetics. A dislocation-based viscoplastic constitutive description for high temperature deformation is used to simulate the stress state evolution in the vicinity of a crack at elevated temperature. The viscoplastic model explicitly accounts for multimodal γ’ particle size distributions. A multicomponent mass transport formulation is used to simulate the formation/evolution of an oxide wedge ahead of the crack tip, where stress-assisted vacancy diffusion is assumed to operate. The resulting set of constitutive and mass transport equations have been implemented within a finite element scheme. Comparison of predicted compositional fields across the matrix/oxide interface are compared with experiments and shown to be in good agreement. Simulations indicate that the presence of a fine γ’ size distribution has a strong influence on the predicted ow stress of the material and consequently on the relaxation in the vicinity of the crack-tip/oxide wedge. It is shown that a unimodal dispersion leads to reduced oxide growth rates (parabolic behavior) when compared to a bimodal one. Stability conditions for oxide formation are investigated and is associated with the prediction of compressive stresses within the oxide layer just ahead of the crack tip, which become progressively negative as the oxide wedge develops. However, mechanical equilibrium requirements induce tensile stresses at the tip of the oxide wedge, where failure of the oxide is predicted. The time taken to reach this critical stress for oxide failure has been calculated, from which dwell crack growth rates are computationally derived. The predicted rates are shown to be in good agreement with available experimental data.

Solute segregation and precipitation across damage rates in dual-ion–irradiated T91 steel

Dual-ion irradiations using 5.0 MeV defocused Fe2+ ions and co-injected energy degraded 2.00 to 2.85 MeV He2+ ions were conducted on a Fe9CrMo ferritic-martensitic steel T91 to 17 dpa at a damage rate range of 5 × 10−5 dpa/s to 3 × 10−3 dpa/s at 445°C, followed by characterization of the microstructure using conventional and scanning transmission electron microscopy. Radiation induced Ni/Si clusters and radiation induced segregation were quantified using energy dispersive X-ray spectroscopy at each condition and were compared with the same material irradiated in the BOR-60 reactor and in the as-received condition. No significant Cr segregation was found at lath boundaries after dual-ion irradiation, while Ni and Si enrichments both decreased with increasing damage rate leading to a sharp decrease in the density of Ni/Si clusters with damage rate. Increased point defect recombination at higher ion damage rates likely reduced the Ni/Si cluster density compared with BOR-60. Although the overall vacancy concentration and diffusion are enhanced by the irradiation damage rate, the lack of time for thermal diffusion and ballistic displacements of solutes are significant limiting factors for Ni/Si cluster formation. This work demonstrates the effect of irradiation damage rate on elemental segregation and clustering when using ion irradiation to simulate reactor irradiation.

Effect of Carbide Precipitation on “Precipitation Plasticity” and Residual Stress during Tempering

Materials with residual stress can experience irreversible plastic strains as carbides precipitate during the tempering process, thereby relaxing the residual stress. In previous studies, this plastic behavior is named “precipitation plasticity.” Although precipitation plasticity plays a key role in the relaxation of the residual stress, the micromechanism involved is not yet elucidated. Here in this study, experiments are performed on low‐carbon steel to study precipitation plasticity and the residual stress during tempering. The results show that precipitation plasticity is mainly produced by the generation of interfacial vacancies at the carbide–matrix interface and the diffusion and evolution of these vacancies under temperature and stress fields. Two precipitation plasticity regimes are observed, namely, Precipitation Plasticity I, which is associated with vacancy diffusion, and Precipitation Plasticity II, which is associated with dislocation proliferation. Precipitation Plasticity I is found to be controlled by the volume mismatch and carbide size and is the main cause of residual stress relaxation. The finer and more dispersed the carbide precipitates are, the more significant the precipitation plasticity is. Precipitation Plasticity II causes a slight increase in the residual stress, which, however, is considerably smaller than the stress relaxation corresponding to Precipitation Plasticity I.

Oxygen vacancy dynamics in monoclinic metallic VO2 domain structures

It was demonstrated recently that the nano-optical and nanoelectronic properties of VO2 can be spatially programmed through the local injection of oxygen vacancies by atomic force microscope writing. In this work, we study the dynamic evolution of the patterned domain structures as a function of the oxygen vacancy concentration and the time. A threshold doping level is identified that is critical for both the metal–insulator transition and the defect stabilization. The diffusion of oxygen vacancies in the monoclinic phase is also characterized, which is directly responsible for the short lifetimes of sub-100 nm domain structures. This information is imperative for the development of oxide nanoelectronics through defect manipulations.

Diffusion rates of hydrogen defect species associated with site-specific infrared spectral bands in natural olivine

We investigated hydrogen transport in naturally occurring, iron-bearing samples of San Carlos olivine that were hydrogenated at confining pressures of 200 or 300 MPa and 1173 to 1303 K or dehydrogenated at room pressure and 1191 to 1358 K. Chemical diffusion coefficients were determined from diffusion profiles for individual O-H-stretching bands from series of infrared spectra in orthogonal directions across each sample. Within experimental uncertainty, the diffusivities associated with all the individual bands are in good agreement with one another in both the hydrogenation and the dehydrogenation experiments. Hydrogenation proceeds by two diffusion mechanisms, as reported previously. The faster process involves interstitial diffusion of protons coupled with a counter-flux of polarons, with proton diffusion rate-limiting hydrogenation. For this mechanism, diffusion is faster along the olivine [100] direction than along [010] and [001], consistent with the anisotropy reported for proton diffusion and conductivity in olivine. The slower process involves interstitial proton diffusion coupled with a parallel flux of metal vacancies, with vacancy diffusion rate-limiting hydrogenation. For this mechanism, diffusion is faster along [001] than along [100] and [010], consistent with the anisotropy previously reported for the diffusion of metal cations in olivine. Diffusivities from our new dehydrogenation experiments are identical in both magnitude and anisotropy to those determined in our earlier hydrogenation experiments. This agreement demonstrates the validity of studies that used the results of our hydrogenation experiments to analyze dehydrogenation profiles in olivine xenocrysts and olivine in mantle xenoliths to determine rates of magma ascent from the source regions in Earth's interior.

A new diffusion mechanism for self-compensating impurities in α-alumina

Abstract Cation diffusion in α-alumina has been the subject of recent investigations, self-diffusion by the radiotracer technique and impurity diffusion by the SIMS technique. A review of the data is carried out to clarify the bulk diffusion mechanisms. Aluminium, cobalt, chromium, and yttrium exhibit a “normal” behaviour, which can be accounted for by an extrinsic vacancy diffusion mechanism. Silver, copper, and platinum behave as “fast diffusers”, although their activation energies are close to those of the other elements. From new experiments on magnesium diffusion reported in this paper, it is suggested that fast diffusion can be traced to self-compensating impurities. A diffusion mechanism which reconciles all the observations is proposed for these elements.

Defects properties and vacancy diffusion in Cu2MgSnS4

Cu2ZnSnS4 (CZTS) is a promising photovoltaic absorber material, however, efficiency is largely hindered by potential fluctuation and a band tailing problem due to the abundance of defect complexes and low formation energy of an intrinsic CuZn defect. Alternatives to CZTS by group I, II, or IV element replacement to circumvent this challenge has grown research interest. In this work, using a hybrid (HSE06) functional, we demonstrated the qualitative similarity of defect thermodynamics and electronic properties in Cu2MgSnS4 (CMTS) to CZTS. We show SnMg to be abundant when in Sn- and Cu-rich condition, which can be detrimental, while defect properties are largely similar to CZTS in Sn- and Cu-poor. Under Sn- and Cu-poor chemical potential, there is a general increase in formation energy in most defects except SnMg, CuMg remains as the main contribution to p-type carriers, and SnMg may be detrimental because of a deep defect level in the mid gap and the possibility of forming defect complex SnMg+MgSn. Vacancy diffusion is studied using generalized gradient approximation, and we find similar vacancy diffusion properties for Cu vacancy and lower diffusion barrier for Mg vacancy, which may reduce possible Cu-Mg disorder in CMTS. These findings further confirm the feasibility of CMTS as an alternative absorber material to CZTS and suggest the possibility for tuning defect properties of CZTS, which is crucial for high photovoltaic performance.

Role of Ni on the helium diffusion, stability and energetics of vacancy–type clusters in Fe–6.25Cr–3.13Ni (at.%) ternary alloys: From first-principles

Ab initio methods based on DFT are utilized to study the role of Ni addition on the interstitial helium diffusion, energetics and stability of HenVm (n = 0–6, m = 0–4) clusters in austenitic Fe–6.25Cr–3.13Ni (at.%) ternary alloys. The limit vacancy concentration considered in our simulation will be as close to the real irradiation conditions as possible. The presence of Ni obviously hinders the long-range migration of an individual helium between tetrahedral and octahedral interstitials, or between tetrahedral gaps, but greatly promotes the movement between octahedral gaps, and the travel between octahedral (tetrahedral) gaps cannot cross the tetrahedral (octahedral) interstitials. The addition of Ni enhances the attractions between interstitial helium, drives the short-range migration and tends to form small helium bubbles; on the other hand, it seriously hinders the trapping of helium by multiple-vacancies, and reduces the risk of helium bubble nucleation. In addition, the effects of Ni on the stability and energetics of helium vacancy complexes are evaluated, and it is confirmed that Ni improves the stability of the cluster models and enhances the irradiation expansion resistance of the alloy configurations, although it is not conducive to the formation of HenVm clusters. Our findings provide a theoretical reference for understanding the thermal helium desorption spectra and helium-induced embrittlement or hardening under irradiation conditions.

Dislocation dynamics simulation of thermal annealing of a dislocation loop microstructure

Thermal evolution and elevated temperature annealing of the dislocation microstructure of an irradiated metal, represented by an ensemble of elastically interacting interstitial dislocation loops, is explored using discrete dislocation dynamics simulations. The two fundamental microscopic processes driving the evolution of dislocations are the pipe diffusion of atoms along the dislocation lines, giving rise to the dislocation self-climb, and bulk diffusion of vacancies, resulting in the conventional dislocation climb. Simulations show that the coalescence and coarsening of the prismatic dislocation loop microstructure, observed at lower temperatures, is driven primarily by the dislocation self-climb. In tungsten, dislocation self-climb gives rise to a pronounced change in the dislocation loop microstructure at temperatures close to 800 °C, see Ferroni et al. (2015) [1], whereas a similar microstructural transformation of the dislocation network driven by self-climb in α-iron is predicted to occur at ∼270 °C. Simulations also show that the diffusion of vacancies in the crystal bulk is able to explain the observed annihilation rates of interstitial loops in tungsten.

Behavior of helium cavities in ion-irradiated W-Ni-Fe ductile-phase toughened tungsten

This study reports on the distribution of helium (He) cavities in a hot-rolled W-Ni-Fe ductile-phase toughened tungsten (DPT W) composite irradiated to a dose and a helium concentration that are comparable to those in the material after 5-year irradiation in a conceptual fusion power plant. The DPT W sample consists of W particles embedded in a ductile-phase NiFeW matrix with a nominal composition of 90W-7Ni-3Fe by weight. It was hot-rolled to a thickness reduction by 87% (87R DPT W). Sequential and individual irradiations of the material with 1.2 MeV Ni+ ions to a fluence of 2.15 × 1016 Ni+/cm2 and 90 keV He+ ions to 6.5 × 1015 He+/cm2 was performed at 973 K. Larger He cavities with a lower number density are observed in NiFeW than W. Helium cavities are aggregated preferentially along the NiFeW/W interphase boundary. This behavior is not observed along the W/W grain boundary under the same irradiation conditions. The average corrected cavity diameter or volume appears to be smaller in the sequentially irradiated sample than He+ ion irradiated sample. There is no evidence for formation of visible voids or Ni precipitates in W phase irradiated with Ni+ ions only. Diffusion, clustering and trapping of vacancies and He atoms during He+ ion irradiation at 973 K may be responsible for the formation of the He cavities.

Simulation of Hot Isostatic Pressing in a Single‐Crystal Ni Base Superalloy with the Theory of Continuously Distributed Dislocations Combined with Vacancy Diffusion

Single‐crystal components made of nickel base superalloys contain pores after casting and homogenization heat treatment. Hot isostatic pressing (HIP), which is carried above the ‐solvus temperature of the alloy, is industrially applied to reduce porosity. A modeling of HIP based on continuously distributed dislocations is developed in a 2D setting. Glide and climb of straight‐edge dislocations, as well as vacancy diffusion, are the deformation mechanisms taken into account. Thereby, dislocation glide is controlled by dragging a cloud of large atoms, and climb is controlled by vacancy diffusion. Relying on previous investigations of the creep behavior at HIP temperatures, it is assumed that new dislocations are nucleated at low‐angle boundaries (LAB) and move through subgrains until they either reach the opposite LABs or react with other dislocations and annihilate. Vacancies are created at the pore surface and diffuse through the alloy until they are either consumed by climbing dislocations or disappear at the LABs. The field equations are solved by finite elements. It is shown that pore shrinking is mostly controlled by vacancy diffusion as the shear stresses at the LABs are too low to nucleate a sufficient amount of dislocations.

Diffusion-induced stress concentrations in diffusional creep

The lattice continuum formulation for diffusional creep is implemented into the computational framework based on the finite difference method for space and the predictor–corrector algorithm for time discretization, to solve the coupled elasticity-diffusion problem with moving boundaries. The numerical scheme is implemented and tested considering 3D periodic structures without grain boundary sliding. Of primary interest in this paper are the stress non-uniformities resulting from nonuniform composition eigenstrains.We consider two regimes: the one where the rate limiting process is bulk diffusion of vacancies, and the one where the rate is controlled by vacancy nucleation/annihilation at grain boundaries (nucleation-controlled creep). We found that the stress concentration factor for diffusion-controlled creep is independent of the applied stress and grain size. No stress concentrations are present for the nucleation-controlled creep case.Stress and grain size dependence of minimum strain creep rates are determined by the present model for a variety of applied stresses, grain sizes, different driving processes (diffusion and nucleation-controlled creep) and compared with the classical theory for diffusional creep. We found that steady-state creep rates varied linearly with applied stresses for both diffusion and nucleation-controlled creep. The numerical results show good correspondence to analytical predictions for idealized diffusional Nabarro-Herring creep. Significantly lower steady-state strain rates were computed for nucleation-controlled creep. These results demonstrate the ability of the present model to reproduce the stress and grain size dependence of the steady-state strain creep rates.

Vacancy formation energies and migration barriers in multi-principal element alloys

Multi-principal element alloys (MPEAs) continue to garner interest as structural and plasma-facing materials due to their structural (phase) stability and increased resistance to radiation damage. Despite sensitivity of mechanical behavior to irradiation and point-defect formation, there has been scant attention on understanding vacancy stability and diffusion in refractory-based MPEAs. Using density-functional theory, we examine vacancy stability and diffusion barriers in body-centered cubic (Mo0.95W0.05)0.85Ta0.10(TiZr)0.05. The results in this MPEA show strong dependence on environment, originating from local lattice distortion associated with charge-transfer between neighboring atoms that vary with different chemical environments. We find a correlation between degree of lattice distortion and migration barrier: (Ti, Zr) with less distortion have lower barriers, while (Mo, W) with larger distortion have higher barriers, depending up local environments. Under irradiation, our findings suggest that (Ti, Zr) are significantly more likely to diffuse than (Mo, W) while Ta shows intermediate effect. As such, material degradation caused by vacancy diffusion can be controlled by tuning composition of alloying elements to enhance creep strength at extreme operating temperatures and harsh conditions.

Effect of the sintering temperature on electrical properties of the Na0.54Bi0.46TiO2.96 compound

The Na0.54Bi0.46TiO2.96 compound was prepared by the sol-gel means and sintered at different temperatures. The effect of sintering temperature on the electrical properties and diffusion behavior of oxygen ions was systematically studied. The bulk conductivity of Na0.54Bi0.46TiO2.96 compound increases first and then decreases with the increase of sintering temperature. At the sintering temperature of 1273 K, the highest bulk conductivity of the Na0.54Bi0.46TiO2.96 sample can reach up to 3.35 × 10-4 S/cm. The total conductivity of the Na0.54Bi0.46TiO2.96 sample shows a monotonous upward trend with increasing sintering temperature. The internal friction spectrum results present that the oxygen vacancy concentration and the relaxation activation energy of the Na0.54Bi0.46TiO2.96 sample show an upward trend, but the mobility of the oxygen vacancy diffusion decreases with the sintering temperature. The Na0.54Bi0.46TiO2.96 sample sintered at 1273 K is characterized by a higher oxygen vacancy concentration and mobility, resulting in the high grain conductivity.

Demonstration of synaptic and resistive switching characteristics in W/TiO2/HfO2/TaN memristor crossbar array for bioinspired neuromorphic computing

In this study, resistive random-access memory (RRAM)-based crossbar arrays with a memristor W/TiO2/HfO2/TaN structure were fabricated through atomic layer deposition (ALD) to investigate synaptic plasticity and resistive switching (RS) characteristics for bioinspired neuromorphic computing. X-ray photoelectron spectroscopy (XPS) was employed to explore oxygen vacancy concentrations in bilayer TiO2/HfO2 films. Gaussian fitting for O1s peaks confirmed that the HfO2 layer contained a larger number of oxygen vacancies than the TiO2 layer. In addition, HfO2 had lower Gibbs free energy (ΔG°=-1010.8 kJ/mol) than the TiO2 layer (ΔG°=-924.0 kJ/mol), resulting in more oxygen vacancies in the HfO2 layer. XPS results and ΔG° magnitudes confirmed that formation/disruption of oxygen-based conductive filaments took place in the TiO2 layer. The W/TiO2/HfO2/TaN memristive device exhibited excellent and repeatable RS characteristics, including superb 103 dc switching cycles, outstanding 107 pulse endurance, and high-thermal stability (104 s at 125 °C) important for digital computing systems. Furthermore, some essential biological synaptic characteristics such as potentiation-depression plasticity, paired-pulse facilitation (PPF), and spike-timing-dependent plasticity (STDP, asymmetric Hebbian and asymmetric anti-Hebbian) were successfully mimicked herein using the crossbar-array memristive device. Based on experimental results, a migration and diffusion of oxygen vacancy based physical model is proposed to describe the synaptic plasticity and RS mechanism. This study demonstrates that the proposed W/TiO2/HfO2/TaN memristor crossbar-array has a significant potential for applications in non-volatile memory (NVM) and bioinspired neuromorphic systems.

Механические напряжения в тонких пленках цирконата-титаната свинца, сформированных на подложках, отличающихся температурными коэффициентами линейного расширения

The article considers the influence of linear and bending stresses on the magnitude of the internal field and self-polarisation in thin lead zirconate titanate (PZT) films formed on silicon and glass-ceramic ST-50 substrates by radio-frequency magnetron sputtering. PZT composition corresponded to the region of the morphotropic phase boundary. It is assumed that bending stresses in “thin PZT film-substrate” bimorph structures lead to the appearance of an internal electric field caused by the diffusion of charged oxygen vacancies (Gorsky effect in ferroelectrics).

An influence of mechanical stresses on the magnitude of the internal field in lead zirconate titanate thin films

In self-polarized lead zirconate titanate thin films formed on platinized silicon substrates, a significant increase in the internal electric field was observed as a result of long-term aging. To explain this phenomenon, a mechanism is proposed for the formation of the internal field associated with the diffusion of charged oxygen vacancies, which, in turn, is due to the action of a mechanical stress gradient. The diffusion coefficient of charged oxygen vacancies was estimated to be ~ 3·10-16 cm2/s. Keywords: thin films, lead zirconate titanate, internal field, Gorsky effect.

Влияние механических напряжений на величину внутреннего поля в тонких пленках цирконата-титаната свинца

In self-polarized lead zirconate titanate thin films formed on platinized silicon substrates, a significant increase in the internal electric field was observed as a result of long-term aging. To explain this phenomenon, a mechanism is proposed for the formation of the internal field associated with the diffusion of charged oxygen vacancies, which, in turn, is due to the action of a mechanical stress gradient. The diffusion coefficient of charged oxygen vacancies was estimated to be ∼ 3 × 10–16 cm2/s.

Spinodal Decomposition of an Alloy with a Strong Concentration Dependence of the Mutual Diffusion Coefficient

It is shown that the kinetics of decomposition of an alloy with a strong concentration dependence of the mutual diffusion coefficient is qualitatively different from the widely known kinetics of spinodal decomposition, since it includes the formation of metastable (in the kinetic sense) precipitates of the intermediate composition. Concentration dependence of the mutual diffusion coefficient is particularly due to the difference in impurity diffusion coefficients, as well as component self-diffusion coefficients, and it increases as temperature decreases. The calculations use an expression for the flow of component atoms in the absence of the substance flow, obtained previously under the kinetic theory of vacancy diffusion ("hole gas method"). Keywords: spinodal decomposition, mutual diffusion coefficient, precipitates of intermediate composition.