Vacancy Condensation Research Articles

Microstructural Insights into Performance Loss of High-Voltage Spinel Cathodes for Lithium-ion Batteries.

Spinel-structured LiNix Mn2-x O4 (LNMO), with low-cost earth-abundant constituents, is a promising high-voltage cathode material for lithium-ion batteries. Even though extensive electrochemical investigations have been conducted on these materials, few studies have explored correlations between their loss in performance and associated changes in microstructure. Here, down to the atomic scale, the structural evolution of these materials is investigated upon the progressive cycling of lithium-ion cells. Transgranular cracking is revealed to be a key feature during cycling; this cracking is initiated at the particle surface and leads to the penetration of electrolytes along the crack path, thereby increasing particle exposure to the electrolyte. The lattice structure on the crack surface shows spatial variances, featuring a top layer of rock-salt, a sublayer of a Mn3 O4 -like arrangement, and then a mixed-cation region adjacent to the bulk lattice. The transgranular cracking, along with the emergence of local lattice distortion, becomes more evident with extended cycling. Further, phase transformation at primary particle surfaces and void formation through vacancy condensation is found in the cycled samples. All these features collectively contribute to the performance degradation of the battery cells during electrochemical cycling.

Void nucleation at dislocation boundaries aided by the synergy of multiple dislocation pile-ups

Void nucleation is of great significance in understanding ductile fracture. Recent experiments have shown that voids are nucleated via vacancy condensation and dislocation boundaries are the main nucleation sites. However, it is unclear what role is played exactly by dislocation boundaries in promoting void nucleation. Here we propose a new mechanism for dislocation boundary-induced void nucleation and develop a corresponding model based on the classical nucleation theory and vacancy diffusion theory. The model suggests that void nucleation is mainly influenced by hydrostatic stress, temperature, and relative vacancy concentration, whose contributions are systematically studied. It is also suggested that the vacancy formation energy and the interaction energy of hydrostatic stress and vacancy, which are absent in the previous models and introduced in ours, exhibit a clear tendency to lower the activation free energy barrier. Analysis of the nucleation kinetic suggests that the growth rate of void depends on the vacancy diffusion coefficient and vacancy concentration; the higher the values of these parameters, the faster the growth rate of the void. The kinetic feasibility of the newly proposed mechanism is examined using three-dimensional discrete dislocation dynamics simulations. The results predict that the size of incipient voids nucleated at the dislocation boundary is ∼35 nm, which is consistent with the experimental characterization value of ∼50 nm. Finally, when the relaxation of the dislocation boundary is considered, the synergistic effect is weakened.

Study on the Deformation and Fracture Mechanisms of Plastic Metals Considering Void Damage

Fracture initiation in plastic metals is attributed to the development of voids. Analyzing the nucleation and growth processes of voids facilitates the study of plastic deformation and fracture mechanisms in metal materials. Uniaxial tensile tests were conducted on two high-quality carbon structural steels, and the microfracture surface morphology of the tensile specimens was observed by using a scanning electron microscope (SEM). From the perspective of vacancy condensation, the nucleation mechanism of voids in the absence of inclusions or particles was analyzed. Based on the continuum damage mechanics theory and the Rice–Tracy (R-T) model, a damage parameter considering the void volume fraction was derived, and a plastic potential function, hardening curve, and constitutive model for the plastic deformation process of the plastic metal material were established. Based on the uniaxial tensile test data of the two sheets of high-quality carbon steel, the strain range data in the hardening stage were converted into true stress–plastic strain data, and the established hardening curve was used to fit the true stress–plastic strain data. The results showed good agreement between the established hardening curve and the experimental results, which effectively reflected the deformation process of ductile fractures in plastic metal materials.

A coupled diffusional-mechanical model accounting for hydrogen enhancements of strain-induced dislocations and vacancies

Informed by the current understanding of hydrogen-dislocations/vacancies interactions, a coupled diffusional-mechanical model accounting for hydrogen-enhanced strain-induced dislocations and vacancies is developed. This model is applied to capture hydrogen diffusion and trapping in the uniaxially tensioned column and type-I loaded crack specimens of nickel alloy, with special attentions paid to the influence of hydrogen-vacancies interaction on them, which is seldom discussed in previous studies. The results show that the hydrogen-enhanced hardening rate and the hydrogen-promoted vacancy enrichment for the nickel alloys observed in experiments can be addressed well by the present model. The strain-induced vacancies rather than dislocations dominate the hydrogen trapping behavior, due to their higher hydrogen binding energies for the nickel alloy. The hydrogen-saturated state of vacancies can heavily impact hydrogen diffusion/trapping, which could be characterized by the effective trap concentration. The supersaturated vacancies caused by hydrogen-enhanced strain-induced vacancies can promote void growth through vacancy condensation, thus accelerating plastic localization or interface rupture. For the type-I loaded blunt crack specimen, the hydrogen-enhanced dislocation multiplication contributes to intergranular cracking in the pressure valley, while hydrogen-enhanced vacancy generation facilitates void growth in the plastic zone, both of which can exacerbate plastic localization ahead of crack tip. A quantitative model for plasticity localization that accounts for the influences of hydrogen-induced IG cracking and void growth would be helpful in physically based modeling of hydrogen embrittlement.

Chloride Induced Secondary Passive Film Failure for Laser Additive Manufacturing Nickel-Based Superalloys during Electrochemical Machining

We investigated the influence of chloride on the secondary passive film (SPF) on laser directed energy deposition Alloy 718 during electrochemical machining. The results show that SPF formed in chloride-containing electrolyte is more defective than that formed in chloride-free solution, due to the stepped-up cation ejection by chloride. Chloride accelerates the SPF failure via enhanced cation vacancy condensation, SPF dissolution and possibly via surface vacancy pairs’ coalescence, restraining the formation of CrO3 and inducing a better surface quality than does the chloride-free electrolyte. Based on the Point Defect Model, a mechanism describing the influence of chloride on SPF was developed.

Time-Dependent Passivation Performance of Plasma Sprayed FeCrMoCBY Amorphous Coating

The relationship between passive film growth behavior and passivation time for plasma-sprayed Fe48Cr15Mo14C15B6Y2 amorphous coating in borate buffer solution has been thoroughly studied. The morphological characteristic and structural feature of as-spayed amorphous coating were estimated by scanning electron spectroscopy (SEM), X-ray diffraction (XRD) and transmission electron spectroscopy (TEM). The influence of passivation time on the film evolution properties was measured by electrochemical impedance spectra (EIS), Mott–Schottky curves, atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The results revealed that both corrosion resistance and self-repairing capacity of passive film greatly increased with time based on high electric field assumption. Reductions in donor density and flat band potential were accountable for a lower conductivity of passive film. An increment in Cr2O3 oxide as the inner barrier layer derived from the dehydration reaction of Cr(OH)3 contributed to the gradually densified structure of passive film. The extracted passive film thickness d increment with passivation time t conformed to the logarithm law on the basis of effective capacitance hypothesis: d=0.43lnt+52.06−2.18 (nm). Passivation mechanism within 600 s was ascribed to the adsorption of mechanical mixtures between metal ions and electrolytes, possibly leading to mechanical stress and rupture of passive film in the later growth procedure. The cation vacancy condensation process at the interface of coating/film was propitious in stabilizing the growth rate of passive film.

Intragranular thermal fatigue of Cu thin films: Near-grain boundary hardening, strain localization and voiding

In order to obtain a fundamental understanding of the phenomena accompanying thermomechanical fatigue of Cu metallization used in power electronics, as well as the resulting deterioration of electric properties, there is a need to assess intragranular microstructure and strain evolution within individual Cu grains and near grain boundaries. Here, synchrotron dark field X-ray microscopy (DFXM) is used to characterize as-deposited and 5 × 104 times thermally-cycled 20 µm thick Cu films. The cycling was performed using a dedicated test chip in the range of 100–400 °C applying a heating rate of 106 K/s. The thermomechanical treatment results in severe shear deformation of Cu grains, film surface roughening, gradual grain microstructural refinement, the emergence of microscopic voids preferably at high angle grain boundaries (HAGBs) and finally in the voids' percolation, as revealed by in-situ and ex-situ scanning electron microscopy. DFXM provides experimental evidence that mosaicity of Cu grains, residual tensile and compressive elastic strain concentrations and full width at half maximum of Cu 111 reflections increase simultaneously in the vicinity of the HAGBs. The latter is interpreted as resulting from vacancy condensation in front of the HAGBs, after dislocations have moved across cycled grains and partly annihilated. Moreover, the observed HAGB decohesion and gradual void formation are correlated with the supposed hardening of regions near HAGBs, which thus lose their ductility during cyclic elasto-plastic deformation. The results are complemented with the ex-situ X-ray nanotomography data, which document the voids' percolation across the film's thickness with a “crack” width of up to ∼2 µm. In general, the study identifies the inhomogeneous intragranular microstructural refinement and a gradual condensation of structural defects near HAGBs as driving forces for void formation in thick Cu metallizations during fast thermo-mechanical fatigue.

Coupling stress fields and vacancy diffusion in phase-field models of voids as pure vacancy phase

High vacancy concentrations in crystals may lead to formation and growth of voids, which is connected to swelling under irradiation and degradation of material properties. Recent experimental work suggests that vacancy condensation may also be involved in nucleation and evolution of porosity in early stages of ductile fracture. Mostly in the realm of irradiation effects, void formation and growth has been simulated with phase-field methods, where voids are treated as pure vacancy phases. Since vacancies induce an eigenstrain field, it is well-known that the evolution of vacancy concentrations is coupled to the elastic stress field. However, the few existing elastically coupled diffuse interface models of void growth seem to face a conceptual problem in the diffuse interface; in the center of which they predict the highest eigenstrains, which results in unrealistically high, fluctuating stresses. In the current work, we present a new model for coupling elastically driven vacancy diffusion with a diffuse interface model of void surfaces, which overcomes the named short-comings and closely reproduces the sharp interface solution. This is achieved by making the eigenstrain a function of the non-conserved order parameter used to distinguish the crystal and void phase. The model is verified for two-dimensional example problems by comparison to the analytical solution of the according sharp interface model. Eventually, the model is used to show the impact of elasto-diffusional coupling on void growth in mechanically loaded systems. We analyze the model with regard to the bi-stable energy landscape and discuss limitations and future prospects of the approach.

Planar Gliding and Vacancy Condensation: The Role of Dislocations in the Chemomechanical Degradation of Layered Transition-Metal Oxides

Stacking faults driven by dislocations have been observed in layered transition-metal oxide cathodes both in cycled and uncycled materials. The reversibility of stacking-sequence changes directly impacts the material performance. Irreversible glide due to lattice invariance or local compositional changes can initiate a catastrophic sequence of degradation mechanisms. In this study we compare the chemomechanical properties of LiCoO2 and LiNiO2 by combining density functional theory and anisotropic linear elasticity theory. We calculate stacking fault energies as a function of Li content and quantify the extent to which excess Ni hinders stacking-sequence changes. We then characterize screw dislocations, which mediate stacking-sequence changes, and find a peculiarly compliant behavior of LiNiO2 due to the interaction of Jahn–Teller distortions with the dislocation strain field. Finally, we analyze the tendency of vacancies to segregate along dislocation lines. This study represents the first instance of explicit ab initio atomistic dislocation models in layered oxides and paves the way for the understanding and optimization of the chemomechanical behavior of cathode active materials during battery operation.

Unveiling the transpassive film failure of 3D printing transition alloys

The failure of transpassive films on transition metal alloys was investigated. The results show that the failure of the transpassive film on 3D printing nickel-based superalloy in flowing NaNO3 solution is dominated by self-induced cracking of the film on niobium-segregated phases and the erosion-induced degradation on the film assisted by cation vacancy condensation at the metal/barrier layer interface. Secondary passivation induces the formation of a transpassive film, the nature of which has been debated for decades. The findings are consistent with a bi-layer structure comprising a point-defective barrier layer that grows into the substrate alloy and a precipitated outer layer.

Metastable pitting corrosion behavior of laser powder bed fusion produced Ti-6Al-4V in Hank’s solution

The metastable pitting corrosion behavior of laser powder bed fusion (LPBF) produced Ti-6Al-4V is still unclear. Therefore, this work investigated the metastable pitting corrosion of LPBF-produced Ti-6Al-4V in Hank’s solution by electrochemical methods. The LPBF-produced sample (dominant by α′ phase in the microstructure) shows a higher frequency of pit nucleation than the annealed counterpart (composed by α + β dual phase). The passive films formed on the LPBF-produced sample exhibit a higher flux of oxygen vacancies, resulting in the absorption of more aggressive ions (e.g., Cl-) thereby producing more cation vacancies. The condensation of excessive cation vacancies contributes to the pit nucleation.

Cavity Nucleation and Growth in Nickel-Based Alloys during Creep

The number of fossil fueled power plants in electricity generation is still rising, making improvements to their efficiency essential. The development of new materials to withstand the higher service temperatures and pressures of newer, more efficient power plants is greatly aided by physics-based models, which can simulate the microstructural processes leading to their eventual failure. In this work, such a model is developed from classical nucleation theory and diffusion driven growth from vacancy condensation. This model predicts the shape and distribution of cavities which nucleate almost exclusively at grain boundaries during high temperature creep. Cavity radii, number density and phase fraction are validated quantitively against specimens of nickel-based alloys (617 and 625) tested at 700 °C and stresses between 160 and 185 MPa. The model’s results agree well with the experimental results. However, they fail to represent the complex interlinking of cavities which occurs in tertiary creep.

In-depth study on diffusion of oxygen vacancies in Li(NixCoyMnz)O2 cathode materials under thermal induction

It is generally believed that an increase in Ni content will severely reduce the lattice structure stability of the cathode material, causing it to release more oxygen during thermal induction. However, the thermal degradation mechanism at the microscopic level remains unclear, which hinders the safety design of cathodes. In this work, we focus on the secondary particles of different Li(NixCoyMnz)O2 cathodes, report the formation and condensation of oxygen vacancies on the surface in detail, and quantitatively analyze the oxygen vacancy concentration distribution inside the particles after thermal failure. The results reveal that the increase of Ni content promotes the oxygen vacancies to diffuse deeply from the surface of the secondary particles to the bulk under thermal induction, resulting in an increase of oxygen vacancy concentration in the bulk of the secondary particles and the release of more oxygen. When the Ni content x in Li(NixCoyMnz)O2 increases to 0.8, both the surface layer and bulk exhibit high oxygen vacancy concentrations, leading to an overall failure of the secondary particles. Furthermore, we suggest that the formation and evolution of intergranular cracks inside the secondary particles depend on the oxygen vacancy concentration gradient. Our work provides a new idea for future research on the thermal stability of cathode and the thermal safety design of Ni-rich cathode materials.

Progressive growth of the solid-electrolyte interphase towards the Si anode interior causes capacity fading.

The solid-electrolyte interphase (SEI), a layer formed on the electrode surface, is essential for electrochemical reactions in batteries and critically governs the battery stability. Active materials, especially those with extremely high energy density, such as silicon (Si), often inevitably undergo a large volume swing upon ion insertion and extraction, raising a critical question as to how the SEI interactively responds to and evolves with the material and consequently controls the cycling stability of the battery. Here, by integrating sensitive elemental tomography, an advanced algorithm and cryogenic scanning transmission electron microscopy, we unveil, in three dimensions, a correlated structural and chemical evolution of Si and SEI. Corroborated with a chemomechanical model, we demonstrate progressive electrolyte permeation and SEI growth along the percolation channel of the nanovoids due to vacancy injection and condensation during the delithiation process. Consequently, the Si-SEI spatial configuration evolves from the classic 'core-shell' structure in the first few cycles to a 'plum-pudding' structure following extended cycling, featuring the engulfing of Si domains by the SEI, which leads to the disruption of electron conduction pathways and formation of dead Si, contributing to capacity loss. The spatially coupled interactive evolution model of SEI and active materials, in principle, applies to a broad class of high-capacity electrode materials, leading to a critical insight for remedying the fading of high-capacity electrodes.

Steam oxidation of Cr-coated Sn-containing Zircaloy solid rod at 1000 °C

In this study, the steam oxidation behavior of Cr-coated Zry-4 alloy at 1000 °C and the various interesting phenomena observed during the oxidation process were investigated. During oxidation, Sn in the Zry-4 substrate diffused into the coating and precipitated in the Cr2O3 scale. The Kirkendall effect of these Sn-containing precipitates then generated microcavities. Furthermore, the outward diffusion behavior of Zr and the selective oxidation mechanism of the Zr particles were studied. Voids detected at the Cr2O3/Cr interface result mainly from vacancy condensation. The amorphization of Cr2O3 was also observed adjacent to the voids, and the amorphization mechanism was discussed.

Pitting of carbon steel in the synthetic concrete pore solution

AbstractPitting corrosion is a possible mode of failure of the carbon steel overpack of the Belgian supercontainer concept for the isolation of high‐level nuclear waste (HLNW). However, no firm experimental data are currently available to estimate the probability of failure over the extended storage time (100,000 years). Extensive work shows that passivity breakdown results from the condensation of cation vacancies (CVs) at the metal/barrier layer (m/bl) interface, in response to the absorption of Cl− into oxygen vacancies at the surface of the barrier oxide layer. The CVs migrate across the bl to the m/bl interface where they condense, leading to the separation of the bl from the metal. The resulting blister prevents the growth of bl into the metal and dissolution results in blister rupture, marking a passivity breakdown event. Stabilization via differential aeration produces a potentially damaging, stable pit. We review our work on passivity breakdown and the nucleation of pits on P355 QL2 carbon steel in high‐pH aqueous media typical of concrete pore solution, with emphasis on the mechanistic aspects. We conclude that failure of the carbon steel overpack containing the HLNW over a storage horizon of 100,000 years is improbable.

Vacancy-induced θ′ precipitation during ultrasonic-affected friction stir welding of Al–Cu alloy

The precipitation strengthening and tensile properties in ultrasonic-affected friction stir welding (UFSW) were investigated using Al–Cu alloy and compared with another joint produced by the same setup but without applying the ultrasonic. Both the nucleation rate and growth rate of θ' phase during UFSW were significantly promoted by the ultrasonic. More importantly, the precipitate distribution was extremely homogeneous in the UFSW. The ultrasonic inducing excess vacancies via reducing the vacancy formation energy was experimentally confirmed by positron annihilation spectroscopy and molecular dynamics simulation. The vacancy formation energies with and without the effect of the ultrasonic at 300 K were 0.60 ± 0.01 eV and 0.67 ± 0.01 eV, respectively. A quantitative estimation suggested that the vacancy concentrations during UFSW and CFSW were about 3 × 10–4 and 5 × 10–5, respectively, indicating that it was increased by almost one order of magnitude due to the application of the ultrasonic. On this basis, a vacancy-induced θ′ precipitation mechanism was proposed. Considerable Cu-vacancy pairs due to the high-density vacancies induced by the ultrasonic contributed to copious nucleation sites for θ′ phases. The vacancy migration and condensation onto the pre-existing phase interface can relieve the coherency strains, permitting a rapidly unrestricted growth of the precipitates. Finally, simultaneous enhancement in tensile strength and elongation was confirmed by applying the ultrasonic.

Performance of protective oxide films on Fe–Ni alloy anodes in molten KF–AlF3–Al2O3 salts at 700 °C

An oxide film containing NiFe2O4 was prepared by oxidizing Fe–Ni alloy in hot air, and the performance of the protective oxide film in molten KF–AlF3–Al2O3 salts at 700 °C was characterized by electrochemical methods, including electrochemical impedance spectroscopy, capacitance measurements, linear sweep voltammetry, and chronoamperometry to clarify the breakdown and growth process of the oxide film. For the pre-oxidized 57Fe–43Ni, 50Fe–50Ni, and 43Fe–57Ni alloys, the oxide films, with a thickness of 5 μm, are composed of Fe2O3 and NiFe2O4, according to X-ray diffraction and scanning electron microscopy analysis. The transfer resistance of ions in the oxide film increases with increasing nickel content, and reaches its maximum value of 91.39 Ω⋅cm2 for the pre-oxidized 43Fe–57Ni alloy based on electrochemical impedance spectroscopy measurements. Moreover, the carrier density in the oxide film determined from Mott–Schottky plots decreases as the nickel content increases. Under anodic polarization, the breakdown of oxide film follows the generation of metal cation vacancies and condensation of excess cation vacancies. Concurrently, a new oxide layer forms along the substrate owing to the annihilation of oxygen vacancies and diffusion of oxygen anion in the oxide film. After oxygen evolution, a new oxide film layer containing NiFe2O4 forms on the alloy substrate, leading to the passivation observed in the anodic polarization curves. The maximum polarization resistance in the passivation region is 4.86 Ω⋅cm−2 for the pre-oxidized 43Fe–57Ni alloy, indicating that the pre-oxidized 43Fe–57Ni alloy possesses better corrosion resistance to molten fluorides than the other two pre-oxidized Fe–Ni alloys. Chronoamperometry measurements show that the oxide film on the 43Fe–57Ni alloy is in a dynamically stable state during oxygen evolution.

Segregation-assisted climb of Frank partial dislocations: An alternative route to superintrinsic stacking faults in L12-hardened superalloys

In this work, we present the first observation and high-resolution analysis of Frank partial dislocations formed upon high temperature creep inside the L12 structure of a superalloy. The dislocations and associated planar faults are observed in a multinary Co-based superalloy after [001] tensile creep at 850 °C and 400 MPa up to a plastic strain of 4.6%. With their a/3〈111〉 Burgers vectors the Frank partial dislocations create superlattice intrinsic stacking faults (SISFs) by positive climb motion via vacancy condensation. High-resolution analysis of the opposite end of such an SISF revealed a terminating a/6〈112〉 Shockley partial dislocation, which is connected to an antiphase boundary (APB). This indicates that the whole defect configuration resulted from splitting of a Lomer-type a/2〈110〉 matrix dislocation which became incorporated into the γ′ phase. All observed SISFs exhibit solute segregation effects with enrichment of Co, Cr and W and depletion of Ni and Al. Leading Frank partial dislocations show pronounced Cottrell atmospheres of Co and Cr and asymmetric concentration profiles. The climb motion of the Frank partial is driven by an osmotic force induced by vacancy supersaturation and is assisted by solute segregation lowering the SISF energy. We discuss the forces acting on the aforementioned dislocations and their motion through the microstructure. Moreover, we propose a natural mechanism by which Lomer-type matrix dislocations can be generated enabling the formation and observed climb of Frank partial dislocations.

Unexpected partial dislocations within stacking faults in a cold deformed Mg−Bi alloy

Stacking faults and the partial dislocations associated with them in a cold-rolled Mg–3wt%Bi alloy have been systematically characterized using transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. Intrinsic stacking faults I2 and I1 are both detected. The I2 fault results from dissociation of a basal 〈a〉 dislocation into two Shockley partial dislocations and exists in both matrix and {101¯2} tension twins. Most of the I1 faults are from dissociation of 〈c + a〉 dislocations, but a small fraction has a structure similar to that of growth I1, and they might result from condensation of vacancies, interstitial precipitation, or coalescence of 〈c + a〉 ribbons. The I1 faults are distributed densely in {101¯2} tension twins but sparsely in the magnesium matrix. They are either bounded by Frank partials or linked to twin boundaries. Unexpectedly, Shockley partials are often found lying at the edge of steps within the I1 faults. Three cases are categorized in terms of the number and the sign of the Shockley partials located in each single I1 fault: one Shockley partial, two Shockley partials having the same sign, and two Shockley partials having opposite signs. The fault bounded by two Shockley partials is explicitly I1 but not I2, which is different from the dissociation of the basal 〈a〉 dislocation. Based on the geometric analysis, two possible mechanisms are proposed to explain the origin of Shockley partials formed in the I1 faults. The mechanism in which basal dislocations react with the bounding Frank partial of I1 is more likely to operate, and this mechanism is supported by molecular dynamics simulations.