Accumulation Of Point Defects Research Articles

Influence of γ-radiation on magnetoelectrical properties of Bi0.85Sb0.15 (0.001≤x≥0.05) solid solution

Abstract The influence of γ-quantum on the magnetoelectric properties of Bi0.85Sb0.15 <Pbx> (0.001≤x≥0.05) thermoelectrics doped with lead atoms at ~77÷300K and magnetic field induction up to 1.0 Tesla has been studied. It was found that at low doses of gamma irradiation of the Bi0.85Sb0.15 <Pb> solid solution, point defects are formed, which lead to an increase in the concentration of free electrons n, specific electrical conductivity σ and a decrease in the absolute value of the thermo EMF coefficients α and RH. As the dose of γ-quanta increases in the process of subsequent restructuring, point defects accumulate, i.e. accumulation of point defects, which leads to a decrease in the concentration of current carriers and a corresponding change in σ, α. and RH.

An insight into annealing mechanism of graphitized structures after irradiation

Graphite components are constantly subjected to a combined actions of annealing and irradiation due to high temperatures and thermal spike caused by irradiation when reactors are operating, resulting in complex microstructures along with matching changes in the material properties and dimensions. This study investigates the evolution process of initial irradiation defects at different annealing temperatures, which is difficult to captured by experiment. The findings indicate that 923K reactor-temperature annealing can also quickly restore isolated self-interstitial atoms and small-sized point defect clusters to intralayer locations on the nanosecond scale. However, multi-interlayer penetration damages and localized point defect aggregation from high-energy irradiation can lead to the disappearance of layered structural information, which allows these damage structures to develop into interlayer dislocations during annealing process. These interlayer dislocations further exacerbate the interlayer expansion of graphite crystal and may elucidate the mechanism behind volume expansion in nuclear graphite within reactors. Fully amorphized regions can also regain a layered structure approximately when guided by residual layered structures at 2000K, while chaotic regions or disordered layered structures form in the absence of guidance. These chaotic regions significantly exacerbate the volume expansion of graphite model, which may be related to the rapid volume increase observed in nuclear graphite components at the end of reactor life. The study provides insights into the transformation of initial irradiation defects into different types of defects under different temperatures and damage states, which serve as a critical foundation for assessing the evolution of irradiation defects in nuclear graphite for reactor applications and annealing studies.

Deciphering the Atomic-Scale Structural Origin for Photoluminescence Quenching in Tin-Lead Alloyed Perovskite Nanocrystals.

The development of tin-lead alloyed halide perovskite nanocrystals (PNCs) is highly desirable for creating ultrastable, eco-friendly optoelectronic applications. However, the current incorporation of tin into the lead matrix results in severe photoluminescence (PL) quenching. To date, the precise atomic-scale structural origins of this quenching are still unknown, representing a significant barrier to fully realizing the potential of these materials. Here, we uncover the distinctive defect-related microstructures responsible for PL quenching using atomic-resolution scanning transmission electron microscopy and theoretical calculations. Our findings reveal an increase in point defects and Ruddlesden-Popper (RP) planar faults with increasing tin content. Notably, the point defects include a spectrum of vacancies and previously overlooked antisite defects with bromide vacancies and cation antisite defects emerging as the primary contributors to deep-level defects. Furthermore, the RP planar faults exhibit not only the typical rock-salt stacking pattern found in pure Pb-based PNCs but also previously undocumented microstructures rich in bromide vacancies and deep-level cation antisite defects. Direct strain imaging uncovers severe lattice distortion and significant inhomogeneous strain distributions caused by point defect aggregation, potentially breaking the local force balance and driving RP planar fault formation via lattice slippage. Our work illuminates the nature and evolution of defects in tin-lead alloyed halide perovskite nanocrystals and their profound impact on PL quenching, providing insights that support future material strategies in the development of less toxic tin-lead alloyed perovskite nanocrystals.

Simulation Study on Defect Damage Behavior of Fe under Irradiation Environment

Abstract As a commonly used structural material in nuclear power plants, Reactor pressure vessel (RPV) steel is subjected to high-energy neutron irradiation from the reactor core for a long time, and the service environment is harsh. The accumulation of point defects (interstitial atoms and vacancies) generated by irradiation over a long period can seriously affect the microstructure of the material. The macroscopic properties of the material changed until the material failed, which threatened the safety and operation stability of nuclear power plants. This paper used the method of molecular dynamics to simulate the cascade collision process of Fe in the irradiation environment. The relationship between different factors and radiation damage defects was studied. Firstly, the quantity of the Frenkel pairs increases rapidly in the irradiation environment, and then recombination occurs after reaching the peak, which makes the quantity of the Frenkel pairs decrease rapidly. Finally, the quantity of Frenkel pairs is in a stable trend. When PKA energy and temperature increase, the higher the quantity of Frenkel pairs at the peak is, the higher the recombination rate of defects is. Meanwhile, the larger the cluster size of interstitial atoms and vacancies is, the greater the corresponding quantity of clusters is. As PKA energy increases, the quantity of residual Frenkel pairs at stability increases, while the number of residual Frenkel pairs at stability decreases with temperature. This is attributed to the intense thermal motion of molecules at high temperatures, resulting in the quantity of Frenkel pairs decreasing at the stable stage. Therefore, by simulating the irradiation damage process of Fe, the prediction of material irradiation damage under a neutron irradiation environment is provided, and the service life of materials can be provided with theoretical guidance.

Assessing the impact of irradiation-induced defects on the hardening of reactor pressure vessel materials for sustainable development of nuclear energy

The operational longevity of a nuclear power plant, specifically of the light water reactor type, hinges on the assessment of component degradation, with a particular focus on the reactor pressure vessel (RPV) materials. Among the most prevalent defects responsible for irradiation-induced hardening in RPV materials are point defects (PDs) and dislocation loops (DLs). However, their respective contributions to the overall hardening of RPV materials have not been clearly determined. To address this issue, this study was conducted using iron alloys with equivalent Mn and Ni content to those used in nuclear RPV in Japan. The study employed transmission electron microscopy (TEM), atom probe tomography (APT), and nanoindentation to assess the roles of PDs and DLs in contributing to the hardening behaviour of RPV materials. The results indicated that both solute clusters (SCs), which are formed by the aggregation of PDs, and DLs play equally important roles in the hardening behaviour of RPV materials. However, an in-depth analysis of three-dimensional images using the APT method revealed that there may have been errors in evaluating SCs and DLs, leading to double counting when assessing the impact of irradiation-induced defects on the hardening behaviour of RPV materials. These findings highlight the need for improved accuracy in evaluating these defects to better understand the behaviour of RPV materials under irradiation.

Interaction of Stacking Faults with point/extended defects in Fe-He irradiated 6H-SiC

The study explored the microstructure evolution of 6H-SiC that underwent sequential iron and helium ion irradiation with energies of 2.5 MeV and 500 keV, respectively, at room temperature, followed by annealing at 1500°C for two hours. Following irradiation, the entire damaged layer underwent amorphization. However, during subsequent annealing, epitaxial recrystallization took place, resulting in the formation of defected polycrystalline 6H-SiC characterized by the presence of Fe-rich clusters, cavities, and stacking faults. Fe-rich cavities were found to predominantly form at the edges of the stacking faults, as revealed by XTEM. The interaction of microstructural defects is further investigated via first-principles calculations. The periphery of the stacking faults has been identified as the primary location for the emergence of vacancy clusters, serving as favorable sites for the accumulation of point defects, including Fe atoms. This behavior can be attributed to the combined effects of mechanical and electronic energy relaxation mechanisms. Mechanically, the presence of stacking faults allows for the release of elastic energy that had been stored at the boundary. Electronically, the energy relaxation arises from the saturation of C- and Si-dangling bonds. Both of these processes contribute to the observed behavior, highlighting the intricate interplay between mechanical and electronic factors in the system. The low point defect migration energy barriers in the vicinity of the stacking faults promise high recombination, which can limit cavity growth and enhance radiation resistance. The study not only offers valuable insights into the mechanism of cavity/stacking faults interaction, contributing to a better understanding of radiation damage in 6H-SiC but also demonstrates that 6H-SiC material containing stacking faults could serve as a viable alternative to 3C-SiC for nuclear application.

Elastic Strain Relaxation of Phase Boundary of α' Nanoscale Phase Mediated via the Point Defects Loop under Normal Strain.

Irradiation-induced point defects and applied stress affect the concentration distribution and morphology evolution of the nanophase in Fe-Cr based alloys; the aggregation of point defects and the nanoscale precipitates can intensify the hardness and embrittlement of the alloy. The influence of normal strain on the coevolution of point defects and the Cr-enriched α' nanophase are studied in Fe-35 at.% Cr alloy by utilizing the multi-phase-field simulation. The clustering of point defects and the splitting of nanoscale particles are clearly presented under normal strain. The defects loop formed at the α/α' phase interface relaxes the coherent strain between the α/α' phases, reducing the elongation of the Cr-enriched α' phase under the normal strains. Furthermore, the point defects enhance the concentration clustering of the α' phase, and this is more obvious under the compressive strain at high temperature. The larger normal strain can induce the splitting of an α' nanoparticle with the nonequilibrium concentration in the early precipitation stage. The clustering and migration of point defects provide the diffusion channels of Cr atoms to accelerate the phase separation. The interaction of point defect with the solution atom clusters under normal strain provides an atomic scale view on the microstructure evolution under external stress.

Atomic-scale study of He ion irradiation-induced clustering in α-Zirconium

Zircaloy-4 alloy specimens consisting of α-Zr grains were irradiated on a tandem accelerator with 5 MeV He ions to a fluence of 5 × 1021 ions m − 2 at different temperatures. Defect clusters and He bubbles were observed in the TEM micrographs of the irradiated microstructure. The small defect clusters and He bubbles were found to exhibit spherical shapes whereas the large defect clusters showed plate-like shapes. Molecular dynamics simulations and crystallography analyses revealed that the defect clusters are mainly composed of Zr interstitials. During He-ion irradiation, the clustering of Zr dumbbell interstitials results in the formation of small and spherical defect clusters. With further irradiation, the Zr interstitials prefer to occupy the octahedral sites that align along the {12¯14} planes with other site interstitials, forming cluster sections. These initial planar clusters then grow by extending along the <101¯0> directions and by forming more cluster sections along the <12¯16> directions, leading to large plate-like defect clusters on the {1¯21¯1}planes. He bubbles nucleate by the clustering of He atoms and vacancies that are formed by kicking out the lattice atoms, and then grow by the migration and coalescence of helium-vacancy clusters. Small He atoms have many interstitial sites to occupy in the α-Zr HCP crystal, allowing He bubbles to grow to spherical shapes. In addition, grain boundaries induce the accumulation of primary point defects and He atoms and high irradiation temperature accelerates diffusion, and hence larger defect clusters and He bubbles form in the grain boundary regions and at higher irradiation temperature.

Influence of point defect accumulation on in-pile thermal conductivity degradation: Fuel rod defect distribution and deviation between in-pile and post irradiation thermal conductivity

As nuclear fuel burn-up increases, its thermal conductivity degrades due to the accumulation of defects that lead to increased phonon scattering rates. This results in a rise in the centerline temperature of the fuel rod, whereby heat generation must be decreased to avoid undesired behavior such as fuel melting and extensive fission gas release. Fuel performance codes are utilized to optimize the fuel's operational conditions; while they are based on established physical principles, their empirical nature limits their predictive capabilities. Recently, an effort has been made to develop predictive fuel performance codes for commonly used nuclear fuels, as well as accelerated qualification of advanced nuclear fuels. In this report, we elaborate on the importance of careful analysis of point defects’ impact on thermal conductivity in fuel performance analysis. A model is presented where point defect concentration is estimated based on Rate Theory modeling and used as input to the Klemens-Callaway model to calculate their contribution to the degradation of thermal conductivity in UO2 under prototypical irradiation conditions. This analysis suggests that point defect concentration is significant at the rim of the fuel pellet and neglecting this leads to underestimation of the centerline temperature, which may have consequences on fission gas behavior.

Generation of sidewall defects in InGaN/GaN blue micro-LEDs under forward-current stress

This work investigates the effect of current stress on InGaN/GaN multiple-quantum-well flip-chip blue micro light-emitting diodes (μ-LEDs) with a mesa size of 30 × 30 μm2 and describes the stress-related mechanisms: defect aggregation and generation, which cause the change in optoelectronic performance of μ-LEDs. A forward-current stress is applied at 75 A/cm2 (0.7 mA) for 200 h. The device performance degrades with increasing stress time except until 25 h. During the initial 25 h of aging, the light output power and the external quantum efficiency (EQE) increase due to the improved crystal quality caused by aggregation of point defects in the active region, which are supported by the ideality factor and the S-parameter. The high-resolution emission-microscope images reveal that the generation of point defects at mesa sidewalls rather than the active region is crucial in performance degradation. We highlight, in particular, that the aging test generates sidewall point defects even though the sidewalls were properly passivated by a thick SiO2 layer. The mechanisms of defect aggregation and generation due to aging are consistently described by the ideality factor, the S-parameter, and the EQE.

Radiation damage in ion-irradiated CeO2 and (Ce, Gd)O2 sinters: Effect of the Gd content

The radiation damage induced in cerium dioxide CeO2 and cerium-gadolinium mixed oxides (Ce, Gd)O2-x was studied by micro-Raman spectroscopy and diffuse UV-visible reflectance spectroscopy. Sintered samples of undoped CeO2 and (Ce, Gd)O2-x for 1 mol% and 5 mol% of Gd2O3 were irradiated at room temperature with 2.5-MeV Ar2+ and 12-MeV Ar4+ ions up to high fluences (i.e. 5 × 1014 cm−2 and 2.1 × 1014 cm−2 respectively). The Raman spectra show that no amorphization of those oxides occurs whatsoever, regardless of the Gd content. No clear change of the damage cross section with the Gd3+ content is deduced from the decay of F2g peak intensity versus fluence. Moreover, the reflectivity spectra show a clear decrease of the band-gap energy and increase of the Urbach energy arising from point defect accumulation. Similar effects are found for 1 mol% and 5 mol% Gd2O3, whereas a larger effect on band tailing is seen for undoped ceria. The respective roles of nuclear collisions and electronic excitations are discussed for both ion energies knowing that the two kinds of measurements do not probe the same sample depth. The substitution of Gd3+ ions for Ce4+ ions is likely reducing the damage and the formation of band tails related to the formation of Ce3+ ions by electronic excitation processes.

Study of Radiation Resistance to Helium Swelling of Li2ZrO3/LiO and Li2ZrO3 Ceramics

The key aim of this paper is to study the presence effect of LiO impurity phases in Li2ZrO3 ceramics on the resistance to helium swelling and structural degradation during implanted helium accumulation in the near-surface layer structure. The hypothesis put forward is based on a number of scientific papers, in which it was reported that the presence of two or more phases in lithium-containing ceramics led to a decrease in the rate of radiation damage and gas swelling due to the presence of additional interfacial boundaries that prevent the point defect accumulation. As a result of the evaluation of the crystal structure deformation, it was found that the presence of the LiO impurity phase in the structure of Li2ZrO3 ceramics led to a threefold decrease in the deformation of the crystal lattice due to helium swelling at doses of 5 × 1017–5 × 1018 ion/cm2. At the same time, the nature of the crystal lattice deformation for different ceramic types is different: in the case of Li2ZrO3 ceramics, an anisotropic distortion of the crystal structure is observed, in the case of Li2ZrO3/LiO ceramics, the crystal lattice deformation has an isotropic nature.

Phase-field simulation of void evolution in UO&lt;sub&gt;2&lt;/sub&gt; under applied stress

Owing to the migration and aggregation of point defects produced by cascade collision, a large number of cavities form easily during irradiation of the uranium dioxide (UO&lt;sub&gt;2&lt;/sub&gt;) that is an important nuclear fuel. In addition, cavities are also inevitably introduced into the ceramic fuel during sintering. Moreover, the creep strain and thermal strain, caused by the extreme environment of high temperature and strong irradiation, significantly increase the applied stress of nuclear fuel. Therefore, it is crucial to investigate the microstructure evolution of the cavities in UO&lt;sub&gt;2&lt;/sub&gt; fuel under applied stress. In this work, a phase-field model of void evolution in UO&lt;sub&gt;2&lt;/sub&gt; under applied stress is established. Firstly, the elastic equilibrium equation is solved by the perturbation-iterative method, and the stress distribution around a single void under applied stress is calculated. The results show that the stress concentration is observed at the edge of the void, and the simulated stress distribution is consistent with the theoretically analytical results. Then, the evolution processes of a single void under different applied stresses are simulated by the phase-field model. The results show that the growth rate of void increases with the augment of applied stress. Finally, the effect of applied stress on grain growth and void evolution in polycrystalline are also studied. The results show that the applied stress will accelerate the void growth. With the increase of the applied stress, the effect of the applied stress on accelerating the void evolution increases.

The effects of temperature and pressure on the physical properties and stabilities of point defects and defect complexes in B1-ZrC

With high stiffness, B1-ZrC in rocksalt structure is a hard structural material for various applications in harsh environments, whereas its tolerance of defects is elusive. Herein, we have investigated the stability and physical properties of B1-ZrC with point defects under high temperature and pressure by means of first-principles calculations accompanied with the quasi-harmonic Debye model. Our results illustrated that the introduction of point defects usually degrades the thermophysical properties. The formations of different point defects with the effects of temperature and pressure are characteristic and classifiable. The accumulation of point defects is in an orderly manner. Carbon vacancies and their complexes are the major point defects in population. The Gibbs energies of formation and concentrations of point defects are sensitive to pressure in the lower pressure range, suggesting an effective routine to manipulate point defects in ZrC. The nearest neighbor defect pair is the most stable at high pressure. The insights from our first-principles calculations might be helpful in material design and defects engineering for ZrC-based materials and its applications.

Size-dependent radiation damage mechanisms in nanowires and nanoporous structures

Nanostructures with a high density of interfaces, such as in nanoporous materials and nanowires, resist radiation damage by promoting the annihilation and migration of defects. This study details the size effect and origins of the radiation damage mechanisms in nanowires and nanoporous structures in model face-centered (gold) and body-centered (niobium) cubic nanostructures using accelerated multi-cascade atomistic simulations and in-situ ion irradiation experiments. Our results reveal three different size-dependent mechanisms of damage accumulation in irradiated nanowires and nanoporous structures: sputtering for very small nanowires and ligaments, the formation and accumulation of point defects and dislocation loops in larger nanowires, and a face-centered-cubic to hexagonal-close-packed phase transformation for a narrow range of wire diameters in the case of gold nanowires. Smaller nanowires and ligaments have a net effect of lowering the radiation damage as compared to larger wires that can be traced back to the fact that smaller nanowires transition from a rapid accumulation of defects to a saturation and annihilation mechanism at a lower dose than larger nanowires. These irradiation damage mechanisms are accompanied with radiation-induced surface roughening resulting from defect-surface interactions. Comparisons between nanowires and nanoporous structures show that the various mechanisms seen in nanowires provide adequate bounds for the defect accumulation mechanisms in nanoporous structures with the difference attributed to the role of nodes connecting ligaments in nanoporous structures. Taken together, our results shed light on the compounded, size-dependent mechanisms leading to the radiation resistance of nanowires and nanoporous structures.

Tomographic Study of Mesopore Formation in Ceria Nanorods.

Porosity in functional oxide nanorods is a recently discovered new type of microstructure, which is not yet fully understood and still under evaluation for its impact on applications in catalysis and gas/ion storage. Here we explore the shape and distribution of pores in ceria in three dimensions using a modified algorithm of geometric tomography as a reliable tool for reconstructing defective and strained nanoobjects. The pores are confirmed as “negative-particle” or “inverse-particle” cuboctahedral shapes located exclusively beneath the flat surface of the rods separated via a sub-5 nm thin ceria wall from the outside. New findings also comprise elongated “negative-rod” defects, seen as embryonic nanotubes, and pores in cube-shaped ceria. Furthermore, we report near-sintering secondary heat treatment of nanorods and cubes, confirming persistence of pores beyond external surface rounding. We support our experiments with molecular modeling and predict that the growth history of voids is via diffusion and aggregation of atomic point defects. In addition, we use density functional theory to show that the relative stability of pore (shape) increases in the order “cuboidal” < “hexagonal-prismatic” < “octahedral”. The results indicate that by engineering voids into nanorods, via a high-temperature postsynthetic heat treatment, a potential future alternative route of tuning catalytic activities might become possible.

Mechanical and optical property assessment of irradiated SiC with displaced atoms

Explicit method to emulate neutron radiation effects on key ceramic materials is lacking, with most literatures stressing direct comparison between radiation parameters while underestimating the defect-property correlation. Herein, the evolutionary correlation between defects of displaced atoms and properties in 6H-SiC was systematically investigated, aiming to found the basis for performance assessment of neutron irradiated material. The point defect accumulation, defect cluster growth, and homogeneous crystalline-amorphous (c-a) transition were recognized, with different stages of Young’s modulus reduction and optical absorption enhancement accordingly triggered. Specifically, the relaxation process accompanying the c-a transition was believed to induce the final reduction stage of Young’s modulus and account for the conflict between existing research results. Average lattice disorder, regarded as the average density of displaced atoms, was proved to be the success reference in the property assessment approach, paving the way for the extension into high temperature radiation where more defect types coexist.

Stress-induced transition from vacancy annihilation to void nucleation near microcracks

The accumulation of point defects and defect clusters in materials, as seen in irradiated metals for example, can lead to the formation and growth of voids. Void nucleation is derived from the condensation of supersaturated vacancies and depends strongly on the stress state. It is usually assumed that such stress states can be produced by microstructural defects such dislocations, grain boundaries or triple junctions, however, much less attention has been brought to the formation of voids near microcracks. Here, we investigate the coupling between point-defect diffusion/recombination and concentrated stress fields near mode-I crack tips via a spatially-resolved rate theory approach. A modified chemical potential enables point-defect diffusion to be partially driven by the mechanical fields in the vicinity of the crack tip. Simulations are carried out for microcracks using the Griffith model with increasing stress intensity factor KI. Our results show that below a threshold for the stress intensity factor, the microcrack acts purely as a microstructural sink, absorbing point defects. Above this threshold, vacancies accumulate at the crack tip. These results suggest that, even in the absence of plastic deformation, voids can form in the vicinity of a microcrack for a given load when the crack’s characteristic length is above a critical length. While in ductile metals, irradiation damage generally causes hardening and corresponding quasi-brittle cleavage, our results show that irradiation conditions can favor void formation near microstructural stressors such as crack tips leading to lower resistance to crack propagation as predicted by traditional failure analysis.

The effect of flux on ion irradiation-enhanced precipitation in AISI-316L: An in-situ TEM study

Thin foils of AISI 316L stainless steel were irradiated in-situ in a transmission electron microscope (TEM) with 325 keV Xe ions at 550 °C at three different fluxes to study flux effects. The kinetics of the radiation-enhanced precipitation (REP) and the evolution of the precipitates were found to be correlated with the irradiation flux. At lower fluxes (1 and 2 × 1012 ions∙cm−2∙s−1), cascade mixing played an important role in the accumulation of point defects within the austenite matrix, facilitating the formation of clusters which act as sinks for heterogeneous nucleation of precipitates with high areal density. At the highest flux (4 × 1012 ions∙cm−2∙s−1) the cascade mixing favours the recombination of vacancies and interstitials which supresses the growth of existing precipitates beyond a certain total damage level. The results agree with a previous radiation-enhanced precipitation model proposed by Wiedersich, Okamoto and Lam and further studied by Bruemmer, but a small modification is proposed when the flux is close to the vacancy-interstitial recombination limit.

In Situ Microstructural Evolution in Fcc and Bcc Complex Concentrated Solid-Solution Alloys Under Heavy Ion Irradiation

This study characterizes the microstructural evolution of single-phase complex concentrated solid-solution alloy (CSA) compositions under heavy ion irradiation with the goal of evaluating mechanisms for CSA radiation tolerance in advanced fission systems. Three such alloys, Cr18Fe27Mn27Ni28, Cr15Fe35Mn15Ni35, and equimolar NbTaTiV, along with reference materials (pure Ni and E90 for the CrFeMnNi family and pure V for NbTaTiV) were irradiated at 50 K and 773 K with 1 MeV Kr++ ions to various levels of dpa using in-situ TEM. Cryogenic irradiation resulted in small defect clusters and faulted dislocation loops as large as 12 nm in FCC CSAs. With thermal diffusion suppressed at cryogenic temperatures, defect densities were lower in all CSAs than in their less compositionally complex reference materials indicating that point defect accumulation is reduced during the displacement cascade stage. High temperature irradiation of the two FCC CSA resulted in the formation of interstitial dislocation loops which by 2 dpa grew to an average size of 27 nm in Cr18Fe27Mn27Ni28 and 10 nm in Cr15Fe35Mn15Ni35. This difference in loop growth kinetics was attributed to the difference in Mn-content due to its effect on the nucleation rate by increasing vacancy mobility or reducing the stacking-fault energy.