Average Number Density Research Articles

Laser powder bed fusion of oxide dispersion strengthened FeCrAl alloy: Processing and microstructural evolution

Optimizing and regulating the process parameters to form a uniform distribution of nanosized dispersoids remains a major challenge in additive manufacturing oxide dispersion strengthened (ODS) alloys. This study introduces the influence of scanning speed and laser power on the microstructure evolution of laser powder bed fusion (LPBF) ODS FeCrAl alloy. Low laser power and high scanning speed reduce the size of oxide particles and increase their number density. The energy density threshold is 83 J/m3, below which the powder fails to achieve complete melting, leading to the formation of defects during solidification. High density and large number of oxide particles were obtained for the printed samples under the parameters of 140 W laser power and 600 mm/s scanning speed. The average size and number density of oxide particles were ∼ 21.61 nm and ∼ 1.8 × 1020/m3, respectively. High scanning speed have a rapid cooling rate which affects the dislocation density and the average width of the columnar grains. Low laser power significantly weakens the 〈100〉 fiber texture, which is related to the degree of recrystallization in the remelted region. The core-shell oxides consisting of Y-Al-O and Y-Ti-O were distributed in the samples deposited at low and high laser powers, respectively. The formation of core-shell oxide particles in LPBF ODS FeCrAl alloys involves solid-state amorphization and recrystallization. This study can be a guideline to achieve materials with specific microstructure for ODS alloys.

Optimizing silver nanowire dimensions by the modification of polyol synthesis for the fabrication of transparent conducting films

Transparent conducting films (TCFs) made by the assembly/deposition of silver nanowires (Ag NWs) are widely used to manufacture flexible electronics such as touch screens, heaters, displays, and organic light-emitting diodes. Controlling the dimensions (length and diameter) of the nanowires is key in obtaining TCFs with the desired optoelectronic properties, namely sheet resistance and optical transparency. This work describes a combined experimental and theoretical investigation on the optimization of the NW dimensions to fabricate high-quality TCFs. Ag NWs of different dimensions are synthesized by the modified polyol method and the average diameter and length of the wires are tailored over a wide range, 35–150 nm and 12–130 μm respectively, by controlling the synthesis parameters such as reaction conditions, stabilizing agents, and growth promoters. The synthesized NWs are spin coated on glass substrates to form TCFs. Comparing the films with different lengths, but identical diameters, enabled the quantification of the effect of length on the optoelectronic properties of the TCFs. Similarly, the effect of NW diameter is also studied. A non-uniformity factor is defined to evaluate the uniformity of the TCF and the transmittance of the NW network is shown to be inversely proportional to its area coverage. The sheet conductance versus the normalized number density is plotted for the different concentrations of NWs to extract a conductivity exponent that agrees well with the theoretical predictions. For thin film networks, the relation between the transmittance and sheet resistance provides the percolative figure of merit (FoM) as a fitting parameter. A large FoM is desirable for a good-performing TCF and the synthesis conditions to achieve this are optimized.

Identification of Superclusters and Their Properties in the Sloan Digital Sky Survey Using the WHL Cluster Catalog

Superclusters are the largest massive structures in the cosmic web, on tens to hundreds of megaparsec scales. They are the largest assembly of galaxy clusters in the Universe. Apart from a few detailed studies of such structures, their evolutionary mechanism is still an open question. In order to address and answer the relevant questions, a statistically significant, large catalog of superclusters covering a wide range of redshifts and sky areas is essential. Here, we present a large catalog of 662 superclusters identified using a modified friends-of-friends algorithm applied on the WHL (Wen–Han–Liu) cluster catalog within a redshift range of 0.05 ≤ z ≤ 0.42. We name the most massive supercluster at z ∼ 0.25 as the Einasto Supercluster. We find that the median mass of superclusters is ∼5.8 × 1015 M ⊙ and the median size ∼65 Mpc. We find that the supercluster environment slightly affects the growth of clusters. We compare the properties of the observed superclusters with the mock superclusters extracted from the Horizon Run 4 cosmological simulation. The properties of the superclusters in the mocks and observations are in broad agreement. We find that the density contrast of a supercluster is correlated with its maximum extent with a power-law index, α ∼ −2. The phase-space distribution of mock superclusters shows that, on average, ∼90% of part of a supercluster has a gravitational influence on its constituents. We also show the mock halos’ average number density and peculiar velocity profiles in and around the superclusters.

The spatial density distribution of H2O2 in the effluent of the COST-Jet and the kINPen-sci operated with a humidified helium feed gas

Abstract Cold atmospheric plasma jets operated with a helium feed gas containing small admixtures of water vapour are excellent sources of H2O2 for direct biomedical applications. However, H2O2 is typically distributed non-uniformly throughout the effluent region, meaning the dosage received by a patient or substrate is dependent on their positioning relative to the plasma source. This study presents the spatial distribution of absolute H2O2 number densities in the effluent of two popular plasma jets, the COST-Jet and the kINPen-sci plasma jet, when operated with a humidified helium feed gas. The measurements were performed using continuous wave cavity ring-down spectroscopy with a tunable, mid-infrared laser. The H2O2 number density measured close to the jet nozzle is 2.3 × 1014 cm−3 for the kINPen-sci plasma jet and 1.4 × 1014 cm−3 for the COST-Jet. The average number density of H2O2 in the effluent of the kINPen-sci plasma jet is a factor of two higher than in the effluent of the COST-Jet. The distribution of H2O2 in the COST-Jet effluent is initially highly uniform and suggests negligible mixing of H2O2 with the ambient air up to 15 mm from the jet nozzle, although it is rapidly diluted at further distances. In the case of the kINPen-sci plasma jet, the number density of H2O2 has a more pronounced radial distribution close to the nozzle, while the mixing with the ambient air is more gradual at further distances from the nozzle. It is evident that a detailed understanding of the H2O2 production in the plasma source, as well as of the transport of H2O2 to the substrate through the effluent, is required in order to optimise the intended effects. This work serves to highlight the difference of the distinct spatial distribution of H2O2 in the effluent of both types of plasma jets when considering their direct application in biomedicine.

Effect of temperature on the anisotropic distribution of defects in self-ion irradiated α-SiC: In-situ TEM investigation

As a structural material suitable for high temperatures and with excellent performance, silicon carbide (SiC) has great potential for application in nuclear systems. However, irradiation-induced anisotropic distribution of defects in hexagonal α-SiC will severely affect its mechanical properties. The effect of temperature on the anisotropic distribution of defects and associated mechanisms is not well understood. In this work, the formation and growth of defects in α-6H–SiC at different temperatures during self-ion irradiation was investigated by in-situ transmission electron microscopy (TEM), and the effect of temperature on the anisotropic distribution of defects was explored. Irradiation temperature significantly affects the average size and number density of defect clusters, the critical irradiation damage at which defect clusters appear, and the growth rate of defect clusters. Moreover, irradiation temperature had a significant effect on the anisotropic distribution of defects in α-SiC and exhibited a critical temperature effect. The possible mechanism dominating the temperature-dependent effects on the number density of defects and anisotropy was discussed, and the effect of recovery rate on the generation of defects was explored.

Enhanced irradiation tolerance of a medium entropy alloy via precipitation and dissolution of nanoprecipitates

High/medium entropy alloys (HEAs/MEAs) are good candidates for nuclear applications due to the excellent mechanical properties, good corrosion resistance and radiation resistance. In this work, a novel cobalt-free MEA was developed by introducing L12 nanoprecipitates. The microstructure evolution and radiation tolerance were evaluated after bombarded using 3 MeV Fe11+ ions at 500 °C. The evolution of nanoprecipitates was closely related to the irradiation dose, and dominated by irradiation-enhanced diffusion and ballistic dissolution mechanism. For the solid-solution MEA (without L12 nanoprecipitates), irradiation hardening occurred due to the irradiation-induced formation of precipitates, voids and dislocation loops. However, in MEA with L12 nanoprecipitates after aging, irradiation induced dissolution and reprecipitation of nanoprecipitates were observed. Different from the solid-solution MEA, the hardness kept almost unchanged in the aging sample after irradiation. The swelling rate of the solid-solution sample after irradiation is 9.4 × 10−6 %/dpa, while no swelling occurs in the aging sample under the same irradiation condition. Besides, the average size and number density of dislocation loops in the aging sample decreased by ∼ 40% and ∼ 28%, compared with the solid-solution sample. The precipitation and dissolution of nanoprecipitates substantially improved the radiation tolerance of the cobalt-free MEA.

Effect of the surface water content on dry saltation cloud dynamics: A wind tunnel simulation with particle tracking velocimetry

AbstractThe mass transport rate of wind‐borne particles, and indeed the fluid stress required for their entrainment, is strongly governed by inter‐particle cohesion arising from water retained through adsorption and capillary force. This paper reports on a series of wind tunnel experiments in which high‐speed photography was used to record the motion of dry sand particles as they impinged on test beds of systematically varied target gravimetric pore water content (0% ≤ W ≤ 10%). The wind friction velocity was preset to 0.33 m s−1, sufficient to maintain a saltation cloud above an upwind strip of dry sand, which then was blown over the wetted surface. Discrete particle trajectories were identified in the camera images using expected particle area searching–particle tracking velocimetry (EPAS‐PTV). Adding progressively more water produced an exponential decrease in the normalized particle number density over the test surface. The largest response was achieved by wetting the bed surface to just 2%. To reduce the number of particles by a further 40%, it was necessary to add eight times more water, signifying a diminishing return regarding water use. Relative to particles either rebounding or splashed from a dry bed, the total particle velocity increased incrementally by a factor between 1.5 and 2 with increasing water content. Increasing amounts of pore water were associated with progressively higher saltation trajectories, reaching a plateau beyond W ~8%. The spatio‐temporal adjustment in the sand cloud was observed to be extremely rapid. There is no consensus in the literature on how to measure the water content that effectively governs aeolian transport. In this study, all approaches to sampling W produced strong correlation (R2 ≥ 0.85). Sampling the topmost grains, however, provided the most accurate prediction of the normalized number density over the full range of water content.

Grain coarsening during liquid phase sintering of zircon–glass composites

This study examined zircon grain growth during isothermal sintering of a zircon–glass composite with a zircon volume fraction of 0.525 zircon volume/solids volume, at a temperature of 1200°C, with different residence times, ranging from 5min to 1500min. Scanning electron microscopy and image analysis were used to determine zircon grain size and number in the fired test specimens. The residual glass composition was determined by energy dispersive X-ray spectroscopy. For each firing condition, the grain size number distribution was fitted to a lognormal function and to a Weibull function. In every case, the lognormal distribution described the results better. Grain size grew and distribution width decreased with sintering time in accordance with non-steady-state grain growth models. At high sintering times, the variation of average grain diameter and grain number density per unit solids cross-section fitted well to steady-state growth models. Moreover, when grain size distribution was scaled by the average size, it was practically time independent. In contrast, scaled grain distribution width was much wider than that predicted by these models. A model was developed that satisfactorily predicts zircon grain evanescence throughout the sintering process.

Spectroscopical Characterization of Copper–Iron (Cu-Fe) Alloy Plasma Using LIBS, ICP-AES, and EDX

In this present work, we demonstrated a spectral characterization of copper–iron (Cu-Fe) alloy using optical emission spectroscopy. The Cu-Fe alloy plasma was generated on the target sample surface by directing the laser pulse of Q-switched Nd: YAG of the second harmonic (2ω) with a 532 nm optical wavelength. The optical emission spectrum was acquired using five miniature spectrometers that lie within the wavelength range of 200–720 nm. The emission plasma was characterized by validating the local-thermodynamical equilibrium (LTE) as well as optically thin (OT) plasma condition. In addition, the LTE condition was verified using the McWhirter criterion, and the OT condition was validated by comparing theoretically calculated intensity ratios with experimental ones. Plasma parameters, including electron number density as well as plasma temperature, were estimated. In the first stage, the plasma temperature was estimated using the Boltzmann-plot method and the two-line method. The average calculated value of the plasma temperatures were 8014 ± 800 K and 8044 ± 800 K using the Boltzmann-plot and two-line methods, respectively. In the second stage, electron number density was estimated using the Saha–Boltzmann equation and stark-broadening method (SBM). The average number density calculated from the SBM was 2.73×1016 cm−3 and from the Saha–Boltzmann equation was 3.9×1016 cm−3, showing a good agreement. Finally, the comparative compositional analysis was performed using CF-LIBS, Boltzmann Intercept Method, EDX, and ICP-AES, which showed good agreement with that of the standard composition.

Life beyond 30: Probing the −20 < M UV < −17 Luminosity Function at 8 < z < 13 with the NIRCam Parallel Field of the MIRI Deep Survey

We present the ultraviolet luminosity function and an estimate of the cosmic star formation rate density at 8 < z < 13 derived from deep NIRCam observations taken in parallel with the MIRI Deep Survey of the Hubble Ultra Deep Field (HUDF), NIRCam covering the parallel field 2. Our deep (40 hr) NIRCam observations reach an F277W magnitude of 30.8 (5σ), more than 2 mag deeper than JWST public data sets already analyzed to find high-redshift galaxies. We select a sample of 44 z > 8 galaxy candidates based on their dropout nature in the F115W and/or F150W filters, a high probability for their photometric redshifts, estimated with three different codes, being at z > 8, good fits based on χ 2 calculations, and predominant solutions compared to z < 8 alternatives. We find mild evolution in the luminosity function from z ∼ 13 to z ∼ 8, i.e., only a small increase in the average number density of ∼0.2 dex, while the faint-end slope and absolute magnitude of the knee remain approximately constant, with values α = − 2.2 ± 0.1, and M * = − 20.8 ± 0.2 mag. Comparing our results with the predictions of state-of-the-art galaxy evolution models, we find two main results: (1) a slower increase with time in the cosmic star formation rate density compared to a steeper rise predicted by models; (2) nearly a factor of 10 higher star formation activity concentrated in scales around 2 kpc in galaxies with stellar masses ∼108 M ⊙ during the first 350 Myr of the universe, z ∼ 12, with models matching better the luminosity density observational estimations ∼150 Myr later, by z ∼ 9.

The magnetic field design of a solenoid for the cold-cathode Penning ion source of a miniature neutron tube

A high-yield and beam-stable neutron tube can be applied in many fields. It is of great significance to the optimal external magnetic field intensity of the cold-cathode Penning ion source (PIS) and precisely controls the movement of deuterium (D), tritium (T) ions and electrons in the source of the neutron tubes. A cold-cathode PIS is designed based on the solenoidal magnetic field to obtain better uniformity of the magnetic field and higher yield of the neutron tube. The degree of magnetic field uniformity among the magnetic block, double magnetic rings and solenoidal ion sources is compared using finite element simulation methods. Using drift diffusion approximation and a magnetic field coupling method, the plasma distribution of hydrogen and the relationship between plasma density and magnetic field intensity at 0.06 Pa pressure and a solenoid magnetic field are obtained. The results show that the solenoidal ion source has the most uniform magnetic field distribution. The optimum magnetic field strength of about 0.1 T is obtained in the ion source at an excitation voltage of 1 V. The maximum average number density of monatomic hydrogen ions (H+) is 1 × 108 m−3, and an ion-beam current of about 14.51 μA is formed under the −5000 V extraction field. The study of the solenoidal magnetic field contributes to the understanding of the particle dynamics within the PIS and provides a reference for the further improvement of the source performance of the neutron tube in the future.

Neutron yield scaling law in laser-cluster fusion experiments

We present a scaling law () of fusion neutron yields (Y) for laser pulse energy (E) in laser-cluster fusion experiments. We compare the available neutron yield data from previous deuterium cluster fusion experiments with those calculated using the cylindrical fusion plasma model. The calculated neutron yields are shown as functions of the incident laser pulse energy, average number density, and ion temperature. Although the deuterium–deuterium fusion reactivity is known to increase rapidly with ion temperature, the neutron yield shows a modest increase above ∼10 keV for a given laser pulse energy. We find the scaling exponent β approaching 1.0 as the ion temperature increases from 1 keV to 100 keV. We explain the observed temperature dependence of β by examining the temperature dependence of the beam–beam and beam–target fusion neutron yields separately. Our scaling law differs from previously reported scaling laws from individual experiments, but it shows an excellent agreement with the scaling law determined by the maximum neutron yields of individual experiments.

Low-Energy Ion Scattering Intensities from Supported Nanoparticles: The Spherical Cap Model

Supported nanoparticles are of great importance to many technologies like fuel processing and chemical synthesis using catalysts and electrocatalysts, energy storage and generation using fuel cells and batteries, electrochemistry, magnetic information storage, and more. Low-energy ion scattering spectroscopy (LEIS) with noble gas ions like He+ is a powerful tool for the characterization of nanoparticles dispersed across flat support surfaces due to its ability to probe the elemental composition in the topmost atomic layer of a surface, providing quantitative information regarding the size and number density of nanoparticles. In this work, we present a derivation of the LEIS intensities expected from nanoparticles and the support material as a function of the average particle size, their number per unit area, and their contact angle with the support when modeled as spherical caps of the nanoparticle material dispersed over the surface of a flat support. The model assumes that the ion intensities are determined only by the physical blocking of linear ion trajectories and independent of the tilt angle of the local surface relative to the incident and scattered ion directions, an assumption we support by quantitative modeling of published data which tested tilt-angle effects. The model is a generalization to arbitrary contact angles of the hemispherical cap model, which assumes 90° contact angle and has been widely used to model spectroscopic signals in LEIS (and also in Auger and photoelectron spectroscopies) during nanoparticle growth. This new model quantitatively reveals how LEIS signals are sensitive not only to the diameter and number density of the nanoparticle but also to their contact angle (or height/diameter ratio). With the use of additional data (e.g., from microscopy or adsorption microcalorimetry), the model presented here will enable more accurate determination of the average size, shape, and number density of supported nanoparticles based on LEIS intensity measurements.

Single-size and cluster dynamics modeling of intra-granular fission gas bubbles in UO[formula omitted

We perform simulations of intra-granular fission gas bubble evolution in UO2 using both a relatively simple, computationally inexpensive single-size model and a detailed cluster dynamics model. Simulations encompass 36 experimental cases from 4 different databases, covering various temperature and burnup levels. We systematically compare results from the two models to each other and to post-irradiation experimental data of bubble average size and number density. Overall, the model-to-model comparisons reveal an excellent agreement across the set of simulations. This outcome indicates that, in spite of the underlying assumptions, the single-size model provides a good approximation of the complex physical behavior that is more rigorously described by the cluster dynamics model. Qualitatively, both models reproduce the trends of the experimental data with temperature and burnup correctly. Quantitatively, calculated results are either in good agreement with the data or within errors that appear consistent with the inherent uncertainties. Moreover, for the single-size model, we demonstrate and assess a multiscale approach whereby values for the fission gas atom diffusion coefficient from separate atomistic calculations are used. Systematic comparisons to experimental data point out a credible accuracy of the multiscale model.

Comparison between 1D radial and 0D global models for low-pressure oxygen DC glow discharges

This work focuses on the comparison between a zero-dimensional (0D) global model (LoKI) and a one-dimensional (1D) radial fluid model for the positive column of oxygen DC glow discharges in a tube of 1 cm inner radius at pressures between 0.5 Torr and 10 Torr. The data used in the two models are the same, so that the difference between the models is reduced to dimensionality. A good agreement is found between the two models on the main discharge parameters (gas temperature, electron density, reduced electric field and dissociation fraction), with relative differences below 5%. The agreement on other species average number densities, charged and neutral, is slightly worse, with relative differences increasing with pressure from 11% at 0.5 Torr to 57% at 10 Torr. The success of the 0D global model in describing these plasmas through volume averaged quantities decreases with pressure, due to pressure-driven narrowing of radial profiles. Hence, in the studied conditions, we recommend the use of volume-averaged models only in the pressure range up to 10 Torr.

Effect of different pre-existing dislocation densities on the microstructure evolution of W-0.5ZrC alloy during in-situ He+ irradiation

The evolution of dislocation loops and helium bubbles in W-0.5ZrC alloy with different pre-existing dislocation densities was investigated by in-situ transmission electron microscopy during 30 keV He+ irradiation at 800 °C. The nucleation, migration, aggregation, and growth of dislocation loops and helium bubbles occurred with the whole irradiation process. The variation trends of average size and volume number density of dislocation loops and helium bubbles with the increase of irradiation dose in the regions with different pre-existing dislocation densities were obtained. It was found that the growth of dislocation loops was inhibited in the region with a low pre-existing dislocation density (about 5.2 × 1013 /m2), and the nucleation of dislocation loops was inhibited in the region with the dispersed medium density (about 1.6 × 1014 /m2, dislocations were crossed but dispersed), while both the nucleation and growth of dislocation loops were inhibited in the region with the compact medium density (about 1.5 × 1014 /m2, dislocations were crossed and compact). During the irradiation process, the pre-existing dislocations would interact with the irradiation-induced dislocation loops and helium bubbles, but their influence weakened with the increase of irradiation dose, and finally resulted in the disappearance or deformation of pre-existing dislocations. The irradiation hardening caused by dislocation loops and helium bubbles was increased with the increase of irradiation dose, and the corresponding mechanism was analyzed.

Effect of LHS-1269 on morphology and angiogenesis of Lewis lung carcinoma

Background. The new anticancer chemical compound LHS-1269 from the class of indolocarbazoles is undergoing preclinical studies at the N.N. Blokhin National Medical Research Center of Oncology, Ministry of Health of Russia. LHS-1269 has a high antitumor activity on transplanted tumors of mice, showed high cytotoxic activity on many tumor cells lines in vitro, and has shown an inhibitory effect on vasculogenic mimicry in tumors in vitro. LHS-1269 does not affect the catalytic activity of human topoisomerases I and IIα. An antiangiogenic mechanism of the drug’s antitumor action is suggested.Aim. To evaluate the effect of LHS-1269 on the morphological features and angiogenesis of transplanted Lewis lung carcinoma (LLC) in BDF1 mice.Materials and methods. In BDF1 mice (n = 20) with transplanted LLC tumor on days 1 and 3 after 5-fold intraperitoneal administration of LHS-1269 in a single dose of 60 mg (total dose – 300 mg/kg), mice (n = 20) on days 5 and 8 after a single intravenous injection of LHS-1269 at a dose of 100 mg/kg, mice (n = 20) with transplanted LLC tumor without administration of LHS-1269 (control). The assessment of the antitumor effect was carried out according to the criterion of inhibition of tumor growth (%) and the study of the morphological features of the tumors. To assess the effect of LHS-1269 on tumor angiogenesis in LLC tumor sections, a visual calculation of the average number (density) of blood vessels and immunohistochemical detection of expression of the CD31+ endothelial marker were performed.Results. The LHS-1269 compound in animal groups with 5-fold and 1-fold use caused tumor growth inhibition – 59–70 and 67–79 %, with pronounced morphological changes and tumor cell death. There is an uneven distribution of blood vessels in tumors and surrounding tissues in all groups. LHS-1269 caused a statistically significant decrease in the average number of blood vessels in the tumor both after 5-fold and after 1-fold administration. The statistically significant decrease in the average number of blood vessels in the surrounding connective tissue tumor was observed only after 5-fold administration of the drug. The decrease in the average number of CD31+ endothelial cells after five times intraperitoneal administration was statistically insignificant compared to control (83.1 ± 8.7 and 59.6 ± 18.9, respectively). The increase in this indicator after a single intravenous injection of LHS-1269 was statistically significant.Conclusion. LHS-1269, when administered five times and once in mice, caused pronounced morphological changes in the form of damage and death of LLC tumor cells. The results of a study of angiogenesis in a tumor do not allow an unambiguous conclusion about the inhibitory effect of LHS-1269 on angiogenesis in LLC tumors in mice.

Effect of Cu Additions on the Evolution of Eta-prime Precipitates in Aged AA 7075 Al–Zn–Mg–Cu Alloys

In the present study, after solid solution treatment, four different artificial aging treatments (100, 120, 140 and 160 °C) were performed on Al-5.98Zn-2.86Mg-1.61Cu (wt.%) alloy, denoted as 7075-LCu, and Al-5.91Zn-2.83Mg-1.98Cu (wt.%) alloy, denoted as 7075-HCu. Peak aging conditions were determined for each aging temperature at various hold time intervals of up to 24 h. It was found that both alloys possessed the optimal strengths after artificial aging at 120 °C for 24 h. Under this condition, the ultimate tensile strengths (UTSs) were 618 MPa (7075-LCu) and 623 MPa (7075-HCu), respectively. Moreover, a method was used to calculate the average sizes and number density of the major strengthening precipitates, η′, under peak aging conditions in these two alloys from transmission electron microscopy (TEM) images and electron energy loss spectroscopy (EELS). The above results indicated that for the 7075-LCu and 7075-HCu samples with the optimal UTS strengths, the former possessed an average thickness of 2.15 nm, and a number density of 3.27 × 1017 cm−3; the latter, 2.04 nm and 3.52 × 1017 cm−3.

Nano-sized Cu precipitation and microstructural evolution in aged ultralow and medium carbon steels

The nanoscale Cu precipitation exhibits considerable potential for enhancing strength and cyclic hardening potential in steels. In the present work, Cu-alloyed ferritic steels with different C contents (X0.5: 0.005 wt.%, X21: 0.21 wt.%) were investigated to systematically manifest the influences of C contents on the microstructural evolution, the precipitation behavior of nano-sized Cu precipitates and the resulting effects on cyclic hardening behavior. The investigated materials were subjected to austenitization at 900 °C for 10 min and subsequently aged at 600 °C for different time durations (120, 720, 7200 s). The 3D atom probe tomography rendered the investigation of nanoscale Cu precipitates in terms of average size, local chemistry, number density and spatial distribution. In X21 steel, the Cu precipitates exhibited a large nucleation density upon the onset of aging and further grew into a larger precipitate size. Compared to X0.5 steel, the higher nucleation density and the inhomogeneous precipitate distribution in X21 steel were attributed to the heterogeneous nucleation and high C content in the matrix. Cyclic indentation tests revealed a strong dependency of hardness and cyclic hardening potential on precipitation size and number density for both steels. Furthermore, for both steels aging had a similar effect on the mechanical properties. However, the increased C content induced a higher hardness and lower cyclic hardening exponentCHT of X21 steel.

A combined rate theory-population balance model of the evolution of irradiation-induced helium bubbles in metals during annealing

A multiscale model describing the evolution of helium bubbles in the irradiated materials is established based on the rate equations and the population balance equation. The size distribution of the helium bubbles and the evolution law of the statistical average size and number density of the bubbles with time changing predicted by the model are in good agreement with the experimental statistics. The influence of annealing temperature, annealing time, irradiation energy, irradiation flux, and coarsening mechanisms on the evolution of helium bubbles are numerically simulated and discussed, taking FeCrAl alloy as the irradiated material. The results show that the bubble evolution is dominated by the Ostwald ripening mechanism under high temperature annealing conditions (≥ 1073 K), and the time dependence of the average size and density of helium bubbles is consistent with the existing theoretical results. And the number of shrinking helium bubbles caused by Ostwald ripening is effectively reduced by the bubble coalescence effect, leading to further growth of the proportion of large-sized helium bubbles. Consequently, a Gaussian bubble size distribution has been obtained modeling the coupling of the Ostwald ripening mechanism and the bubble coalescence mechanism. The generation rates of helium and vacancy in the material during irradiation, as well as their ratio, are significant factors that can affect the nucleation and evolution of helium bubbles.