Diffusion Of Interstitial Atoms Research Articles

Effect of temperature on dynamic strain aging and intermediate temperature embrittlement of nickel-based alloy N06625

Due to its excellent mechanical properties and good corrosion resistance at low to intermediate temperatures, nickel-based alloy N06625 has gradually become an ideal material for high-temperature gas-cooled reactor evaporators in nuclear power plants. It is currently receiving more and more attention. In this work, the tensile test of nickel-based alloy N06625 was carried out at room temperature to 900℃. The dynamic strain aging (DSA) process and microstructure evolution were studied by electron back-scatter diffraction (EBSD). The dynamic strain aging and intermediate temperature embrittlement of nickel-based alloy N06625 in the temperature range 25–900℃ were systematically studied. The results show that the stress-strain curve exhibits a serrated shape at 200–700℃, and with the increase in temperature, the serrated type changes from type A to type A+B and type C. The type A and type (A+B) serrations are related to the diffusion of interstitial carbon atoms. The interaction between substitutional elements Cr and Mo and movable dislocations causes the type C serration. As the temperature increases, high-density dislocations accumulate at the grain boundaries, and the proportion of substructural grains increases. A brittle fracture occurs at 600–800℃. The fracture mechanism may be due to the stress concentration at the grain boundary caused by the appearance of micropore, which weakens the grain boundary's strength and promotes the grain boundary's early failure. The continuous increase of strain makes the micropore fuse and grow to form a crack source.

Creep-recovery deformation of 304 stainless-steel springs under low forces

The deformation behavior of materials at high temperatures determines the structural stability of mechanical structures under high-temperature service conditions. In this work, we prepare helical springs from 304 stainless-steel wires and study creep-recovery deformation of the helical springs under small forces at temperatures of 475–575 °C. In contrast to most methods reported in the literature, we use Kelvin representation of the Burgers model with a nonlinear dashpot in series connection to analyze the creep-recovery deformation of the helical springs and suggest the deformation mechanisms of the diffusion of interstitial atoms and dislocation generation/annihilation in transient creep and recovery of the helical springs under small forces. For the creep deformation, the stress exponent and activation energy for the plastic flow of the nonlinear dashpot are 2 and 57.8 kJ/mol, respectively, and the activation energy for the deformation flow of the linear dashpot is 41.8 kJ/mol. The nonlinear dashpot does not play any role in the recovery deformation, and the formation energy of defects for the recovery deformation of the helical springs is 51.4 kJ/mol. The approach used in this work provides a simple method to use a phenomenological model with a nonlinear dashpot to capture the power-law creep deformation of metallic materials.

Improving the Mechanical Properties of Low‐Carbon Steel by In Situ Strain Aging

A novel continuous process that utilizes the concept of strain aging not only in the automotive industry but also in structural materials as a whole is developed. The principle behind achieving in situ strain aging is as follows. 1) Dislocations are generated through compressive deformation during continuous pressing. 2) The processing conditions at the strain aging temperature create a favorable environment for the diffusion of interstitial atoms, leading to the formation of Cottrell atmospheres. The in situ strain aging‐processed low‐carbon steel demonstrates a significant increase in strength compared to the unprocessed sample (increased yield strength by ≈37.6 MPa), which can be attributed to the strain aging effect, as well as the combined effects of grain refinement and pre‐existing dislocations. Additionally, the generation of dislocations during compressive deformation suppresses void nucleation during pre‐strain, preventing a loss of elongation (reduced uniform elongation by ≈1.5%). The in situ strain aging‐processed low‐carbon steel exhibits a superior strength–elongation combination compared to both the unprocessed low‐carbon steel and strain aging‐simulated counterparts obtained through tensile deformation.

Machine Learning for Semiconductor Process Simulation Described by Coupled Partial Differential Equations

AbstractTechnology computer‐aided design (TCAD) simulation is an important tool for the development of semiconductor devices. Based on coupled partial differential equations (PDEs) for behaviors, TCAD can calculate objects such as impurities, point defects, and electronic carriers in semiconductors. However, over recent years semiconductor devices have become increasingly miniaturized and complicated, resulting in much longer calculation times for TCAD. Machine learning is one technology that may be used to overcome this simulation cost problem. In this study, a neural network architecture is proposed that considers the structure of the coupled PDEs. Features representing each concentration distribution of the calculation objects are extracted by convolution operations and their reaction is modeled by channel attention. The performance of the proposed architecture and of conventional neural network models is evaluated using a simulation dataset generated by 1D coupled PDEs that models the diffusion and reaction of vacancies and interstitial atoms. In addition, the advantage of the method is discussed through the analyses of error correlations of the two predictions and attention coefficients. The machine learning method developed in this study will be applicable to other physics described by coupled PDEs and is expected to speed up the computation of simulations in various fields.

LOCALIZATION OF NITROGEN ATOMS IN Si–SiO2 STRUCTURES

Studies have been carried out by time-of-flight mass spectroscopy of secondary ions of subcutaneous silicon
 oxides, nitridation by ion implantation (II) or nitrided by high-temperature annealing in an atmosphere of N2. Nitrogen
 AI was produced with an energy of 40 keV, implantation doses of 2,5×1014 and 1×1015 cm-2. High-temperature
 annealing was carried out at a temperature of 1200 C for 2 hours or at 1100 C for 30 minutes. It is established
 that at the Si–SiO2 interface, after nitriding by II or high-temperature annealing, a maximum with a high concentration
 of nitrogen atoms is observed. It is shown that after conducting nitrogen AI with a dose of 2,5 ×1014 cm-2
 through a protective SiO2 with a thickness of 23 nm and RTA at 1000 C for 15 seconds, the main maximum
 of nitrogen distribution (1×1019 cm-3) is observed at the Si–SiO2 interface, which indicates the presence of a saturation
 concentration of the Si–SiO2 interface.
 
 A charge-based one-dimensional Fermi model is proposed to describe the accelerated diffusion of nitrogen
 atoms. The main mechanism is the diffusion of interstitial atoms, which can occur with the preliminary displacement
 of nodal nitrogen atoms by their own embedding atoms. It is shown that nitrogen atoms can act as annihilation
 centers of point defects in the silicon crystal lattice.

Microstructural control and reinforcement mechanism of the Mo and B inserts in the SiCf/SiC composites/Ni-based superalloy joint

The SiCf/SiC composites/Ni-based superalloys dissimilar joining encountered the problem of huge thermal mismatch and uncontrollable interfacial reactions. Two novel composite fillers were designed for the dissimilar joining. An AuCuTi/Mo/AuCuTi filler enabled a complete hindrance on the atomic exchange by a 50 μm Mo interlayer and an effective stress relief by multiple layers of the ductile (Au, Cu) solid solution and rigid Mo insert. Furthermore, the addition of B introduced an extra dimensionality to control the diffusion of interstitial atoms (Ni, Cr, and Si) and some TiB whispers to add extra nucleation sites for the Ni-compounds, by which way an easier and more accurate method to control the distribution of metallic atoms were obtained. The load-carrying of joints increased by 17.7% when using an AuCuTiB/Mo/AuCuTiB filler, because of the distribution of intermetallic compounds was homogenized and fined, and the interfacial region changed from Ti3SiC2 to TiC + Ni-compounds.

The Theory of Diffusion Zone Formation during Surface Modification of Materials with Allowance for the Thermodiffusion Effects

Using asymptotic methods in the theory of differential equations, an analytical approximate solution to the problem of diffusion of interstitial atoms in a restricted medium under the condition of a constant temperature gradient is obtained. As follows from the analysis, this solution is applicable to plate samples undergoing thermal and ionic modification of one of the surfaces, whose thickness does not exceed the diffusion penetration depth of impurities, provided that a stationary temperature distribution over the thickness is established. For the certification of the proposed method, a comparison of the approximate solution with the numerical solution of the thermal diffusion problem is carried out. Conditions are obtained under which the contribution of thermal diffusion effects to the total diffusion mass transfer of the impurity is significant.

Dynamic strain aging and serrated flow behavior of Cr-Ti-B low carbon steel during warm deformation

The compression behavior of Cr-Ti-B low carbon steel was studied over the temperature range from 250 to 650 °C at three strain rates: 0.01 s−1, 0.1 s−1, and 1 s−1. Moreover, the serrated plastic flow was analyzed during compression, which revealed that type A serrations occurred at high strain rates, while types B or B + C serrations formed at low strain rates. The temperature range (250–350 °C) of negative strain rate sensitivity (NSRS) was slightly lower than that (300–400 °C) of dynamic strain aging (DSA). A pronounced increase of peak stress was observed at 350 °C deformation temperature and a 1 s−1 strain rate, which was attributed to the interactions between the interstitial atoms or atmospheres and movable dislocations. The average diffusion activation energy (73 kJ/mol) at low temperature (250–350 °C) also indicated that the serration mechanism was associated with the diffusion of interstitial atoms. The serration behavior at high temperatures (550–650 °C) was attributed to the pinning of dislocations by precipitates. The steel tended to exhibit planar slip of dislocations in the DSA region, and dynamic recovery was dominant during deformation at 550 °C and above.

Структура і властивості сталі 25ГСЛ, модифікованої наночастинками заліза

The results of the study of the effect of nanoparticles in the shell of iron oxide Fe2O3, which when injected into the melt and heated up to melt temperature is converted into oxide Fe3O4, on the microstructure of hardened and tempered steel 25GSL and its properties. It is shown that in modified steel martensite crystals thickness is reduced compared to the original steel in average 1.9 times after the quenching, tempered martensite crystal length after tempering hardened steel at 200oC - 3 times, the plate cementite length in troostite after tempering hardened steel at 450 оС – 1,4 times, the carbide length of after tempering of steel at 550 оС – 1,6 times. It has been found that modification of iron nanoparticles in a shell of Fe3O4 oxide leads to dispersion of the subgrain structure (regions of coherent scattering) and to the decrease of the crystal lattice parameters of solid solutions. The mechanism of nanoparticle influence on the reduction of the crystal lattice parameters of solid solutions, which is to accelerate the diffusion of interstitial atoms of the elements into the zones of high stress on the nanoparticles surface, is proposed. It is determined that the optimal improvement mode of the modified steel is quenching at 900 °C and tempering at 550 °C. The yield, tensile strengths and toughness of the modified steel are 894 MPa, 993 MPa and 92 J/mm2, respectively, on 62, 41 and 14% higher than the original steel. The injection of iron nanoparticles in a refractory shell into the steel melt before crystallization predetermines the effective modification and enhancement of mechanical properties. Keywords: iron nanoparticles, structure, martensite, troostite, sorbite, carbides, stresses, phase transformations.

First-principles study of chromium diffusion in the ferritic Fe-Cr alloy

Ferritic Fe-Cr alloy is an important interconnect material for solid oxide fuel cell (SOFC) stacking. The Cr2O3 scale growth and Cr-induced cathode poisoning both related to the Cr diffusion have become the main concerns affecting the efficiency and life of the interconnect. In this study, a systematic theoretical study on the point defects and Cr diffusion process in the ferritic Fe-Cr alloy is performed by first-principles calculations. The different crystal models with vacancy or impurity Cr atoms are set up and optimized. The formation energies of two types of Fe vacancy and one type of Cr vacancy are obtained and compared. The adsorption of an impurity Cr atom in the Fe-Cr alloy is investigated and one type of stable tetrahedral interstitial and two types of metastable octahedral interstitials are found. Different diffusion pathways are explored including the vacancy diffusion and the interstitial Cr atom diffusion in the pure Fe layer and in the Fe-Cr mixed layer of the crystal, respectively. It is found that the interstitial Cr atom is easier to diffuse in the pure Fe layer in the studied Fe-Cr alloy. The activation energy and the diffusion coefficient of each diffusion pathway is obtained and the magnitude of Cr diffusion coefficient mainly varies from 10−15 to 10−12 m2/s under the realistic SOFC working temperature of 600 °C-800 °C. Our results provide a profound understanding of the microscopic diffusion mechanism of Cr atom in the ferritic Fe-Cr alloy, which could give theoretical guidance for the inhibition of Cr poisoning in the application of SOFC interconnect.

Tunable chemical complexity to control atomic diffusion in alloys

In this paper we report a new fundamental understanding of chemically-biased diffusion in Ni–Fe random alloys that is tuned/controlled by the intrinsic quantifiable chemical complexity. Development of radiation-tolerant alloys has been a long-standing challenge. Here we show how intrinsic chemical complexity can be utilized to guide the atomic diffusion and suppress radiation damage. The influence of chemical complexity is shown by the example of interstitial atom (IA) diffusion that is the most important defect in radiation effects. We use μs-scale molecular dynamics to reveal sluggish diffusion and percolation of IAs in concentrated Ni–Fe alloys. We develop a mean field diffusion model to take into account the effect of migrating defect energy properties on diffusion percolation, which is verified by a new kinetic Monte Carlo approach addressing detailed processes. We demonstrate that the local variations in the ground state energy of IA configurations in alloys, reflecting the chemical difference between alloying components, drives the percolation effects for atomic diffusion. Percolation, chemically-biased and sluggish diffusion are phenomena that are directly related to the chemical complexity intrinsically to multicomponent alloys.

First-principles study of the insertion and diffusion of interstitial atoms (H, C, N and O) in nickel

Solubility and diffusivity of interstitials in Ni is of prime importance to understand and quantify reactivity of Ni-base structural alloys at high temperature. We thus present a first-principles study of the insertion and diffusivity of interstitial species in nickel. We put an emphasis on carbon, nitrogen and oxygen atoms and compare our results with those found for hydrogen and in the literature. The interactions with the metal and the relative stability of sites are discussed in detail in terms of phonon, electronic, elastic, magnetic properties and charge transfers. As a result, we identified one new stable interstitial position for C and N atoms, and we also showed that the tetrahedral site can be considered as unstable from an elastic standpoint for carbon. In the light of these new results, diffusion mechanisms were reviewed and diffusion coefficients were calculated. The effects of temperature on enthalpy and migration energies were investigated. We thus showed that C and N atoms on one hand, H and O atoms on the other hand, show the same behavior.

Hydrogen Transport and Metal Embrittlement Risk in Storage and Industrial Applications

A physical model studying the diffusion of interstitial atoms has been used in the study of hydrogen redistribution, in order to predict the risk of hydrogen damage in a range of manufacturing processes. In this work, conditions representative of hydrogen storage and some scenarios in the nuclear or chemical industries are considered. A singular advantage of this model is that, contrary to some simplified commercial and academic models, it contemplates diffusion in its most comprehensive description, i.e., with the driving force for atom diffusion being the gradient in chemical activation instead of simply considering it occurs down a composition gradient. Because the model also incorporates thermal history, microstructure, matrix solubility, multiple trapping distributions, interaction with the atmosphere and others, it is ideally suited to study real industrial applications. In this work, several simulations of hydrogen permeation are considered. Hydrogen permeation in industrial applications may introduce damage within the metal structure, leading to delayed failure. In the cases studied hydrogen is transported through a metal wall separating one volume with high hydrogen pressure and/or high temperature from another volume with low hydrogen pressure and temperature. By using such comprehensive physical model, it is possible to study the effects of hydrogen pressure and temperature gradient, wall thickness, metal microstructure and trap distribution on the flux across the wall and on the accumulation of hydrogen within the metal. Furthermore, it makes possible to estimate the embrittlement risk and when necessary the time to fracture. Keywords: hydrogen, steel, permeation, physical model, hydrogen storage, nuclear industry

First-principles study of the diffusion of Li in bcc Fe

First-principles calculations based on density functional theory were performed to investigate the diffusion behavior of lithium atoms in bcc iron crystals. The site-preference was verified primarily. The interactions with vacancies and self-interstitial atoms were analyzed. Subsequently, the diffusion mechanisms including migration path, energy barrier and diffusion coefficient were evaluated and discussed. The results show that the substitutional site is the favorite of lithium atoms since lithium atoms are even larger than iron atoms. Among all the interstitial sites, <111> dumbbell position has the lowest formation energy. The binding effects of substitutional lithium atoms for both vacancies and self-interstitial atoms were confirmed by our calculations. The high binding energy and low migration barrier for vacancies suggest the diffusion through vacancy mechanism is plausible. The reaction between a substitutional lithium atom and a self-interstitial atom will form a mixed <111> dumbbell eventually. The diffusion of interstitial lithium atoms is accomplished by the rotation of the <111> dumbbell, which has a very low energy barrier of 0.063 eV.

Plasma nitriding of gradient structured AISI 304 at low temperature: Shot peening as a catalyst treatment

This study involves the capability of severe shot peening (SSP) as a catalyst intake for plasma nitriding process and the probability of reducing the requirement of thermal energy individually on the diffusion of interstitial atoms. To this end, combination of mechanical-thermal energy is run with pure thermal plasma assisted energy. Therefore, SSP is exposed to AISI 304 austenitic stainless steel as a former treatment and gradient structured surface (nanograined zone, ultrafine grain martensite-twin intersections zone and twin densed zone) is created. Then, plasma nitriding at 4000C-4h and 4750C-2h temperature-duration conditions. The condition of 4750C-2h provides the requirements of diffusion individually however former treatment SSP increases the diffusion layer effectively. In contrast, 4000C-4h condition is not adequate for diffusing atoms and white layer formation on the surface. The support of SSP as a mechanical energy triggers the process and white layer and the interstitial atoms diffusion take place influentially.

Serrated yielding in an advanced stainless steel Fe-25Ni-20Cr (wt%)

Understanding serrated yielding behavior resulting from dynamic strain aging (DSA) is essential for design and safety considerations. In this work, uniaxial tensile tests were carried out at temperatures ranging from 298 to 1073 K and strain rates 10−5 – 10−3 s−1 followed by microstructural examination of Fe-25Ni-20Cr (wt%) austenitic stainless steel (Alloy 709), a candidate structural material for Sodium-cooled Fast Reactors. Serrated yielding was found to occur in this alloy in two temperature regimes; low-temperature serrated flow (LT-SF) from 498 to 648 K and high-temperature serrated flow (HT-SF) from 648 to 973 K (depending on the strain rate) with activation energies of 103 ± 13 kJ/mole and 204 ± 11 kJ/mole respectively. The critical strain for the occurrence of serrations was found to increase with strain rate as an exponential function with exponent (m + β) of 0.78 ± 0.1 and 1.56 ± 0.2 for the LT-SF and HT-SF regimes respectively. Based on the activation energies and m + β values, diffusion of interstitial atoms has been suggested to be responsible for serrated flow in the LT-SF regime while Cr atom migration was inferred to be responsible for DSA in the HT-SF regime. Manifestations of DSA in the Alloy 709 were observed including peaks and/or plateaus in flow stresses along with negative strain rate sensitivity at intermediate temperatures. However, no loss in ductility was observed within DSA regime attributed to the relatively high work hardening rate and strain-hardening exponent. The samples deformed in DSA regime showed planar substructure while equiaxed subgrains formed at higher temperatures. The fraction of low angle grain boundaries after deformation exhibited a valley at intermediate temperatures believed to be another manifestation of the DSA.

Influence of substitutional atoms on the diffusion of oxygen in dilute iron alloys

A multiscale approach including density functional theory (DFT) and atomistic kinetic Monte Carlo (AKMC) simulations is applied to investigate the diffusion of interstitial oxygen atoms in bcc Fe under the influence of substitutional foreign atoms (Al, Si, P, S, Ti, Cr, Mn, Ni, Y, Mo, and W). The substitutional atoms can be assumed to be immobile since their diffusion coefficient is much smaller than that of oxygen. First, jumps of oxygen in pure bcc Fe between first-, second-, and third-neighbor octahedral interstitial sites are investigated. It is found that the first-neighbor jump is most relevant, with the tetrahedral site as the saddle point. The second-neighbor jump consists of two consecutive first-neighbor jumps. The barrier for a direct third-neighbor jump is too high to be significant for the diffusion process. In the presence of substitutional atoms the most important migration paths are first-neighbor jumps between modified octahedral sites with modified tetrahedral sites as saddle points. Calculations show that Si, P, Ni, Mo, and W cause some modifications of the migration barriers of oxygen and their interaction with O is mainly repulsive. Al, Cr, and Mn have a significant influence on the barriers and they exhibit strong attractive interactions with O. The most important modification of the barriers is found for S, Ti, and Y where deep attractive states exist. Based on the migration energies obtained by DFT, AKMC simulations on a rigid lattice are employed to determine the diffusion coefficient of oxygen in a dilute iron alloy containing the different substitutional atoms. It is found that Si, P, Ni, Mo, and W have almost no influence on the diffusivity of O. The presence of Al, Cr, Mn, S, Ti, and Y causes a significant reduction of the mobility of oxygen. In these cases the temperature dependence of the oxygen diffusion coefficient shows considerable deviations from an Arrhenius law. These phenomena are discussed in detail by considering the occupation time for the different states. The present findings on the strong dependence on the kind of substitutional atoms change the picture of oxygen diffusion in dilute iron alloys substantially.

Stress-controlled carbon diffusion channeling in bct-iron: A mean-field theory

Diffusion of interstitial carbon atoms in iron is the rate-limiting phenomenon of a number of phase transitions in body-centered (bcc) and body-centered tetragonal (bct) phases such as ferrite and martensite. These phases being rarely stress-free and undeformed, the influence of stress/strain on the diffusivity of carbon is essential, although scarcely documented. We developed a model of carbon elastodiffusion in bct-iron. We combined anisotropic linear elasticity theory of point defects, the dilute approximation of regular solutions and the multisite model of random walk into a coherent mean-field theory. The model allows predicting the effects of composition, temperature and mechanical loading on the anisotropy of carbon diffusion. Density functional theory calculations have provided most of the materials parameters. The predictions were successfully tested against kinetic Monte Carlo simulations. Our results show that compression of the crystal increases carbon diffusivity, while tension has the opposite effect. Axial straining is accompanied by a large anisotropy of diffusion. This effect could be exploited to produce stress-controlled diffusion channeling for the engineering of anisotropic microstructures during thermal ageing of martensitic Fe-C alloys.

Theoretical prediction and atomic kinetic Monte Carlo simulations of void superlattice self-organization under irradiation

Nano-structured superlattices may have novel physical properties and irradiation is a powerful mean to drive their self-organization. However, the formation mechanism of superlattice under irradiation is still open for debate. Here we use atomic kinetic Monte Carlo simulations in conjunction with a theoretical analysis to understand and predict the self-organization of nano-void superlattices under irradiation, which have been observed in various types of materials for more than 40 years but yet to be well understood. The superlattice is found to be a result of spontaneous precipitation of voids from the matrix, a process similar to phase separation in regular solid solution, with the symmetry dictated by anisotropic materials properties such as one-dimensional interstitial atom diffusion. This discovery challenges the widely accepted empirical rule of the coherency between the superlattice and host matrix crystal lattice. The atomic scale perspective has enabled a new theoretical analysis to successfully predict the superlattice parameters, which are in good agreement with independent experiments. The theory developed in this work can provide guidelines for designing target experiments to tailor desired microstructure under irradiation. It may also be generalized for situations beyond irradiation, such as spontaneous phase separation with reaction.

Evolution of 3He bubble microstructure in TiT2 films during aging

TEM observation was performed on titanium tritide films at He:Ti ratios of 0.036–0.356. Most helium atoms remained in the TiT2 lattice until at a He/Ti ratio of 0.06, almost all helium atoms entered the bubbles. The density of helium bubbles in Ti tritide increased with the increase in helium content, while their size distribution remained almost unchanged until the bubbles in the samples begin to interconnect at a He/Ti ratio of 0.247. Helium bubbles preferred to distribute in Ti {111} planes for all the samples. The morphology of helium bubble distribution changes not only with the density of bubbles but also with the strain in the films. Helium tended to migrate to the grain boundaries, and plenty of cracks were developed along the grain boundaries. The extension of cracks to the film surface is the cause for the large bursts of 3He into the atmosphere. The observed microstructural evolution and its dependence on helium concentration, density of bubble, and stress distribution are discussed in terms of diffusion of interstitial He atoms and their clustering.