Migration Of Interstitial Atoms Research Articles

Trapping and thermal migration of the first- and second-row atoms in Ar, Kr and Xe crystals.

Trapping and temperature-induced migration (TIM) of the first- and second-row atoms A from H to Ne in the face-centered cubic rare gas RG = Ar, Kr and Xe crystals are investigated within the classical crystal model parameterized by the empirically modified pairwise potentials. New ab initio coupled cluster A-RG potentials computed in a uniform way for all the atoms A are used to represent the atom-crystal interactions. Absolute and relative stabilities of the substitutional and interstitial trapping sites, their structures, interstitial migration pathways, related activation energies and rough estimates of the TIM rates are obtained. The isotropic model, which neglects non-zero atomic electronic orbital momentum, reveals that migration of interstitial atoms along the network of conjugated fcc octahedral voids is the generic case for atomic mobility. Anisotropic interactions with a crystal inherent to P-state atoms B, C, O and F are accounted for using the non-relativistic diatomics-in-molecule method. Depending on its sign, interaction anisotropy can alter the structures of interstitial trapping sites and transition states remarkably. This, in turn, can dramatically affect the TIM rates. Comparison with reliable experimental data available for oxygen and hydrogen indicates a systematic overestimation of the measured activation energies, by 30% at worst. A comprehensive literature review accomplished for other atoms reveals a lack of information on the TIM processes and rates, though makes it possible to verify a part of the present results on the trapping site energies and structures.

C and N Atom Redistribution Behaviors in 0.30C-15.0Cr-1.0Mo-0.50N Martensitic Stainless Steel During Cryogenic Treatment

The redistribution of C and N interstitial atoms i n a martensitic stainless steel combined alloying with 0.3 wt.% C and 0.5 wt.% N after cryogenic treatment was investigated by the transition electronic microscope (TEM), X-ray diffractometer (XRD), and atom probe tomography (APT). C atoms accumulated to form clusters which were enhanced by the decrease of cryogenic temperature. N atoms as Mo-N and Cr-N pairs were found when the cryogenic temperature was deep to -190 o C. It seems that the redistribution behavior of C and N atoms is considerably different, and these interstitial atoms migrated more strenuously despite of the cryogenic temperature decreasing. The migration of interstitial atoms is supposed to associate with the microstructure evolution combining the increase of dislocation density and the martensite transformation from the retained austenite during cryogenic treatment.

Differing behaviors of point defects in Fe–36Ni alloy during annealing after low-temperature deformation and irradiation

Fe–36Ni alloy is studied in states in which it is quenched, irradiated with electrons at 70 K, and deformed at 77 K. It is shown that similar structure-phase transformations occur in the investigated alloy during annealing at the temperature range of 70–570 K both after preliminary low-temperature electron irradiation and after preliminary low-temperature deformation. Migration of interstitial atoms (70–140 K) and vacancies (180–570 K), which induce phase transformations and are the results of atomic separation and ordering of zones with increased nickel content, is observed. Mechanisms of structure-phase changes under deformation and annealing are discussed.

The Accumulation and Annealing of Radiation-Induced Defects and the Effect of Hydrogen on the Physicomechanical Properties of the V−4Ti−4Cr and V−10Ti−5Cr Vanadium-Based Alloys under Low-Temperature (at 77 K) Neutron Irradiation

The processes of the accumulation and annealing of radiation-induced defects that occur under low-temperature (at 77 K) irradiation (with an energy E > 0.1 MeV) of V−4Ti−4Cr and V−10Ti−5Cr bcc alloys both nonmodified and modified with hydrogen isotopes in a concentration of 200 ppm, as well as the effect of these processes on the physicomechanical properties of these alloys, have been studied. It has been found that the saturation of these alloys with hydrogen leads to slight changes in their strength and ductility characteristics. The irradiation of the alloys at the temperature of 77 K results in a substantial increase in their yield stress and ultimate strength, as well as a decrease in their ductility. In the course of the postradiation annealing of the alloys at a temperature of 130 K, the stage related to the migration of interstitial atoms is observed. At temperatures of 290–320 K, the recovery stage occurs due to the formation of vacancy clusters. The stage that occurs at a temperature of 470 K can be attributed to the formation of impurity-vacancy clusters. Possible mechanisms of the radiation-induced strengthening of the alloys during irradiation and subsequent annealing have been discussed.

The Energy of Generation and Migration of Point Defects in the Ordered CuPt and CuPt3 Alloys

Using molecular dynamics simulations, the energies of formation and migration of vacancies and interstitial atoms in the ordered CuPt and CuPt3 alloys are estimated. The energy-favorable configurations of the interstitial atoms are determined. In CuPt, high diffusion anisotropy is revealed during migration of an interstitial atom: in the ordered CuPt alloys it generally migrates along (111) planes containing Cu atoms.

Specificity of stage III in electron-irradiated Fe-Cr alloys

It has been found that no typical features of stage III are traced in the resistivity recovery (RR) and positron lifetime (PL) data of electron-irradiated Fe-Cr alloys (4–10 at.%). None of the observed RR stages has shifted its temperature position with changing concentration of defects, which is characteristic of stage III. A new quantity was considered–the difference between RRs (DRR) of samples having different defect concentrations. The onset of free migration is indicated in the DRR plot by a peak, which corresponds in physical meaning to a partial (separated) RR peak of the free migration stage. Such a single peak was found in Fe-9Cr at 205–210 K and identified as the sign of stage III, however, the peak amplitude turned out inverted, i.e. it corresponded to a resistivity rise instead of the usual resistivity drop. In Fe-4Cr the peak amplitude is about zero. Such anomalous RR behaviour is related to a very high resistivity contribution of immobile di-vacancies formed in stage III. This contribution masks the resistivity reduction and makes stage III invisible in conventional RR spectra. Close PLs in mono- and di-vacancies make stage III poorly detectable by PL spectroscopy as well. The RR stage and PL increase at 220 K, considered earlier as the signs of stage III, are actually connected with the onset of long-range migration of interstitial atoms. Recombination and a release of mobile mono-vacancies resulted from this onset give rise, correspondingly, to a conventional RR stage and clearly detectable enlargement of the small vacancy aggregates formed in stage III.

On the concentration maxima of nonequilibrium point defects in irradiated copper with an impurity of gold

Results of a numerical solution based on a model that describes the process of accumulation of radiation-induced point defects in metals with impurities are presented. It has been established that a twofold increase in the energy of migration of interstitial atoms in copper with an impurity of gold results in a 40-fold increase in the excess concentration of interstitial atoms.

Microstructure of high-pressure-synthesized diamonds under rapid quenching and electron irradiation

A type of synthetic diamond single crystal about 0.4–0.5 mm in dimension prepared under high pressure–high temperature (HPHT) in the presence of a FeNi molten catalyst was quenched from HPHT and irradiated with 300 keV electrons at room temperature. Transmission electron microscopy was employed to examine the microstructure of the diamond single crystal before and after electron irradiation. It was found that there exists a large amount of cellular interfaces in the quenched diamond sample, which indicates the growth condition of the diamond under HPHT. Hexagonal dislocation loops about several tens of nanometers in dimension were observed in the high-pressure-synthesized diamond single crystal before electron irradiation, which strongly suggests that a number of vacancies were quenched-in due to rapid quenching from high temperature at the end of diamond synthesis, and were aggregated in the synthetic diamond to form vacancy disks on the (111) plane, the collapse of such vacancy disks forming vacancy-type dislocation loops. After electron irradiation, it was found that defect clusters present as interstitial-character dislocation loops were formed in the electron-irradiated region of the diamond. The interstitial dislocation loops grow with increase of the irradiation time. The present study, in comparison to previous work on ion implantation on diamond, indicates that electron irradiation does not induce a phase transformation but produces interstitial dislocation loops due to the migration of interstitial atoms and vacancies. The result of the study directly indicates that interstitials and vacancies in diamond are mobile at room temperature under electron irradiation. Nitrogen, as the most important kind of impurity contained in the HPHT as-grown diamond, probably acts as nucleation of the interstitial loops.

Effect of electric current on the migration of hydrogen interstitial atoms in the region of a crack tip in a crystal

The effect of a constant electric current on the migration of interstitial atoms dissolved in a crystal in the region of a tensile crack tip is estimated. The calculation takes into account plastic deformation that is produced in the vicinity of the crack tip in the loaded sample by dislocation motion in active slip planes of the crystal under the action of mechanically and electrically induced shear stresses, Joule heat release, the Thomson effect, and ponderomotive forces and allows for the effect of gas exchange near the crack edges on the evolution of the distribution of interstitial impurity atoms. The time dependence of the stress intensity factor is found for both the cases of the presence and absence of a constant electric current near the crack tip. Numerical calculations are performed for an α-Fe crystal.

Stress evolution in Ti-C:H films

Time evolution of stress in diamond-like films with small quantities(10-20 at%) of titanium (Ti-C:H) was investigated together with theirstructure, electrical resistivity, optical constants, Raman spectra, andcomposition. The compressive stresses for all films were found to increase,regardless of their structure (either amorphous or a-C:H with TiCnanocrystalline inclusions) and composition (10-20 at% Ti) over a period ofseveral months. The conductivity and optical constants (n and k) decreasedwith time, and Raman spectra indicated no change of the film structure. Theobservations can be explained by a migration of interstitial atoms (probablyhydrogen) resulting in a saturation of dangling bonds and, consequently,leading to a reduction in the number of free carriers. Vacancy clustering andeffects of hydrogen atoms on stress increase are discussed.

Atomic processes during damage production and defect retention

Fundamental atomic processes during production and retention of radiation damage in structural materials are reviewed. Firmly established damage mechanisms are outlined, and some of the major uncertainties exposed. Damage production under neutron and ion irradiation proceeds through the elementary step of collision cascade formation, with further modification of the nascent cascade structure by inter-, and intra-cascade interaction events. Under fusion neutron irradiation, single cascade damage production occurs within 10 ps in most metals, with spatial extent of several tens to hundreds of nano-meters. Damage accumulation takes place over much longer time and length scales, during which multiple cascades interact leading to further re-structuring of the incipient cascade. Areas which require further investigation are: (1) the mechanisms of vacancy cluster collapse at the cascade core; (2) the athermal nucleation of interstitial loops at the periphery; (3) the migration of interstitial atoms and clusters emanating from the cascade; (4) the replacement of the dpa with a more physical measure of damage retention; (5) cascade structure for high PKA energies (above 20 keV); and (6) Inter- and Intra-cascade interactions, (7) and finally, cascade dynamics in damaged materials at high dose.

Vacancies and Interstitial Atoms in Irradiated Silicon

ABSTRACTDifferently doped Si-wafers were irradiated with 2.5MeV electrons and subsequently investigated by X-ray diffraction methods. We show that a high concentration of defects (>1016 cm−3) can be frozen in at 4K and that a large fraction of the defects is stabilized in the form of close Frenkel pairs. These Frenkel pairs are characterized by the nearly perfect cancellation of the long-range displacement fields of the interstitial atom and the vacancy. We discuss the absolute size of these displacements as well as the introduction rate of the defects, which is of the order of Σ = 1cm−1. Similar results are obtained for weakly doped Cz-Si or FZ-Si and for degenerate p-type Si(B) and n-type Si(As), i.e., the results are independent of the oxygen content of the samples and of the position of the Fermi level. The high defect introduction rates are at variance with the results of electrical and EPR investigations and indicate that these methods detect only a few percent of the total defect concentration which is produced and frozen in at 4K. The consequences for the understanding of the defect production in Si and for the assumption of an athermal migration of interstitial atoms are discussed. In addition we compare the defect patterns observed after 4K irradiation to those observed after room-temperature irradiations and discuss the thermally activated defect reactions up to the final annealing at 1000K.

Positron lifetime study of hafnium nitride films

AbstractFilms of HfN can have expanded lattices which are restored by tempering, reflecting the presence of a high concentration of defects. The present study was made to get a qualitative understanding of the lattice relaxation on an atomic scale. The results support the proposal that the dominant mechanism is the migration of interstitial atoms into lattice vacancies.

Lattice vacancies in TiN and HfN films: A study by positron annihilation

Films of the group IVB nitrides can have expanded lattices which are restored by tempering, reflecting the presence of a high concentration of defects. This positron annihilation study is a first attempt to obtain a qualitative understanding of the lattice relaxation on an atomic scale. The spectra measured before and after tempering were resolved into three components, showing more than one type of defect to be present. The major contribution is attributed to the trapping of positrons in vacancies. The results of all samples taken together support the proposal that the dominant mechanism is the migration of interstitial atoms into lattice vacancies. The nature of the defects leading to the longer-lived positron states, possibly voids and/or dislocations, is still uncertain.

Mobile interstitials in niobium near 4.5 K

Abstract New resistivity recovery experiments including radiation doping and damage-rate measurements on high purity niobium following 3 MeV electron irradiation indicate long range migration of interstitial atoms already near 4.5 K.

Nature and thermal stability of radiation-induced defects in zirconium hydride

1. On irradiating ZrH1.9 with an integrated flux of 3.2·1021 neutrons/cm2 (1.8·1020 neutrons/cm2 with energies over 1 MeV) at 50°C, zirconium vacancies accumulate, possibly together with small vacancy complexes (≤10 A) and also defects in the hydrogen sublattice. The defects of the metallic sublattice cause swelling, an increase in hardness, and increased thermal and electrical resistance. The defects of the hydrogen sublattice increase the electrical resistance and reduce the lattice tetragonality. 2. The recovery of the properties on annealing the irradiated hydride at 50–540°C takes place in three stages; the first is associated with the migration of interstitial atoms, the second with vacancy migration, and the third with the annealing of complex defects. There are two temperature ranges of complex-formation: Close to 180°C aggregates of interstitial atoms are apparently formed, and at 270–450°C vacancy cavities (voids). 3. The recovery of the hydrogen sublattice takes place in the second and third stages. 4. The melting point of zirconium hydride (Tm=2155±75°K) may be estimated by comparing the annealing temperatures of similar defects in the hydride and fcc metals.

Free Migration of Interstitial Atoms in Metals

Free Migration of Interstitial Atoms in Metals

Nature of the failure process in metals in a composite stress state

1. It has been established that failure in a composite stress state is a kinetic process and is described by an equation such as (1). 2. In tension, torsion, and their combined effect, the activation energy of the rupture process may be considered to be one of two values: one near the sublimation energy, the other near the energy of self diffusion. 3. It is suggested that, in the range of changes in stress state from torsion to tension, failure may be controlled by two kinetic processes: for tension, and stress states near it, a thermal fluctuation process obtains, occasioned by the rupture of bonds or, according to [6, 7, 20, 21, 22], by the migration of interstitial atoms; and for stress states in the range of torsion the process is associated with diffusion of vacancies. 4. In the range of changes in stress state in which the mechanism controlling failure remains unchanged, there may exist a single criterion of failure having a clear physical significance. For aluminum alloy it has been established that in the stress states 2 ≥ σ1/tmax ≥ 1.55 the criterion of failure is represented by the greatest tensile stresses σ1 in the states 1 ≤ σ1/tmax ≤ 1.42 it is represented by the maximum tangential stresses tmax. 5. For tension and torsion, a relationship has been found between the coefficients γ in Eq. (1), permitting us to determine service life in torsion from the service life in tension, and vice versa.

The migration of interstitial atoms in gold I. experimental

Abstract Direct information about the migration energy of single interstitial atoms in gold was obtained using a combination of low energy ion bombardment and transmission electron microscope techniques. Both annealed and quenched gold foils, containing vacancy tetrahedra, were bombarded with 200 ev argon ions inside an electron microscope at temperatures between -130°c and 30°c. The foils were observed both at the bombardment temperature and during subsequent annealing. The bombardment produced focused replacement sequences which deposited interstitials about 100 A beneath the incident surface. The observations of the formation of interstitial clusters and of the removal of vacancy tetrahedra as a function of temperature and foil thickness give strong support to the hypothesis that a type of single interstitial is produced which migrates over distances of several hundred angstroms only above 0°c. On this model the energy of migration of this interstitial is not less than 0·7 ev.

The migration of interstitial atoms in gold II. analysis and interpretation of experiments

Abstract In the previous paper, the results of a series of experiments were reported, whose most plausible interpretation was that low energy ion bombardment produces a type of single interstitial which is immobile below about 0°c. In this paper this model is analysed quantitatively, and is shown to yield values for the interstitial diffusion coefficient, the migration energy of the interstitial, and the binding energy of small interstitial clusters. All the experiments are shown to have a self-consistent interpretation in terms of the migration of a single interstitial which becomes mobile near 0°c with a migration energy of 0·75±0·06 ev.