Vacancy-type Dislocation Loops Research Articles

Molecular dynamics simulations of cascade overlap with Void/Helium bubble

When tungsten (W) is used as a plasma-facing material in fusion reactors, it will suffer irradiation from the high-energy neutron and high-flux low-energy helium (He) plasma, affording voids and He bubbles with varying helium-to-vacancy ratios (He/V). With the accumulation of irradiation dose, an energetic neutron is very likely to trigger collision cascades close to the pre-existing voids/He bubbles. Therefore, in this study, we ran systematic molecular dynamics simulations of the cascades that overlapped with voids/He bubbles in W. We investigated the relationship between the evolution of the stress field and defect formation. Cascades overlapping with the bubbles with He/V values between 1 and 2 produced the least defects; alternatively, other bubbles could facilitate defect formation. Furthermore, two mechanisms for generating dislocation loops, the cascade-destruction and the over-pressured bubble-assist types, were identified and can be attributed to subcascade formation. When there are voids or under-pressured bubbles, the cascade-destruction mechanism can yield vacancy-type dislocation loops; however, the existence of He atoms can inhibit this behavior. In contrast, when the bubbles are over-pressured, the bubble-assist mechanism can contribute to the formation of self-interstitial atom-type dislocation loops. This study helps to improve our understanding of radiation damage in a complicated environment in experiments with multiple irradiations or fusion reactors.

Effect of Mn addition on the formation of vacancy-type dislocation loops in α-Fe

It has recently been established that vacancy loops of ~100 nm in maximum size could form in hydrogen pre-implanted α-Fe (Huang et al. Fusion Eng. Des., 2010). The formation temperature of these large loops is composition dependent, varying from ~500 °C in α-Fe, to ~450 °C in FeNi and ~ 620 °C in FeCr (Wan et al. J. Nucl. Mater., 2014; Du et al. Acta Metall. Sin., 2019). In this work, the effect of Mn addition upon vacancy loop formation has been investigated. Pure Fe and Fe-1.4 wt% Mn alloy specimens were irradiated with 30 keV H+ ions at room temperature. Radiation-induced loop production, size, population and nature were characterized based on diffraction contrast imaging using a combination of 1 MV high voltage electron microscope (HVEM) and 200 kV conventional transmission electron microscope (CTEM). Results of observation indicate that Mn addition gives rise to the increase of vacancy loop formation temperature in α-Fe, similar to the effect of Cr. A new criterion has been thus proposed, to classify the common alloying elements (Cr, Ni, Mn etc) found in Fe.

Evolution of dislocation loops and effect of annealing temperature on hydrogen-ion-implanted Fe-based binary alloys

Reduced activation ferritic/martensitic (RAFM) steels have been considered as a family of prime candidate structural materials for fusion reactors due to low radioactivity and good resistance to irradiation swelling. Various types of defects such as dislocation loops can form in these materials during irradiation. Effects of alloying elements in iron on the formation and migration of dislocation loops have been widely investigated. However, most studies dealt with interstitial-type dislocation loops in iron alloys, while very few focused on vacancy-type dislocation loops. Previous high voltage electron microscope (HVEM) studies from the authors' group have shown that interstitial loops are fully eliminated in hydrogen-ion-implanted α-Fe at 500 ℃, only vacancy loops remain and can achieve up to 100 nm in size. The addition of Ni in α-Fe can reduce the formation temperature of vacancy-type dislocation loops (<i>T</i><sub>c</sub>) to ~450 ℃, while the addition of Cr can increase the temperature to above 600 ℃. However, these experiments are usually difficult to perform due to the scarce resource of HVEM facilities. In this work, in-situ observations by conventional transmission electron microscope (CTEM, 200 kV) are systematically carried out on the hydrogen-ion-implanted α-Fe and Fe-based binary alloys (Fe-3wt.%Cr, Fe-1.4wt.%Ni and Fe-1.4wt.%Mn). The evolutions of morphology and average size of dislocation loops under different annealing temperatures are investigated. The formation temperatures of vacancy-type dislocation loops are determined from the change of average loop size with annealing temperature. The results are consistent with previous studies by HVEM. The effect of Mn atoms in α-Fe is similar to that of Cr atoms, which leads to <i>T</i><sub>c</sub> increase, and the addition of Ni in α-Fe can reduce <i>T</i><sub>c</sub>. Furthermore, the results of D thermal desorption spectrum analysis show that <i>T</i><sub>c</sub> is affected by the binding and release process of hydrogen isotopes to vacancies in α-Fe. Alloying element Ni promotes the binding and release of hydrogen isotopes to vacancies, which leads to<i> T</i><sub>c</sub> decrease. Cr and Mn inhibit the binding and release of hydrogen isotopes to vacancies, causing <i> T</i><sub>c</sub> to increase.

Primary damage production in the presence of extended defects and growth of vacancy-type dislocation loops in hcp zirconium

Production rates in long-term predictive radiation damage accumulation models are generally considered independent of the material's microstructure for reactor components. In this study, the effect of pre-existing microstructural elements on primary damage production in alpha-Zr -- and vice-versa -- is assessed by molecular dynamics (MD) simulations. a-type dislocation loops, c-component dislocation loops and a tilt grain boundary (GB) were considered. Primary damage production is reduced in the presence of all these microstructural elements, and clustering behavior is dependent on the microstructure. Collision cascades do not cause a-type loop growth or shrinkage, but they cause c-component loop shrinkage. Cascades in the presence of the GBs produce more vacancies than interstitials. This result, as well as other theoretical, MD and experimental evidence, confirm that vacancy loops will grow in the vacancy supersaturated environment near GBs. Distinct temperature-dependent growth regimes are identified. Also, MD reveals cascade-induced events where a-type vacancy loops are absorbed by GBs. Fe segregation at the loops inhibits this cascade-induced absorption.

Defect structures and statistics in overlapping cascade damage in fusion-relevant bcc metals

Most experimental work on radiation damage is performed to fairly high doses, where cascade overlap effects come into play, yet atomistic simulations of the primary radiation damage have mainly been performed in initially perfect lattice. Here, we investigate the primary damage produced by energetic ion or neutron impacts in bcc Fe and W. We model irradiation effects at high fluence through atomistic simulations of cascades in pre-damaged systems. The effects of overlap provide new insights into the processes governing the formation under irradiation of extended defects. We find that cascade overlap leads to an increase in the numbers of large clusters in Fe, while in W such an effect is not seen. A significant shift in the morphology of the primary damage is also observed, including the formation of complex defect structures that have not been previously reported in the literature. These defects are highly self-immobilized, shifting the damage away from the predominance of mobile 1/2〈111〉 loops towards more immobile initial configurations. In Fe, where cascade collapse is extremely rare in molecular dynamics simulations of individual cascades, we observe the formation of vacancy-type dislocation loops from cascade collapse as a result of cascade overlap.

Formation of Vacancy-Type Dislocation Loops in Hydrogen-Ion-Implanted Fe–Cr Alloy

Fe–10 at.%Cr alloy was implanted with hydrogen ions at room temperature, followed by annealing at high temperatures. The annealing process made the defects develop into large dislocation loops. The nature of the dislocation loops formed after annealing was studied by the evolution of loops under in situ electron irradiation in high-voltage electron microscope. It indicated that only interstitial-type loops were observed when annealed at 550 °C and below, but vacancy-type loops started to form at the temperature higher than 600 °C. According to the previous study of our group, the presence of chromium element made the formation temperature of vacancy-type loops higher than that in pure iron. The effect of alloying elements on the formation temperature of the vacancy-type loops was discussed.

Effects of intragranular defects on helium accumulation damage in nanocrystalline nitrides

Nanomaterials with a high density of interfaces, which absorb helium (He) atoms and suppress the expansion of He bubbles, can exhibit a superior neutron radiation tolerance. However, the intragranular defects formed under long-term radiation can interact strongly with the He atoms, which would bring more uncertainties to the reliability of materials served in neutron radiation environment. In this work, He atoms and intragranular defects were introduced uniformly into the nanocrystalline (NC) ZrN and TaN films for investigating the effects of the intragranular defects on the He accumulation damage in nanomaterials. The microstructure, lattice parameter and surface morphology of the NC nitrides were investigated systematically. The results showed that the He accumulation damage was very sensitive to the structure of intragranular defects in the He-implanted materials. The cross-grained stacking faults in the ZrN films may act as diffusion channels for the He atoms, facilitating the He accumulation at the grain boundaries, while the vacancy-type dislocation loops in the TaN films can act as small cracks trapping the He atoms, resulting in the formation and unstable expansion of the He platelets in the interior of the grains.

Primary Ion-Irradiation Damage of BCC-Iron Surfaces

Using MD simulations, the influence of crystallographic orientation of an irradiated surface on the evolution of atomic displacement cascades is investigated in BCC-iron. The energy of atomic displacement cascades is varied from 1 to 20 keV. It is found that irradiation of the (111) surface results in crater formation while irradiation of the (110) surface gives rise to the formation of vacancy-type dislocation loops with the Burgers vectors а or a/2 . Both processes are associated with anisotropic propagation of the shock waves generated by the atomic displacement cascades. In the case of low energies of the latter, the number of point defects survived near the (110) surface is larger than that near (111). At higher energies, the influence of the free surface orientation on the number of survived defects decreases.

The nucleation and growth of H blisters in dislocation loops in W{110}

We propose a mechanism for nucleation and growth of H blisters in vacancy-type dislocation loops (DLs) in tungsten (W), based on the first principles calculations. We find that vacancy-type DLs are energetically preferred to form on {110} planes. The H atoms nearby are readily to migrate to and accumulate in the DLs with energy cost less than 0.24 eV. With the increasing number of the trapped H atoms, the DL expands rapidly, consequently, the nucleation and growth of H blisters is completed in the DL gradually. In addition, H2 molecules are observed in the H blisters when the H areal density in DL reaches to be of 6.3 × 1015 atoms/cm2 (the number of H atoms per the cross-sectional area of the DL). Our proposed formation mechanism for H blisters in W material may also be applicable to other metals and metal alloys.

The behavior of vacancy-type dislocation loops under electron irradiation in iron

Two different types of dislocation loops were found in hydrogen ion implanted pure iron after annealing. The natures of these dislocation loops have been characterized by the inside–outside method of transmission electron microscope (TEM). It is found that the dislocation loops in specimens annealed at 670K are interstitial type with b=1/2〈111〉, n≈〈112〉, and the dislocation loops in the specimens annealed at 770K are vacancy type with b=1/2〈111〉, n≈〈011〉 or b=〈100〉 and, n≈〈112〉. Adding nickle element in iron could decrease the formation temperature of the vacancy loops.

High-energy collision cascades in tungsten: Dislocation loops structure and clustering scaling laws

Recent experiments on in situ high-energy self-ion irradiation of tungsten (W) show the occurrence of unusual cascade damage effects resulting from single-ion impacts, shedding light on the nature of radiation damage expected in the tungsten components of a fusion reactor. In this paper, we investigate the dynamics of defect production in 150 keV collision cascades in W at atomic resolution, using molecular-dynamics simulations and comparing predictions with experimental observations. We show that cascades in W exhibit no subcascade break-up even at high energies, producing a massive, unbroken molten area, which facilitates the formation of large defect clusters. Simulations show evidence of the formation of both and interstitial-type dislocation loops, as well as the occurrence of cascade collapse resulting in vacancy-type dislocation loops, in excellent agreement with experimental observations. The fractal nature of the cascades gives rise to a scale-less power-law–type size distribution of defect clusters.

Vacancy migration process in F82H and Fe-Cr binary alloy using positron annihilation lifetime measurement

Microstructral evolution of electron-irradiated F82H and Fe-8%Cr at 77 K was studied using positron annihilation lifetime measurements. Irradiation-induced vacancies started to migrate at 300 K and 180 K in F82H and Fe-8%Cr, respectively. Solute Cr atoms did not suppress vacancy migration, but they made di-vacancies more stable. Microvoids were not formed by annealing. In F82H, solute atoms acted as trapping site of irradiation-induced defects and annihilation of vacancies and interstitials was facilitated. Pre-existing dislocations and precipitates were also their sinks. These lead to the suppression of microvoids formation. In Fe-8%Cr, small vacancy-type dislocation loops were formed by isochronal annealing test.

Dislocation loop formation under various irradiations of laser and/or electron beams

A high-voltage electron microscope (HVEM) equipped with a laser head (laser-HVEM) was developed at Hokkaido University and is used to investigate the surface modification of semiconductors and the behaviour of lattice point defects in metals under various irradiations of laser (photon) and/or electron beams. In the present study, the annealing effect of pulsed laser irradiation on a face-centred-cubic metal was experimentally investigated and theoretically calculated. The systematic assessment of dislocation loop evolution under laser-electron sequential irradiation and laser-electron dual-beam irradiation was performed. Our results show that the rapid heating and quenching that occurred during pulsed laser irradiation caused vacancies to be introduced at the surface of the specimen and to diffuse to the interior, which led to the formation and growth of vacancy-type (V-type) dislocation loops. These loops gradually shrank and finally disappeared during the subsequent electron irradiation of the sample. During laser-electron simultaneous dual-beam irradiation, the type of loop formed, interstitial-type (I-type) or V-type loops, is determined by the relative intensities of the laser beam and electron beam, which indicates that the loop type can be controlled by changing the relative intensities of the beams. Accordingly, models of dislocation loop formation during various irradiations were proposed. The newly developed laser-HVEM instrument is expected to be employed in the exploration of mechanisms in material science, as well as in other scientific fields.

Formation of stacking-fault tetrahedra in aluminum irradiated with high-energy particles at low-temperatures

Stacking-fault tetrahedra (SFT) are typical vacancy clusters in fcc metals. SFT have been, however, considered to be unstable in aluminum because of its high stacking-fault energy, until the recent finding of SFT in thin aluminum foils subjected to a tensile fracture. This study confirmed that SFT form in aluminum following irradiation with high-energy particles. In electron irradiation, SFT with an average size of 2 nm formed below 203 K, while a larger irradiation intensity at lower temperature induced SFT with a larger number density. Irradiation with 60-keV Al + ions induced SFT below room temperature, although the defect yield (the ratio of the number of defect clusters to the number of incident ions) was about 10 3 , considerably smaller than that in the other fcc pure metals. With neutron irradiation below 15 K to a fluence of 2 × 10 2 1 neutrons m - 2 , SFT were not observed at room temperature. Instead, dislocation loops were observed to form and to disappear during observation with 120-kV electrons that do not cause atomic displacements. Tensile fracture of aluminum thin foil induced SFT at up to 400 K, which is close to the temperature at which SFT become unstable in isochronal annealing experiments. These results suggest that a high concentration of vacancies at lower temperature tends to cause SFT to form rather than vacancy-type dislocation loops in aluminum.

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.

Elongation Fracture of Metals Containing Pre-introduced Secondary Defects

High-speed, high-strain tensile deformation of thin foils of fcc metals has recently been found to result in the formation of an anomalously high density of small vacancy clusters, probably in the absence of dislocations. In the present study, deformation of secondary-defect-pre-introduced pure Al is performed; at strain rates ranging from 10 m 4 to 10 5 /s and a deformation temperature of m 196 or 25°C. Microstructures in the deformed regions are examined by transmission electron microscopy. Vacancy-type dislocation loops and voids are eliminated by the deformation. He-bubbles are observed to elongate under a tensile stress, mainly in the absence of dislocations. Rows of bubbles oriented close to directions of elongation are found, and might result from extreme elongation and division of bubbles. The bubble rows are parallel to particular crystallographic directions, either d 001 ¢ or d 112 ¢ . This indicates that displacement of atoms during high-speed, high-strain tensile deformation progresses while conforming with the nature of a crystal, even in the absence of dislocations.

Indistinct Defect Images in Topographs of Nearly Perfect Aluminum Crystals Just Prior to Appearance of Dislocation Loops

Vacancy generation and annihilation mechanisms by the growth of two types of dislocation loops, the interstitial and the vacancy type, in nearly perfect aluminum single crystals at fairly high temperatures were investigated by synchrotron radiation topography. Topographs were continuously taken with white beam X-ray at an elevated temperature. Just prior to the appearance of the loops, an indistinct image topograph, in which not only defect images but also fringes were unclear, was taken for both types of loops in spite of a short exposure time (<3 s). This phenomenon was interpreted as being due to the scattering of the diffracted X-ray beam by very small clusters of interstitial atoms or vacancies which are invisible on the X-ray topograph. Moreover, some of them grow into interstitial or vacancy type dislocation loops as revealed by X-ray topography.

Correlation factor and concentration profiles for interstitial—substitutional mechanism with impurity—vacancy interaction in the FCC lattice by means of Monte Carlo method

The defect structure induced by zinc diffusion at 1170 K into undoped and indium-doped, semi-insulating GaAs single crystals was characterized for various diffusion times by analytical transmission electron microscopy of cross-sectional specimens. The results were compared with zinc concentration profiles obtained by spreading-resistance measurements on the same samples. Corresponding to zones of different zinc concentrations, three regions of characteristically different defect structures were distinguished. In the diffusion front region, interstitial-type dislocation loops, dislocation networks and gallium precipitates with voids in spatial correlation with the dislocations were found independent of diffusion conditions. The defect structure in the surface region depends on the diffusion conditions and is dominated by zinc-rich precipitates. For longer diffusion times, a transition region is observed where faceted voids and large vacancy-type dislocation loops coexist. A model for the formation of the diffusion-induced defects is presented which is based on the creation of a supersaturation of interstitial gallium atoms by the interstitial-substitutional zinc exchange and a resulting supersaturation of arsenic vacancies during diffusion.

Modeling of nucleation and growth of voids in silicon

During Si crystal growth, nucleation and growth of voids and vacancy-type dislocation loops under Si vacancy supersaturation conditions have been modeled. From nucleation barrier calculations, it is shown that voids can be nucleated, but not dislocation loops. The homogeneous nucleation rate of voids has been calculated for different temperatures by assuming different enthalpy of Si vacancy formation. The void growth process has been calculated using a moving boundary formulation. Matching the results of void nucleation and growth simulations and taking into account the competition between the two processes, limited time available, and the crystal cooling rate, it is shown that the experimentally observed void density and size data can be explained if the Si vacancy formation enthalpy is in the range of 2.8–3.4 eV and the void nucleation temperature is in the range of 970–1060 °C.

Nucleation and Growth of Voids in Silicon

ABSTRACTNucleation of voids and vacancy-type dislocation loops in Si under vacancy supersaturation conditions has been considered. Based upon nucleation barrier calculations, it has been found that voids can be nucleated, but not dislocation loops. The homogeneous nucleation rate of voids has been calculated for different temperatures by assuming different enthalpy values of Si vacancy formation. The process of void growth due to precipitation of vacancies has been numerically simulated. Comparing results of the nucleation and the growth modeling and taking into account the competition between the two processes, the limited time available, and the crystal cooling rate after growth, it has been shown that homogeneous nucleation of voids to experimentally observed densities and void growth to observed sizes is possible if enthalpy of Si vacancy formation is within the range of 2.9 to 3.6 eV with the nucleation temperature in the range of 980–1080 °C.