He-vacancy Clusters Research Articles

Evolution and change of He bubbles in He-containing Ti films upon thermal treatment studied by small-angle X-ray scattering and transmission electron microscopy

Evolution and change of He bubbles in magnetron sputtering prepared He-containing Ti films under thermal treatment are studied by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and X-ray diffraction. Incorporation of He introduces a large number of He-vacancy clusters and some voids in the films, and significantly increases SAXS intensity and causes anisotropic scattering. The change of He induced defects during annealing is affected by thermal diffusion and migration of trapped He to the surface and between interfaces of He induced defects within the films. Annealing at 200 and 400°C reduces intensity and anisotropy of SAXS, in accord with observed shrinking and disappearance of the voids. The simultaneous growth of non-uniformly distributed He bubbles to the sizes of 1–2nm and a population level of 105/μm3 are detected in the temperature range. The changes are explained by migration and coalescence mechanisms, which requires low apparent activation energy. Inconsistence between TEM and SAXS observations is noted and attributed to thinning induced internal stress relaxation of TEM specimen. Remarkable enlargement of He bubbles, associated with increased SAXS intensity and fractal dimension, is observed after 600°C annealing, indicating involvement of Ostwald Ripening (OR) mechanism. The OR process dominates at 800°C, where the high temperature provides activation energy for accelerated He dissociation from small bubbles into larger ones, and generating textured microstructure and agglomerated bubble clusters. The inhomogeneous bubble size distribution observed at this temperature covers a broad range of about 10–50nm and possessing a population density level of 103/μm3.

He migration and bubble formation in Ga stabilised δ-Pu

The migration and formation of He into bubbles in Ga stabilised δ-Pu has been investigated using molecular dynamics simulation. Formation energy calculations indicate that isolated He interstitial atoms are unfavourable and that it is preferential for He to reside as a substitutional atom at the expense of producing a Pu self-interstitial. Migration energy barrier calculations and on-the-fly kinetic Monte Carlo simulations support this result establishing that an interstitial He atom soon becomes substitutional, after which migration is unlikely unless assisted by local vacancies. He-vacancy cluster formation energies show that as the void size increases, a He:vacancy ratio up to 2:1 becomes energetically favourable over isolated He substitutional atoms and vacancies.

First-principles study of helium, carbon, and nitrogen in austenite, dilute austenitic iron alloys, and nickel

An extensive set of first-principles density functional theory calculations have been performed to study the behavior of He, C, and N solutes in austenite, dilute Fe-Cr-Ni austenitic alloys, and Ni in order to investigate their influence on the microstructural evolution of austenitic steel alloys under irradiation. The results show that austenite behaves much like other face-centered cubic metals and like Ni in particular. Strong similarities were also observed between austenite and ferrite. We find that interstitial He is most stable in the tetrahedral site and migrates with a low barrier energy of between 0.1 and 0.2 eV. It binds strongly into clusters as well as overcoordinated lattice defects and forms highly stable He-vacancy (V${}_{m}$He${}_{n}$) clusters. Interstitial He clusters of sufficient size were shown to be unstable to self-interstitial emission and VHe${}_{n}$ cluster formation. The binding of additional He and V to existing V${}_{m}$He${}_{n}$ clusters increases with cluster size, leading to unbounded growth and He bubble formation. Clusters with $n/m$ around 1.3 were found to be most stable with a dissociation energy of 2.8 eV for He and V release. Substitutional He migrates via the dissociative mechanism in a thermal vacancy population but can migrate via the vacancy mechanism in irradiated environments as a stable V${}_{2}$He complex. Both C and N are most stable octahedrally and exhibit migration energies in the range from 1.3 to 1.6 eV. Interactions between pairs of these solutes are either repulsive or negligible. A vacancy can stably bind up to two C or N atoms with binding energies per solute atom up to 0.4 eV for C and up to 0.6 eV for N. Calculations in Ni, however, show that this may not result in vacancy trapping as VC and VN complexes can migrate cooperatively with barrier energies comparable to the isolated vacancy. This should also lead to enhanced C and N mobility in irradiated materials and may result in solute segregation to defect sinks. Binding to larger vacancy clusters is most stable near their surface and increases with cluster size. A binding energy of 0.1 eV was observed for both C and N to a [001] self-interstitial dumbbell and is likely to increase with cluster size. On this basis, we would expect that, once mobile, Cottrell atmospheres of C and N will develop around dislocations and grain boundaries in austenitic steel alloys.

Effect of Alloy Composition & Helium ion-irradiation on the Mechanical Properties of Tungsten, Tungsten-Tantalum & Tungsten-Rhenium for Fusion Power Applications

ABSTRACTModel alloys have been made of pure W and 1% & 5% W-Ta and W-Re. Indentation hardness and modulus data were obtained by nanoindentation to assess the effect of composition on mechanical properties. Results showed that both the Ta and Re compositions hardened with increasing alloy content, greater in the W-5%Ta composition which showed an increase of 1.03GPa (17%), compared to a 0.43GPa (7%) increase in W-5%Re. The samples also showed very small increases in modulus of ∼ 25GPa (6%) in both W-5%Re and W-5%Ta. The samples were implanted with 3000appm concentration of helium. All samples show a substantial increase in hardness of up to 107% in the case of pure W. An appreciable difference in modulus is also seen in all samples. Initial TEM work has shown no visible He bubbles, suggesting that the mechanical properties changes are due to He-vacancy cluster formation below the resolvable limit.

Atomistic studies of nucleation of He clusters and bubbles in bcc iron

Atomistic simulations of the nucleation of He clusters and bubbles in bcc iron at 800K have been carried out using the newly developed Fe–Fe interatomic potential, along with Ackland potential for the Fe–Fe interactions. Microstructure changes were analyzed in detail. We found that a He cluster with four He atoms is able to push out an iron interstitial from the cluster, creating a Frenkel pair. Small He clusters and self-interstitial atom (SIA) can migrate in the matrix, but He-vacancy (He-V) clusters are immobile. Most SIAs form <111> clusters, and only the dislocation loops with a Burgers vector of b=1/2<111> appear in the simulations. SIA clusters (or loops) are attached to He-V clusters for He implantation up to 1372appm, while the He-V cluster–loop complexes with more than one He-V cluster are formed at the He concentration of 2057appm and larger.

Computer Simulation of Helium Effects in Plutonium During the Aging Process of Self-Radiation Damage

AbstractDue to α radioactive decay Pu is vulnerable to aging. The behavior of He in Pu is the foundation for understanding Pu self-radiation damage aging. Molecular dynamics technique is performed to investigate the behavior of defects, the interaction between He and defects, the processes of initial nucleation and growth of He bubble and the dependence of He bubble on the macroscopical properties of Pu. Modified embedded atom method, Morse pair potential and the Lennard-Jones pair potential are used for describing the interactions of Pu-Pu, Pu-He and He-He, respectively. The main calculated results show that He atoms can combine with vacancies to form Hevacancy cluster (i.e., the precursor of He bubble) during the process of self-radiation as a result of high binding energy of an interstitial He atom to vacancy; He bubble’s growth can be dominated by the mechanism of punching out of dislocation loop; the swelling induced by He bubble is very small; grain boundaries give rise to an energetically more favorable zone for the interstitial He atom and self-interstitial atom accumulation than for vacancy accumulation; the process of He release can be identified as the formation of release channel induced by the cracking of He bubble and surface structure.

Defect structures and hardening mechanisms in high dose helium ion implanted Cu and Cu/Nb multilayer thin films

Helium (He) exerts a significant influence on the mechanical behavior of irradiated materials. The microstructural evolutions and hardening mechanisms of pure 1μm thick Cu film and Cu/Nb multilayers of individual layer thickness of 70nm, 5nm and 2.5nm were investigated after 1at.% and 7at.% He ion implants at room temperature. Implantation of 7at.% He produces a uniform dispersion of bubbles throughout the film in all samples and bubble pressure increases and volume fraction decreases with reducing layer thickness. For 5nm layer thickness approximately 32% He atoms are trapped at Cu–Nb interface, grain boundaries or dislocations in the form of He-vacancy clusters, which cannot be detected by electron microscopy. For a 1 at.% He implantation, He bubbles are barely detectable in Cu/Nb multilayers with 5nm individual layer thickness or less, suggesting the extraordinary capability of the Cu–Nb interface in absorbing and annihilating point defects. Hardness measurement indicates for coarse multilayers (h≥70nm) and pure Cu, the hardening from He bubbles is significant and increases with increasing He content, which can be described by Orowan hardening mechanism. However, when h is small (h≤5nm), the hardening is significantly mitigated, regardless of He concentration. The strengthening mechanism is dependent upon the resistance of the defect loaded interface to the transmission of single dislocation.

Atomistic simulation of interactions of fracture with defect clusters in delta-Pu

The Pu-He pair potential fitted by ab initio data, and the Pu-Pu and He-He modified embedded atom method (MEAM) potentials have been implemented to perform multi-scale simulations for the interactions of fracture with the self-interstitial atom (SIA), He interstitial atom and He-vacancy clusters. The simulation results indicate that Pu atoms around the fracture agglomerate into an elliptic self-interstitial loop. Interstitial He atoms evolve into separate interstitial atoms, small He atom clusters and some substitutional He atoms. The He-vacancy cluster forms a spheric structure with a 1:1 He-to-vacancy ratio. Finally, the existence of self-interstitial atoms will lead to the local change of Pu lattice and an increasing disorder, and the whole simulation cell shows a melting state at about 10.0 ps.

Cr interactions with He and vacancies in dilute Fe-Cr alloys from first principles

We have performed density functional theory calculations to study the effect of Cr in the formation of He-vacancy (He${}_{n}$V${}_{m}$) clusters in a dilute Fe-Cr system. We find a significant repulsive interaction between Cr and He at neighboring tetrahedral sites. The octahedral He close to Cr becomes unstable falling to a nearby tetrahedral site, unlike in pure systems. We note a slight electronic interaction between an interstitial He and the metal atoms, stronger with Cr and responsible of their repulsion. The effect of a Cr atom in the formation of He${}_{n}$V${}_{m}$ clusters is small except for cases with high He-to-vacancy ($n/m$) ratio. In systems with multiple Cr atoms we find that, while interstitial He always repels Cr, a vacancy or a substitutional He may screen the ferromagnetic Cr coupling and promote their clustering around them. Two different behavior regimes may therefore be suggested. When the $n/m$ ratio is close to 1 or lower the Cr atoms seem to be attracted to the He${}_{n}$V${}_{m}$ cluster while when the $n/m$ ratio is greater than 1 the Cr atoms are in general repelled from it. Concerning diffusion, the energy barrier for a tetrahedral He to hop toward a neighboring position of Cr is found to be twice as large as its migration barrier in pure Fe (0.12 vs 0.06 eV). On the other hand, the diffusion of a substitutional He via the dissociative mechanism has a slightly higher dissociation barrier for first nearest neighbor sites of a Cr atom. For the vacancy mechanism we find similar diffusion properties in dilute Fe-Cr systems and in pure Fe except a slightly lower barrier close to Cr that may induce small trapping effects at low temperatures.

Strain and stress build-up in He-implanted UO2 single crystals: an X-ray diffraction study

The strain and stress build-up in 20-keV He-implanted UO2 single crystals have been determined by means of X-ray diffraction through reciprocal space mapping, with the use of a model dedicated to the analysis of the strain/stress state of ion-irradiated materials. Results indicate that the undamaged part of the crystals exhibits no strain or stress; on the other hand, the implanted layer undergoes a tensile strain directed along the normal to the surface of the crystals and a compressive in-plane stress. The build-up of both strain and stress with He fluence exhibits a two-step process: (i) a progressive increase up to a maximum level of ~1% for the strain and ~−2.8 GPa for the stress, followed by (ii) a dramatic decrease. The origin of the strain and stress build-up is the formation of both self-interstitial defects and small He-vacancy clusters. The strain, and stress relief is tentatively attributed to the formation of extended defects (such as dislocations) that induce a plastic relaxation.

Thermal behaviour of helium-implanted spinel single crystals

The study of the microstructural modifications induced in spinel implanted with 4He + at 4.7 at.% and subsequently annealed at 1075 K is addressed in this paper. The combination of three analysis techniques Rutherford backscattering spectrometry in channeling geometry (RBS/C), X-ray diffraction and transmission electron microscopy was used in order to gain information about the damage depth distribution, the nature of radiation defects, and the occurrence of microstructural modifications. In as-implanted crystals the disorder level is weak, and the damage principally consists of small helium-vacancy clusters. These defects induce a tensile strain in the direction normal to the implanted crystal surface. After annealing, a surprising increase of the disorder level is measured by RBS/C. This increased backscattering yield is due to the formation of a particular type of He-vacancy clusters, namely He platelets, which also induce a relaxation of the strain.

Development of a pair potential for Fe–He by lattice inversion

A new pair potential for helium in bulk iron was developed using a method based on the Chen–Möbius lattice inversion in order to study the effect of He in irradiated iron. By means of molecular dynamic (MD), we have examined this interatomic potential. Comparing with the ab initio calculation results, the stability of He-vacancy clusters at zero temperature and migration energy was well reproduced by this potential.

Implementation of a new Fe–He three-body interatomic potential for molecular dynamics simulations

A recently developed interatomic potential for He–Fe interactions includes a three-body term to stabilize the interstitial He defect in the tetrahedral position in the Fe bcc matrix and provides simultaneous agreement with the forces and energies of different atomic configurations as computed by first principles. This term makes a significant contribution to the static and dynamic properties of He in Fe. The implementation of this potential for atomistic simulations using molecular dynamics (MD) presented certain challenges which are discussed here to facilitate its further use in materials research, particularly to investigate the behavior of iron-based alloys that may be employed in fusion energy systems. Detailed results of an MD study comparing the new potential and alternate He–Fe pair potentials with different iron matrix potentials have been presented elsewhere to illustrate the impact of the He–Fe potential on He diffusion, helium clustering and the dynamics of He-vacancy clusters.

Dynamic interactions of helium-vacancy clusters with edge dislocations in α-Fe

The effects of He-vacancy (He-V) clusters on the mobility of an a/2〈1 1 1〉{1 1 0} edge dislocation in α-Fe are investigated by atomic simulation with empirical potentials at 0 K. A number of small He n V m ( n/ m 0–4) clusters initially placed at the same position on the slip plane are comparatively studied. The results show that the interaction of He-V clusters with edge dislocations depends on not only the helium-to-vacancy (He/V) ratio, but also the cluster size. The small He-V clusters with low He/V ratios have a small effect on the dislocation mobility, but the larger clusters with higher He/V ratios significantly increase the critical resolved shear stress for dislocation glide. One of interesting results is that the He-V clusters almost stay at their original positions, and do not move along with the dislocation.

Influence of defects on the coalescence of helium in titanium: An atomistic simulation

Molecular dynamics was employed to study the migration and coalescence of He atom and He dimmer in Ti lattice which contains He-vacancy cluster. The influence of the ratio of helium to vacancy （He density） of the cluster and the He number of the cluster on coalescence of He and He-vacancy cluster in Ti lattice is studied. It is found that anisotropic diffusion is independent of the He-vacancy cluster in Ti lattice. The coalescence of helium and He-vacancy cluster is mainly along the ［001］ direction. The coalescence of He dimmer and He-vacancy cluster is faster than the coalescence of He atom and He-vacancy cluster.

Influence of Self-Interstitial Mobility on He-Vacancy Cluster Nucleation and Growth in Nickel

The presence of impurities could affect the results observed in pure Ni. In particular, impurities could interact with self-interstitial atoms, which are highly mobile, effectively reducing their mobility. In this work we study the influence of the mobility of self-interstitials on He desorption. The nucleation of He-vacancy complexes is studied depending on the mobility of these self-interstitials in terms of He to vacancy content as well as concentration of these complexes.

Multiscale Modeling of Helium-Vacancy Cluster Nucleation under Irradiation: A Kinetic Monte-Carlo Approach

AbstractWe applied ab initio calculation and an object kinetic Monte Carlo modeling to the study of He-vacancy cluster nucleation under irradiation in bcc and fcc Fe, which are surrogate materials for ferritic/martensitic and austenitic steels, respectively. The ab initio calculations provided parameters for the object kinetic Monte Carlo model, such as the migration energies of point defects and the dissociation energies of He and vacancy to He-vacancy clusters. We specially focused on the simulation of high He/dpa irradiation such as He-implantation into the materials and tracked the nucleation of clusters and the fate of point defects such as SIAs, vacancies, and He atoms. We found no major difference of He-vacancy cluster nucleation between bcc and fcc Fe when we ignore the intracascade clustering even if the migration energies of point defects are significantly different between the two crystals.

Migration of vacancies, He interstitials and He-vacancy clusters at grain boundaries in α-Fe

The dimer method for searching transition states has been used to systematically study possible migration paths of vacancies, He interstitials and He-vacancy (He/V) clusters at Σ11<1 1 0> {3 2 3} and Σ3<1 1 0> {1 1 1} grain boundaries (GBs) in α-Fe. Vacancies trapped at the GBs diffuse along the GBs with migration energies much less than that within the perfect crystal. Long-time dynamics simulations of diffusion pathways reveal that vacancies migrate one-dimensionally along specific directions in both GBs: directly along close-packed rows in the Σ3 GB, and in zigzag paths within the Σ11 GB. Also, dimer saddle point searches show that He interstitials can diffuse along the GBs with migration energies of 0.4–0.5 eV, similar to those of individual vacancies at the GBs, and the corresponding mechanisms are determined. The rate-controlling activation energy for migration of a He-divacancy cluster in the GBs determined using the dimer method is about 0.9 eV. This is comparable to the migration energy for a He-divacancy cluster in bulk α-Fe.

Effects of interatomic potential on He bubble creation by cascades in α-iron

The effects of using different interatomic potentials in molecular dynamics (MD) simulations of the formation of He-vacancy clusters within displacement cascades in α-Fe are investigated using two sets of potentials. Simulations of cascades produced by primary knock-on atoms of energy Ep=1–20keV were performed in α-Fe containing a concentration of substitutional He atoms varying from 1to5at.% at an irradiation temperature of 100K. Although the effects of interatomic potentials on the nucleation of He-vacancy clusters induced by cascades are relatively small, the number and size of He-vacancy clusters produced are significantly different for the different potentials employed in this study. Thus, these differences may influence the microstructural evolution predicted in damage accumulation models that use the results from MD cascade simulations as input. The observed differences in postcascade configurations can be attributed mainly to the differences in the Fe–Fe and Fe–He potentials.

A Molecular Dynamics Study of Helium-Vacancy Clusters Production due to Cascades in α-Iron

Molecular dynamics (MD) methods are utilized to study the displacement cascades in α-Fe containing different concentrations of substitutional He atoms. Primary knock-on atom (PKA) energies, Ep, from o.5 keV to 20 keV are considered at a temperature of 100 K and 600 K, and the results are compared with those performed in pure α-Fe. There are distinct differences in the number and size of defect clusters within displacement cascades with and without substitutional helium atoms. Particularly, the number and size of helium-vacancy clusters generally increase with increasing helium concentration and PKA energy. However, the number of He-vacancy (He-V) clusters increases with increasing temperature, the mean size of He-V clusters is independent on temperature for the same He concentration and energy recoils.