Kazunori Morishita

Thermal stability of helium-vacancy clusters in iron

Molecular dynamics calculations were performed to evaluate the thermal stability of helium–vacancy clusters (HenVm) in Fe using the Ackland Finnis–Sinclair potential, the Wilson–Johnson potential and the Ziegler–Biersack–Littmark–Beck potential for describing the interactions of Fe–Fe, Fe–He and He–He, respectively. Both the calculated numbers of helium atoms, n, and vacancies, m, in clusters ranged from 0 to 20. The binding energies of an interstitial helium atom, an isolated vacancy and a self-interstitial iron atom to a helium–vacancy cluster were obtained from the calculated formation energies of clusters. All the binding energies do not depend much on cluster size, but they primarily depend on the helium-to-vacancy ratio (n/m) of clusters. The binding energy of a vacancy to a helium–vacancy cluster increases with the ratio, showing that helium increases cluster lifetime by dramatically reducing thermal vacancy emission. On the other hand, both the binding energies of a helium atom and an iron atom to a helium–vacancy cluster decrease with increasing the ratio, indicating that thermal emission of self-interstitial atoms (SIAs) (i.e. Frenkel-pair production), as well as thermal helium emission, may take place from the cluster of higher helium-to-vacancy ratios. The thermal stability of clusters is decided by the competitive processes among thermal emission of vacancies, SIAs and helium, depending on the helium-to-vacancy ratio of clusters. The calculated thermal stability of clusters is consistent with the experimental observations of thermal helium desorption from α-Fe during post-He-implantation annealing.

Microstructure evolution in tungsten during low-energy helium ion irradiation

In situ transmission electron microscopy (TEM) study was performed to investigate the microstructural changes in tungsten during low-energy He+ ion irradiations in an electron microscope linked with an ion accelerator. The irradiations were carried out with 8 and 0.25 keV He+ ions at 293, 873 and 1073 K. In the case of the 8 keV irradiation, irradiation-induced vacancies act as nucleation sites for dislocation loops and helium (He) bubbles. Accordingly, such defects were formed even at the higher temperatures. With increasing irradiation temperature, the growth rates of dislocation loops and He bubbles rise remarkably. Although no vacancies are produced during 0.25 keV irradiation, He platelets, interstitial loops and He bubbles were formed. Impurity atoms may act as trapping centers for He atoms, which form bubbles by ejecting W atoms from their lattice site.

Effects of helium bombardment on the deuterium behavior in tungsten

The effects of pre-irradiation with helium ions of fusion relevant energy on trapping of injected deuterium in W was studied by thermal desorption spectrometry technique using high-resolution quadrupole mass spectrometer. Pre-irradiation with He ions caused remarkable effects on the trapping of injected deuterium. Most of the injected deuterium is desorbed between 400 and 600 K for the case without helium pre-irradiation, while additional desorption occurs between 600 and 800 K for the helium pre-irradiation case. Total amount of the trapped deuterium for irradiations of 2.0×1021 He/m2 and 1.0×1022 D2/m2 is 6.2×1020 D2/m2, which is more than three times higher than that in the case of no helium pre-irradiation. The present result indicates that irradiation effects of He bombardment must be taken into account to understand and evaluate the behavior of hydrogen isotopes in fusion environment.

High resistance to helium embrittlement in reduced activation martensitic steels

Abstract Reduced activation martensitic steels (RAMSs) are the prime candidate structural material for the DEMO reactor and beyond where the material has been considered to suffer severe embrittlement caused by high-dose neutron irradiation and several thousands appm of transmutated helium. However, recent several works show high resistance to helium embrittlement of RAMS. Good performance of RAMS in the presence of rather high concentration of helium is considered to be due to high trapping capacity for helium atoms in the martensitic structure that consists of dislocations, lath boundaries, grain boundaries and carbide/matrix interfaces. To make clear the role of dislocations in trapping helium, thermal helium desorption spectra were measured for iron specimens annealed at different temperatures after cold work. A desorption peak, which increased its height with increasing dislocation density, was observed at around 550 °C, suggesting that dislocations trap helium atoms. A molecular dynamics simulation study for investigating the helium trapping behavior at helium–vacancy complexes suggests that helium is rather strongly bound to the complexes and increases the binding energy of vacancy to the complex, resulting in increasing stability of the complexes at elevated temperatures by reducing thermal emission of vacancies.

Effects of dislocation on thermal helium desorption from iron and ferritic steel

Abstract Thermal desorption measurements were performed to investigate helium trapping in α-iron and a reduced activation martensitic steel (RAMS) bombarded at room temperature with mono-energetic He + ions. Incident energies were both 8 keV and 150 eV. Prior to helium implantation, the samples of α-iron were plastically deformed by rolling at room temperature followed by annealing at 673, 873 and 1073 K for 2, 12 and 2 h, respectively. These samples are hereafter called PR1, PR2 and FA, respectively, and the as-rolled sample is called CW. The dislocation densities of the samples decrease in the order of CW, PR1, PR2 and FA. Thermal desorption spectra show a clear peak at around 800 K, only for the CW, PR1, PR2 and RAMS samples, which may correspond to desorption of helium atoms trapped by dislocations. The effects of dislocations on micro-structural evolution in iron and the steel during helium implantation are discussed.

Thermal stability of helium-vacancy clusters and bubble formation: Multiscale modeling approach for fusion materials development

Abstract The recent progress on our multiscale modeling to understand radiation damage processes in materials during irradiation is reviewed. The energies of He-V cluster formation in Fe were evaluated using a molecular dynamics (MD) simulation technique that employed interatomic potentials partially developed by first-principle (FP) calculations. Using the calculated energies, the longer timescale behavior of He-V clusters in Fe was investigated using a kinetic Monte-Carlo (KMC) simulation technique. The FP-MD-KMC scheme provided us significant information on the thermal stability of a He-V cluster in Fe as a function of the helium-to-vacancy ratio of the cluster.

Nucleation path of helium bubbles in metals during irradiation

A thermodynamic formalization is developed for description of the nucleation and growth of helium bubbles in metals during irradiation. The proposed formalization is available for evaluating both microstructural changes in fusion first wall materials where helium is produced by (n, α) nuclear transmutation reactions, and those in fusion diverter materials where helium particles with low energy are directly implanted. The calculated nucleation barrier is significantly reduced by the presence of helium, showing that a helium bubble with an appropriate number of helium atoms depending on bubble size can nucleate without any large nucleation barriers, even at a condition where an empty void has very large nucleation barriers without helium. With the proposed thermodynamic formalization, the nucleation and growth process of helium bubbles in iron during irradiation is simulated by the kinetic Monte Carlo (KMC) technique. It shows the nucleation path of a helium bubble on the (N He, N V) space as functions of temperatures and the concentration of helium in the matrix, where N He and N V are the numbers of helium atoms and vacancies contained in the helium bubble, respectively. Bubble growth rates depend on the nucleation path and suggest that two different mechanisms operate for bubble growth: one is controlled by vacancy diffusion and the other is controlled by interstitial helium diffusion.

Helium Accumulation Behavior in Iron Based Model Alloys

Helium desorption from Fe-based model alloys irradiated by energetic helium ions was measured during post-irradiation annealing to investigate the energetics and kinetics of formation and annihilation of helium-related defects. Desorption temperatures were observed to be widely ranged from 450 to 1500 K, indicating that helium is bound to a wide variety of trapping sites such as vacancies and dislocations at various binding states. Such a feature is also observed in fusion ferritic steel. A comparison of helium desorption spectra obtained using Fe, Fe-Cr and Fe-Cr-Ni alloys showed that helium is more strongly trapped in bcc Fe than fcc Fe. It indicates that the long distance migration of helium takes place less frequently in bcc matrix, which may reduce the probability of helium clustering. Fusion ferric steel has a lot of trapping sites for helium such as dislocations, solute atoms, the interface of precipitates, impurities and lath boundaries, and so on, and in addition, it has bct matrix, indicating that most of helium atoms must be dispersed in the matrix and therefore it is difficult for them to cluster as a bubble. This may be a reason for higher helium resistance of the steel.

Super-saturated hydrogen effects on radiation damages in tungsten under the high-flux divertor plasma irradiation

Tungsten is a prime candidate as the divertor material of the ITER and DEMO reactors, which would be exposed to unprecedentedly high-flux plasmas as well as neutrons. For a better characterization of radiation damages in the tungsten under the divertor condition, we examine influences of super-saturated hydrogen on vacancies in the tungsten. The present calculations based on density functional theory (DFT) reveal unusual phenomena predicted at a super-saturated hydrogen concentration: (1) strongly enhanced vacancy concentration with the super-saturated hydrogen concentration is predicted by a thermodynamics model assuming multiple-hydrogen trapping, i.e. hydrogen clusters formation, in the vacancies; and (2) DFT molecular dynamics revealed that hydrogen clusters can prevent a vacancy from recombining with the neighboring crowdion-type self-interstitial-atom. This suggests that neutron damage effects will be increased in the presence of the hydrogen clusters.

Mobility of self-interstitial atom clusters in vanadium, tantalum and copper

Abstract Molecular dynamics (MD) simulations were performed to investigate the mobility of isolated self-interstitial atoms (SIAs) and their clusters in V, Ta and Cu. The migration of an isolated SIA is accompanied by rotation of a dumbbell axis to the close-packed direction of metals. The migration of an SIA cluster strongly depends on its structure. A relatively smaller-size cluster can migrate with simultaneous rotation of the axes of SIA pairs in the cluster to the same close-packed direction, which is the glissile configuration of the cluster. The transformation to the glissile configuration takes place more frequently than the dumbbell rotation of an isolated SIA in V and Ta, while it takes place less frequently in Cu. The smaller cluster can still change its diffusion direction. A greater-size cluster in the bcc metals, on the other hand, has the thermally stable form of densely-packed, parallel crowdions. It migrates without any changes of diffusion direction. The migration behavior of 7-SIAs clusters in Ta was also evaluated as a function of tensile and compressive strains.