T. Tsutsumi

Positron lifetime calculations on vacancy clusters and dislocations in Ni and Fe

Abstract Positron lifetime calculations have been performed on vacancy clusters (stacking fault tetrahedra (SFT), vacancy loops), such clusters on dislocation line, interstitial clusters, such clusters on a dislocation line and dislocation line itself in order to investigate the so-called intermediate lifetimes observed in the experiments, namely, positron lifetimes between that at a matrix and that at a single vacancy. Before lifetime calculations, various defects were constructed in the model lattices and were relaxed completely to obtain the stable atomic structure by using N -body potentials. Then positron lifetime calculation was carried out for each defect. It was shown that positron lifetime for a SFT in Ni dependes on its size and becomes smaller with increasing the size. The positron wave function is mainly localized at the corner of a SFT, which gives rather lifetime, e.g., 130 ps for V 28 , but when the cluster size is small, e.g., less than 10 vacancies, it gives a rather longer lifetime, e.g., 177 ps for V 6 because of the wave function localized at the inner space of a cluster. These behaviours are consistent with the experimental results. It was also found that the positron lifetime on a dislocation line and that at a jog are short (113 and 119 ps, respectively for Ni, 117 and 117 ps, respectively for Fe), close to the lifetime at matrix (110 ps for both Ni and Fe) and in these cases trapping potentials for a positron are shallow both for Ni and Fe.

Computer simulation of defects interacting with a dislocation in Fe and Ni

In order to investigate the fundamental aspects of damage evolution under irradiation computer simulations of the interaction between a dislocation and point defects such as a self-interstitial atom (SIA), a vacancy and interstitial clusters have been performed for various configurations. It is found both in Fe and Ni that a crowdion with axis parallel to the Burgers vector of an edge dislocation interacts more strongly than those with other axis orientations. The same tendency was seen for the dumbbell. The capture range (capture area) within which SIAs are trapped by an edge dislocation is larger for Ni than for Fe, and that for vacancies is much smaller than that for SIAs, suggesting that the bias factor in Ni is larger than that in Fe.

Computer simulation of the interaction between an edge dislocation and interstitial clusters in Fe and Ni

Atomic structures of interstitial type dislocation loops and the interaction between these loops and an edge dislocation have been investigated for Fe and Ni by means of computer simulation in order to understand the basic feature of the damage structure evolution during irradiation which provides cascade formation of defects. Clusters of crowdions or dumbbells were placed in the model lattices and final structures were observed after full relaxation. It is found that in the case of clusters of crowdions, relaxation of the structure, namely, split of strain concentration for each crowdion in the cluster occurs with increasing the number of crowdions in one cluster, i.e., beyond about ten, and this split structure indicates the transition to dislocation loops, because a straight edge dislocation has the same split nature both in Fe and Ni. In the study of the interaction between an initial cluster and an edge dislocation it is found that a stacking fault of an extended edge dislocation is heavily deformed by the presence of an interstitial cluster just below the slip plane. Finally the critical stress for the motion of an interstitial loop, that is, Peierls stress, for the dislocation loop was investigated and it is found that as the loop size increases, Peierls stress decreases and approaches the level for a straight edge dislocation.

Computer simulation of the bias factor in void swelling in metals

Abstract In order to obtain information on the bias factor in void swelling in metals, computer simulation in a model lattice has been performed for the interaction between a dislocation, or a dislocation loop and point defects in both fcc (Au) and bcc (Fe) metals. Not only a straight dislocation but also a jogged dislocation was chosen for the calculation of the interaction. In fcc metal, the calculation was made for the interaction between an extended dislocation and point defects. The effective interaction range, L, was determined for all the cases, and from these values the so-called bias factor was obtained as 55 and 19% for fcc (Au) and bcc (Fe) metals, respectively in the case of a straight dislocation. Almost no difference for this range L was seen between a dislocation without a jog and that with a jog. However, sink efficiency is considered to be higher for a jogged dislocation than a straight one. A comparatively small bias factor, about 8%, was obtained for the case of a dislocation loop and point defects in the fcc lattice.

Positron lifetime calculation in FeCu binary alloy with lattice relaxation

Abstract Using the embedded atom type many body potentials for iron and copper, the lattice relaxation around copper impurity atoms in iron has been performed. The stable configuration of atoms around copper and copper vacancy complexes in FeCu alloy was simulated. By introducing a positron into the relaxed lattice of FeCu binary alloy with and without a vacancy, positron lifetimes have been calculated. The positron lifetime in a relaxed FeCu matrix was 119 ps, while in a relaxed Cu-vacancy pair it was 168 ps. The latter, which is shorter than the positron lifetime in a single vacancy, is in good agreement with the experimental lifetime observed in FeCu alloy irradiated with electrons at low temperature.

Computer simulation of atomic properties and dynamic behavior of interstitial clusters in Ni

The atomic structure and dynamic behavior of interstitial clusters, i.e., a bundle of <110> crowdions, have been investigated in a model Ni lattice. An extended dislocation loop was obtained after full relaxation of a loop of hexagonal shape, consisting of four dislocation segments lying on {111} slip planes and two dislocation segments on {100} slip planes. The dislocation segments on {111} slip planes are extended, but the segments on {100} slip planes are not extended. By observing the motion of a dislocation loop under axially symmetrical shear stress, the Peierls stress for the dislocation loop was obtained. Also, a diamond-shaped dislocation loop was constructed in the model lattice, consisting of four dislocation segments on {111} slip planes and no segments on {100} slip planes. The Peierls stress for this diamond-shaped dislocation loop was found to be less than that for the hexagonal-shaped dislocation loop.

Atomic structure and dynamic behavior of small interstitial clusters in Fe and Ni

Atomic structure and dynamic behavior of small interstitial clusters (dislocation loops) in Fe and Ni have been studied by means of both the static relaxation method and the molecular dynamics (MD) method in order to clarify their role in the evolution of damage structure during irradiation, especially in the so-called production bias effect through one-dimensional migration and sink efficiency to dislocations. Model crystals were constructed by using N-body potentials and stable atomic structures of small interstitial clusters, i.e., bundle of crowdions were obtained. It was found that each crowdion in the cluster has a split structure when the number of crowdions in the cluster is larger than 10. Dynamic behavior of the loop, e.g., the interaction with a crowdion on a (I I 1 loop axis was also investigated as a function of time by the MD method. It was shown that a small interstitial cluster, e.g., I 19 in Fe is very mobile under the interaction with a crowdion, which shows that this interstitial cluster I 19 has already the property of a dislocation loop of edge character and low Peierls potential for the motion of this loop, which is consistent with the straight edge dislocation in Fe.

Study of fundamental features of bias effect in metals under irradiation

Abstract Studies on bias mechanisms, such as the dislocation bias and production bias have so far been made in the Fe and Ni model crystals. In the present work the interaction between an edge dislocation and interstitial clusters has been mainly studied by computer simulation from the viewpoint of the production bias for Fe and Ni. Capture zones for interstitial clusters In (bundles of n crowdions, n=1, 2, 3, 5) to an edge dislocation line have been obtained, with the definition of the capture zone as the region where the binding energy is larger than kT=0.067 eV (T=500 °C). This shows increasing capture areas on the expansion side of the edge dislocation line with increasing size of interstitial clusters both in Fe and Ni. The energy change from the as-trapped state of the interstitial cluster to the absorbed state into the edge dislocation core was also calculated. This showed that the final stable configuration is a jogged state in Fe. This is also correct in Ni but only when the size of the interstitial cluster is larger than a certain value. These results give a confirmed basis to the production bias mechanism.

Finnis-Sinclair ポテンシャルを用いたBCC結晶中のらせん転位の運動の計算機シュミレーション

Motion of a screw dislocation in a bcc lattice has been investigated by computer simulation technique using Finnis-Sinclair potential (Ta). Motion under pure phear stress was compared with that under both shear and normal stress, i. e., conventional tensile condition. It was made clear that existence of normal stress reduced critical shear stress, i. e., Peierls stress for a screw dislocation. The total energy change was calculated during the screw dislocation motion overcoming Peierls potential, and sh6wed double peaks of the total energy exist within one atomic distance motion. This showed that Peierls potential has a camel－hump shape (double peak shape), which is desirable to explain the characteristic temperature dependence of the yield stress observed in bcc metals.

Computer Simulation of the Interaction between a Screw Dislocation and Point Defects in α-Iron Lattice

Abstract Computer simulation was performed in order to understand atomistically the softening-hardening transition due to a post-irradiation annealing at 150K, which was observed in high-purity iron single crystals irradiated by 28MeV electrons at 77K. According to the results of simulations, in a stressed model iron crystal a screw dislocation motion was enhanced by the side motion of three kinks formed by absorbing a self-interstitial atom (SIA). However, if a moving screw dislocation was made to encounter a di-interstitial, a complex kink configuration was formed and an enhancement of a screw dislocation was not observed. These results can explain the experimentally observed softening-hardening transition, because 150K annealing removed away all the SIAs and introduced a di-interstitial to specimens.