Yoshiharu Shimomura

Computer Simulation of Displacement Damage Cascade Formation near Sigma 5 Twist Boundary in Silver

A computer simulation of molecular dynamics is carried out to study the effects of a sigma 5 twist boundary on the formation of displacement damage cascades in silver. When a displacement damage cascade forms near the boundary, interstitial atoms are attracted and segregate on the boundary. The number of Frenkel defects after the formation of damage cascade near the boundary is more than that in a perfect crystal. The segregation of interstitial atoms is due to the interaction with the boundary through the strain field. The movement of interstitial clusters in a strain field is due to a crowdion migration. The larger interstitial clusters are attracted to the boundary at the larger separation. When a damage cascade forms on the boundary, the crystal structure of sigma 5 boundary is modified during the solidification of its molten core. The segregation of interstitial atoms on stacking fault and twin boundary on the formation of damage cascade was also studied. The segregation of interstitial to thes...

Molecular dynamics calculations of properties of the self-interstitials in copper and nickel

Molecular dynamics calculations are performed to study the properties of self-interstitials in copper and nickel using semi-empirical, many-body potentials based on Embedded Atom Method. A set of values of formation, binding and migration energy calculated for single, di-, tri-, tetra-, penta- and hexa-interstitial in copper and nickel are reported. The stable configurations and migration mechanism of them are also presented. The results show that the small interstitial clusters have a significant lower migration energy than the single interstitials and move in the form of one-dimensional crowdion migrations. The clusters in copper are very mobile and the tri-, penta-interstitial in nickel are less mobile because they are difficult to relax to the crowdion dumbbell configurations. The results are also compared with available experimental investigations and previous interstitial calculations using pair potentials.

Microstructure in pure copper irradiated by simultaneous multi-ion beam of hydrogen, helium and self ions

Abstract Pure copper was irradiated at 300–500°C by 5 MeV Cu ions (single beam) and Cu ions plus gas atoms (H and He) (dual beam irradiation) simultaneously. The high energy ion irradiation was carried out with the accelerator TIARA at the Takasaki-establishment of JAERI. The ions stop within a few microns from surface level and damage was formed up to this depth. The damage structure was observed as a function of the depth utilizing a focused ion beam (FIB) device. Below 300°C irradiation with a single beam produced a high density of stacking fault tetrahedra (SFT) but void formation was not observed. Large voids were observed with single beam irradiation at 500°C. In specimen irradiated with a dual beam of helium and Ni ions, the number density of voids was increased significantly. In copper irradiated with hydrogen and Ni ions, the number density of voids was not so large. Experimental results show that helium atoms promote void formation. Hydrogen atoms have less effect on void formation than helium atoms in pure copper.

Vacancy generation in deformed thin metal

Abstract Kiritani found that vacancy clusters of large number density form in f.c.c. thin metal of 100 nm thickness deformed to their fracture. In the present work, a computer simulation of deformation of thin metal is carried out to investigate how vacancies of high concentration generate during a deformation of thin specimens of Al and Cu. A crystal of 4000 atoms whose size is (10a0xa0×xa010a0xa0×xa010a0) is elongated to z-axes. Two modes of simulation are carried out. In the mode 1, surface which are normal to x-axes and y-axes are kept free. In the mode 2, the periodic boundary condition is applied for all surfaces. The mode 2 is equivalent to the deformation of bulk metal. In the simulation of mode 1, the tilting of 〈1 1 0〉 atom row initiates on the surface. A tilting of rows to the same direction expands on a (1xa01xa01) plane and arrives to other side of surface. Dislocations do not form during the tilting. The tilting of atom rows occurs due to easy movement of atoms on surface responding to stress. In highly deformed thin metal, the tilting of atom rows occurs on multi-layers of parallel planes. Subsequently a tilted row split into two rows. A new row initiates by moving an atom on surface to the interstitial position. A transportation of atoms from the normal row to the new row occurs during deformation, which contributes to the reduction of thickness. Vacancies of high concentration are not generated in the case of the mode 1 deformation. In the simulation of the mode 2, the formation of domain in which atom rows tilt to the same direction occurs. At the domain boundary ordered array of atom rows becomes disordered in an instant and grows to a small crack of vacancy cluster. The formation process of vacancy clusters which were observed in deformed thin metal is due to the combination of the processes of the modes 1 and 2.

Point defects and their clusters in bcc metals

Abstract Recently Doyama and Kogure have developed new EAM potentials of bcc metals (DK_EAM). The validity of DK_EAM potential of Fe and V is examined by calculating a thermal expansion coefficient, a phonon dispersion and properties of point defects. In Fe, a calculated thermal expansion coefficient is negative and its absolute value is much smaller than that of experimental data. The phonon dispersion relation shows fair agreements at several branch. The calculated density of state of phonon shows a lack of a peak at high frequency determined experimentally. In V, the thermal expansion coefficient is negative and the crystal volume decreases significantly during a molecular dynamics (MD) simulation at 1200 K. This means that the embedding energy and the repulsive pair potential are not calculated correctly at the position which deviates from the stable site of atoms. We calculate the property of point defects in Fe. The relaxation volume of vacancies in Fe is very small which leads to the void formation as a vacancy cluster grows to a larger cluster. A MD simulation shows that two atoms in interstitial clusters approach very closely. This suggests the repulsive pair potential in DK_EAM is weak. By combining the pair potential of DK_EAM with the classical pair potential, the DK_EAM is modified. The results are compared with the previous calculation and the experimental data.

Annihilation of interstitial atoms to dislocations in neutron-irradiated Cu and Ni at high temperature

Abstract When Cu and Ni were irradiated at 300°C by neutrons in a reactor, interstitial clusters decorate dislocations at a low fluence such as 5×10 17 n / cm 2 . Computer simulation suggests that this is due to the accumulation of interstitial atoms in the dislocation strain field and also the difficulty of absorption of interstitials to extended dislocation which existed prior to irradiation. Interstitial clusters accumulated along dislocations coagulated to form dislocation loops in Cu and dipole loops which were along 〈1xa01xa02〉 direction with a 〈1xa01xa00〉 Burgers vector in Ni. These newly grown loops and dipoles can absorb interstitial clusters due to the unextended nature of these dislocations, and cause disappearance of decoration of interstitial clusters. Dislocations grew to a bow-out shape and was pinned at voids.

Computer simulation of the clustering of small vacancies in nickel

Abstract Small clusters of vacancies were introduced in a crystal of 4000 atoms at randomly selected positions in Ni. Computer simulations were carried out with the isotropic potential of embedded atom method (EAM) (M.S. Daw and M.I. Baskes, Phys. Rev. B 29 (1984) 6443) at 1200, 1400, 1500, 1600 and 1700 K. After MD simulation, the results were output in an interval of 0.2 ps for 40–600 ps to observe the structural relaxation with AVS. In the present work, forty single vacancies were observed to cluster at 1700 K after MD run for 100 ps whereas four single vacancies were not observed for 600 ps. Further MD run is needed for vacancies to agglomerate. Triangular tri-vacancy (3v) also grew to a larger cluster from two tri-vacancies which were initially positioned closer than the other 3v. Four clusters of triangular 15-vacancy were introduced in a crystal. The clusters at first relaxed stacking fault tetrahedron (15v-sft). To change the structure of clusters played an important role for the direction to agglomerate. At low temperature agglomeration did not occur during MD calculation.

Development of vacancy clusters in neutron-irradiated copper at high temperature

Pure copper was fission-neutron-irradiated at 473 and 573 K from 0.0003 to 0.14 dpa. In copper, which was irradiated at 573 K to 0.0003 dpa, the number of vacancies which were accumulated in the largest stacking fault tetrahedron (SFT) was 276, while it was 470 in the smallest voids. This is explained by a model in which at 573 K a SFT converts to a void when the number of vacancies exceeds about 400. In 573 K irradiated copper, the number of vacancies in a SFT and a void of average size increases with the neutron fluence. The number of vacancies in a void increases more rapidly than that in a SFT. The reason appears to be that small vacancy clusters relax at 573 K to a string-like cluster, move as a cluster and coalesce. Experimental results are presented which show the movement of voids. In copper which was neutron-irradiated at 573 K, the number density of SFTs and voids peaked at 0.0003 dpa and decreased with fluence. The reason appears to be a low sink efficiency of dislocations for point defect absorption at 0.0003 dpa. Due to a low sink efficiency straight extended dislocations were decorated with many interstitial clusters. After jogs are formed on dislocations by joining with grown interstitial loops, the absorption efficiency of point defects increases significantly which lowers the density of SFTs and voids with increasing of dpa.

Atomistic processes of damage evolution in neutron-irradiated Cu and Ni at high temperature

Abstract This paper consists of two parts. In part 1, the experimental results of damage evolution of neutron-irradiated Cu and Ni are described. In part 2, results of computer simulations are described with linkage of experimental data to explore the atomistic process of damage evolution. To study experimentally the atomistic processes of damage evolution in neutron-irradiated Cu and Ni in part 1, we prepare two types of specimens for both metals. One is as-received specimen from manufacturer. Another is a residual-gas-free specimen which is prepared by melting as-received metals in highly evacuated vacuum at 10−5 Pa. Specimens are irradiated with fission neutrons in the temperature-controlled-irradiation capsule at JMTR (Japan Materials Testing Reactor). TEM (Transmission Electron Microscope) observation shows that the dislocation structure is developed by the aggregation of interstitial clusters in irradiated metals. It is found that the number density of void which are observed in specimens, both as-r...

Damage evolution in neutron-irradiated Cu during neutron irradiation

Abstract We fabricate Cu of residual-gas-free by melting them in vacuum of 10 −5 Pa. Both residual-gas-free specimen and as-received specimens which were estimated to contain 4 ppm hydrogen atoms were neutron-irradiated at 200°C and 300°C with temperature-controlled rig in Japan Material Testing Reactor (JMTR). Neutron fluence ranges from 5.3xa0×xa010 18 to 1.0xa0×xa010 20 n/cm 2 . Irradiated specimens were observed by electron microscopy. In copper, both stacking fault tetrahedron (SFT) and voids were observed. The number density of voids decreased with increasing the fluence. The size of voids increased with the fluence. The voids formed uniformly in specimens at the low fluence, while some of voids were observed near dislocations at the high fluence. The number density of SFT increased with the fluence at 200°C. The number of vacancies which are accumulated in a void is 350 times larger than that in a SFT in a specimen irradiated to 5.3xa0×xa010 18 n/cm 2 at 200°C. At low fluence the number density of voids is same for as-received specimens and residual-gas-free specimens. The difference of the number density of voids between these two specimens was observed at high fluence in which the density is low in the residual-gas-free copper. Results are modeled as follows. Small vacancy clusters move during an irradiation. Voids nucleate when the coalescence of small vacancy clusters occurs. The mobility of voids with gas atoms is lower than that without gas-atom. This causes the number density of voids in as-received copper to be larger than that in residual-gas-free copper.