Kenjiro Sugio

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...

Formation of vacancy clusters in deformed thin films of Al–Mg and Al–Cu dilute alloys

Abstract In the present study, vacancy clusters in elongated Al–Mg and Al–Cu thin films (Mg/Cu concentration=0.05–1.70 at.%) were examined by electron microscopy. No dislocations were observed in these films. In Al–Mg thin films deformed at room temperature, a large number of stacking fault tetrahedra (sft) were observed alongside a few vacancy loops. The opposite was true for Al–Cu thin films, where well-grown loops predominated, and only a few sft were observed. The Al–Cu film results show that the majority of vacancies form loops larger than sft. We also deformed Al–0.05at.% (Mg or Cu) alloys in liquid nitrogen and cold-transferred to an electron microscope. In Al–Mg, a large number of dotted defects (possibly sft) were observed, while very few such defects were observed in Al–Cu. This indicates that loops observed in Al–Cu thin films deformed at room temperature, grew during/after deformation. The likely contribution of strain-induced vacancies in deformed Al thin films to the voiding in VLSI interconnect wires due to electro-migration were discussed.

The influence of dynamical structural relaxation of point defect clusters on void formation in irradiated copper

Abstract In the neutron-irradiation experiment with a temperature controlled capsule at JMTR, residual-gas-free copper was irradiated at 200°C and 300°C together with as-received copper. The fluences were 5 × 10 18 n/cm 2 (the low fluence) to 1 × 10 20 n/cm 2 (the high fluence). TEM observation of the irradiated specimens showed that interstitial clusters form a colony at the low fluence which develops into a dislocation structure at the high fluence. Between the colonies only vacancy clusters in the form of voids and stacking fault tetrahedra (sft) were observed. There are no effects of residual gas atoms on the formation of voids at the low fluence although the effects become appreciable at the high fluence. The number of vacancies which are accumulated in a void is 350 times larger than that in a sft at the low fluence. The number density of voids decreased with increasing neutron fluence while the number density of sft increased. The voids form uniformly in copper irradiated to the low fluence while they were observed along dislocations at the high fluence. Computer simulations by molecular dynamics show that small interstitial clusters relax to a bundle of 〈110〉 crowdions and move long distances in response to small strain fields. Interstitial clusters move along a 〈110〉 direction and can switch to other 〈110〉 directions, and form groups of clusters. At high temperature, a dense colony of the clusters forms and develops into a dislocation structure. It is shown that small vacancy clusters relax to movable structures at high temperature. The structure consists of vacancies which are connected in a curved string shape. Along the vacancy strings, many relaxations of a tri-vacancy of Damask- Dienes-Weizer type (3v-sft) were observed. Such a relaxation to the 3v-sft type makes it difficult for a single vacancy evaporation. Small vacancy clusters move and coalesce into larger vacancy clusters. The linkage of the results of experiments and computer-simulations suggests that voids nucleate as a metastable defects at coalesced vacancy clusters at high temperature. The nucleation of voids is not affected by the influence of gas atoms dissolved in the material. Micro-voids migrate in the specimens after their nucleation. During their movement, gas atoms are trapped in the voids. The trapping of a larger number of gas atoms limits the movement of voids. This leads to a higher number density of voids in the as-received copper than in residual-gas-free specimens at the high fluence. Voids form uniformly in specimens at the low flunce and they migrate to dislocation lines. Dislocations are also trapped at voids during climbing by absorbing interstitial clusters. These finally lead to the preferential formation of voids along dislocation lines.

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.

Fabrication and characterization of unidirectional CF/Al composites

Abstract The unidirectional carbon fiber (CF) reinforced aluminum (Al) composites have been fabricated by the low pressure infiltration (LPI) of molten Al into porous CF preform. Prior to the fabrication of the unidirectional CF/Al composites, the unidirectional CF preform was prepared by sintering of CFs and copper (Cu) particles under the spark plasma sintering (SPS). The compression strength of CF preform was examined to determine the infiltration pressure of molten Al into CF preform. The effects of the different infiltration pressures and sizes of Cu particles on the densification of unidirectional CF/Al composites have also been investigated. The compression strength of CF preform increased with increasing of the contact area between CFs and Cu particles. The density of CF/Al composites improved with the increase of the infiltration pressure. The CF/Al composites, into which the bimodal Cu particles are added, have average particle sizes of 2.55 μm and 11.79 μm, with a high relative density of about 95% with the nearly homogeneous fiber distribution.

Crystal Structure of Elongated Thin Foils of Fe and V

Kiritani et al. have recently reported that a large number of vacancy clusters are formed in plastically elongated thin foil of fcc metals and that no dislocations at all are observed in the thin portion of the elongated film. These findings suggest that elongation proceeds by atom transportation accompanied by generation of vacancies. In the present work, strips of prototype bcc metals of iron and vanadium were elongated to fracture, and thin portions that can be observed under TEM; i.e. , fractured edges, were observed by electron microscopy. The results show that iron thin foils that had been elongated in pure helium gas reacted with residual oxygen atoms such that the thin portions transformed to α-Fe 2 O 3 . Iron thin foil that had been elongated under a 10 −6 Pa vacuum showed usual bcc structure, and void-like defects were observed. Strips of vanadium were elongated to fracture at 77 K and 293 K, and large number of void-like defects were observed in as-elongated specimens. In thin vanadium foil that had been elongated at 673 K, TEM visible defects were not observed initially, and void like defects appeared after 200 keV electron illumination for 60 minutes. Stereo-microscopic examination showed that void-like defects had formed inside vanadium thin foil.

Fabrication of Vapor Grown Carbon Fiber Reinforced Aluminum Composites by Spark Sintering

As vapor grown carbon fiber (VGCF) possesses the good mechanical properties, high thermal conductivity, high electrical conductivity and low thermal expansion, VGCF/ Al composites are expected to be suitable materials for a high performance radiator. In this study, VGCF were dipped and treated with ultrasonic vibration in some kinds of solution at first. Then, the mixed powders including three kind of average particle size of Al (1, 3 and 30µm) was milled by wet process with three kinds of solution, which are acetone, ethanol and butanol. Ethanol is most suitable for mixing solution because of homogeneous distribution of VGCF and Al powders. The mixed powders were spark-sintered in order to obtain dense VGCF/Al composites. The densification mechanism of VGCF/Al composites was divided into the plastic deformation (2nd stage) and creep deformation (3rd stage) after the 1st stage of rectangular wave pulse discharge. The densification rate of VGCF/Al composite powder depended on Al powder for matrix, but independent on VGCF. VGCF are dispersed uniformly in the VGCF/1 µm Al composite. But the aggregations of VGCF exhibited a preferential orientation in VGCF/30 µm Al composite, which results from the deformation of Al powders under the uniaxial pressure during the hot pressing in sintering process.

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...

Computer simulation on the void formation in neutron-irradiated Cu and Ni at high temperature

Abstract Atomistic processes of evolution of damage structure in neutron-irradiated Cu and Ni at high temperature are investigated on the linkage of experimental results and computer-simulation. Interstitials and their clusters move to form a grouping of interstitial clusters which subsequently evolve to dislocation at high temperature. Vacancies aggregate to stacking fault tetrahedron (SFT) and void. At high temperature gas atoms do not make a significant contribution to the nucleation of voids at low fluence of neutron irradiation such as 1018 n/cm2. SFT relax at a high temperature to a string of vacancy clusters, in which vacancies are connected one-dimensionally. The break-up of SFT occurs by jumping of an atom into a SFT. A loosely bound vacancy cluster of a string shape moves with low activation energy of 0.2 eV. Voids are formed by the coalescence of moving vacancy clusters of string shape at high temperature. This is due to low Helmholtz free energy of void-containing crystal than that of SFT-containing crystal. The role of gas atoms in the formation of voids is to degrade the mobility of movable vacancy clusters when captured by the clusters.

Effect of the Interfacial Thermal Resistance on Effective Thermal Conductivity of Al/SiC Particle-Dispersed Composites

Steady state thermal conductivity measuring device was designed to measure the effective thermal conductivity of composites. Computer simulations of thermal conduction revealed that the designed device over estimates the effective thermal conductivity, and the correction coefficient was suggested. With this designed device, the effective thermal conductivities of Al/SiC particle-dispersed composites were measured by changing the size of SiC particles from 0.3 μm to 3 μm. The critical element size which could determine the optimal size of reinforcements have been suggested, and validity of the critical element size for Al/SiC composites was confirmed. The thermal conductivity of the composites including small SiC particles was degraded by the interfacial thermal resistance between the matrix and the reinforcement. On the other hand, the thermal conductivity of the composites including large SiC particles was not affected by the interfacial thermal resistance. These results suggest that consideration of the critical element size is valid.