Search

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.

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.

The unidirectional carbon fiber (CF) preform for carbon fiber reinforced aluminium (CF/Al) composites has been investigated in terms of the fabrication condition and the strength property. The CF preform which consists of CFs and Cu particles was fabricated by spark plasma sintering (SPS) process with different fabrication temperatures. In order to determine the infiltration pressure of molten Al to the CF preform, the compression test was performed to the CF preform. The unidirectional CF preform by SPS process was well formed with the fabrication temperature condition at 1123 K in accordance with the formation of Cu particle bridging between fibers and Cu deposition on fibers. The compression strength of the CF preform increased with increasing fabrication temperature. Besides, the CF preform was deformed such as fiber micro-buckling or fiber kinking phenomena over the maximum compression strength. The infiltration pressure of the molten Al can be decided under the maximum compression strength. [doi:10.2320/matertrans.L-MZ201110]

In this study, the fabrication of carbon containing aluminum composites was attempted by using low-pressure infiltration method. At first, porous preform containing vapor grown nano-fiber (VGCF) and pure aluminum powder was fabricated by spark plasma sintering (SPS) method. Porosity in preform was controlled by changing the applied pressure during plasma sintering. Consequently, the porous preform with 40-50vol％ in porosity was obtained, which has enough compression strength for low-pressure infiltration (

We defined a 3-dimensional local number, LN3D, and its probability distribution to describe the spatial distribution of new grains in phase transformation, and suggested a modification of Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation for clustered nucleation using parameters of the probability distribution of LN3D. This modification was validated by computer experiments assuming the isotropic growth of new grains but changing their spatial and size distribution. It was demonstrated that modified KJMA equation could be applied to clustering spatial distribution of new grains, regardless of the size distribution of them. In addition, the effective surface area was derived from the modified KJMA equation and was compared with the previous model.

{1 1 1} Σ3 and {1 1 2} Σ3 twin boundaries and their junctions in a phosphor-doped cast polycrystalline silicon were investigated by EELS using HREM to study the local electrical properties and impurity effects at these boundaries and junctions. FWHMs of the silicon plasmon-loss peaks are wider at the grain boundary junctions and {1 1 2} Σ3 twin boundary as compared with other area partly because of the overlapped effect of plasmon loss of SiO 2 and carbon. In the inner-shell edge part of EELS spectrum, the grain boundary junction having distorted structure shows slightly strong intensity at around 110 eV, suggesting the formation of nano-size SiO 2 , even though the presence of the SiO 2 could not be observed by HREM. The effects of carbon K-edge can be recognized in the EELS spectra acquired at {1 1 1} Σ3 and {1 1 2} Σ3 boundaries and their junctions, suggesting the grain boundary segregation of carbon atoms.

A newly developed model was proposed to predict the flow and strain-hardening behavior of the heterogeneously dispersed Al-10 vol%SiC composites with particle damage in tensile deformation. The frequency of local number of particles was decomposed into several Poisson distributions and the frequency of local number of damaged particles was approximated by one Poisson distribution. Then, the flow stress was obtained by summation of strain-hardening rate for each decomposed distribution and/or relieved stress for the distribution of damaged particles. This model gave a good agreement with the experimental results. [doi:10.2320/matertrans.MER2007284]

The effect of the volume fraction of second-phase particles on the distribution of damaged particles and its relation to the tensile deformation behavior were investigated in the particle reinforced metal-matrix composites, Al-SiC. The spatial distributions of all SiC particles and the delaminated SiC particles were evaluated from their two-dimensional local number (LN2D). During tensile deformation, larger SiC particles in more highly clustered regions were more easily delaminated at the particle/matrix interface. The amount of particle delamination in the more highly clustered region increased with increasing volume fraction and strain during tensile deformation. A model was developed to explain the flow behavior of the composites; this showed a good fit to the experimental results in the presence of particle damage, especially when the spatial distribution of damaged particles was taken into consideration. [doi:10.2320/matertrans.MER2007237]

We defined 3-dimensional local number, LN3D, 2-dimensional local number, LN2D, and their probability distribution to describe the spatial distribution of second phase particles, and then suggested the statistical relationship of probability distributions between LN2D and LN3D concerning uniform random and clustering spatial distributions. The relationship was validated by computer experiments using particles of overlap permissive spheres, and was applied to the real microstructures of Al-10 vol%SiC composites. Using the relationship, probability distributions of either LN3D or LN2D could be predicted from the measured relative frequency distributions of another dimension in computer experiments with satisfactory accuracy. Using the relationship, the probability distributions of LN3D was approximately predicted from the relative frequency distributions of LN2D that were obtained by measurements of spatial distributions of SiC particles in Al-SiC composites. [doi:10.2320/matertrans.MER2007137]

Three-dimensional local number, LN3D, and two-dimensional local number, LN2D, were defined as the number of gravity centers (GCs) of second phase particles in the measuring sphere and circle with specially determined radiuses, respectively, whose centers were put on GCs of noticed particles. LN3D and LN2D represent local number density including a noticed and its neighboring particles and each particle has a specific value. We suggested the quantitative method to evaluate the particle spatial distribution using the relative frequency distributions of LN3D and LN2D, and this method was examined by computer experiments using overlap permissive spheres. It was shown that randomness of second phase particles was correctly evaluated in 3- and 2-dimensions by this method, and the average and variance of LN3D and LN2D are proper descriptors to evaluate the spatial distribution randomness of second phase particles. It was also shown that spatial distribution randomness of 2-dimensional particles appeared on cut planes of 3-dimensional particles having uniform random arrangement can be evaluated by this method regardless of both the particle volume fraction and the particle size distribution.

The effect of particle clustering distribution on the damage accumulation and the deformation behavior is investigated in the particle reinforced metal-matrix composites. The clustering tendency of the Al-10 vol%SiC composite is evaluated by the normalized 2-dimensional local number density (LND2D) of the particle. It is found that the uniform material having a spatial distribution close to the random distribution has higher flow stress and larger elongation. The particle/matrix delamination is easy to occur preferentially at the particles of larger size in the more clustered regions in tensile deformation. Composites with lower clustering tendency have larger strain hardening capacity than those with more clustering one.

Molecular dynamics simulation using Tersoff potential was carried out to investigate the formation and the migration of (010) E5 twist boundary in silicon. Effects of carbon atoms on the grain boundary formation and the grain boundary migration were also investigated. Amorphous thin layers remained at the twist boundary even after crystallization, and changes in the thickness of this layers caused grain boundary migration. When carbon atoms were segregated at the twist boundary, these atoms prevented shrinkage of an amorphous thin layer, and the grain boundary migration was retarded. Precipitated carbon atoms within the grain produces a strain field and this strain field possibly became driving force for the grain boundary migration.

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.

Irradiation of high purity silver with 14 MeV D–T neutrons was carried out at the fusion neutron source facility in Japan Atomic Energy Research Institute. The range of neutron fluence was 6.1×1017 to 1.1×1021 n/m2, which is lower than in earliest D–T neutron irradiations. Thin foil and bulk specimens were irradiated at 288, 423 and 573 K, and observed using transmission electron microscopy. For irradiation at 288 K, the fraction of interstitial clusters in bulk is higher than that in thin foil. In irradiation experiments at 288 and 432 K, the number density of defect clusters is proportional to the neutron fluence to the power of 1.3. In irradiation experiments at 573 K, the defects are mostly stacking fault tetrahedrons (SFTs) and their number density is proportional to the neutron fluence. During isochronal annealing of specimens irradiated at 288 K, disappearance and coalescence of defect clusters were observed, and SFTs were mobile.

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 tha...

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...

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.

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 (10a0 × 10a0 × 10a0) 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 (1 1 1) 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.

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.

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...

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.

20vol% titanium boride particle (TiB2) dispersed aluminum composites were prepared by spark sintering process. By controlling the mixture method, the composites with different dispersibility of the particle were obtained, and the dispersibility of particles in composites was estimated by the spatial distribution of particle number in local area quantitatively. In addition, an electrical conductivity of the composites at the room temperature was estimated. As a result, with improving the dispersibility of particle, the electrical conductivity of the composites decreased. And, it is found the electrical conductivity of composites is determined by a geometric electrical conductive circuit and the quantity of the plastic deformation region of aluminum (Al) around the particle.

The molecular dynamics (MD) simulations of small point defect clusters were carried out to study their dynamical behavior and atomistics of structural relaxation. The MD simulation implements an isotropic many body potential of EAM (embedded atom method) due to Daw and Baskes[l]. Movements of vacancies whose size is below 3v in pure copper is examined at elevated temperature. Tri-vacancies 3v in copper can move with the activation energy which are close to that of di-vacancy. The structure of vacancy clusters whose size are below 14v is found to be the combination of hexa-vacancy of stacking fault tetrahedron (6v-sft) and tri-vacancy of the Damask-Dienes type (3v-sft). The structure of these vacancy clusters showed the fluctuations which are caused by atoms which move with the resonant vibration of large amplitude at low frequency of 6v-sft and 3v-sft. The fluctuating structure of small vacancy clusters such as 10v leads to the coalescence of two vacancy clusters whose locate nearby for each other. The relaxed interstitial clusters whose size is below ten interstitial (10i) is examined. A di-interstitial shows two parallel crowdions and migrates to the stable position under the existence of strain field. The penta- and hexa-interstitial relax to the parallel interstitial and moves to stable position where they relax towards directions.