Vol. 5 No. 4 CONTENTS
Special issue on Hierarchical Estimations of Materials Strength
Regular Articles
Special issue on Hierarchical Estimations of Materials Strength (Vol.5 No.4)
Preface (Vol.5 No.4)
Preface |
Hiroshi KITAGAWA |
223 |
Review Papers (Vol.5 No.4)
Atomic-Level Description of Material Strength of a-Fe |
 |
Yoji SHIBUTANI, Genrich L. KRASKO, Mojmir SOB and Sidney YIP |
225 |
Cooperative Dislocation Generation at Finite Temperatures in Loaded Solids -Transition From Brittle to Ductile Regime- |
|
Mahadevan KHANTHA, Mang-mang LING and Vaclav VITEK |
234 |
Connecting Molecular Dynamics and Dislocation Dynamics to Continuum in Hierarchical Simulations of Microcracks in Solids |
|
Akihiro NAKATANI, Hiroshi KITAGAWA and Sidney YIP |
241 |
General Papers (Vol.5 No.4)
Lattice Trapping on he {011} Cleavage Plane in NiAl |
|
Matthias LUDWIG |
248 |
Molecular Dynamics Study of he Stress Distribution near a Crack Tip in -Silicon Nitride |
|
Naoto HIROSAKI, Shigenobu OGATA and Hiroshi KITAGAWA |
253 |
Mesoscopic Dynamics on Dislocation Patterning in Fatigued Material by Cellular Automata |
|
Yoji SHIBUTANI |
258 |
Regular Articles (Vol.5 No.4)
General Papers (Vol.5 No.4)
QUANTITATIVE EVALUATION OF PINHOLE DEFECTS IN TiN FILMS PREPARED BY R.F.REACTIVE SPUTTERING |
|
Hitoshi UCHIDA, Shozo INOUE, Yasuhide NAKANO and Keiji KOTERAZAWA |
265 |
Topochemistry of SiO2 Wood-Inorganic Composites for Enhancing Water-Repellency |
|
Hisashi MIYAFUJI and Shiro SAKA |
270 |
Interfacial Microstructure and Fracture Characterization of Al18B4O33 /Al Alloy Composites |
|
Gen SASAKI, Makoto YOSHIDA, Jin PAN and Hideharu FUKUNAGA |
276 |
Effect of SiC Volume Fraction on Creep Behavior of SiCp/2124Al Metal Matrix Composite |
|
Ho Jin RYU, Woong Hee SOHN and Soon Hyung HONG |
280 |
Characterization of Integrated Data on Inherent Flaws in Engineering Ceramics |
|
Toshihiko HOSHIDE and Hidehiko HIRAMATSU |
285 |
Deformation Analysis of Laser Processed Grain Oriented Silicon Steel Sheet Using Moire Interferometry |
|
Xie HUIMIN, Satoshi KISHIMOTO, Li JINDONG, Zou DAQING, Dai FULONG and Norio SHINYA |
291 |
Vol.5 No.4 ABSTRACT
Atomic-Level Description of Material Strength of a-Fe
Yoji SHIBUTANI, Genrich L. KRASKO, Mojmir SOB and Sidney YIP
Abstract:Considering the current developments in multiscale materials modeling and longstanding interest in the mechanical behavior in Fe, we propose a new atomistic description of a-Fe which is an improvement over existing models that do not consider explicitly the magnetic interactions. The proposal is to combine a many-body potential fitted to a database of ab-initio electronic structure calculations with a Stoner model treatment of itinerant ferromagnetism. Using selected results on deformation behavior from the literature, we illustrate the kind of material strength studies that can benefit from the new description, as well as other significant applications which are beyond the capabilities of first-principles approach.
Key Words:Atomic-level description, a-Fe, Material strength, Elastic properties, Vibrational properties, Stress-induced phase transformation, Bifurcation, Ferromagnetism, Ab-initio calculation, Many-body potential
Cooperative Dislocation Generation at Finite Temperatures in Loaded Solids -Transition From Brittle to Ductile Regime-
Mahadevan KHANTHA, Mang-mang LING and Vaclav VITEK
Abstract:This paper describes a theoretical model and a Monte Carlo simulation of cooperative dislocation generation in loaded crystals at finite temperatures. The theoretical work, employing a mean-field approach, is presented for the three-dimensional case when dislocation loops are formed. It describes how extensive plasticity can be initiated in
dislocation-free crystals above a critical temperature due to 'homogeneous' cooperative nucleation and expansion of many dislocation loops. The Monte Carlo simulation is carried out to demonstrate the feasibility of the theoretical model. It is performed for a two dimensional case when dislocation dipoles are formed on an underlying lattice under applied loads.
The application of this model to the brittle-to-ductile transition and the yielding of whiskers is discussed.
Key Words:Kosterlitz-Thouless transition, Monte Carlo, Mean field, Critical temperature, Brittle-ductile transition
Connecting Molecular Dynamics and Dislocation Dynamics to Continuum in Hierarchical Simulations of Microcracks in Solids
Akihiro NAKATANI, Hiroshi KITAGAWA and Sidney YIP
Abstract:Multiscale approach to crack tip plasticity involves length scales from electronic structure to the continuum, as well as deformation behavior from single dislocation nucleation to plastic-zone shielding.
A current challenge is to connect the different levels and methods of simulation in order to study ductile fracture in a more holistic fashion, a goal that cannot be accomplished through any one single simulation.@We present two specific examples of potentially useful connections, (1) using molecular dynamics to determine a stress-displacement relation for direct use in continuum-level analysis, and (2) a comparative study of dislocation microstructure evolution by discrete dislocation dynamics and finite-element method. Applications to understanding brittle-ductile behavior in an important metal, -Fe, are particularly emphasized.
Key Words:Molecular dynamics, Discrete dislocation dynamics, Brittle-ductile transition, Multiscale modeling, Kinetic Monte Calro, Mesomechanics, Micromechanics, Fracture, Finite element method
Lattice Trapping on he {011} Cleavage Plane in NiAl
Matthias LUDWIG
Abstract:The crack tip response of cracks on the {110} cleavage plane for five different propagation directions in NiAl was studied atomistically under mode I opening loading conditions. The region near the crack tip was described by an Embedded Atom Method (EAM) potential, and the FEAt (Finite Element - Atomistic) coupling scheme provided the atomistic core region with realistic boundary conditions. In agreement with experimental observations, perfectly brittle cleavage is observed for the {110} crack plane. A simple spring lattice trapping model was developed which allows qualitative predictions on the lattice trapping in dependence of the projected bonding distance along the crack propagation direction.
Key Words:NiAl, Lattice trapping, Embedded atom method, Crack, Brittle
cleavage, Atomistic, Finite Element, Cleavage plane, Spring model
Molecular Dynamics Study of he Stress Distribution near a Crack Tip in -Silicon Nitride
Naoto HIROSAKI, Shigenobu OGATA and Hiroshi KITAGAWA
Abstract:The stress distribution near a crack tip and crack propagation in a beta-silicon nitride crystal are investigated by molecular dynamics (MD) simulations. A crack propagates when KI is greater than 0.7 Mpa root(m). On the other hand, it shrinks when KI is smaller than 0.65 Mpa root(m), indicating that the KIC value is about 0.7 Mpa root(m). This value agrees well with that calculated using Griffith's theory. The stress distribution near the crack tip for KI is 0.7 Mpa root(m), I.e. the case when the crack stops, is calculated from the MD results, assuming that the stress is the average of the atomic stresses. The calculated stress distribution is in good agreement with the linear elastic solution.
Key Words:beta-Si3N4, Molecular dynamics, Simulation, Stress intensity factor, Stress,Linear lastic solution
Mesoscopic Dynamics on Dislocation Patterning in Fatigued Material by Cellular Automata
Yoji SHIBUTANI
Abstract:A typical pattern formation of collective dislocations under cyclic loading, the persistent slip bands (PSBs), is simulated by the cellular automata (CA) method. The local rule of CA taken in the present paper is the weighted addition modulo two rule. Each weight is chosen referring to the reaction-diffusion coupled differential equations with respect to immobile and mobile dislocation densities. The mesoscopic evolution of the immobile dislocation density is driven by the space-oriented bifurcation (Turing instability) and results in the ladder-type self-organization. Instability related to the space- and the time-oriented bifurcations is in detail discussed in a one-dimensional model. A stress amplitude-like parameter and two diffusion coefficients of the immobile and mobile dislocation densities highly affect the sequential
evolution process. Finally, a two-dimensional hexagonal cell model representing a {111} glide plane of the fcc crystal is proposed. The same local rule is applied along the three preferential sliding directions for easy description of mutual interaction between cells. The evolution yields a localized isotropic and a one-directional anisotropic flow patterns of
dislocation densities according to the combination of the three diffusion coefficients.
Key Words:Dislocation density flow patterning, Cellular automata, Persistent slip bands (PSBs), Cyclic loading, Reaction-diffusion system, Turing instability
QUANTITATIVE EVALUATION OF PINHOLE DEFECTS IN TiN FILMS PREPARED BY R.F.REACTIVE SPUTTERING
Hitoshi UCHIDA, Shozo INOUE, Yasuhide NAKANO and Keiji KOTERAZAWA
Abstract:Pinhole defects of TiN-coated stainless steels were evaluated potentiodynamically in a deaerated 0.5kmol/m3H2SO4 + 0.05kmol/m3SCN solution at 298K. The TiN films prepared by radio frequency (r.f.) reactive sputtering exhibited the columnar structure with the preferential <220> or <111> orientation, and they contained more or less pinhole defects. The
critical passivation current density icrit in the TiN-coated specimens decreased considerably with increasing film thickness. Above 1.5mm in thickness, however, there was an increasing tendency in icrit with cracking and/or peeling. The area ratio of pinhole defects was evaluated by the ratio of icrit of a coated and a non-coated specimen. The result coincided
comparatively well with the true defect area ratio based on the optical micrographs before and after anodically polarized. Such electrochemical method was concluded to be a reliable evaluation technique for the pinhole defects of corrosion-resistive coating.
Key Words:Dry coating, Titanium nitride, R.f. reactive sputtering, Pinholedefect, Anodic polarization curve, Quantitative evaluation
Topochemistry of SiO2 Wood-Inorganic Composites for Enhancing Water-Repellency
Hisashi MIYAFUJI and Shiro SAKA
Abstract:Methyltrimethoxysilane (MTMOS) was used to prepare the SiO2 wood-inorganic composites. To enhance the properties of these composites, 2-heptadecafluorooctylethyltrimethoxysilane (HFOETMOS) as a property enhancer was added to this reaction system for SiO2 wood-inorganic composites. To elucidate the mechanism for enhancing the water-repellent property in the HFOETMOS-SiO2 wood-inorganic composites, the composites with various weight percent gains (WPGs) were prepared, and the prepared composites with the lower WPG revealed the better property in water-repellency, whereas those with the higher WPG could not attain the high water-repellent property. SEM-EDXA analysis on these composites revealed that the HFOETMOS-derived residues were concentrated on the boundary between the cell wall and cell lumen in both composites with the lower and higher WPGs, while SiO2 gels were almost uniformly distributed within the cell walls in the composites with lower WPG. SiO2 gels in the composites with higher WPG were distributed in a similar manner but slightly higher in its concentration at the boundary between the cell wall and cell lumen. From these results, it was assumed that for the composites with lower WPG, the cell wall would be covered with the long hydrophobic alkyl residue of the HFOETMOS, whereas for the composites with higher WPG, SiO2 gels formed in the cell lumen would have prevented HFOETMOS-derived residues from being exposed uniformly over the surface of cell wall. This may be why the different water-repellent properties were shown between the composites with higher and lower WPGs. These results suggest that the topochemical effects of the SiO2 gels and HFOETMOS-derived residues exist for the enhancement of water-repellent property.
Key Words:Wood-inorganic composites, Sol-gel process, Topochemistry, Property-enhancement, Water-repellency, SiO2 gels, HFOETMOS
Interfacial Microstructure and Fracture Characterization of Al18B4O33 /Al Alloy Composites
Gen SASAKI, Makoto YOSHIDA, Jin PAN and Hideharu FUKUNAGA
Abstract:In order to investigate the relationship between the mechanical properties and the microstructure of aluminum borate (Al18B4O33) whisker reinforced Al-1% Mg alloy composites fabricated by the squeeze casting process, the microstructure and fractured structure were observed by scanning and transmission electron microscopy (SEM and
TEM). The interface between the matrix and whisker in an as-cast composite is smooth. By heat treatment at 550 deg for 2h, the MgAl2O4 reaction layer with a spinel structure is produced at the whisker/matrix interface and appears trapezoidal. The interface between Al18B4O33 and MgAl2O4 has a certain preferred orientation with very little mismatch.
Cracks tend to propagate along the whisker/matrix and reaction product/matrix interfaces. The interface between the whisker and reaction product seems to have good bonding strength. There are many microcracks in the whiskers in fractured composites, which propagate along the (001) plane in the whisker. Many dislocations and strain fields were observed in the matrix around the reaction products. The bending strength decreases with increasing amount of reaction product. The degradation of the composite strength by heat treatment is caused by weakening of the strength of the interface between the reaction product and matrix or strain concentration at the convexity formed by the interfacial reaction.
Key Words:Metal matrix composites, Fracture, Transmission electron microscopy, Mechanical@roperties, Interface structure
Effect of SiC Volume Fraction on Creep Behavior of SiCp/2124Al Metal Matrix Composite
Ho Jin RYU, Woong Hee SOHN and Soon Hyung HONG
Abstract:The high temperature creep behavior of SiCp/2124Al metal matrix composites containing 10~30 vol.% of SiC particulate reinforcement was investigated to clarify the effect of the volume fraction of SiC particles on creep deformation. The SiCp/2124Al composites were fabricated by mixing 8?m SiC particles and 20?m 2124 Al powders and were followed by hot pressing at 570oC and hot extrusion at 500oC with an extrusion ratio of 25:1. The high temperature creep behavior of SiCp/2124Al composites was investigated by constant stress creep tests at 300 deg. The load transfer phenomena of a spherical particle in metal matrix composite were analyzed based on the shear-lag model. The minimum creep rate of SiCp/2124Al composite decreased with increasing the volume fraction of SiC particles. The increase in the volume fraction of SiC particles reduces the effective stress for creep deformation of the Al matrix by the load transfer from the matrix to SiC particles. The minimum creep rates of SiCp/2124Al composites with different volume fractions of SiC particles were found to be similar under an identical effective stress on the matrix, which is calculated by the modified shear-lag model. It is suggested that the role of SiC particles is to increase the creep resistance by reducing the effective stress acting on the matrix in metal matrix composites.
Key Words:Metal matrix composite, Creep behavior, Load transfer, Volume fraction, Particulate reinforcement
Characterization of Integrated Data on Inherent Flaws in Engineering Ceramics
Toshihiko HOSHIDE and Hidehiko HIRAMATSU
Abstract:Integrated data on inherent flaws in engineering ceramics were characterized for quantitative analyses of them. For this purpose, a ceramic strength database utilized on a personal computer was temporarily constructed so that collected data on flaws could be compiled as records in the database. It was clarified that distributions of the equivalent crack length l for several ceramics were nearly fitted to normal distribution functions. The distribution
of l was investigated for every type of flaw, and it was found to shift toward a longer length region in the order of embedded, surface, and corner flaws. The flaw data were also correlated with strength and microstructural parameters. In the relation to the strength, the strength was reduced as a whole with the decrease in the crack length, while the lower bound of strength scatter was predicted by a modified relation of the fracture mechanics criterion for long cracks. It was revealed that the flaw size increased with the increase in the grain size or the porosity. The relation of a Petch-Hall type was observed between the grain size and the mean strength estimated by using the mean value of l. Some useful suggestions were offered for the improvement of ceramic strength.
Key Words:Ceramics, Inherent flaw, Data characterization, Strength, Flaw size, Flaw type, Statistical distribution, Grain size, Porosity, Database
Deformation Analysis of Laser Processed Grain Oriented Silicon Steel Sheet Using Moire Interferometry
Xie HUIMIN, Satoshi KISHIMOTO, Li JINDONG, Zou DAQING, Dai FULONG and Norio SHINYA
Abstract:In this study, the residual deformation of silicon steel@sheets with lowered core loss by strong power laser was investigated in combination of two deposited metal layers grating for high temperature with laser moire in-terferometry method. The silicon steel sheet was processed with different processing parameters under a CO2 and a YAG laser, respectively. The distributions of residual deformation in x and y axes were obtained. The experimental results showed that the silicon steel sheet was under the state of compressive residual
strain, and were analyzed in detail.
Key Words:Laser processing, Silicon steel, Moire interferometry