Journal of the Society of Materials Science, Japan

ATOMIC-LEVEL DESCRIPTION OF MATERIAL STRENGTH OF α-Fe

Yoji SHIBUTANI, Genrich L. KRASKO, Mojmír ŠOB, Sidney YI ...

1999 Volume 48 Issue 12Appendix Pages 225-233
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_225

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Considering the current developments in multiscale materials modeling and longstanding interest in the mechanical behavior in Fe, we propose a new atomistic description of α-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.

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COOPERATIVE DISLOCATION GENERATION AT FINITE TEMPERATURES IN LOADED SOLIDS

Transition from Brittle to Ductile Regime

Mahadevan KHANTHA, Mang-Mang LING, Vaclav VITEK

1999 Volume 48 Issue 12Appendix Pages 234-240
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_234

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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 meanfield 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.

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CONNECTING MOLECULAR DYNAMICS AND DISLOCATION DYNAMICS TO CONTINUUM IN HIERARCHICAL SIMULATIONS OF MICROCRACKS IN SOLIDS

Akihiro NAKATANI, Hiroshi KITAGAWA, Sidney YIP

1999 Volume 48 Issue 12Appendix Pages 241-247
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_241

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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, α-Fe, are particularly emphasized.

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LATTICE TRAPPING ON THE {011} CLEAVAGE PLANE IN NiAl

Matthias LUDWIG

1999 Volume 48 Issue 12Appendix Pages 248-252
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_248

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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.

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MOLECULAR DYNAMICS STUDY OF THE STRESS DISTRIBUTION NEAR A CRACK TIP IN β-SILICON NITRIDE

Naoto HIROSAKI, Shigenobu OGATA, Hiroshi KITAGAWA

1999 Volume 48 Issue 12Appendix Pages 253-257
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_253

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The stress distribution near a crack tip and crack propagation in a β-silicon nitride crystal are investigated by molecular dynamics (MD) simulations. A crack propagates when K_I is greater than 0.7MPa√m. On the other hand, it shrinks when K_I is smaller than 0.65MPa√m, indicating that the K_IC value is about 0.7MPa√m. This value agrees well with that calculated using Griffith's theory. The stress distribution near the crack tip for K_I is 0.7MPa√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.

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MESOSCOPIC DYNAMICS ON DISLOCATION PATTERNING IN FATIGUED MATERIAL BY CELLULAR AUTOMATA

Yoji SHIBUTANI

1999 Volume 48 Issue 12Appendix Pages 258-263
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_258

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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.

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QUANTITATIVE EVALUATION OF PINHOLE DEFECTS IN TiN FILMS PREPARED BY R.F. REACTIVE SPUTTERING

Hitoshi UCHIDA, Shozo INOUE, Yasuhide NAKANO, Keiji KOTERAZAWA

1999 Volume 48 Issue 12Appendix Pages 265-269
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_265

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Pinhole defects of TiN-coated stainless steels were evaluated potentiodynamically in a deaerated 0.5kmol/m³ H₂SO₄+0.05kmol/m³ KSCN 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 i_crit in the TiN-coated specimens decreased considerably with increasing film thickness. Above 1.5μm in thickness, however, there was an increasing tendency in i_crit with cracking and/or peeling. The area ratio of pinhole defects was evaluated by the ratio of i_crit 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.

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TOPOCHEMISTRY OF SiO₂ WOOD-INORGANIC COMPOSITES FOR ENHANCING WATER-REPELLENCY

Hisashi MIYAFUJI, Shiro SAKA

1999 Volume 48 Issue 12Appendix Pages 270-275
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_270

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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.

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INTERFACIAL MICROSTRUCTURE AND FRACTURE CHARACTERIZATION OF Al₁₈B₄O₃₃/Al ALLOY COMPOSITES

Gen SASAKI, Makoto YOSHIDA, Jin PAN, Hideharu FUKUNAGA

1999 Volume 48 Issue 12Appendix Pages 276-279
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_276

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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°C 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.

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EFFECT OF SiC VOLUME FRACTION ON CREEP BEHAVIOR OF SiCp/2124Al METAL MATRIX COMPOSITE

Ho Jin RYU, Woong Hee SOHN, Soon Hyung HONG

1999 Volume 48 Issue 12Appendix Pages 280-284
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_280

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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 8μm SiC particles and 20μm 2124Al powders and were followed by hot pressing at 570°C and hot extrusion at 500°C 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°C. 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.

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CHARACTERIZATION OF INTEGRATED DATA ON INHERENT FLAWS IN ENGINEERING CERAMICS

Toshihiko HOSHIDE, Hidehiko HIRAMATSU

1999 Volume 48 Issue 12Appendix Pages 285-290
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_285

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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.

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DEFORMATION ANALYSIS OF LASER PROCESSED GRAIN ORIENTED SILICON STEEL SHEET USING MOIRE INTERFEROMETRY

Huimin XIE, Satoshi KISHIMOTO, Jindong LI, Daqing ZOU, Fulong DAI, Nor ...

1999 Volume 48 Issue 12Appendix Pages 291-295
Published: December 15, 1999
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.48.12Appendix_291

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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 interferometry 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.

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