• Proceedings of the Materials Research Society of Korea Conference
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• 2003.11a
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• pp.39-39
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• 2003

Molecular dynamics (MD) simulation was performed to study the stress induced grain boundary migration caused by the interaction of dislocations with a gain boundary. The simulation was carried out in a Ni block (295020 atoms) with a ∑ = 5 (210) grain boundary and an embedded atom potential for Ni was used for the MD calculation. Stress was provided by indenting a diamond indenter and the interaction between Ni surface and diamond indenter was assumed to have a fully repulsive force to emulate a faction free surface. Results showed that the indentation nucleated perfect dislocations and the dislocations produced stacking faults in the form of a parallelepiped tube. The parallelepiped tube consisted of two pairs of parallel dislocations with Shockley partials and was produced successively during the penetration of the indenter. The dislocations propagated along the parallelepiped slip planes and fully merged onto the ∑ = 5 (210) grain boundary without emitting a dislocation on the other grain. The interaction of the dislocations with the grain boundary induced the migration of the grain boundary plane in the direction normal to the boundary plane and the migration continued as long as the dislocations merged onto the grain boundary plane. The detailed mechanism of the conservative motion of atoms at the gram boundary was associated with the geometric feature of the ∑ = 5 (210) grain boundary.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2004.11a
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• pp.215-220
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• 2004

Molecular Dynamics(MD) method was used to investigate the change of friction force due to interaction between dislocations and a grain boundary when a Ni tip was scratched on a Cu bicrystal. The substrate comprised a Cu bicrystal containing a vertical$\Sigma=5(210)$ grain boundary. The moving tip for scratching simulation was consisted of fixed Ni atoms emulating a rigid tip. The indentation depth was $3.6\AA$ and the scratching was performed along <110>direction in the first grain. As the scratching was continued, nucleation and propagation of dislocations were observed. In the early stage, the grain boundary played as a barrier to moving dislocations and interrupting further dislocation movement with no dislocation resulting in no propagation across the grain boundary. As the Ni tip approached the grain boundary, dislocations were nucleated at the grain boundary and propagated to the second grain. However, stick-slip phenomena that were observed on a single crystal scratching were not observed in the bicrystal. And, instead, irregular oscillation of friction force was observed during the scratching due to the presence of a grain boundary.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.6 no.2
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• pp.194-202
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• 1996

Transmission electron microscopy has been used to characterize the dislocation structure in hot-pressed titanium diboride. The thin foil samples were prepared by the conventional ion beam thinning technique and reveal the main features associated with the dislocations ; low-angle grain boundaries with dislocation arrays, high-angle grain boundaries with ledges/steps on the boundary planes. The ledges/steps on the grain boundaries were characterized as the origin of defect structures such as dislocation formation or crack propagation near grain boundaries. A fraction of the high angle grain boundaries contained periodic arrays of grain boundary dislocations. The Burger's vectors of the dislocations in the $TiB_{2}$specimens were determined.

• Korean Journal of Materials Research
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• v.20 no.1
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• pp.37-41
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• 2010

Creep tests were conducted under a condition of constant stress on two aluminum-based alloys containing particles: Al-5% Mg-0.25% Fe and Al-5% Zn-0.22% Fe. The role of grain boundary sliding was examined in the plane of the surface using a square grid printed on the surface by carbon deposition and perpendicular to the surface using two-beam interferometry. Estimates of the contribution of grain boundary sliding to the total strain, $\varepsilon_{gbs}/\varepsilon_t$ reveal two trends; (i) the sliding contribution is consistently higher in the Al-Mg-Fe alloy, and (ii) the sliding contribution is essentially independent of strain in the Al-Mg-Fe alloy, but it shows a significant decrease with increasing strain in the Al-Zn-Fe alloy. Sliding is inhibited by the presence of particles and its contributions to the total strain are low. This inhibition is attributed to the interaction between the grain boundary dislocations responsible for sliding and particles in the boundaries.

• Korean Journal of Crystallography
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• v.7 no.2
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• pp.156-164
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• 1996

The atomic configurations of GaAs ∑19, {331}/{331} [110] and ∑3 {111}/{111} [110] tilt grain boundaries were investigated by HRTEM (High-resolution Transmission Electron Microscopy). The boundary planes lie parallel to the crystallographic planes with high number density of coincident lattice sites for given misorientations, exhibiting particular atomic structural units. Secondary boundary dislocations with a core diameter of about 2nm were observed at boundary steps when a slight deviation from exact ∑-related misorientations occurs in the bicrystal system. The relation between the secondary boundary dislocation and the boundary step was discussed based on a DSC lattice model.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.04a
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• pp.45-45
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• 2006

• Proceedings of the KSME Conference
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• 2007.05a
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• pp.326-331
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• 2007

Large-scale molecular dynamics simulations are performed to verify the deformation characteristics of grain boundaries in nanolithography process. The copper substrate made of 200,000 atoms is constructed by two grains in different crystal orientations using dynamic relaxation method. The grain boundary is located in the middle of the substrate with $45\sim135$ degree angles. The plowing tip is made of diamond-like-carbon atoms in a variety of shapes. In the simulations, the generation, propagation, and accumulation of dislocations are observed inside the substrate. From the numerical results, we address the dynamic behavior of the grain boundaries as well as the frictional characteristics in terms of the morphology of initial grain boundaries.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.25 no.1
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• pp.13-19
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• 2015

Among the various possible factors affecting the Minority Carrier Life Time (MCLT) of the mc-Si crystal, dislocations formed during the cooling period after solidification were found to be a major element. It was confirmed that other defects such as grain boundary or twin boundary were not determinative defects affecting the MCLT because most of these defects seemed to be formed during the solidification period. With a measurement of total thickness variation (TTV) and bow of the silicon wafers, it was found that residual stress remaining in the mc-Si crystal might be another major factor affecting the MCLT. Thus, it is expected that better quality of mc-Si can be grown when the cooling process right after solidification is carried out as slow as possible.

• Korean Journal of Materials Research
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• v.28 no.12
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• pp.702-708
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• 2018

This study deals with the effect of microstructure factors on the strain aging properties of API X70 pipeline steels with different microstructure fractions and grain sizes. The grain size and microstructure fraction of the API pipeline steels are analyzed by optical and scanning electron microscopy and electron backscatter diffraction analysis. Tensile tests before and after 1 % pre-strain and thermal aging treatment are conducted to simulate pipe forming and coating processes. All the steels are composed mostly of polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite. After 1 % pre-strain and thermal aging treatment, the tensile test results reveal that yield strength, tensile strength and yield ratio increase, while uniform elongation decreases with an increasing thermal aging temperature. The increment of yield and tensile strengths are affected by the fraction of bainitic ferrite with high dislocation density because the mobility of dislocations is inhibited by interaction between interstitial atoms and dislocations in bainitic ferrite. On the other hand, the variation of yield ratio and uniform elongation is the smallest in the steel with the largest grain size because of the decrease in the grain boundary area for dislocation pile-ups and the presence of many dislocations inside large grains after 1 % pre-strain.

• Transactions of Materials Processing
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• v.6 no.3
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• pp.213-220
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• 1997

The microstructure of the inconel 690 alloy was varied by the solution treatment and the thermal treatment. The specimens having different microstructures were examined in order to understand the strengthening mechanism of the inconel 690. The level of supersaturation of carbon in the solid solution was increased by applying a longer solution treatment at 115$0^{\circ}C$. As increased carbon content in the solid solution, more carbides precipitated during the thermal treatment at $700^{\circ}C$. Since the carbides played a role of obstacle on the movement of dislocations, a higher tensile strength was obtained in the sample having a large number of carbider. The accumulation of dislocations at the grain boundary carbides caused the development of intergranular fracture which led to a lower elongation.