• Journal of the Korean Ceramic Society
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• v.40 no.1
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• pp.11-17
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• 2003

The effect of$ZrO_2$addition on the microstructure and electrical properties of Ni-Mn oxide NTC thermistors was studied. Major phases present in the sintered bodies of $Ni_{1.0}Mn_{2-x}Zr_xO_4$ were the solid solutions of Ni-Mn-Zr oxides with a cubic spinel structure and the $ZrO_2$ with a tetragonal structure. The $ZrO_2$ was formed by the partial decomposition or incomplete formation of the Ni-Mn-Zr oxides during sintering. With increasing the amount of added $ZrO_2$, the $ZrO_2$ phase increased. The relationship between log resistivity (log p) and the reciprocal of absolute temperature (1/T) of the NTC thermistors prepared was linear, indicative of NTC characteristics. The resistivity, B constant and activation energy of the thermistors increased with increasing $ZrO_2$ content.

• Corrosion Science and Technology
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• v.5 no.6
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• pp.218-221
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• 2006

Influence of microstructure on the corrosion property of Mg-Al-Zn alloy was investigated using potentiodynamic polarization experiments, galvanic coupling experiments, and scanning electron microscopy in sodium chloride solutions. Pitting was the most common form of attack in chloride solution, and filiform corrosion was also occurred in AZ91D-T4 alloy. On the contrary, filiform attack in the bulk matrix was predominant corrosion form in AZ91D-T6 alloy, and the number and size of pit were decreased than those of AZ91D-T4 alloy. Galvanic coupling effect between $Mg_{17}Al_{12}$ and matrix was existed, but the propagation of galvanic corrosion was localized only near the $Mg_{17}Al_{12}$ phase in AZ91D-6T alloy. The corrosion resistance of Mg-Al matrix increased with decreasing Al content in the matrix. And, it could be regarded that Al content in the matrix is decreased by precipitation of $Mg_{17}Al_{12}$ during the aging treatment and it decreases the anodic reaction rate of the matrix and galvanic effect in AZ91D-T6 alloy. It could be considered that the composition and microstructure of surface protective layer would be varied by precipitation of $Mg_{17}Al_{12}$ and subsequent decreasing of Al content in the matrix. And it would contribute the corrosion resistance of AZ91D-T6 aging alloy.

• Korean Journal of Materials Research
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• v.17 no.2
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• pp.100-106
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• 2007

The effects of $MgO-P_2O_5$ based sintering additive on the microstructure and material and biological properties of hydroxyapatite $(HAp,\;Ca_{10}(PO_4)_6(OH)_2)$ ceramic were investigated using XRD, SEM and TEM techniques. The $MgO-P_2O_5$ sintering additive improved the material properties and increased the grain size in the sintered HAp bodies. As the content of sintering additive increased over 4 wt%, a small amount of the HAp phase was decomposed and transformed to ${\beta}-TCP$. In the 2 wt% $MgO-P_2O_5$ content HAp sintered body, the maximum values of density and hardness were respectively about 3.10 gm/cc and 657 HV. However, the maximum fracture toughness in the HAp body containing 8 wt% $MgO-P_{2}O_{5}$ was about $1.02MPa{\cdot}m^{1/2}$ due to the crack deflection effect. Human osteoblast like MG-63 cells and osteoclast like raw 264.7 cells were well grown and fully covered all of the HAp sintered bodies. The osteoblast cells were grown with spindle-shaped and the osteoclast cells had a grape-like round shape.

• Journal of Welding and Joining
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• v.32 no.1
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• pp.93-101
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• 2014

This study aims to investigate the effect of heat input of outside SAW weld on low temperature toughness($-20^{\circ}C$) of inside SAW weld for API 5L X70 with sour gas resistance. As increasing heat input of the outside weld, low temperature toughness of the inside weld was decreased. Especially, in spite of the same heat input, the value of low temperature toughness was fluctuated. On the basis of fracture and microstructure analysis, the low temperature toughness is correlated with the fracture area ratio of shear lips and four kinds of fracture sections. These sections were divided with size and shape of dimple correlated with grain boundary ferrite and cleavage correlated acicular and polygonal ferrite in grain. Therefore, it was seen that these sections were two of final solidification area in the inside weld and the outside weld, no reheated zone and reheated zone in the inside weld. In conclusion, it is thought that the difference of low temperature toughness at the same heat input is due to the fact that each of impact test specimens could have the different microstructure, even though the notch was machined under the error tolerance of 1mm. It is because the final solidification area of the inside weld is very narrow.

• Journal of Korea Foundry Society
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• v.33 no.4
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• pp.171-180
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• 2013

The effects of alloying elements on the solidification characteristics, microstructure, thermal conductivity, and tensile strength of Al-Zn-Mg-Fe alloys were investigated for the development of high strength and high thermal conductivity aluminium alloy for die casting. The amounts of Zn and Mg in Al-Zn-Mg-Fe alloys had little effect on the liquidus/solidus temperature, the latent heat for solidification, the energy release for solidification and the fluidity of Al-Zn-Mg-Fe alloys. Thermo-physical modelling of Al-Zn-Mg-Fe alloys by the JMatPro program showed $MgZn_2$, AlCuMgZn and $Al_3Fe$ phases in the microstructure of the alloys. Increased amounts of Mg in Al-Zn-Mg-Fe alloys resulted in phase transformation, such as $MgZn_2{\Rightarrow}MgZn_2+AlCuMgZn{\Rightarrow}AlCuMgZn$ in the microstructure of the alloys. Increased amounts of Zn and Mg in Al-Zn-Mg-Fe alloys resulted in a gradual reduction of the thermal conductivity of the alloys. Increased amounts of Zn and Mg in Al-Zn-Mg-Fe alloys had little effect on the tensile strength of the alloys.

• Journal of Periodontal and Implant Science
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• v.38 no.1
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• pp.67-74
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• 2008

Purpose: The aim of this study was to evaluate the effect of Er:YAG laser on microstructure and roughness of $TiO_2$ blasting implant surface. Materials and Methods: Ten $TiO_2$ blasting implant were used in this experiment. One implant was control group, and nine $TiO_2$ blasting implant surfaces were irradiated with Er:YAG laser under 100 mJ/pulse, 140 mJ/pulse, and 180 mJ/pulse condition for 1 min, 1.5 min, and 2 min respectively. Optical interferometer and scanning electron microscopy was utilized to measure roughness and microstructure of specimens. Results: The surface roughness was decreased after Er:YAG laser irradiation in all groups, but there was no significant difference. 100 mJ/pulse and 140 mJ/pulse group did not alter the $TiO_2$ blasting implant surface in SEM study while 180 mJ/pulse group altered the $TiO_2$ blasting implant surface. Implant surfaces showed melting, microfracture and smooth surface in 180 mJ/pulse group. Conclusion: Detoxification of implant surface using Er:YAG laser must be irradiated with proper energy output and irradiation time to prevent implant surface alteration.

• Korean Journal of Materials Research
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• v.25 no.8
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• pp.403-409
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• 2015

Metastable phase characteristics of beta Ti alloys were investigated to consider the relationship of the microstructure and diffraction pattern in TEM. TEM analysis showed that the microstructure was mottled as a modulated structure, and the diffraction pattern was composed of spot streaks between the main spots of a stable beta phase with a specific lattice relationship. The modulated structure may be induced by short distance slip or atom movement during a very short interval of solution treated and quenched (STQ) materials. The athermal ${\omega}$ phase, which could be precipitated at low temperature aging, is also analysed by the metastable phase. The metastable phases including athermal ${\omega}$ phase had a common characteristic of hardened and brittle behavior because the dislocation slip was restricted by a super lattice effect due to short distance atom movement at the metastable state.

• Journal of Powder Materials
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• v.27 no.4
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• pp.311-317
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• 2020

In this study, the effects of Co content on the microstructure and Charpy impact properties of Fe-Cr-W ferritic/martensitic oxide dispersion strengthened (F/M ODS) steels are investigated. F/M ODS steels with 0-5 wt% Co are fabricated by mechanical alloying, followed by hot isostatic pressing, hot-rolling, and normalizing/tempering heat treatment. All the steels commonly exhibit two-phase microstructures consisting of ferrite and tempered martensite. The volume fraction of ferrite increases with the increase in the Co content, since the Co element considerably lowers the hardenability of the F/M ODS steel. Despite the lowest volume fraction of tempered martensite, the F/M ODS steel with 5 wt% Co shows the highest micro-Vickers hardness, owing to the solid solution-hardening effect of the alloyed Co. The high hardness of the steel improves the resistance to fracture initiation, thereby resulting in the enhanced fracture initiation energy in a Charpy impact test at - 40℃. Furthermore, the addition of Co suppresses the formation of coarse oxide inclusions in the F/M ODS steel, while simultaneously providing a high resistance to fracture propagation. Owing to these combined effects of Co, the Charpy impact energy of the F/M ODS steel increases gradually with the increase in the Co content.

• Transactions of Materials Processing
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• v.28 no.1
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• pp.34-42
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• 2019

In this study, various alloying elements (Cr, Sr, Ca, Cd) were added to improve the mechanical properties of ADC12 fabricated by a die casting process. The effect of alloying elements on the microstructure and mechanical properties were investigated. The phase analysis results of the modified ADC12 alloy with conventional ADC12 alloy, showed the similar characteristics of Al matrix, Si phase, $CuAl_2$ phase and the Fe intermetallic phase. As a result of the microstructure observation, the secondary dendrite arm spacing (SDAS) was shown to have decreased after the addition of the alloying elements. The eutectic Si phase, which existed as flake form in the conventional ADC12 alloy, was modified finely as a fiber form in the modified ADC12 alloy. It was observed that the $CuAl_2$ phase as the strengthening phase was relatively finely distributed in the modified ADC12 alloy. The Fe intermetallic appeared as a Chinese script shaped $Al_6$ (Mn,Fe) which is detrimental to mechanical properties in conventional ADC12 alloy. On the other hand, in the modified ADC12 alloy, polyhedral ${\alpha}-Al_{15}Si_2$ $(Fe,Mn,Cr)_3$ was observed. The tensile properties were improved in the modified ADC12 alloy. The yield strength and tensile strength increased by 12.4% and 10.0%, respectively, in the modified ADC12 alloy, and the elongation was also seen to have been increased. As a result of the pin on disk wear test, the wear resistance properties were also improved by up to about 7% in the modified ADC12 alloy. It is noted that the wear deformation microstructures were also observed, and it was found that the fine eutectic Si and strengthening phases greatly improved abrasion resistance.

• Korean Journal of Materials Research
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• v.31 no.10
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• pp.539-545
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• 2021

The effect of Sm₂O₃ doping on the microstructure and electrical properties of the ZPCCA-based varistors is comprehensively investigated. The increase of doping content of Sm₂O₃ results in better densification (from 5.70 to 5.82 g/cm³) and smaller mean grain size (from 7.8 to 4.1 ㎛). The breakdown electric field increases significantly from 2568 to 6800 V/cm as the doping content of Sm₂O₃ increases. The doping of Sm₂O₃ remarkably improves the nonlinear properties (increasing from 23.9 to 91 in the nonlinear coefficient and decreasing from 35.2 to 0.2 µA/cm² in the leakage current density). Meanwhile, the doping of Sm₂O₃ reduces the donor concentration (the range of 2.73 × 10¹⁸ to 1.18 × 10¹⁸ cm^-3) of bulk grain and increases the barrier height (the range of 1.10 to 1.49 eV) at the grain boundary. The density of the interface states decreases in the range of of 5.31 × 10¹² to 4.08 × 10¹² cm^-2 with the increase of doping content of Sm₂O₃. The dielectric constant decreases from 1594.8 to 507.5 with the increase of doping content of Sm₂O₃.