• Korean Journal of Materials Research
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• v.17 no.8
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• pp.397-401
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• 2007

Fe based SOFC(Solid Oxide Fuel Cell) interconnector was fabricated by using the spark plasma sintering process and its microstructure and mechanical properties were investigated in this study. To fabricate the interconnector, the Fe-26Cr powder was mixed with the Ag (5, 10, 20wt.%) particles. In the Fe-26Cr-Ag sintered bodies, the Ag particles were almost dispersed at the grain boundary of the Fe-26Cr. The sintered bodies have the density of 87.2-97.5%, the density increases with increasing Ag content at sintering temperature of $850^{\circ}C$. Also, the compressive yield strength increases with increasing Ag content at the same sintering temperature.

• Transactions of Materials Processing
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• v.30 no.4
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• pp.195-200
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• 2021

In this study, we investigated the dominant processing variables that would influence the microstructural development of AA6061 deformed by differential speed rolling (DSR) at ambient temperature. For this purpose, we carried out a series of DSR on the samples to investigate the effects of roll speed ratio, sample rotation, and number of operation under specific rotation. Among these, the condition with a height reduction of 75% at a speed ratio of 1:4 through rotation along rolling direction (RR) displayed the pronounced results of more homogenous matrix-structure and superior mechanical properties than the others tested in this study. This was mainly due to the cross shearing of macro-shear pattern in segment where dynamic recrystallization took place with ease throughout the sample. Thus, an average microhardness value of 101 Hv was obtained in the present sample deformed by 4-pass DSR with RR where macro cross-shearing was effectively applied.

• The korean journal of orthodontics
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• v.40 no.1
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• pp.40-49
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• 2010

Objective: The aim of this research was to evaluate the mechanical properties (MP) and degree of the phase transformation (PT) of martensitic (M-NiTi), austenitic (A-NiTi) and thermodynamic nickel-titanium wire (T-NiTi). Methods: The samples consisted of $0.016\;{\times}\;0.022$ inch M-NiTi (Nitinol Classic, NC), A-NiTi (Optimalloy, OPTI) and T-NiTi (Neo-Sentalloy, NEO). Differential scanning calorimetry (DSC), three-point bending test, X-ray diffraction (XRD), and microstructure examination were used. Statistical evaluation was undertaken using ANOVA test. Results: In DSC analysis, OPTI and NEO showed two peaks in the heating curves and one peak in the cooling curves. However, NC revealed one single broad and weak peak in the heating and cooling curves. Austenite finishing ($A_f$) temperatures were $19.7^{\circ}C$ for OPTI, $24.6^{\circ}C$ for NEO and $52.4^{\circ}C$ for NC. In the three-point bending test, residual deflection was observed for NC, OPTI and NEO. The load ranges of NC and OPTI were broader and higher than NEO. XRD and microstructure analyses showed that OPTI and NEO had a mixture of martensite and austenite at temperatures below Martensite finishing ($M_f$). NEO and OPTI showed improved MP and PT behavior than NC. Conclusions: The mechanical and thermal behaviors of NiTi wire cannot be completely explained by the expected degree of PT because of complicated martensite variants and independent PT induced by heat and stress.

• Corrosion Science and Technology
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• v.13 no.1
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• pp.28-35
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• 2014

Recently, the fuel oil of diesel engines of marine ships has been increasingly changed to heavy oil of low quality as the oil price is getting higher and higher. Therefore, the spiral gear attached at the motor of the oil purifier which plays an important role to purify the heavy oil is also easy to expose at severe environmental condition due to the purification of the heavy oil in higher temperature. Thus, the material of the spiral gear requires a better mechanical strength, wear and corrosion resistance. In this study, the heat treatment(tempering) with various holding time at temperature of $500^{\circ}C$ was carried out to the alloy of Cu-7Al-2.5Si as centrifugal casting, and the properties of both hardness and corrosion resistance with and without heat treatment were investigated with observation of the microstructure and with electrochemical methods, such as measurement of corrosion potential, cathodic and anodic polarization curves, cyclic voltammogram, and a.c. impedance. in natural seawater solution. The ${\alpha}$, ${\beta}^{\prime}$ and ${\gamma}_2$ phases were observed in the material in spite of no heat treatment due to quenching effect of a spin mold. However, their phases, that is, ${\beta}^{\prime}$ and ${\gamma}_2$ phases decreased gradually with increasing the holding time at a constant temperature of $500^{\circ}C$. The hardness more or less decreased with heat treatment, however its corrosion resistance was improved with the heat treatment. Furthermore, the longer holding time, the better corrosion resistance. In addition, when the holding time was 48hrs, its corrosion current density showed the lowest value. The pattern of corroded surface was nearly similar to that of the pitting corrosion, and this morphology was greatly observed in the case of no heat treatment. It is considered that ${\gamma}_2$ phase at the grain boundary was corroded preferentially as an anode. However, the pattern of general corrosion exhibited increasingly due to decreasing the ${\gamma}_2$ phase with heat treatment. Consequently, it is suggested that the corrosion resistance of Cu-7Al-2.5Si alloy can be improved with the heat treatment as a holding time for 48 hrs at $500^{\circ}C$.

• Korean Journal of Materials Research
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• v.23 no.4
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• pp.227-232
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• 2013

Al-based alloys have recently attracted considerable interest as structural materials and light weight materials due to their excellent physical and mechanical properties. For the investigation of the potential of Al-based alloys, a surface porous $Al_{88}Cu_6Si_6$ eutectic alloy has been fabricated through a chemical leaching process. The formation and microstructure of the surface porous $Al_{88}Cu_6Si_6$ eutectic alloy have been investigated using X-ray diffraction and scanning electron microscopy. The $Al_{88}Cu_6Si_6$ eutectic alloy is composed of an ${\alpha}$-Al dendrite phase and a single eutectic phase of $Al_2Cu$ and ${\alpha}$-Al. We intended to remove only the ${\alpha}$-Al phase and then the $Al_2Cu$ phase would form a porous structure on the surface with open pores. Both acidic and alkaline aqueous chemical solutions were used with various concentrations to modify the influence on the microstructure and the overall chemical reaction was carried out for 24 hr. A homogeneous open porous structure on the surface was revealed via selective chemical leaching with a $H_2SO_4$ solution. Only the ${\alpha}$-Al phase was successfully leached while the morphology of the $Al_2Cu$ phase was maintained. The pore size was in a range of $1{\sim}5{\mu}m$ and the dealloying depth was nearly $3{\mu}m$. However, under an alkaline NaOH, aqueous solution, an inhomogeneous porous structure on the surface was formed with a 5 wt% NaOH solution and the morphology of the $Al_2Cu$ phase was not preserved. In addition, the sample that was leached by using a 7 wt% NaOH solution crumbled. Al extracted from the Al2Cu phase as ${\alpha}$-Al phase was dealloyed, and increasing concentration of NaOH strongly influenced the morphology of the $Al_2Cu$ phase and sample statement.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.12
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• pp.1805-1810
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• 2010

We present a simple, cost-effective, and fast fabrication process for three-dimensional (3D) microstructures; this process is based on multi-step electrochemical etching of metal foils which facilitates the mass production of 3D microstructures. Compared to electroplating, this process maintains uniform and well-controlled material properties of the microstructure. In the experimental study, we perform single-step electrochemical etching of aluminum foils for the fabrication of 2D cantilever arrays. In the single-step etching, the depth etch rate and bias etch rate are measured as $1.50{\pm}0.10 {\mu}m/min$ and $0.77{\pm}0.03 {\mu}m/min$, respectively. Using the results of single-step etching, we perform two-step electrochemical etching for 3D microstructures with probe tips on cantilevers. The errors in height and lateral fabrication in the case of the fabricated structures are $15.5{\pm}5.8% $ and $3.3{\pm}0.9%$, respectively; the surface roughness is $37.4{\pm}9.6nm$.

• 연구논문집
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• s.26
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• pp.103-112
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• 1996

$Al_2O_3/SiC$ Hybrid-Composite has been fabricated by conventional powder process. The addition of $\alpha-Al_2O_3$ as seed particles in the transformation of $\gamma-Al_2O_3 to $\alpha-Al_2O_3$ provided a homogeneity of the microstructure, resulting in increase of mechanical properties. The grain growth of $Al_2O_3$ are significantly surpressed by the addition of nano-sized. SiC particles, increasing in fracture strength. The addition of SiC plates to $Al_2O_3$ nano-composite decreased the fracture strength, but increased the fracture toughness. Coated SiC plates with nitrides such as BN and /SiC$Si_3N_4$ enhanced fracture toughness much more than uncoated SiC plates by inducing crack deflection.

• Geomechanics and Engineering
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• v.36 no.6
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• pp.619-629
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• 2024

To improve the utilization rate of phosphate tailings (PTs) and widen the sources of subgrade filler, the PTs is employed to modify clay, forming a PTs modified clay, applied in the subgrade. Accordingly, the environmental friendliness of PTs was investigated. Subsequently, an optimal proportion was determined through compaction and California Bearing Ratio (CBR) experiments. Afterward, the stability of mixture with the optimal proportion was further evaluated through the water stability and dry-wet stability experiments. Finally, via the gradation and microstructure experiments, the strength mechanism of PTs modified clay was analyzed. The results show that the PTs were classified in the non-hazardous solid wastes, belonging to Class A building materials. With the increase of PTs content and the decrease of clay content, the optimum water content and the swelling degree gradually decrease, while the maximum dry density and CBR first increase and then decrease, reaching their peak value at 50% PTs content, which is the optimal proportion. The resilient modulus of PTs modified clay at the optimal proportion reaches 110.2 MPa. The water stability coefficient becomes stable after soaking for 4 days, while the dry-wet stability coefficient decreases with the increase of cycles and tends to be stable after 8 cycles. Under the long-term action, the dry-wet change has a greater adverse impact than continuous soaking. The analysis demonstrates that the better strength mainly comes from the skeleton role of PTs and the cementation of clay. The systematic laboratory test results and economic analysis collectively provide data evidence for the advantages of PTs modified clay as a subgrade filler.

• Journal of Powder Materials
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• v.30 no.6
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• pp.463-469
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• 2023

Aluminum alloys, known for their high strength-to-weight ratios and impressive electrical and thermal conductivities, are extensively used in numerous engineering sectors, such as aerospace, automotive, and construction. Recently, significant efforts have been made to develop novel aluminum alloys specifically tailored for additive manufacturing. These new alloys aim to provide an optimal balance between mechanical properties and thermal/electrical conductivities. In this study, nine combinatorial samples with various alloy compositions were fabricated using direct energy deposition (DED) additive manufacturing by adjusting the feeding speeds of Al6061 alloy and Al-12Si alloy powders. The effects of the alloying elements on the microstructure, electrical conductivity, and hardness were investigated. Generally, as the Si and Cu contents decreased, electrical conductivity increased and hardness decreased, exhibiting trade-off characteristics. However, electrical conductivity and hardness showed an optimal combination when the Si content was adjusted to below 4.5 wt%, which can sufficiently suppress the grain boundary segregation of the α-Si precipitates, and the Cu content was controlled to induce the formation of Al₂Cu precipitates.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.27 no.11
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• pp.712-717
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• 2014

Glass ceramic has a high mechanical strength and low sintering temperature. So, it can be used as a thick film substrate or a high strength insulator. A series of glass ceramic samples based on MgO-$Al_2O_3-SiO_2-ZrO_2$ (MASZ) were prepared by melting at $1,600^{\circ}C$, roll-quenching and heat treatment at various temperatures from $900^{\circ}C$ to $1,400^{\circ}C$. Dependent on the heat treatment temperature used, glass ceramics with different crystal phases were obtained. Their nucleation behavior, microstructure and mechanical properties were investigated with differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Vicker's hardness testing machine. With increasing the heat treatment temperature of MASZ samples, their hardness and toughness initially increase and then reach the maximum points at $1,300^{\circ}C$, and begin to decrease at above this temperature, which is likely to be due to the softening of glass ceramics. As the content of $ZrO_2$ in MAS glass ceramics increases from 7.0 wt.% to 13 wt.%, Vicker's hardness and fracture toughness increase from $853Kg/mm^2$ to $878Kg/mm^2$ and $1.6MPa{\cdot}m^{1/2}$ to $2.4MPa{\cdot}m^{1/2}$ respectively, which seems to be related with the nucleation of elongated phases like fiber.