• Journal of Korea Foundry Society
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• v.21 no.1
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• pp.15-23
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• 2001

The changes in the microstructures and $D0_3{\leftrightarrow}B2$ transition temperature were investigated for Fe-30at.%AI-5at.%Cr alloy with the ordered $D0_3$, structure when Ti, Hf and Zr were added respectively. The addition of Cr has no effect on the microstructure. However, as the amount of Ti addition increased, the grain size became smaller. Addition of Ti+Hf, Ti+Zr and Ti+Hf+Zr also showed the similar effect. When 20at.% of Ti was added, the second phase precipitates on the substrate. The addition of Cr, Hf and Zr alone has no effect on $D0_3{\leftrightarrow}B2$ transition temperature. However, as the amount of Ti addition increased by 5, 10, 15 and 20at.%, the transition temperatures showed 929, 930, 960 and $930^{\circ}C$ respectively.

• Progress in Superconductivity and Cryogenics
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• v.26 no.2
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• pp.24-30
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• 2024

Metal 3D printing is utilized in various industrial fields due to its advantages, such as fewer restrictions on production shape and reduced production time and cost. Existing research on 3D printing metal materials focused on changes in material properties depending on manufacturing conditions and was mainly conducted in a room temperature environment. In order to apply metal 3D printing products to cryogenic applications, research on the properties of materials in cryogenic environments is necessary but still insufficient. In this study, we evaluate the properties of stainless steel (STS) 316L and CuCr1Zr manufactured by Laser Powder Bed Fusion (LPBF) in a cryogenic environment. CuCr1Zr is a precipitation hardening alloy, and changes in material properties were compared by applying various heat treatment conditions. The mechanical properties of materials manufactured using the LBPF method are evaluated through tensile tests at room temperature and cryogenic temperature (77 K), and the thermal properties are evaluated by deriving the thermal conductivity of CuCr1Zr according to various heat treatment conditions. In a cryogenic environment, the mechanical strength of STS 316L and CuCr1Zr increased by about 150% compared to room temperature, and the thermal conductivity of CuCr1Zr after heat treatment increased by about 6 to 10 times compared to before heat treatment at 40 K.

• Journal of Korea Foundry Society
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• v.22 no.6
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• pp.293-298
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• 2002

The effects of alloying elements added on the microstructure and high temperature oxidation behavior of the electro-slag remelted Fe-22Cr-5Al alloy were investigated. The amount of casting defect was makedly reduced by the electro-slag remelting. The electro-slag remelted ingot had a directionally solidified structure and cleaner surface than that of air-melted one. The high temperature oxidation reststance was greatly improved by the addition of Be and Zr.

• Nuclear Engineering and Technology
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• v.54 no.9
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• pp.3272-3282
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• 2022

To improve the safety and life extension qualities of nuclear fuel rods which is currently made of zirconium (Zr) alloy, research on the application of chromium (Cr) coating was conducted. Cr coating has advantages such as increased corrosion resistance and reduced oxidation rate, but non-destructive thickness evaluation studies are needed to ensure the reliability of the steps taken to provide uniform coating thickness. Eddy current testing (ECT) is a representative non-destructive technique for such as thickness evaluation and surface defect inspection. To inspect changes in thickness at micron scale, the Swept Frequency Eddy Current Testing (SFECT) method was applied to select a frequency range sensitive to changes in thickness. The coating thickness was evaluated using changes in signals, such as that for impedance. In this study, basic research was performed to evaluate the thickness of the Cr coating on a rod using an encircling sensor and the SFECT technique. The sensor design parameters were determined through simulation, after which the new sensor was manufactured. A sensor capable of measuring the thickness of a non-uniformly Cr-coating rod was selected through an experiment evaluating the performance of the manufactured sensor. This was done using the impedance-difference of a Cr-coating rod and a Zr alloy rod. The possibility of evaluation of the Cr coating thickness was confirmed by comparing the experimental results with the selected sensor and the signals of the measured Cr-coating rod. All simulation results were verified experimentally.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.48 no.8
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• pp.570-576
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• 1999

$LaCrO_3$-dispersed Cr alloys for metallic interconnector of solid oxide fuel cell have been studied as function of $LaCrO_3$ content in the range of 5 to 25 vol.% in order to examine the electric conductivity, the oxidation property and the thermal expansion behavior of these alloys. The $LaCrO_3$-dispersed Cr alloys showed high electrical conductivities of $3~5\times10^4$ S/cm at room temperature, and as the $LaCrO_3$content increased the conductivity decreased slightly. During the cyclic oxidation test at $1100^{\circ}C$, the weight change of the Cr alloys decreased with increasing number of oxidation cycle except first cycle, which is attributed to the vaporization of the oxide scale. More addition of the $LaCrO_3$ content reduced also the weight change of the Cr alloys. These mean that the oxide scale formed at the surface of the Cr alloy becomes stable with increasing number of oxidation cycle and$LaCrO_3$ content. The measured thermal expansion of the Cr alloy was well fitted to that of 8 mol% $Y_2O_3$-stabilized $ZrO_2$ electrolyte. These results demonstrate that $LaCrO_3$-dispersed Cr alloy is a useful material for metallic interconnector of solid oxide fuel cell.

• Transactions of the Korean hydrogen and new energy society
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• v.2 no.1
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• pp.29-37
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• 1990

The effect of pressure cycling of $Zr_{0.9}Ti_{0.1}Cr_{0.7}Fe_{1.3}$ on the hydrogenation properties was investigated using the P-C-Isotherm curves and hydrogen absorption rate curves in the isotherm condition. The reversible hydrogen absorption capacity was decreased about 45 % after 3300 cycles. In the case of activated sample, the rate controlling steps of hydriding reaction changed from the surface reaction to the hydrogen diffusion process through hydride phase sequentially as reaction proceeded. After 3300 cycles, the sequential change of rate controlling step was same as activated one. However, the hydrogen absorption rate significantly decreased. It is suggested that the degradation of $Zr_{0.9}Ti_{0.1}Cr_{0.7}Fe_{1.3}$ can be interpreted with the formation of $ZrFe_3$ phase at the particle surface.

• Journal of Powder Materials
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• v.30 no.6
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• pp.478-483
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• 2023

High-entropy alloys (HEAs) are characterized by having five or more main elements and forming simple solids without forming intermetallic compounds, owing to the high entropy effect. HEAs with these characteristics are being researched as structural materials for extreme environments. Conventional refractory alloys have excellent high-temperature strength and stability; however, problems occur when they are used extensively in a high-temperature environment, leading to reduced fatigue properties due to oxidation or a limited service life. In contrast, refractory entropy alloys, which provide refractory properties to entropy alloys, can address these issues and improve the high-temperature stability of the alloy through phase control when designed based on existing refractory alloy elements. Refractory high-entropy alloys require sufficient milling time while in the process of mechanical alloying because of the brittleness of the added elements. Consequently, the high-energy milling process must be optimized because of the possibility of contamination of the alloyed powder during prolonged milling. In this study, we investigated the high-temperature oxidation behavior of refractory high-entropy alloys while optimizing the milling time.

• Journal of the Korean Society for Heat Treatment
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• v.30 no.6
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• pp.279-284
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• 2017

Mechanical properties of titanium alloy can be improved by controlling microstructure through heat treatment. In this study, Ti-3Al-8V-6Cr-4Mo-4Zr metastable beta titanium alloy, was controlled for excellent mechanical property and sound formability through various high temperature heat treatment and aging conditions and the optimum heat treatment conditions were determined. The specimens were heat-treated at $950^{\circ}C$, followed by various aging treatments from $430^{\circ}C$ to $500^{\circ}C$ for 1 to 24 h. As aging temperature and holding time increased, hardness increased by ${\beta}^{\prime}$ phase formation and precipitation of secondary ${\alpha}$ phase in ${\beta}$ matrix. However, the optimum aging temperature and holding time for mechanical properties were at $450{\sim}470^{\circ}C$ for 8~16 hr. Hardness values of the specimen aged at $450^{\circ}C$ for 8 h were found to be the highest. These results can be effectively applied to fabrication of spring with better formability and mechanical property.

• Korean Journal of Materials Research
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• v.8 no.11
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• pp.1031-1037
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• 1998

To investigate the effect of cooling rate and annealing temperature on the corrosion of Zircaloy-4 and Zr-2. 5Nb alloys, autoclave corrosion tests were performed at $500^{\circ}C$ for the specimens prepared by various heat treatments. The specimens were heat-treated at $1050^{\circ}C$ for 30 minutes and cooled by ice-brine quenching, water quenching, oil quenching, air cooling, and furnace cooling. To investigate the effect of annealing temperature, the specimens were annealed at $\alpha$, ($\alpha$+$\beta$)-, and $\beta$-temperatures. It was observed from the $500^{\circ}C$ corrosion test that nodular corrosion occurred on the Zircaloy-4 alloy but did not occur on the Zr-2.5Nb alloy. The corrosion resistance of Zircaloy-4 increased with increasing the cooling rate. On the other hand, the corrosion resistance of Zr-2.5Nb decreased with increasing the cooling rate and the annealing temperature. It is suggested that corrosion resistance of Zircaloy-4 would be controlled by the distribution of Fe and Cr element in the matrix and precipitates, while that of Zr-2.5Nb alloy the niobium concentration and $\beta_{-Nb}$ phase.

• Journal of the Korean Society for Heat Treatment
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• v.18 no.5
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• pp.312-317
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• 2005

The precipitation process in Cu-0.2 wt.%Cr-0.05 wt.%Zr alloys has been studied by electrical electrical resistivity measurements. The kinetics of precipitation could be well described by Johnson-Mehl-Avrami equation, $f(t)=1-\exp(-kt^n)$. The values of n were found to be in the range of 0.36~0.42 at first stage and 1.3~1.6 at second stage. The activation energy was determined by cross-cut method and was 80~89 kJ/mol. The value is similar to the energy for the migration of either a vacancy or a vacancy-solute complex through the lattice.