• Journal of the Korean Society for Heat Treatment
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• v.37 no.2
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• pp.58-65
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• 2024

In the present study, the effect of annealing condition on the microstructures and mechanical properties of the cold-rolled CoCrFeMnNi high entropy alloys were studied. Annealing treatment was performed under six different temperatures. Microstructural analyses confirmed that annealing below 800℃ resulted in the formation of intermetallic sigma (σ) phase within face-centered cubic (FCC) matrix, and this σ phase has beneficial effects on the formation of fine-grained structures through retardation of grain growth and recrystallization due to Zener pinning effect. This led to the enhanced yield strength and tensile strength of ~646 and ~855 MPa, respectively. The microstructures annealed above 800℃ demonstrated single FCC phase, and fully-recrystallized single FCC microstructure resulted in a slight increase in ductility with a considerable decrease in strength. The evolution of mechanical properties, such as strength, ductility, and strain hardening exponent, will be discussed.

• Journal of Welding and Joining
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• v.19 no.5
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• pp.534-539
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• 2001

The formation mechanism of microconstituents in brazed joints of duplex stainless steel and Cr-Cu alloy which is an essential process of rocket engine manufacturing was investigated using Cu base insert metal. $SUS329J_3L$ and C18200 were used for base metal and AMS 4764 was used for insert metal. The brazing was carried out under various conditions. There were various phases in the joints, because of reaction between liquid insert metal and base metals. Since liquid insert metal reacts with duplex stainless steel, liquid Cu from insert metal infiltrated into the $\alpha/\beta$ interface of duplex stainless steel. Through the process of Cu infiltration, isolated stainless steel pieces come into the liquid insert metal. Since liquid insert metal reacts with Cr-Cu alloy. Cr precipitates from C18200 come into the liquid insert metal. With increment of bonding temperature and holding time, amounts and sizes of phases increased. but Cr-Mn compounds decreased at 1303k for 1.2ks and Mn-rich phases disappeared Fe-Cr compounds formed.

• Proceedings of the Korean Environmental Sciences Society Conference
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• 2001.05a
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• pp.236-237
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• 2001

In this study, experiments were carried out to measure the variations in the corrosion potential and current density of polarization curves with amorphous $Fe_{70}Cr_5Si_{10}B_{15}$ and $Co_{70}Cr_5Si_{10}B_{15}$ alloy ribbon. The results were particularly examined to identify the influences of corrosion potential including various conditions such as hydrochloric acid, temperature, salt, pH, and oxygen. The optimum conditions were established with variations including temperature, salt, pH, oxygen, corrosion rate, and resistance of corrosion potential. The mass tranfer coefficient(${\alpha}$) value was determined with the Tafel's slope for the anodic dissolution based on the polarization effect with optimum conditions. The second anodic current density peak and maximum passive current density were designated as the critical corrosion sensitivity($I_{r}/I_{f}$).

• Korean Journal of Materials Research
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• v.23 no.7
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• pp.385-391
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• 2013

The influence of Cu and Ni on the ductile-brittle transition behavior of metastable austenitic Fe-18Cr-10Mn-N alloys with N contents below 0.5 wt.% was investigated in terms of austenite stability and microstructure. All the metastable austenitic Fe-18Cr-10Mn-N alloys exhibited a ductile-brittle transition behavior by unusual low-temperature brittle fracture, irrespective of Cu and/or Ni addition, and deformation-induced martensitic transformation occasionally occurred during Charpy impact testing at lower temperatures due to reduced austenite stability resulting from insufficient N content. The formation of deformation-induced martensite substantially increased the ductile-brittle transition temperature(DBTT) by deteriorating low-temperature toughness because the martensite was more brittle than the parent austenite phase beyond the energy absorbed during transformation, and its volume fraction was too small. On the other hand, the Cu addition to the metastable austenitic Fe-18Cr-10Mn-N alloy increased DBTT because the presence of ${\delta}$-ferrite had a negative effect on low-temperature toughness. However, the combined addition of Cu and Ni to the metastable austenitic Fe-18Cr-10Mn-N alloy decreased DBTT, compared to the sole addtion of Ni or Cu. This could be explained by the fact that the combined addition of Cu and Ni largely enhanced austenite stability, and suppressed the formation of deformation-induced martensite and ${\delta}$-ferrite in conjunction with the beneficial effect of Cu which may increase stacking fault energy, so that it allows cross-slip to occur and thus reduces the planarity of the deformation mechanism.

• Nuclear Engineering and Technology
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• v.56 no.5
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• pp.1738-1746
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• 2024

The effect of europium-driven corrosion behavior of Alloy 709 in FLiNaK molten salt was investigated by static immersion tests at 700 ℃. It was found that the corrosion of Alloy 709 increased after the addition of EuF₃, even though the standard reduction potential of Eu(III)/Eu(II) was negative than those of Fe(II)/Fe, Ni(II)/Ni and Cr (II)/Cr. The presence of Eu(III) led to deeper corrosion attack layers and more pits on the steel surface in comparison with corrosion in blank FLiNaK. However, the addition of Eu(III) seemed to have a role in reducing surface cracking that was explored in corrosion by blank FLiNaK, which depended on Eu(III) concentration.

• Journal of the Korean institute of surface engineering
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• v.35 no.6
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• pp.371-376
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• 2002

Steam turbine components of power generators are subjected to severe damages from the particle erosion by iron oxides (mainly $Fe_3$$O_4$) which are formed due to the oxidation of boiler tubes, causing high costs for maintaining and repairing. One of the practical ways to minimize the particle erosion is to apply the erosion resistant boride coating on the turbine components which is composed of boride apply. But the evaluation of its performance has not been carried out. A particle erosion tester, which can offer the erosion condition of steam turbine components, was developed to evaluate the performances of the boronized Cr/Mo alloy. The result showed that the boronized Cr/Mo alloy showed superior resistance to particle erosion to the bare Cr/Mo alloy in all test conditions.

• Transactions of Materials Processing
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• v.33 no.1
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• pp.36-42
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• 2024

Additive manufacturing with 3XX austenitic stainless steels has been widely investigated during a decade due to its high strength, good corrosion resistance, and fair weldability. However, in recently, Ni price drastically increased due to the high demand of secondary battery for electric mobilities. Thus, it is essential to substitute the Ni with Mn for reducing stainless steels price. Meanwhile, the chemical composition changes in stainless steels not only affect to its properties but also change the optimal processing parameters during additive manufacturing. Therefore, it is necessary to optimize the processing parameters of each alloy for obtaining high-quality product using additive manufacturing. After processing optimization, mechanical properties and microstructure of the laser-powder bed fusion processed Fe-15Cr-7Ni-3Mn alloy were investigated in both room (298 K) and cryogenic (77 K) temperatures. Since the temperature reduction affects to the deformation mechanism transition, multi-scale microstructural characterization technique was conducted to reveal the deformation mechanism of each sample.

• Proceedings of the Korean Nuclear Society Conference
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• 1995.05a
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• pp.723-728
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• 1995

Alloy 600 표면에 레이저 빔을 이용하여 Ni, Cr 흔합분말 및 순수 Cr 분말로 표면합금층을 만들었다. 표면 합금층은 모재와 정합계면를 이루고 있으며 레이저 표면용용 시편에서와 같이 크게 면선단 응고부와 셀룰라 응고부로 나눌 수 있고, 모재에는 레이저 처리과정에서 생긴 수십 $\mu\textrm{m}$ 두께의 열영향 부위가 존재하였다. 그리고 합금층 내부에는 레이저 표면 용용된 시편과 달리 구형의 커다란 기공(pores)이 존재하였다. 레이저 표면 합금층에서 합금원소의 조성 분포를 조사하기 위해 레이저 처리된 시편에 대해 WDX 분석을 하였고, 합금층 내부에 Ni, Cr, Fe 원소의 조성 분포는 매우 균일하였다.

• Journal of the Korean Society of Safety
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• v.14 no.1
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• pp.208-215
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• 1999

The creep behavior of a rapidly solidified and consolidated Al-9.45wt%Fe-4.45wt%Cr alloy were investigated in the stress range 40 to 115 MPa and temperature range 300(0.53Tm) to 441$^{\circ}C$(0.66Tm). It is of use to available aerospace and automobile industries for the improved performance of materials used at high temperature. Because Al alloys with improved creep resistance offer the potential for lower weight and reduced costs in aerospace and automobile components (e.g., structural members and engine parts) through the replacement of heavier and more costly materials, the safety in use at high temperature is good. The alloy is characterized by high stress exponents and activation energies for creep, which are greatly dependent on the stress and temperature. Because the creep stress is seen to cause a strongly significant enhancement of coarsening, the coarsening rate of the dispersed particles in all crept specimens is faster than that in isothermally annealed specimens. Dislocations connecting dispersoids are observed more cofrequently in crept specimens with higher stress and lower temperature. The creep strain rates in the power law creep regime were found to be predicted much better by the Shorty and Rosler/Arzt equation with the inclusion of a threshold stress and dislocation detachment mechanism. The dispersoids in this alloy were acting a source of void nucleation that finally leaded to ductile fracture within the grain so called intergranular. Each void was initiated, grown and failed at the dispersoids in the aluminium matrix. Grain boundary accommodation of the slip produced, which result in initiation of the void and then final transgranular fracture. Therefore, it was confirmed that these dispersoids played an important role in the fracture mechanism by the formation of $Al_{13}Fe_4$, $Al_{13}Cr_2$ and $Al_2O_3$.

• Journal of Magnetics
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• v.21 no.1
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• pp.35-39
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• 2016

$Cr_{100-x}Ti_x$ amorphous texture-inducing layers (TIL) were investigated to realize highly (002) oriented $L1_0$ FePt-C granular films through hetero-epitaxial growth on the (002) textured bcc-$Cr_{80}Mn_{20}$ seed layer (bcc-SL). As-deposited TILs showed the amorphous phase in Ti content of $30{\leq}x(at%){\leq}75$. Particularly, films with $40{\leq}x{\leq}60$ kept the amorphous phase against the heat treatment over $600^{\circ}C$. It was found that preference of the crystallographic texture for bcc-SLs is directly affected by the structural phase of TILs. (002) crystallographic texture was realized in bcc-SLs deposited on the amorphous TILs ($40{\leq}x{\leq}70$), whereas (110) texture was formed in bcc-SLs overlying on crystalline TILs (x < 30 and x > 70). Correlation between the angular distribution of (002) crystal orientation of bcc-SL evaluated by full width at half maximum of (002) diffraction (FWHM) and a grain diameter of bcc-SL indicated that while the development of the lateral growth for bcc-SL grain reduces FWHM, crystallization of amorphous TILs hinders FWHM. $L1_0$ FePt-C granular films were fabricated under the substrate heating process over $600^{\circ}C$ with having different FWHM of bcc-SL. Hysteresis loops showed that squareness ($M_r/M_s$) of the films increased from 0.87 to 0.95 when FWHM of bcc-SL decreased from $13.7^{\circ}$ to $3.8^{\circ}$. It is suggested that the reduction of (002) FWHM affects to the overlying MgO film as well as FePt-C granular film by means of the hetero-epitaxial growth.