• 한국분말재료학회지
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• 제16권5호
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• pp.358-362
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• 2009

Ti$_{50}$Cu$_{20}$Ni$_{20}$Al$_{10}$ quaternary amorphous alloy was prepared by high-energy ball milling process. A complete amorphization was confirmed for the composition of Ti$_{50}$Cu$_{20}$Ni$_{20}$Al$_{10}$ after milling for 30hrs. Differential scanning calorimetry showed a large super-cooled liquid region ($\Delta$T$_x$ = T$_x$ T$_g$, T$_g$ and T$_x$: glass transition and crystallization onset temperatures, respectively) of 80 K. Prepared amorphous powders of Ti$_{50}$Cu$_{20}$Ni$_{20}$Al$_{10}$ were consolidated by spark-plasma sintering. Densification behavior and microstructure changes were investigated. Samples sintered at higher temperature of 713 K had a nearly full density. With increasing the sintering temperature, the compressive strength increased to fracture strength of 756 MPa in the case of sintering at 733 K, which showed a 'transparticle' fracture. The samples sintered at above 693 K showed the elongation maximum above 2%.

• The Journal of Advanced Prosthodontics
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• 제8권3호
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• pp.224-228
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• 2016

PURPOSE. This study aimed to investigate whether dentin surface preparation with diamond rotary instruments of different grit sizes affects the shear bond strength of resin-bonded restorations. MATERIALS AND METHODS. The buccal enamel of 60 maxillary central incisors was removed with a low speed diamond saw and wet ground with silicon carbide papers. The polished surfaces of the teeth were prepared with four groups of rotary diamond burs with super-coarse (SC), coarse (C), medium (M), and fine (F) grit sizes. Following surface preparation, 60 restorations were casted with nickel-chromium alloy and bonded with Panavia cement. To assess the shear bond strength, the samples were mounted on a universal testing machine and an axial load was applied along the cement-restoration interface at the crosshead speed of 0.5 mm/min. The acquired data was analyzed with one way ANOVA and Tukey post hoc test (${\alpha}=.05$). RESULTS. The $mean{\pm}SD$ shear bond strengths (in MPa) of the study groups were $17.75{\pm}1.41$ for SC, $13.82{\pm}1.13$ for C, $10.40{\pm}1.45$ for M, and $7.13{\pm}1.18$ for F. Statistical analysis revealed the significant difference among the study groups such that the value for group SC was significantly higher than that for group F (P<.001). CONCLUSION. Dentin surface roughness created by diamond burs of different grit sizes considerably influences the shear bond strength of resin bonded restorations.

• 대한기계학회논문집A
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• 제34권12호
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• pp.1909-1915
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• 2010

고온의 연소가스에서 운전되는 국내 가스터빈 부품들은 매일 기동정지를 반복함으로써 열사이클에 의해 재료특성에 변화가 발생한다. 최근 많은 가스터빈 고온부품들이 단결정 초내열 합금으로 제작되어 지지만 재료열화 예측을 통한 부품 교체와 정비에 대한 기준이 없어서 대부분 제작사에 의존하고 있는 실정이다. 본 연구에서는 향후 수명평가와 손상분석의 기초자료로 활용하기 위해 실제 운전된 가스터빈 1 단 블레이드의 열화상태를 조사하였다. 사용한 블레이드는 25,000 및 52,000 의 등가운전시간(EOH : Equivalent Operating Hour)을 가졌으며, 재질은 단결정 초내열합금인 CMSX-4 이다. 사용된 블레이드에서 직접 시험편을 채취하여 기계적특성 시험 및 미세조직을 관찰하였다.

• 대한기계학회논문집A
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• 제34권10호
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• pp.1403-1409
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• 2010

니켈기 초내열합금은 고온 강도를 지속적으로 증가시키며 현재 비행기 엔진, 선박 엔진 및 발전용 가스터빈 고온 부품 등을 만드는 가장 중요한 소재로 오래전부터 사용되어져 왔다. 이러한 부품의 수명을 연장하기 위해서는 사용 환경과 유사한 조건에서의 피로수명 예측이 매우 중요하다. 따라서 본 연구에서는 가스터빈 블레이드 소재인 니켈기 초내열합금 IN738LC에 대하여 실제운전환경과 유사한 조건을 설정하여 다양한 변형률 범위와 온도에서 시험을 수행하였다. 저주기 피로수명을 예측하기 위하여 변형률 에너지 밀도와 파단 사이클과의 관계를 사용하였다. 수명의 예측은 시험결과를 토대로 변형률 에너지법과 Coffin-Manson법에 의하여 예측을 하였다.

• Steel and Composite Structures
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• 제36권2호
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• pp.213-228
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• 2020

Shape Memory Alloys (SMAs) are new materials used in various fields of science and engineering, one of which is civil engineering. Owing to their distinguished capabilities such as super elasticity, energy dissipation, and tolerating cyclic deformations, these materials have been of interest to engineers. On the other hand, the connections of a steel structure are of paramount importance because of their vulnerabilities during an earthquake. Therefore, it is indispensable to find approaches to augment the efficiency and safety of the connection. This research investigates the behavior of steel connections with extended end plates equipped hybridly with 8 rows of high strength bolts as well as Nitinol superelastic SMA bolts. The connections are studied using component method in dual form. In this method, the components affecting the connections behavior, such as beam flange, beam web, column web, extended end plate, and bolts are considered as parallel and series springs according to the Euro-Code3. Then, the nonlinear force- displacement response of the connection is presented in the form of moment-rotation curve. The results obtained from this survey demonstrate that the connection has ductility, in addition to its high strength, due to high ductility of SMA bolts.

• 한국분말재료학회지
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• 제16권1호
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• pp.9-15
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• 2009

Amorphization and crystallization behaviors of $Ti_{50}Cu_{50}Ni_{20}Al_{10}$ powders during high-energy ball milling and subsequent heat treatment were studied. Full amorphization obtained after milling for 30 h was confirmed by X-ray diffraction and transmission electron microscope. The morphology of powders prepared using different milling times was observed by field-emission scanning electron microscope. The powders developed a fine, layered, homogeneous structure with prolonged milling. The crystallization behavior showed that the glass transition, $T_g$, onset crystallization, $T_x$, and super cooled liquid range ${\Delta}T=T_x-T_g$ were 691,771 and 80 K, respectively. The isothermal transformation kinetics was analyzed by the John-Mehn-Avrami equation. The Avrami exponent was close to 2.5, which corresponds to the transformation process with a diffusion-controlled type at nearly constant nucleation rate. The activation energy of crystallization for the alloy in the isothermal annealing process calculated using an Arrhenius plot was 345 kJ/mol.

• 한국분말재료학회지
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• 제18권2호
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• pp.141-148
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• 2011

Carbon material shows relatively high strength at high temperature in vacuum atmosphere and can be easily removed as CO or $CO_2$ gas in oxidation atmosphere. Using these characteristics, we have investigated the applicability of carbon mold for precision casting of high melting point metal such as nickel. Disc shape carbon mold with cylindrical pores was prepared and Ni-base super alloy (CM247LC) was used as casting material. The effects of electroless Nickel plating on wettability and cast parameters such as temperature and pressure on castability were investigated. Furthermore, the proper condition for removal of carbon mold by evaporation in oxidation atmosphere was also examined. The SEM observation of the interface between carbon mold and casting materials (CM247LC), which was infiltrated at temperature up to $1600^{\circ}C$, revealed that there was no particular product at the interface. Carbon mold was effectively eliminated by exposure in oxygen rich atmosphere at $705^{\circ}C$ for 3 hours and oxidation of casting materials was restrained during raising and lowering the temperature by using inert gas. It means that the carbon can be applicable to precision casting as mold material.

• Journal of Welding and Joining
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• 제20권2호
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• pp.82-89
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• 2002

In order to clarify hot cracking phenomena occurred in Inconel 690 welds and it's prevention, in this study, the cracking behavior and the influence of welding variables on cracking in Inconel 690 overlay welds on Mn-Ni-Cr-Mo steel(SA 508 cl.3) for pressure vessel were investigated by using mock-up test. The main results are as follows: The cracks in Inconel 690 overlay welds were mainly generated near the start and the end part of welding beads adjacent to STS 309L welded outside of Inconel 690 welds. Most of the cracks showed typical solidification crack, and also it was assumed that there was possibility of liquation cracking in HAZ. The existence of Nb constituents or concentration of Nb was recognized on the fracture facets of the solidification cracks in the welds by SMAW. Therefore Nb was considered to be the main factor of the solidification cracking. As the weld heat input was more increased and the weld bead length was longer, the extent of cracking was more increased. Moreover the extent of cracking was considerably decreased by changing of welding sequence to the start and the end part of welds. Hot cracking in welds by GTAW was considerably decreased as compared with that of SMAW. And cracks were well generated in the Inconel 690 overlay welds adjacent to 575 309L welds. This means that the hot cracking susceptibility of Inconel 690 welds was largely varied by chemical components and/or compositions of filter metals, base metals and neighboring welds.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2004년도 하계학술대회 논문집 Vol.5 No.2
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• pp.1258-1261
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• 2004

ACSR전력선의 송전용량 증가를 위해 개발된 증용량저이도 송전선인 STACBUAW(Super Thermal-resistant Aluminum alloy Conductors, aluminum-clad Invar-Reinforced)전선은 초내열 Al도체 및 인바강선의 사용으로 비교적 고온에서의 안정적 운전이 가능하다. 그러나 고온 환경에서 장시간 노출된 STACIR/AW전선의 안정적 관리를 위해서는, 열화 된 STACIR/AW 전선의 인장강도, 각 구성소재의 탄성 계수, 비틀림 계수 둥과 같은 기계적 물성이 장기 운전 모의를 위해 설정된 열화온도, 열화시간 등에 대해 종합적으로 평가될 필요가 있다. 또한 크립 등과 같이 고온응력 부하상태에서의 변형거동과 탄성계수 및 선 팽창계수의 온도의존성 등은 전선의 이도관리와 예측을 위해서도 명확히 규명되어야할 중요한 관리 인자이다. 그러나 현재까지는 이들에 대해 수행한 어떠한 연구결과들도 보고 되어 있지 않은 실정이다. 본 연구에서는 STACIR/AW $410mm^2$ 송전선을 장시간 운전의 모의를 위해 가속열화 시키고 가속열화에 따른 STACIR/AW전선 및 그 구성소재의 강도, 비틀림 특성의 변화를 조사하여 장시간 운전에 따른 STACIR/AW전선의 안정성을 평가하여 보고하고자 하였다.

• 대한기계학회논문집A
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• 제35권7호
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• pp.753-758
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• 2011

초내열합금인 GTD-111은 고온강도와 내산화성이 우수하여 가스터빈에서 사용되는 소재이다. 초내열합금의 피로 수명 예측은 가스터빈의 효율을 개선하기 위하여 매우 중요하다. 본 연구에서의 저주기 피로시험은 실제 운전 환경과 유사하게 변형률 범위, 온도를 다양하게 설정하여 시험을 수행하였다. GTD-111의 저주기 피로수명을 예측하기 위하여 변형률 에너지 밀도와 파단 사이클과의 관계를 이용하였다. 시험결과를 토대로 변형률 에너지법과 Coffin-Manson법에 의하여 피로수명을 예측하였다.