• 한국재료학회지
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• 제26권1호
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• pp.13-16
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• 2016

Morphology and formation processes of lamellar grain boundaries in titanium rich binary TiAl intermetallics were studied. TiAl alloys containing aluminum content of 44 to 48 at.% were induction-heated to 1723 K followed by helium-gas-quenching at various temperatures. For the Ti-44%Al, few lamellae were observed in samples quenched from higher than 1473 K. Although small peaks of beta phase were detected using X-ray diffraction, only the ordered hexagonal phase (${\alpha}_2$) with clear APB contrast was observed in TEM observation. For the Ti-48 at.%Al alloy, almost no lamellar structure, and straight grain boundaries were observed in samples quenched from higher than 1623 K. The formation of lamellae along grain boundaries was observed in the sample quenched from 1573 K. The fully lamellar microstructures with serrated boundaries were observed in samples quenched from lower than 1473 K. It was found that the formation of ${\gamma}$ platelets took place at higher temperatures in Ti-48 at.%Al than in Ti-44 at.%Al. Although the size of the serration is different, serrated lamellar grain boundaries could be obtained for all alloy compositions employed. The serration appeared to be due to the grain boundary migration induced by precipitation and growth of ${\gamma}$. Differences in transformation characteristics with aluminum content are discussed.

• 한국분말재료학회지
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• 제4권4호
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• pp.243-251
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• 1997

Engine valve-shaped TiAl-Mn intermetallics containing 43.5 to 47.5at%Al (Mn/Al=0.036) are successively fabricated by reactive sintering the elemental powder mixtures near-net shaped by extrusion and die forging. A duplex structure consisted of lamellar grains and equiaxed $\gamma$ grains is developed for all compositions, and the areal fraction of the lamellar grains(or equiaxed $\gamma$ grains) decreases (or increases) with increasing Al content. As Al content increased, the elongation increases with accompanying decrease in yield strength and ultimate tensile strength at both room temperature and 80$0^{\circ}C$. This indicates that the suitable composition is Ti-45at%Al-1.6at%Mn in considering the balance of ambient and elevated tensile properties. The reactive-sintered Ti-45Al-1.6Mn alloy shows superior oxidation resistance not only to the plasma arc melted one but also to the heat resistance steel STR35(representative exhaust valve head material for automotive engine). The reactive-sintered Ti-45Al-1.6Mn alloy coated with an oxidizing scale exhibits a better wear resistance than induction hardened martensitic steel STR11(representative exhaust valve tip material for automotive engine).

• 한국재료학회지
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• 제27권1호
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• pp.8-11
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• 2017

Binary Ti-Al alloys containing 50 to 60 atomic percent aluminum are rapidly solidified by hammer anvil method under an argon atmosphere. Constituent phases in each alloy are identified by X-ray diffractometry and microstructures of the alloys are investigated using a transmission electron microscope. In alloys with aluminum content between 50 and 54 percent, a second phase exists besides TiAl(${\gamma}$); this second phase is identified as $Ti_3Al$(${\alpha}2$). The ${\alpha}2$ phase is observed in two types of morphology. One is as fine lamellar alternating with ${\gamma}$ and the other is as a particle. It is concluded that the existence of a metastable phase with the morphologies stated above should arise from a higher quenching rate attained by the hammer anvil method as compared to the conventional roll or splat-quench method. Implications of the above observation are discussed with respect to the phase relations in the Ti-Al binary system; these implications are still controversial in many respects.

• 열처리공학회지
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• 제32권6호
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• pp.256-262
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• 2019

The microstructural features and texture development by both hot rolling and hot forging in ${\gamma}-TiAl$ alloy were investigated. In addition, additional heat treatment after hot forging was conducted to recognize change of the microstructure and texture evolution. The obtained microstructural features through dynamic recrystallization after hot deformed ${\gamma}-TiAl$ were quite different because two kinds of formation process were occurred depending on deformation condition. However, analyzed texture tends to be random orientation due to intermediate annealing up to ${\alpha}+{\beta}$ region during the hot deformation process. After additional heat treatment, microstructure transformed into fully lamellar microstructure and randomly oriented texture was also observed due to the same reason as before. Tensile test at room temperature demonstrated that anisotropy of mechanical properties were not appeared and transgranular fracture was occurred between interface of ${\alpha}_2/{\gamma}$. As a result, it could be suggested that microstructural features influenced much more than texture development on mechanical properties at room temperature.

• 한국재료학회지
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• 제15권12호
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• pp.773-779
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• 2005

Properties of Ni-base superalloys of Nimonic 80A alloy system were investigated by the observation of microstructures, precipitates ana hardness as a function of the chemical compositions ana the paths of heat treatment. The higher hardness values showed, the higher Ti/Al ratio among high compositions of Cr and Co element. The lower (Ti+Al) and Fe contents decreased in the same Ti/Al ratio, the higher hardness values showed. This results are considered that coherent deformation was increased with increasing Ti/Al ratio. Hardness showed higher value when Cr contents was $18 wt\%$ less than $21wt\%Cr$. In $3.15 wt\%$Co alloy, $\gamma'$ phase was very fine as around 50nm and, its volume fraction and hardness showed the highest value by 2step-aging treatment.

• 한국결정성장학회지
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• 제27권5호
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• pp.229-234
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• 2017

본 연구에서는 자동차 터보차저(turbo charger)용 터빈휠을 제조하기 위하여 고속원심주조법을 이용하여 TiAl 합금을 주조하였다. 여러 가지 조성의 주형을 사용하여 주조결함이 최소화 되는 최적의 몰드 조건을 찾고자 하였으며, 주조된 TiAl 합금의 결정구조와 미세조직을 고찰하였다. TiAl 샘플의 XRD 분석 결과 ${\gamma}$-TiAl 상과 ${\alpha}_2-Ti_3Al$ 상으로 구성되어 있음을 확인하였으며, 광학현미경과 FE-SEM 분석을 통하여 TiAl 샘플은 6-fold 및 4-fold 대칭성으로 이루어진 두 개의 수지상(dendrite) 형태로 이루어져 있음을 확인할 수 있었다. 또한 샘플의 단면에서 표면과 가까운 지점과 내부에서 측정한 원소함량의 차이를 확인할 수 있었으며, 마이크로비커스 경도계를 이용하여 표면에서 $50{\mu}m$ 미만의 영역에서 경도상승 영역 alpha-case 층이 형성되었음을 확인하였다.

• 한국재료학회지
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• 제10권9호
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• pp.629-635
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• 2000

본 논문에서는 Ti-48.5at%Al-0.96at%Mo 조성을 갖는 $\gamma$-TiAl 합금은 항온 단조 및 후속 조직제어 열처리 시 발달하는 집합조직의 변화에 관해 연구하였다. 특히, 동적 재결정 후 조직제어 열처리 온도와 시간의 증가에 따라 발생하는 lamellar volume fraction 과 집합조직 변화에 주목하여 관찰하였다. 동적 재결정 후 집합조직은 ND$\perp${302) 선분과, TD$\perp${100) 및 이에 비해 상대적으로 다소 약한{111) 성분들이 발달하였으며, 열처리 온도와 시간이 증가함에 따라 lameller volume fraction 은 증가했고 동일한 성분의 집합조직도 점차 강하게 발달함을 알 수 있었다. 하지만 상온인장시험 결과는 lameller volume fraction이 증가할수록 낮은 상온 연신을 보였는데, 이는 준 층상조직을 갖는 $\gamma$-TiAl합금의 상온인장특성이 집합조직의 영향보다는 lameller volume fractio과 같은 미세조직 특성에 더욱 강하게 영향을 받기 때문인 것으로 판단된다.

• 한국재료학회지
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• 제11권6호
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• pp.502-507
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• 2001

일방향응고법으로 IN792+Hf 초내열합금의 응고속도에 따른 응고거동의 변화에 대해 연구하였다. 조직관찰을 통해 각 상의 응고과정과 석출거동을 분석하였다 일방향응고시 응고속도가 감소하면 문자형의 탄화물은 면상 탄화물로 변화하였고 ${\gamma}$상과 탄화물의 결합은 탄화물의 수지상 성장에 의한 것임을 확인할 수 있었다. 긴 막대형상의 탄화물이 0.5$\mu\textrm{m}$/s의 응고속도에서 입계를 따라 형성되었으며 잔류액상지역에서 ${\gamma}$'형성원소가 풍부한 구역과 고갈된 구역이 발견되었다. 공정 ${\gamma}$/${\gamma}$'은 형성원소가 풍부한 구역에서 핵생성하였으며 공정 ${\gamma}$/${\gamma}$'의 형성은 잔류액상지역의 (Ti+Hf+Ta+W)/Al 비율을 높여 η상의 석출을 유발하였다. 느린 응고속도에서는 잔류액상지역으로부터의 충분한 역확산으로 (Ti+Hf+Ta+W)/Al 비율이 낮아져 η상의 석출이 억제되었다.

• 대한기계학회논문집A
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• 제31권5호
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• pp.550-555
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• 2007

The effective fracture toughness testing of materials intended for application in Micro Electro Mechanical Systems (MEMS) devices is required in order to improve understanding of how micro sized material used in device may be expected to perform upon the micro scale. ${\gamma}$-TiAl based materials are being considered for application in MEMS devices at elevated temperatures. Especially, in Alloy 4, both ${\alpha}_2$ and ${\gamma}$ lamellae were altered markedly in 3,000 h, $700^{\circ}C$ exposure. Parallel decomposition of coarse ${\alpha}_2$ into bunches of very fine (${\alpha}_2+{\gamma}$) lamellae. Parallel decomposition of coarse ${\alpha}_2$ into bunches of very fine (${\alpha}_2+{\gamma}$) lamellae. The materials were examined 2 types Alloy 4 on heat exposed specimen($700^{\circ}C$, 3,000 h) and no heat exposed one. Micro sized cantilever beams were prepared mechanical polishing on both side at $25{\sim}30{\mu}m$ and electro final stage polishing to observe lamellar orientation of same colony with EBSD (Electron Backscatter Diffraction Pattern). Through lamellar orientation as inter-lamellae or trans-lamellae, Cantilever beam was fabricated with Focused Ion Beam(FIB). The directional behavior of the lamellar structure was important property in single material, because of the effects of the different processing method and variations in properties according to lamellar orientation. In MEMS application, it is first necessary to have a reliable understanding of the manufacturing methods to be used to produce micro structure.

• 한국재료학회지
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• 제25권4호
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• pp.202-208
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• 2015

Alumina dispersion strengthening copper(ADSC) alloy has great potential for use in many industrial applications such as contact supports, frictional break parts, electrode materials for lead wires, and spot welding with relatively high strength and good conductivity. In this study, we investigated the oxidation behavior of ADSC alloys. These alloys were fabricated in forms of plate and round type samples by surface oxidation reaction using Cu-0.8Al, Cu-0.4Al-0.4Ti, and Cu-0.6Al-0.4Ti(wt%) alloys. The alloys were oxidized at $980^{\circ}C$ for 1 h, 2 h, and 4 h in ambient atmosphere. The microstructure was observed with an optical microscope(OM) and a scanning electron microscope(SEM) equipped with energy-dispersive X-ray spectroscopy(EDS). Characterization of alumina was carried out using a 200 kV field-emission transmission electron microscope(TEM). As a result, various oxides including Ti were formed in the oxidation layer, in addition to ${\gamma}$-alumina. The thickness of the oxidation layer increased with Ti addition to the Cu-Al alloy and with the oxidation time. The corrected diffusion equation for the plate and round type samples showed different oxidation layer thickness under the same conditions. Diffusion length of the round type specimen had a value higher than that of its plate counterpart because the oxygen concentration per unit area of the round type specimen was higher than that of the plate type specimen at the same diffusion depth.