• 한국분말야금학회:학술대회논문집
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• 한국분말야금학회 2006년도 Extended Abstracts of 2006 POWDER METALLURGY World Congress Part2
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• pp.1159-1160
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• 2006

For pure Molybdenum carburized in mixed gases of argon and carbon monoxide, microstructural observations were carried out. X-ray diffraction analysis for carburized specimens revealed that brittle ${\alpha}-Mo_2C$- layer hardly formed in the case of low carbon monoxide concentration. Fracture strength of the specimen carburized at 1673 K for 16 h is about 550 MPa higher than that of the un-carburized specimen. SEM observation revealed that with increasing carburizing temperature, the region demonstrating a transgranular fracture mode progressed towards the center of specimen. This result means that the grain boundaries were strengthened by the grain boundary diffusion of carbon and the strength of grain boundaries exceeded that of grain itself.

• 한국해양공학회지
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• 제14권1호
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• pp.29-36
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• 2000

Surface micro-crack grows along intergranular or transgranular region of crystal grains. But if it meets the barrier such as sessile dislocation and precipitates it loses straightness and deflects. Investigators had many difficulties in estimating fatigue life of smooth specimen because of the random distribution growth and coalescence of surface micro-cracks. The path of surface micro-crack has irregularity due to nonhomogeneous microstructure. Euclidian geometry can't quantify the shape of surface micro-crack but fractal geometry can. Therefore in this paper fractal dimension is measured at various stage of cycle ratio and estimated cycle ratio in 2024-T3 aluminium, alloy.

• 한국세라믹학회지
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• 제36권4호
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• pp.372-379
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• 1999

Hihg-toughened ceria-stabilized tetragonal zirconia ceramics with irregular grain shape and undulated grain boundary was prepared by ceria doping. Irregularity of grain shapes was increased with the amount of doped ceria. But in case of the large amount of doped ceria grain boundary was migrated to the reverse direction of DIGM. Ceria-stabilized zirconia ceramics annealed at 1650$^{\circ}C$ for 2h after twice dippings into cerium nitrate solu-tion of 0.2M and sintering at 1500$^{\circ}C$ for 2h showed the highest grain boundary length with a value of 23.6$\mu\textrm{m}$ Ceria concentration difference between convex and concave sides in irregular grains was observed over 1 mol% but not observed in normal grains, Specimens with normal grain shape showed intergranular fracture mode whereas the specimens with irregular grain shape showed transgranular fracture mode.

• 한국안전학회지
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• 제14권1호
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• pp.41-48
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• 1999

The fatigue life tests were performed in strain control with triangular and hole-time wave-forms at $650^{\circ}C$. The fatigue lifes were investigated according to waveform examining damage mechanisms, which could be used to predict the fatigue life and estimate the remaining life. The results obtained are as follows; The fatigue lifes were in order of the fast-fast>the fast-slow>the slow-fast in the triangular waveforms, and the fatigue lifes in slow-fast waveforms got shorter in the hold-time waveforms. The damage mechanisms of the fracture surfaces were transgranular fracture in the fast-fast, the fast-slow waveforms and intergranular fracture in the slow-fast waveform.

• 대한기계학회논문집A
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• 제31권4호
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• pp.532-539
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• 2007

The 7075-T651 Al alloy was welded by friction stir welding. Microstructure, macro behaviors and fracture type in the nugget, thermo-mechanically affected zone(TMAZ) and heat affected zone(HAZ) of the welded part were compared to base metal. The microsturctures of nugget zone were compared with tool rotation speeds and various tool transition speed. When the rotation speeds were decreased and transition speeds were increased, the hardness of nugget zone were decreased. Also, the optimal microstructure was observed at the low rotation speed of 800rpm and the high transition speed of 124mm/min. The transgranular dimple and quasi-cleavage at fractured part of nugget zone were investigated.

• 한국재료학회지
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• 제18권4호
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• pp.181-186
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• 2008

In this study, tantalum (Ta) compacts were fabricated in a spark plasma sintering (SPS) process and their microstructure and mechanical properties were investigated. Ta compacts with a density of 99% were successfully fabricated by controlling the sintering conditions of the current and the temperature. The density and hardness were increased as the sintering temperature increased. The $Ta_2C$ compound was observed at the surface of the compacts due to the contact between the Ta powder and graphite mold during the sintering process. The main fracture mode showed a mixed type with intergranular and transgranular modes having some roughness.

• 한국세라믹학회지
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• 제29권8호
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• pp.617-622
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• 1992

In order to study on the effect of SiC whisker in flexural strength characterization of macro defect-free (MDF) cement composites, which composed of high alumina cement and polyvinyl alcohol, microstructural characterization of the composite specimens fabricated by the addition of SiC whiskers was investigated. Microproes are created around the SiC whisker, MDF cement didn't react with the SiC whisker. However, flexural strength of the composites have been improved. Fracture morphology of the composites, presents mainly intergranular type fracture passing around the unhydrated particles and siC whiskers, and partially transgranular type fracture. The main strengthening mechanisms of the MDF cement composites reinforced with SiC whiskers are characterized by crack deflection, microcracking, and bridging of cracks.

• 한국재료학회지
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• 제2권5호
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• pp.313-319
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• 1992

The mechanisms of fatigue crack growth (FCG) in a Ti$_3$Al-based (${\alpha}_2$) alloy, Ti-24Al-11Nb (a/o) with acicular microstructure were studied with particular attention focused on the fatigue crack path through the microstructure and on the effects of specimen orientation and crack closure. The results showed that the fatigue cracks of Ti-24Al-11Nb alloy grew much faster than conventional titanium alloys, with little difference in FCG rates for TL and TS orientations. The fatigue crack paths revealed crystallographic transgranular fracture with frequent serrations and branching. This is in agreement with the known effects of slip planarity and microstructure on the FCG behavior. The load-displacement hysteresis loops showed that the crack closure influenced the FCG behavior.

• Corrosion Science and Technology
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• 제18권2호
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• pp.49-54
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• 2019

The purpose of this paper is to present failure analysis, of the heat exchanger tube in a district heating system. SS304 stainless steel is used, as material for the heat exchanger tube. The heat exchanger operates in a soft water environment containing a small amount of chloride ions, and regularly repeats operation and standstill period. This causes concentration of chloride ions on the outer surface of the tube, as well as repeat of thermal expansion, and shrinkage of the tube. As a result of microscopic examination, cracks showed transgranular as well as branched propagation, and many pits were present, at the initiation point of each crack. Energy disperstive spectroscopy analysis showed Fe and O peak, as well as Cl peak, meaning that cracks were affected by Cl ion. Failure of the tube was caused by chloride-induced stress corrosion cracking by thermal stress, high temperature, and localized enrichment of chloride ions.

• 대한기계학회논문집A
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• 제40권6호
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• pp.565-571
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• 2016

본 논문은 Alloy 617 모재와 용접재에 대한 저사이클 피로 시험 후의 파괴 시험편에 대한 거시적 및 미시적 파면해석을 나타낸다. 용접재 시험편은 Alloy 617의 가스텅그스텐아크 용접 패드로부터 채취, 제작하였다. 본 연구의 목적은 Alloy 617의 모재와 용접재 시험편의 저사이클 파괴 모드 및 기구의 거시적 및 미시적 양상을 고찰하는 것이다. 전변형률 제어 피로시험이 상온에서 0.6, 0.9, 1.2 및 1.5%에 대하여 수행되었다. Alloy 617 모재의 거시적 파면은 피로하중 축에 수직인 평평한 형태의 양상을 보였으나, 용접재 시험편의 경우는 상대적으로 전단/별모양의 양상의 파괴를 나타내었다. 두 시험편 모두 피로균열전파 영역에서는 명확한 스트라이에이션이 관찰되었다. 한편, 모재의 피로균열은 피로 하중 축에 수직인 방향으로 결정입내를 따라 전파하였으나, 용접재 시험편의 경우 하중 축에 거의 $45^{\circ}$의 경사진 형태의 결정입내로 나타났다.