• Proceedings of the Korean Nuclear Society Conference
• /
• 1995.05b
• /
• pp.709-714
• /
• 1995

영광 3, 4호기에 사용되고 있는 원전 압력용기용 SA-508 IH 강의 미세구조 및 상온 피로균 열성장 특성을 고찰하였다. 본 강재의 미세구조, 석출물 분포 및 형상을 투과전자현미경을 통하여 관찰하였으며 임계영역에서의 거동 및 균열닫힘에 주목하여 피로균열성장 특성을 연구하였다. 다양한 형태의 (Fe, MR)$_3$C 세멘타이트 및 Mo$_2$C 석출물이 입계, 래스경계면 및 입내에 분포되고 있음을 확인하였다. Paris 영역에서의 피로균열성장 속도는 ASME 기준선과 유사하였으며, 임계 영역에서는 일반적인 저합금강의 경우보다 다소 낮게 나타났다. 파면조사 결과 입내 연성파괴현상이 전 $\Delta$K 영역에서 나타나고 있으며, $\Delta$K$_{th}$ 부터 $\Delta$K가 12 Mpa√m 영역에서는 입계파괴 및 입내 평활면이 관찰되었다.

• Journal of the Korean Society for Precision Engineering
• /
• v.24 no.10
• /
• pp.31-36
• /
• 2007

• 한국전자현미경학회:학술대회논문집
• /
• 2001.05a
• /
• pp.73-75
• /
• 2001

• Nuclear Engineering and Technology
• /
• v.17 no.3
• /
• pp.193-199
• /
• 1985

Iodine-induced stress corrosion cracking tests of Zircaloy-4 cladding were undertaken using the modified infernal pressurization method. The effects of iodine concentration and applied stress were studied. The critical iodine concentration for SCC was found to be about 0.2 mg/$\textrm{cm}^2$ at 603$^{\circ}$K. The threshold stress was dependent on the test temperature and the mechanical properties of the specimen. The fracture surface showed that the crack propagated stepwise iron one grain to others until the material was unstable and then ruptured mechanically. The initial region showed the transgranular feature and the wedge-shaped cracks. As the crack proceeded, the transgranular and ductile-tearing mired feature appeared in the middle region.

• Korean Journal of Materials Research
• /
• v.10 no.2
• /
• pp.111-116
• /
• 2000

Fracture mode and carbide reactions of cast alloy 738LC during thermal exposure and creep at 816$^{\circ}C$/440MPa and 982$^{\circ}C$/152MPa were investigated. Crystallographic transgranular failure was observed in the specimen crept at 816$^{\circ}C$ due to shearing on the slip plane. Because selective oxidation at the grainboundaries which was exposed at the surface leads reduction in surface energy, however, early initiation of crack at the grainboundaries and intergranular failure were observed in the specimen crept at 982$^{\circ}C$/152MPa. As a result of decomposition of MC carbide at the tested temperatures, M(sub)23C(sub)6 carbide precipitated either on the grainboundaries or on the deformation band. The applied stress enhanced decomposition of MC. $\sigma$phase nucleated from Cr(sub)23C(sub)6 then grew to the ${\gamma}$+${\gamma}$\\` matrix. Precipitation of $\sigma$was accelerated by increasing temperature and applied stress.

• Journal of the Korean Crystal Growth and Crystal Technology
• /
• v.10 no.2
• /
• pp.122-127
• /
• 2000

Microstructural evolutions in ceria-stabilized zirconia (Ce-TZP) and alumina-dispersed Ce-TZP ceramics were investigated as functions of doping and annealing conditions. All of sintered specimens showed the relative density over 99 %. Sintered specimens had linear grain boundaries and normal grain shapes, but ceria-doped specimens had irregular grain shapes and nonlinear grain boundaries due to the diffusion-induced grain boundary migration during annealing at $1650^{\circ}C$ for 2 h. Mean grain boundary length of Ce-TZP with irregular grain shapes was higher than that of normal grain shapes, and was a value of 23pm at the maximum. Alumina particles dispersed in Ce-TZP inhibited the grain growth of zirconia particles. $Al_2O_3$Ce-TZP doped with ceria and annealed at $1650^{\circ}C$ for 2 h showed irregular grain shapes as well as small grain size. Added alumina particles showed the grain growth during sintering or annealing, and they changed the position from grain boundary to inside of the grains during the annealing. The specimens with normal grain shapes showed an intergranular fracture mode, whereas the specimens with irregular grain shapes showed a transgranular fracture mode during the crack propagation.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.41 no.3
• /
• pp.193-198
• /
• 2017

Alloy 617 is one of the primary candidate materials to be used in a very high temperature reactor (VHTR) system as an intermediate heat exchanger (IHX). To investigate the low cycle fatigue behavior of Alloy 617 weldments at a high temperature of $850^{\circ}C$, fully reversed strain-controlled fatigue tests were conducted with the total strain values ranging from 0.6~1.5％. The weldment specimens were machined using the weld pads fabricated with a single V-grove configuration by gas tungsten arc welding (GTAW) process. The fatigue life is reduced as the total strain range increases. For all testing conditions, the cyclic stress response behavior of the Alloy 617 weldments exhibited the initial cyclic strain hardening phenomenon during the initial small number of cycles. Furthermore, the overall fatigue cracking and the propagation or cracks showed a transgranular failure mode.

• Analytical Science and Technology
• /
• v.5 no.2
• /
• pp.203-211
• /
• 1992

Fracture behavior and martensite substructure of Ni-36.5at.%Al alloy were investigated with the addition of vanadium which is known as scavenging element of grain boundary. The fracture surfaces were examined by scanning electron microscopy and the EDX spectrometer was applied for composition analysis of fracture surfaces. The substructure of martensite was studied by transmission electron microscopy. By addition of vanadium, fracture surfaces show mixed modes of intergranular and transgranular fracture and more Al content is found on the grain boundaries. For Ni-36.5at.%Al alloy, the planar faults observed in the martensite plates are the internal twins. By increasing the vanadium content, the modulated structure with stacking faults and dislocations dominates while the twinned martensite disappears. The stacking fault is determined to be extrinsic due to the substitution of V for Al. It is concluded that the segregation of sulfur on the high-energy state stacking fault area suppresses the intergranular fracture.

• Proceedings of the Korean Nuclear Society Conference
• /
• 1998.05b
• /
• pp.84-89
• /
• 1998

Alloy 600 및 alloy 690과 Ni-8Cr-lOFe 합금 등의 응력부식(stress corrosion cracking, SCC) 거동을 고온의 염기성 분위기에서 C-ring 시편을 사용하여 연구하였다. Alloy 600과 alloy 690을 여러 조건에서 열처리하여 etching한 후 탄화물의 분포와 입계 주변의 Cr고갈 정도 등의 미세조직을 광학현미경과 주사 전자현미경(SEM)으로 관찰하였다. 이들 재료에 대한 SCC 시험을 315$^{\circ}C$의 40% NaOH 수용액에서 일정한 부하전위(부식전위 + 200㎷)를 가하면서 수행하였으며, 동일 조건에서의 분극거동도 측정하였다. Alloy 600 MA(mill anneal) 및 TT(thermal treatment)의 SCC 저항성은 alloy 690 TT와 Ni-8Cr-10Fe SA(solution anneal)보다 낮았다. Alloy 600 TT 재료는 alloy 600 MA 및 SA 재료에 비해 SCC 저항성이 더 컸다. 고용 탄소농도는 alloy 600의 SCC 저항성에 큰 영향을 주지 못했다. 대부분의 Alloy 600은 균열전파 입계균열을 보였으나, 일부에서는 입계 및 입내 혼합양상(mixed mode cracking)을 보였다. 염기성 분위기에서 Ni기 합금의 SCC 거동을 미세조직, 분극거동의 관점에서 고찰하였다.

• Proceedings of the Materials Research Society of Korea Conference
• /
• 2003.11a
• /
• pp.25-25
• /
• 2003

안정화 지르코니아(Stabilized Zirconia)는 산화물 연료전지 (SOFC: Solid Oxide Fuel Cell), 전기화학식 가스센서 등에 널리 사용되고 있는 대표적인 고체전해질이다. SOFC의 효율향상 및 센서의 저온 동작을 위해서는 높은 이온전도도를 갖는 고체전해질이 요구된다. 안정화 지르코니아의 이온 전도도를 향상시키기 위해 MgO, CaO, $Y_2$O$_3$, Yb$_2$O$_3$, Sc$_2$O$_3$ 등의 안정화제가 첨가된 바 있으며, 이들 첨가제의 변화에 의한 전기전도도 향상 연구는 현재 성숙된 단계이다. 지르코니아 고체전해질의 안정화제가 정해진 상태에서 재료의 전기전도도를 향상시키는 다른 방법은 입계에서의 이온전도도를 높이는 것이다. 안정화 지르코니아는 입계가 입내에 비해 저항이 약 100-10000배 정도크기 때문에, 입계가 얇은 두께에도 불구하고 전해질의 저항에서 큰 역할을 한다고 알려져 있다. 일반적으로 입계의 Si-포함상 편석, 입계액상, 공간전하층등이 입계의 저항에 대한 원인으로 받아 들여 지고 있다.