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Ni-Cr-W-Al-Ti 계 초내열합금 (KM 1557) 의 크리프특성
최주,허성강,백홍구,도정만,정호년 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.7
The creep-rupture properties of a Ni-Cr-W-Al-Ti superalloy(KM 1557) have been investigated in the temperature range of 750℃ and 1000℃. In the stress and temperature ranges studied, the inverse of the rupture life(strain rate) exhibited power law dependence on the applied stress. The stress exponents for this alloy were between 5 and 12, and the activation energy for the creep was determined to be approximately 115 ㎉/mole. Precipitate free zones formed by grain boundary migration or sliding and the accommodation of sliding by the dislocation motion, were observed from the creep-ruptured specimens. Cracks were nucleated at the grain boundary or triple point, and propagated along the grain boundary containing PFZs. The shape of grains remained unchanged for the samples tested at low temperatures of 750 and 800℃. On the contrary, highly elongated grain shape was observed along the direction of applied stress when tested at high temperatures of 850, 900, 950, and 1,000℃.
크리프 변형이 Ni-Cr-W-Al-Ti 계 초내열합금 KM 1557 의 미세조직에 미치는 효과
최주,허성강,백홍구,도정만,유경근 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.3
Creep induced instability of hardening precipitates at the grain boundaries occurs at high temperatures. This paper discusses the effect of tensile creep stress on the microstructural instability of a Ni-Cr-W-Al-Ti superalloy(KM 1557). The tensile creep stress results in the formation of the precipitate free zones(PFZs) of α-W and γ′ near grain boundaries, and PFZs are generally found near grain boundaries inclined to the tensile-stress axis or triple points. These PFZs are not formed at secondary stage, but observed at tertiary stage and in the creep-ruptured specimens. In the TEM observations, it is found that the cutting of γ′ particles by dislocations starts at the secondary stage, and slip lines and bands are formed in the tertiary stage. Many PFZs are observed from the region very close to the fracture surface where severe deformation took place. On the other hand, gage section near the grip of creep specimen in which lightly deformed did not contain PFZs. The area fraction of PFZs is directly proportional to the reduction of area in the creep specimens. These experimental results support that the formation of PFZs is dependent on the plastic deformation.