발간논문

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Vol.36, No.9, 1408 ~ 1415, 1998
Title
The relationship between microstructure and initiation of creep and LCF crack in DS superalloy CM186LC
조창용 , 서성문 , 김두현 , 이재현 , 최승주 , Chang Yong Jo , Seong Moon Seo , Doo Hyun Kim , Je Hyun Lee , Seung Joo Choe , David Knowles
Abstract
The second generation DS alloy CM186LC has comparable creep-rupture life to first generation single crystal alloy CMSX-2 and CMSX-3 in the as-cast and double aged condition. In the assembly of nozzle guide vanes, DS components are joined into pairs by brazing. In order to study mechanical behavior and microstructural evolution of the material at the same condition as production, DS bars were subjected to brazing simulated treatments. Compared with standard treatment a small drop in creep-rupture life was observed but the degree of fall in life was not serious. Creep cracks initiated at the transverse components of the grain boundaries, which were normal to the applied stress direction. Every crack was associated with the γ-γ` eutectic phase at grain boundaries. γ-γ` eutectic was not only located at grain boundaries but also in the interdendritic regions. It also acted as crack initiation site during creep. Low cycle fatigue tests were conducted under fully reversed trapezoidal strain wave at a frequency of 0.25Hz. Relatively large total strain range showed multiple initiation sites. The depth of surface cracks was comparable to that of the secondary dendrite arms from the surface of the specimen. Cracks initiated at all of the interdendritic regions exposed at the surface, especially at the γ-γ` eutectic phase. LCF crack initiation and propagation is therefore proposed to be microstructally dependent.
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