발간논문

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Vol.37, No.3, 261 ~ 266, 1999
Title
Research Paper / Transformations : Sigma Morphology & Precipitation Mechanism in Udimet 720 Li
나영상Y . S . Na, 박노광N . K . Park
Abstract
The precipitation kinetics of the detrimental sigma phase in Udimet 720Li has been characterized and quantified by varying exposure temperatures between 725℃ and 775℃ and exposure period up to 5000 hours. The electrochetemiyal extraction and the X-ray diffraction combined with the Rietveld analysis have been employed for the quantification of sigma precipitation. The sigma formation rate is increased and the incubation time for sigma nucleation is decreased with increasing temperatures. Sigma formation behavior has been analyzed by comparing the activation energy for sigma formation with the activation energy for the diffusion of sigma forming elements such as Cr, Co, Mo. Sigma precipitation is controlled by the diffusion of Cr segregated in the γ matrix adjacent to the primary γ′ in the early stage. and then controlled by the diffusion of Mo in the steady state. Sigma morphology was also investigated by observing the extracted residue. As the aging time increases, sigma morphology is changed from the globular shape to the dendritic. From the analysis mentioned above. a model of sigma precipitation behavior was schematically suggested.
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