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이정구 ( Jung G. Lee ),손기선 ( Kee Sun Sohn ),이성학 ( Sung Hak Lee ),김낙준 ( Nack J. Kim ),김충년 ( Choong Nyun P. Kim ) 대한금속재료학회 ( 구 대한금속학회 ) 2006 대한금속·재료학회지 Vol.44 No.5
The fracture property improvement of Zr-based bulk amorphous alloy containing ductile crystalline particles was explained by direct observation of microfracture processes using an in situ loading stage installed inside a scanning electron microscope (SEM) chamber. Strength of the amorphous alloy containing crystalline particles were lower than that of the monolithic amorphous alloy, while the ductility was higher. According to the in situ microfracture observation, crystalline particles initiated shear bands under low stress levels, acted as blocking sites of shear band or crack propagation, and provided the stable crack growth which could be confirmed by the R-curve analysis, although they negatively affected apparent fracture toughness. This increase in fracture resistance with increasing crack length improved overall fracture properties of the alloy containing crystalline particles. The improvement could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.
In-Situ SEM Loading Stage 를 사용한 파괴인성의 정량적 해석 연구
이성학,손기선 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.5
An in -situ observation of fracture behavior, together with direct measurement of fracture toughness, was adopted to verify microfracture mechanism of a high speed steel (HSS) roll material using a compact tension (CT) type loading stage inside a scanning electron microscope (SEM). The speciment used in the present study was a small and thin CT speciment which had a sharply machined notch and was grooved along its central region. The observation of fracture process indicated that the interfaces between hard primary carbide and matrix were separated first to form microcracks at low stress intensity factor levels. Then, the microoracks were easily propagated along the intercellular carbide networks, leading to relatively low fracture toughness. This in-situ SEM fracture technique was found useful for studying the microstructural aspects of fracture initiation and for correlating the microscopic fracture behavior with macroscopic fracture toughness of brittle materials.
이성학,김낙준,이희춘,이창길,손기선 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.3
The main objective of the present study is to clarify the microstructural factors such as tempered martensite matrix, coarse primary carbides, eutectic structures, and fine secondary carbides, which influence fracture properties of high speed steel (HSS) rolls. Three HSS rolls with different Cr content were manufactured by a centrifugal casting method, and the microstructures of their shell regions were investigated quantitatively. MC type primary carbides, together with eutectic ledeburite structures, were observed in the intercellular regions. The observation of fracture processes indicated that these primary carbides cleaved first to form microcracks at very low stress levels. These microcracks were easily propagated along the intercellular networks, leading to predominantly cleavage mode of fracture on fracture surfaces. To improve microstructures for use in the HSS rolls, therefore, it is suggested that the cell size must be minimized to achieve the homogeneous distribution of primary carbides