Papers by Author: Choong Nyun Paul Kim

Abstract: Microfracture mechanisms of Zr-based bulk metallic glass (BMG) alloy containing ductile crystalline particles were investigated by directly observing microfracture processes using an in situ loading stage. Strength of the BMG alloy containing crystalline particles was lower than that of the monolithic BMG alloy, while ductility was higher. According to the direct microfracture observation, crystalline particles initiated shear bands, 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, and could be explained by mechanisms of blocking of crack or shear band propagation, formation of multiple shear bands, crack blunting, and shear band branching.

Abstract: The formability of several Zr-based bulk metallic glasses in the supercooled liquid region has been estimated. Using the data obtained from compression tests, normalized processing maps based on a dynamic materials model (DMM) have been constructed to evaluate feasible forming conditions. Laboratory-scale hot extrusion of the Zr44Ti11Cu9.8Ni10.2Be25 BMG has also been carried out to clarify the effectiveness of the normalized processing maps established in this study. The influence of thermal properties and microstructural differences on the formability of BMGs is interpreted in terms of a normalized temperature within the supercooled liquid region.

Abstract: Solidification behavior of two Fe-base amorphous alloys during twin-roll strip casting has been investigated in the present study. Continuous cooling transformation (CCT) diagrams of both alloys were calculated using the heterogeneous nucleation theory coupled with thermal data obtained during cooling. Cooling sequence during twin-roll strip casting was also simulated. These were used for the determination of the optimum twin-roll strip casting conditions at which desirable solidification behavior could be achieved. It shows that twin-roll strip casting can continuously fabricate sheet product of Fe-base bulk amorphous alloys, despite their higher liquidus temperatures and larger differences between liquidus and glass transition temperatures (Tg) than those of other easier glass-forming bulk amorphous alloys.