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미세조직학 및 기계적인 안정화를 이용한 비정질 복합재료의 인장연신율의 향상

미세조직학 및 기계적인 안정화를 이용한 비정질 복합재료의 인장연신율의 향상
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Bulk amorphous alloys, which exhibit very high glass forming abilities (GFAs), have been the subjects of extensive R&D due to their attractive mechanical and physical properties (high hardness, strength and, corrosion resistance), which are quite desirable for high performance structural applications. Despite such remarkable properties of bulk amorphous alloys, however, only a few practical applications have been realized mainly due to their lack of tensile ductility at ambient temperatures. In this report, a large improvement in tensile ductility can be achieved by the introduction of crystalline phase during solidification of Ti-base bulk amorphous alloys. Microstructure of the alloys consists of continuously networked amorphous phase with b-Ti existing as isolated particles, resembling the typical liquid-phase sintered ceramic materials. These alloys have high yield strength (~ 1.3 GPa) and large elongation (~ 9.5 %). It has been shown that the deformation behavior and the resultant tensile stress-strain behavior of the alloys depend strongly on the morphology (volume fraction and size) and properties of constituent phases. When the amorphous composite alloy has ductile and stable b phase (Ti12), the plastic deformation is initiated at b phase, and the alloy shows little work hardening with necking occurred just after yielding. On the other hand, when the amorphous composite alloys has meta-stable b phase (Ti22), the plastic deformation starts at the amorphous phase, and the alloy exhibits an extensive work hardening without the occurrence of necking. From a scanning electron microscope (SEM) observation of deformed specimens, Ti12 has ductile dendrites deformed by slip. Shear bands are deformed at amorphous phases, and these are connected with slip bands of dendrites. In the case of Ti22, however, shear bands formation of amorphous phases occurs without any apparent deformation of dendrites. Deformed twins are observed in the dendrites rather than slip bands As the deformation continues Electron back-scatter diffraction (EBSD) analysis has been conducted to study why the soft and ductile dendrites of Ti22 are deformed prior to amorphous phases which are harder elastically. The beta dendrites are transformed to alpha' phases during elastic deformation, i.e. stress-induced phase transformation. The amorphous phases which already received a large amount of elastic deformation eventually reach the elastic limit and plastically deforms by the initiation of shear bands. On the other hand, the newly formed stress-free alpha' phases hardly deform at this stage. As the deformation continues, the phase-transformed alpha' phases are deformed by twins plastically. The plot of strain hardening rate shows that strain hardening rate increases rapidly at the initial stage of deformation. It indicates that deformation twinning in hcp martensite results in the strain hardening of the present BMG composite.
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