| Summary: | 碩士 === 國立臺灣科技大學 === 材料科學與工程系 === 100 === In recent years, metallic glasses or amorphous metals are available not only in bulk, powder, wire, and ribbon but also thin film. Thin film metallic glasses (TFMGs) possess many excellent and unique properties, including high strength, good ductility, excellent wear and corrosion resistance. Like ordinary metallic glasses, the TFMGs in general undergo the glass transition and crystallization at temperatures of Tg and Tx, respectively, upon heating.
In this study, Cu- and Zr-based TFMGs with different thicknesses were deposited on Si substrates. Nanoindentation tests were performed to evaluate the film properties. Surface morphology and microstructure before and after annealing at a temperature between Tg and Tx (or so-called the supercooled liquid region, ΔT) were examined. It revealed that the size of indentation mark decreased after annealing. This study confirmed that TFMGs exhibit shape-recovery ability during annealing at temperatures within ΔT due mainly to the low viscosity flow. Interestingly, Cu-based TFMG exhibits higher recovery, in the range of 14-19% of the indentation depth, compared with only 7-9% for Zr-based TFMG. It may be attributed to the different compositions of the TFMGs exhibit different viscosities during annealing. The percentages of depth recovered after annealing in thicker samples were higher than in thinner samples due to the larger mass in the thicker films contributing to the recovery during annealing.
TFMG also can be used to improve ductility of bulk metallic glass (BMG). As widely known, metallic glasses in the bulk form or BMG are often fractured with very limited plastic strains at room temperature, accompanying with imhomogeneous shear band formation and propagation. In this study, various 200-nm-thick overlay coatings, including TFMG, were deposited onto Zr52.5Cu17.9Ni14.6Al10Ti5 BMG substrates to impede shear-band initiation and propagation. Rockwell indentations with different loadings were carried out on coated and uncoated samples to investigate the effects of coatings on the morphology of deformation zone around the indent on the BMG substrates. As a result, more homogeneous shear bands were found on the coated substrate. The number of radial shear band for uncoated substrate was 30 while 78 shear bands were found on the coated substrate. The increased ability of the shear band multiplication is due to the resistance of the coating to the shear band propagation at the interface between the BMG and the coating.
For comparison, thermal annealing below and above glass transition was also carried out in order to improve ductility of BMG. The result shows that annealing at a temperature below Tg for short period of time caused increasing of plasticity. On the other hand, embrittlement occurred when the temperature was increased to near Tx presumably due to the significant decrease in free volume.
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