| Summary: | 碩士 === 國立臺灣科技大學 === 工程技術研究所 === 99 === Abstract
The primary objectives of this study are to investigate the effects of surface modifications (such as coatings and homogenous scratches) on bending properties of the bulk metallic glass substrate (BMG).
In previous study, the Zr-based BMG substrate was coated with 200 nm thickness Zr-based thin film and a few nanometer thick Ti buffer layer, and its maximum four-point bending stress was improved noticeably from ~2.2 to ~3.9 GPa, an increase of ~77%. The surface energy and surface strain are also significantly improved ~10 folds and to 12~14%, respectively.
As a result, the occurrence of the shear bands on the tension surface can create surface offsets, which serve as potential crack-initiation sites. Subsequently, more primary, secondary and tertiary shear bands are formed to accommodate large bending strains. The shear band spacing is ~50 μm for the MGTF/Ti-coated BMG substrate. The high ductility and strength of MGTF can prevent the shear band formations which can enhance its bending property. The good ductility of the MGTF can be attributed to the multiplication of the shear bands in the substrate, which prevents a catastrophic failure by primary shear band formations and increases the ductility of the Zr-based BMG substrate.
In this study, Annealing below Tg for a short period of time does not change Zr-based BMG microstructure, while it only releases surface residual stress caused by the scratches. It may be the key factor of bending property improvement achieved. The Zr-based BMG substrate was scratched in two directions (angles of 60o and 120o between scratch lines and substrate long side) by #4000 SiC sand paper (fine scratches) and annealed below Tg. This sample obviously shows high bending strength after scratching and annealing.
The Zr-based BMG samples with scratches and annealing treatment have a maximum four-point bending strength increase noticeably from ~1.3 to ~1.9 GPa, an increase of ~46%. The strain energy and surface strain are also significantly improved ~2 folds and to 1.7%, respectively.
As a result, the #4000 scratches absorb energy homogenously and avoid fatal major shear bands to form easily in the substrate. The shear bands this formed follow the scratch marks and propagated homogenously on the tensile surface, yielding increases in the bending strength. The annealing temperature of metallic glass below Tg leads to transformation of its structure towards a more relaxed state. The annealing release the stress concentrated on scratches which may lead to an easily fatal crack formation. The improved bending properties due to the scratches and annealing can be attributed to the multiplication of the shear bands, which prevents a catastrophic failure by primary shear band formations.
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