| Summary: | 碩士 === 國立臺灣科技大學 === 材料科學與工程系 === 102 === Bulk metallic glasses (BMGs) are normally fractured with very limited plastic strain at room temperature, severely restricting their application as engineering materials. Since deformation in BMGs is accommodated by shear bands and shear-offsets, it is important to promote homogenous formation and propagation of shear bands in order to obtain plastic strains.
In this study, various surface modifications were performed on the tensile surface of Zr-based BMG substrates to enhance their four-point bending ductility and fatigue properties. The surface modifications for ductility enhancement include artificial scratch, thin film metallic glass (TFMG) coating as well as femtosecond laser patterning. For the artificial scratch, the pre-existing shear bands are generated and they lead to shear-softening. The stress is concentrated in the scratches upon loading, which causes the multiple shear band formations and offsets. As a result, the scratched samples show at least 1.5 times higher plastic strain than that of as-polished samples. The TFMG coated samples also exhibit significantly enhanced bending plasticity (>2 times improved). A 500 nm-thick TFMG coating with high toughness is capable of promoting the formation of a high density of homogeneously distributed shear bands. Thus, abundant of shear band branches can prevent shear band localization so as to obtain increased plastic strain. In addition, the samples after being subjected to the femtosecond laser patterning show an increased plasticity (>1.8 times improved) which might be attributed to stress redistribution in the grooves and the laser-induced periodic surface structure produced by femtosecond laser.
The shear band spacing and offsets on BMG samples are also examined after bending test. It is found that a high density of shear bands with large shear band offset formed in surface-modified BMG samples can prevent localized shear banding and accommodate large plastic deformation under bending. The results demonstrate that the various surface modifications in this study become promising methods for bending ductility enhancement of large-scale BMG.
On the other hand, a 200 nm-thick TFMG was deposited on the BMG substrate to improve the four-point bending fatigue properties. The fatigue life of a coated BMG substrate is significantly improved by at least 25 times and the fatigue limit is increased by 33% compare with the bare one. The smooth surface, good adhesion and high toughness of film are found to play an important role in superior fatigue properties.
|
|---|
