| Summary: | 碩士 === 國立中央大學 === 材料科學與工程研究所 === 103 === Due to the high specific ratios of strength to weight of 7075 aluminum alloy, it is widely used in light weight sport equipment, automobile bodies, and aircraft frames. However, the 7075 aluminum alloy has poor fatigue property in comparison to steel. In this study, different thicknesses (namely 200, 300, 400, and 500 nm) of Zr-based ((Zr53Cu30Ni9Al8)99.5Si0.5) and 200 nm Ti-based (Ti40Zr10Cu36Nb7Co7) metallic glass thin films (MGTF) were successfully coated on the 7075 aluminum alloy substrates by DC-sputtering method. Then the fatigue properties of these coated and as-polished samples were tested following the standard of ASTM-C1161-02c. The results revealed that the fatigue life and fatigue strength show an opposite trend with increasing the thickness of MGTF due to the changes of surface roughness and residual stress. The sample coated with 200-nm-thick Zr-based MGTF is found to have the best fatigue properties than others thickness-coated samples. Moreover, the fatigue life of 7075-T6 aluminum alloy can be improved about 26.3 and 7.9 times than the bare one at the high stress level of 250 MPa by coating with 200 nm-thick Zr-based and Ti-based MGTF. In parallel, the improvements of Zr-based and Ti-based MGTF-coated samples in fatigue strength were 235 MPa (56.7 %) and 205 MPa (43.3 %) compared with as-polished sample (150 MPa).
Based on the TEM examination, the formation of offsets and cracks from the surface of specimen was revealed to be effectively restricted by both MGTF coatings. This is attributed to the high strength, good flexibility, and strong adhesion of both MGTF coatings, which can provide effective compressive stress to suppress the slip band protruding. In summary, Zr-based MGTF coating is believed to be a promising coating material for improving 7075-T6 aluminum fatigue properties.
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