| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 90 === The effects of Zn content and microstructure on the mechanical property, damping capacity, and resonant vibration fracture of Al-Zn alloys were investigated.
The tensile tests for the as-cast and extruded specimens were performed under initial strain rates 8´10-4s-1, and the tensile temperature was R.T. Experimental results indicated that the flow stress of as-cast and extruded specimens had a positive proportion to Zn content, and the flow stress of extruded specimens were higher than as-cast specimens which were the same Zn content. The total elongation and uniform elongation of as-cast specimens had a negative proportion to Zn content. In particular, Al-11wt%Zn alloy of extruded specimens had maximum total elongation and uniform elongation. Most of the tensile fracture surface of all specimens displayed typical dimple patterns, but it was a brittle inter-granular fracture in Al-16wt%Zn of extruded specimens.
Measurements of the logarithmic decrement d for the as-cast and extruded specimens were increased with increasing Zn content.
The deflection amplitude in resonance can be classified into three stages as a function of vibration cycle. In stage I, the deflection amplitude increases with the number of vibration cycles. The hardness of specimen also increases with vibration cycle during this stage. The test specimens possess a distinct plateau stage of maximum deflection amplitude in second stage. In stage III, the frequency of specimen diverges from the resonance condition. The deflection amplitude decreases with increasing vibration cycles. Experimental results indicate the D-N curves can be classified into three types. For A type, the curve is formed by stage II and stage III. For B type, the curve is formed by stage I、stage II and stage III.
For C type, the curve is formed by stage I and stage III.
In the observation of crack paths, for the as-cast specimens, the crack mainly propagates through grains. The proportion of crack which is propagates through grain boundary in extruded specimens was increases with increasing Zn content. In current study, the vibration resistance of Al-Zn alloy is mainly influenced by crack propagation resistance. The resonant vibration life of Al-16wt%Zn of extruded specimens was best.
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