| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 92 === The Al-Zn alloy has been considered as a high damping material. For practical use, this study investigated the effects of Zn content and precipitation behavior on the vibration fracture characteristics of the Al-Zn alloy under resonance. Al-Zn alloys with different Zn contents were prepared and designated according to their Zn content. To achieve varying degree of precipitation, all the samples were annealed and cooled with various cooling rates to obtain the water-quenched (WQ), air-cooled (AC) and furnace-aged (FC) specimens.
Experimental results show that the damping capacity of the WQ samples with different Zn contents decreases in turn from 83Zn, 7Zn, 11Zn and to 49Zn. Under constant force conditions, although the 83Zn exhibits superior damping capacity, the 7Zn shows the greatest vibration life. In addition, the 49Zn sample has inferior vibration fracture resistance. As for constant initial deflection conditions, the vibration life in the decreasing order is 7Zn, 11Zn, 49Zn and then 83Zn. Differences in vibration properties of the samples with various Zn contents can be attributed to their microstructural characteristics. When the Zn content is raised from 7wt% to 49wt%, the microstructure of Al-Zn alloys consists of alpha-Al grains and the precipitates on the matrix or the grain boundaries. The mechanism for absorbing vibration energy could be considered as dislocation slip. A higher Zn content may result in more precipitates within Al grains or on the grain boundaries, consequently, dislocation is more difficult to slip and the intergranular fracture occurs. The 83Zn specimen shows a typical eutectoid structure of alternate alpha-Al and beta-Zn layers. The phase boundary sliding between alpha and beta can significantly dissipate vibration energy. However, this eutectoid structure possesses poor crack propagation resistance and thus inferior vibration life.
The 7Zn and 83Zn samples are chosen to examine the effect of cooling rate on vibration properties. Results show that for both 7Zn and 83Zn, a higher cooling rate will lead to a greater damping capacity and the WQ specimens possesses longer vibration life under constant force conditions. As for constant initial deflection conditions, the WQ samples of 7Zn and the AC of 83Zn exhibit better vibration fracture resistance. The amount and distribution of the precipitates influenced by cooling rate may account for those phenomena.
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