摘要
本研究的目的在於探討鎂合金AZ91D在半固態時的流變行為。實驗中，利用可在高溫下量測黏度的同心圓柱流變計為量測設備。由實驗結果發現，在穩態實驗中，視黏度會隨著攪拌時間的增長而漸漸變小，並在達到一個最小值後，又會隨著攪拌時間的增長而漸漸變大。而在等溫停留時間實驗中，不同的初始攪拌時間會對接下來的停留時間的視黏度有顯著的影響。在較短的初始攪拌時間時，視黏度會隨著停留時間的延長而漸漸變大；相反的，當給予較長的初始攪拌時間時，視黏度反而會隨著停留時間的延長而漸漸變小。
在暫態實驗中，當剪應變率驟升或驟降時，剪應力也會隨著驟升或驟降，且破裂時間會比回復時間短。而當固相分率比較大時，所對應的剪應力值也會變大，且破裂時間也會變長。且當末剪應變率驟升的幅度比較大時，其所相對應的剪應力值驟升的幅度也會隨著變大，且破裂時間將會縮短；相反的當末剪應變率驟升的幅度比較小時，其所相對應的剪應力值驟升的幅度也就比較小，且破裂時間將會拉長。

Abstract
In this study, the rheological behavior of the semi-solid AZ91D magnesium alloy is researched by a high-temperature Couette type viscometer. The results show that the apparent viscosity of semi-solid AZ91D alloy decrease initially as the isothermal stirring time increase and achieve a minimum value and start increasing in steady state. And the different time of stirring before resting also affects striking on the apparent viscosity. The apparent viscosity was gradually increased as the resting time increased when the shorter stirring time. But the apparent viscosity gradually decreased as the resting time increased when the melt was isothermally stirred for the longer stirring time. And the solid fraction also has greatly influence the apparent viscosity. The apparent viscosity increases with increasing resting time as the higher solid fraction. And the apparent viscosity decreases with increasing resting time as the lower solid fraction. But it has not greatly influence the apparent viscosity when the very low solid fraction.
At transient experimentation, the breakdown time is then described for a shear-rate jump from a lower shear rate to a higher shear rate and the recovery time is described for a shear rate drop from a higher to a lower shear rate. In this study, the recovery time is found to be longer than the breakdown time and the breakdown time is found to be longer at the higher solid fraction. The breakdown time is also found to be shorter as the range of the final shear rate jumps is big.

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