等通道轉角擠型製程對鎂-5錫-1鈣合金抗潛變性質的影響

Abstract

本實驗係以重力澆鑄法製成Mg-5wt.%Sn-1wt.%Ca之鎂合金鑄錠，其平均晶粒大小約為150μm，其主要析出相為Mg2Sn和CaMgSn。將實驗合金於250℃下經ECAE製程後，以OM觀察其顯微組織，並以Instron 8501萬能試驗機測試其高溫潛變性質。 研究結果顯示，經過不同道次ECAE製程的實驗合金，其應力指數n隨著擠型次數的增加有下降的趨勢，從鑄態的n=7.7到擠型四道次的n=5.68，這意味著經ECAE製程後，實驗合金的潛變主導機制從power-law break down的區域漸漸移向晶界滑移區(Grain Boundary Sliding)。 經由ECAE後活化能從鑄態的148.2KJ/mol下降到N=4的95.8KJ/mol，意味著材料擴散機制隨著擠製道次的增加，從差排的滑移轉變為晶界擴散。 抗潛變性質方面，分別在N=1和N=4有最好和最差的抗潛變性質，以溫度200℃，施加應力為350MPa的情況來說，N=1的潛變速率為5.4E-4，遠低於N=4的1.24E-3，這是由於潛變機制朝向晶界滑移，使得具有較小晶粒，更多晶界的N=4合金自然具有較N=1合金差的抗潛變性質。

In this research, the effect of equal channel angular extrusion (ECAE) on the impression creep behavior of magnesium alloy (Mg-5wt.%Sn-1wt.%Ca) was investigated. Energy dispersive system (EDS) was used for chemical composition analysis. The precipitation phases confirmed by X-ray diffraction pattern (XRD) were Mg2Sn and CaMgSn. The microstructures were examined by optical microscopy (OM). Instron 8501 was used for impression creep test. The stress exponent decreases from n=7.7 for the as-cast sample to n=5.68 for the sample after four passes of ECAE process (N=4). It suggests that the dominant creep mechanism of alloy changes from power-law breakdown region to grain boundary sliding region. The creep activation decreases from 148.2KJ/mol for the as-cast sample to 95.8KJ/mol for N=4 sample. It suggests that the dominant diffusion mechanism of alloy changes from dislocation slip to grain boundary diffusion. When the applied stress is 350MPa at 200℃, N=1 sample has the best creep resistance. Its creep rate is 5.4E-4. Under the same test condition , the N=4 sample has the worst creep resistance. Its creep rate is 1.24E-3. It's because of the change of dominant creep mechanism. The N=4 sample has the smallest gains, which means the most grain boundaries, so its creep resistance is worse than the others.