ZA85 鎂合金經等通道轉角擠型後抗潛變性質的研究

Abstract

本實驗係以重力澆鑄法製成ZA85 (Mg-8wt.%Zn-5wt.%Al) 鎂合金鑄錠，其平均晶粒大小約為150μm，而其主要析出相為τ-Mg32(Al,Zn)49，並無β-Mg17Al12相析出。將實驗合金經在180℃下經ECAE製程後，以OM、SEM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ZA85鎂合金，其應力指數n隨著擠型次數的增加有下降的趨勢，從as-cast的n=7.4到擠型四道的n=5.5，這意味著經ECAE製程後，ZA85鎂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的區域(Grain Boundary Sliding)，而N=4的高應力指數推測可能跟其晶粒大小的bi-modal形式有關係。 抗潛變性質方面，在N=1和N=4時，ZA85鎂合金分別有最好和最壞的抗潛變性質，以溫度180℃，施加應力為375MPa的情況來說，N=1的潛變速率為5.2E-6，遠低於N=4的8.51E-5，而再進一步擠型到N=6時，其潛速率則為7.4E-6，優於N=4的試片。 關於活化能方面，N=4的潛變活化能=62.1kJ/mol，可能為有析出相及無析出相兩區域的混合效應，而N=6的潛變活化能=144.8kJ/mol，高於鎂自體擴散的活化能，此高活化能為180℃相對低溫時N=6試片抗潛變性質較佳的原因。

In this research, the effect of equal channel angular extrusion (ECAE) on the impression creep behavior of magnesium alloy ZA85 (Mg-8wt.%Zn-5wt.%Al) was investigated. Energy dispersive system (EDS) was used for chemical composition analysis. The second phase of the as-cast ZA85 alloy confirmed by x-ray diffraction pattern (XRD) was mainly τ-phase (Mg32(Al,Zn)49), instead of β-phase (Mg17Al12). The microstructures were examined by optical microscopy (OM) and scanning electron microscopy (SEM). Instron 8501 was used for impression creep test. The stress exponent decreases from n=7.4 for the as-cast ZA85 sample to n=5.5 for the sample after four passes of ECAE process (N=4). It suggests that the dominant creep mechanism of ZA85 alloy change from power-law break down region to grain boundary sliding region, and the high stress exponent of N=4 ZA85 sample may be related to its bi-modal distribution of grain size. When the applied stress is 375MPa at 180℃, the N=1 ZA85 sample has the best creep resistance. Its creep rate is 5.2E-6. Under the same test condition, the N=4 ZA85 sample has the worst creep resistance. Its creep rate is 8.51E-5. However, under the same test condition, the creep rate of the N=6 ZA85 sample is 7.4E-6, which is better than N=4 ZA85 sample. The creep activation energy of N=4 ZA85 sample is 62.1kJ/mol, and it may be contributed to both the regions with and without precipitation. The creep activation energy of N=6 ZA85 sample is 144kJ/mol, which is slightly higher than the self-diffusion energy of pure magnesium. The high activation energy results in a better creep resistance of the N=6 ZA85 sample at lower temperature such as 180℃.