Effect of Equal Channel Angular Extrusion on the Impression Creep Behavior of Mg Alloy (ZA85)

• Main Authors: Wu, Jeng-Lin, 吳政霖
• Other Authors: Liu, Tzeng-Feng
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/53496027972488787991

碩士 === 國立交通大學 === 材料科學與工程學系 === 99 === 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℃.