Study of Precipitation Hardening and Superplasticity of AZ80 Magnesium Alloy

• Main Authors: Wei-Jen Lai, 賴威任
• Other Authors: Wen-Hsiung Wang
• Format: Others
• Language: en_US
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/23042499467093135086

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 96 === This study mainly focuses on precipitation hardening behavior and superplasticity of AZ80 magnesium alloy. The precipitation hardening experiment was conducted between 125-300℃. The influence of the precipitate on mechanical properties was measured by microhardness test and tensile test. The precipitate was investigated in detail by XRD, OM, SEM and TEM. The superplastic experiment focuses on three different rolling directions. The temperature ranges from 200-400℃ and the strain rates were 3×10-3、1×10-3 and 3×10-4(s-1). The results were analyzed by OM and SEM to further understand the influence of temperature and strain rate on microstructural evolution. The result of the precipitation hardening experiment indicates that each aging temperature between 150-300℃ shows hardening effect. The highest hardness is occurred at 175℃/256 h and the hardness increment is about 38%. The result shows that the hardness increment is low compared with precipitation-hardenable aluminum alloys. This is influenced by the nature of Mg17Al12 precipitates. The precipitates can be divided into continuous and discontinuous precipitates by their forming mechanism. They can be further divided into lamellar, elliptical, intergranular, Widmanstätten structure and irregular slab by their morphologies. Because the precipitate can not effectively resist dislocation moves, the hardening effect is poor. At different aging temperature the morphology, the size, and the distribution density of Mg17Al12 precipitates are also different. These reasons then lead to different hardness. The result of superplastic experiment shows that the material has best elongation (about 350%) at 300℃/3×10-4s-1. The rapid elongation increase at this temperature has two reasons: one is because 300℃ is close to the recrystallization temperature of the material, dynamic recrystallization then occurs and provides enough grains for grain boundary sliding which produces large deformation; the other reason is because Mg17Al12 precipitates rapidly at 300℃ and most of them are formed on grain boundaries, they can effectively impede grain growth to remain the fine grain structure. These two effects appear at the same time and contribute to high elongation.