| Summary: | 碩士 === 國立臺北科技大學 === 材料及資源工程系研究所 === 101 === This study investigates the addition of 0.5~2.0wt% of rare earth element Nd, in AZ71 magnesium alloy. Nd additions are found to suppress the formation of β-Mg17Al12 and refine the as-cast grain size. 1.0 wt.% addition of Nd appears to demonstrate most effective grain refinement effect. The grain size can be further reduced to below 30μm after rotary swaging and annealing. 32% and 42% swaging strains are employed to observe its effect on annealing and mechanical property. The start and finish recrystallization temperatures for 42% swaged samples are lower than those swaged by 32%. However, their grain sizes after recrystallization are similar, and their Vickers hardness do not vary much, either. Both tensile strength and ductility increase with Nd concentration up to 1.0 wt.%. Both solid solution and precipitation strengthening by forming AlxNdy compound give rise to the increase of strength. In addition, Nd increases c/a value, stacking fault energy, and critical resolved shearing stress of the magnesium alloys, which all lead to the increase of ductility. When over 1.0 wt.% of Nd is added, large amount of Al11Nd3 rod precipitates are formed in aggregation, which results in reduction of tensile strength and ductility. In the 1.0 wt.% Nd added AZ71, the precipitates are uniformly distributed, 253 MPa tensile strength and 10.7% elongation could be achieved, showing the optimum toughness combination.
High temperature tensile tests further confirm that Nd suppresses the forma -tion of β-Mg17Al12 eutectic phase formation, which is believed to be the prime cause for improved high temperature tensile strength. Precipitation of high temperature intermetallic compounds is the secondary cause. In summary, addition of 1.0 wt% Nd to AZ71 alloy generates the best toughness in either as-cast or annealed states tested at both room temperature and 170°C.
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