| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 97 === Magnesium alloys have many advantages such as low density, high specific strength, good mechanic damping properties and good radiation absorption of electromagnetic waves. They are commonly applied in traffic and electronic industries. Magnesium alloys with adding lithium element could make the density lower than the pure magnesium. This study focuses on tensile properties and ductile to brittle transition characteristics of the two-phase Mg-10Li-2Al-1Zn alloy, which is consist of HCP a-phase and BCC b-phase, at the temperature range of 248K ~ 523K (−25℃~250℃).
Tensile testing results show that the ultimate tensile stress and the yield stress decreased with increasing temperatures. The uniform elongation has a maximum value at 50℃. The total elongation is significantly increased at the temperature range of RT~50℃, and it further increases to 55% at 100℃. The ductile to brittle transition behavior is occurred at the temperature range of RT~50℃. Fracture sub-surfaces reveal that initial cracks occurred within a-phase and a/b-phase interfaces. At low temperatures, failures are resulted from cracks propagation and inter-connection rapidly. When the temperature raising upon 50℃, cracks grow into pores by tensile stress and inter-connection result in failures. Fracture surfaces coexist ductile and brittle features at lower temperatures, the cleavage failures are observed in a-phase and a lot of ductile fracture features are represented in b-phase. There is no cleavage fracture feature and materials show almost ductile fracture features when the temperature raising upon 100℃.
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