| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系 === 88 === The development of Al-Li alloys spans more than 75 years. Owing to the advantages of having low density and high strength, Al-Li alloys have been applied as the important structural materials of airplanes. During the application, failure of Al-Li alloys may occur under vibration. This study aimed to realize the features of tensile and resonant vibration fracture of Al-3.8Cu-0.8Li-0.26Mg alloy.
The experimental results indicate that the life under resonant vibration decrease with increasing aging time, it trends to the following sequence of solution treatment, under-aging, peak-aging and over-aging. During resonant vibration, the cracks of solution-treated specimens propagate along slip bands. For under-aged specimens, the cracks propagate not only along slip bands but also along grain boundaries and through grains. Moreover, the cracking modes of peak-aged and over-aged specimens are similar and mainly transgranular with some portion of intergranular.
According to the tensile test results of two different initial strain rates (8’10-4 s-1 and 2.1’10-2 s-1), the fracture feature shows no obvious difference. The fracture surfaces of solution-treated specimen display dimple fracture. After aging treatment, the fracture surfaces transform to partially transgranular and partially intergranular. The results also indicate that room temperature tensile deformation is influenced by dynamic strain aging.
TEM observation shows a lot of grain boundary precipitate and precipitate free zone after aging, which may induce intergranular fracture during the tensile and resonant vibration test. Beside, the tensile strength and elongation of have no obviously correlation with resonant vibration fatigue life.
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