Al-Mg-Si-Cu合金时效状态对疲劳过程及断裂特征的影响Effect of precipitation state on fatigue process and fracture characteristics of Al-Mg-Si-Cu alloy

采用疲劳测试、拉伸试验、金相显微电镜、扫描电镜和透射电镜技术研究不同时效处理状态的Al-Mg-  Si-Cu合金在循环载荷作用过程中力学性能的演变和疲劳断口特征。结果表明：过时效合金的疲劳寿命最长，峰值时效合金的疲劳寿命最短；欠时效和峰值时效合金的屈服强度在疲劳过程中出现下降，且欠时效合金表现得尤为显著，欠时效合金的伸长率随循环次数的增加而增加；过时效合金的强度在疲劳过程中得到提高，而伸长率在疲劳寿命后期出现大幅下降。疲劳裂纹在样品表面萌生后倾向于沿晶界增殖并形成裂纹源区，过时效合金在射线状裂纹增殖区与瞬断区之间还出现了沿晶扩展区。

Under cyclic loading the evolution rules of mechanical properties and the fractographs of the Al-Mg-Si-Cu alloy samples aged for different times were investigated by the fatigue test, tensile test, optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the fatigue life of the over-aged alloy is the longest, while that of the peak-aged alloy is the shortest. The yield strengths of the under-aged and peak-aged alloys decrease during the fatigue process, especially for the under-aged alloy, while the elongation of the under-aged alloy increases steadily with the increase of fatigue cycles. For the over-aged alloy, the strength increases during the fatigue process, whereas the elongation drops sharply in the late fatigue stage. The fatigue fracture analysis reveals that the cracks are inclined to grow along the grain boundary after initiating on the specimen’s surface and the crack-source-regions form. Interestingly, grain boundary separation regions were observed between the ray-like crack growth region and the over-loading region in the over-aged alloy.

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