挤压态镁合金ZK60的超高周疲劳行为

ULTRA-HIGH CYCLE FATIGUE BEHAVIOR OF THE AS-EXTRUDED MAGNESIUM ALLOY ZK60

利用超声疲劳实验研究了挤压态镁合金ZK60的超高周疲劳行为．结果表明，合金的疲劳S—N曲线在5×10^6-10^8cyc范围内存在一平台，而在10^8-10^9cyc范围内，疲劳强度逐渐降低，对应于10^9cyc的疲劳强度为（90±5）MPa．SEM断口观察表明，在5×10^6-10^8cyc范围内，疲劳裂纹基本上萌生于试样表面或亚表面，而在10^8-10^9cyc范围内，疲劳裂纹主要萌生于试样内部的非金属夹杂物．通过测定疲劳源区的尺寸，估算的合金疲劳强度与实验结果基本一致．疲劳源的形成是由微裂纹在多个夹杂物处起裂和合并所引起的．因此，合金的疲劳强度不是由最大夹杂物尺寸决定，而是取决于由多个夹杂物组成的“缺陷区”尺寸．通过测定多个部位的“缺陷区”尺寸，可以有效的估算合金的疲劳强度．

The experiments of the fatigue behavior of the as-extruded Mg alloy ZK60 in a regime of 106-109 cyc revealed that a plateau exists in the regime of 5×10^6-10^8 cyc, whereas the fatigue strength gradually decreases in the regime of 10^8-10^9 cyc, and the fatigue strength corresponding to 10^9 cyc is （90±5） MPa. SEM observation indicated that the fatigue crack mostly initiated at the surface or subsurface for the samples failed in the regime of 5×10^6-10^8 cyc, and at the internal non-metallic inclusion for the samples failed between 10^8 and 10^9 cyc. Through determining the size of the fatigue crack initiation area, the fatigue strength of the alloy is estimated, which is basically consistent with the experimental results. Analysis of fractographs indicated that the formation of the fatigue crack initiation area is caused by the crack initiation at several inclusions and subsequent crack coalescence. Therefore, the fatigue strength seems not to be determined by the maximum inclusion, but by the ＂ defect area ＂ consisting of several inclusions. According to determination of the sizes of ＂ defect areas ＂ at different locations, the fatigue strength of the alloy can be effectively evaluated.