Influence of Ca Content on Ultra-High Cycle Fatigue Fracture Behavior of AMCa Magnesium Alloy
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摘要: 为了阐明Ca含量对镁合金疲劳性能的影响，采用旋转弯曲疲劳试验机对两种AMCa镁合金进行超高周疲劳实验，并利用扫描电子显微镜SEM （scanning electron microscope）和X射线能谱仪EDS （X-ray energy dispersive spectroscopy）观察疲劳试样的断口形貌，分析了两种镁合金疲劳S-N（疲劳应力-疲劳寿命）曲线特性和疲劳断裂行为，讨论了Ca元素含量增加对镁合金疲劳寿命和疲劳裂纹萌生机制的影响. 结果表明，AM1.77 Ca镁合金S-N曲线没有传统的疲劳极限，呈现曲线连续下降趋势；AM1.85 Ca镁合金具有双S-N曲线特性，在130 MPa左右出现转折点；Ca元素含量增加导致镁合金产生微观结构缺陷，使材料的疲劳裂纹萌生模式从AM1.77 Ca镁合金的表面萌生模式转变为AM1.85 Ca镁合金的两种疲劳裂纹萌生模式，即表面萌生和次表面萌生模式，这种转变对材料抗疲劳性能的提升不利.Abstract: In order to investigate the effect of Ca content on the fatigue properties of magnesium alloys, the ultra-high cycle fatigue tests were carried out for two AMCa magnesium alloys using a rotating bending fatigue machine. The fracture morphologies of the fatigue specimens were observed by using scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS). Based on the experimental results, the fatigue S-N (fatigue strength-fatigue life) curves features and the fatigue fracture behavior were analyzed. Then, the influence of the Ca content on the fatigue life and fatigue crack initiation mechanism was discussed. The results indicate that the S-N curve of the AM1.77 Ca magnesium alloy shows a trend of continuous decline without a traditional fatigue limit, and the AM1.85 Ca magnesium alloy shows double S-N curve characteristics with a turning point around 130 MPa. The fatigue crack initiation mode is the surface initiation mode for the AM1.77 Ca magnesium alloy. However, the AM1.85 Ca magnesium alloy has two types of fatigue crack initiation modes, namely, the surface initiation mode and sub-surface initiation mode, which is due to the increase in the Ca content. The change that AM1.85 Ca magnesium alloy has two types of fatigue crack initiation modes is not conducive to improve the anti-fatigue performance.
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图 3 AM1.77 Ca镁合金的疲劳断口形貌（σa = 230 MPa）
Figure 3. Fatigue fracture morphologies of AM1.77 Ca magnesium alloy （σa = 230 MPa）
图 4 AM1.77 Ca镁合金的疲劳断口形貌（σa = 110 MPa）
Figure 4. Fatigue fracture morphologies of AM1.77 Ca magnesium alloy （σa = 110 MPa）
图 5 AM1.85 Ca镁合金的疲劳断口形貌（σa=180 MPa）
Figure 5. Fatigue fracture morphologies of AM1.85 Ca magnesium alloy （σa=180 MPa）
图 6 AM1.85 Ca镁合金的疲劳断口形貌（σa=110 MPa）
Figure 6. Fatigue fracture morphologies of AM1.85 Ca magnesium alloy （σa=110 MPa）
图 7 疲劳裂纹萌生区EDS分析（σa=120 MPa）
Figure 7. EDS analysis of fatigue crack initiation site （σa=120 MPa）
图 8 疲劳裂纹萌生区EDS分析（σa=130 MPa）
Figure 8. EDS analysis of fatigue crack initiation site （σa=130 MPa）
表 1 实验材料的化学成分
Table 1. Chemical composition of experimental material
材料 Al Zn Mn Si Cu Fe Ni Ca Mg AM1.77 Ca 6.790 0.003 0.25 0.04 0.002 0.006 <0.001 1.770 余量 AM1.85 Ca 5.950 0.021 0.21 0.02 0.001 0.001 <0.001 1.850 余量 
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表 2 实验材料的力学性能参数
Table 2. Mechanical properties of experimental material
材料 抗拉强度/MPa 屈服强度/MPa 伸长率/% 硬度/HV AM1.77 Ca 299 215 12 60.5 AM1.85 Ca 308 250 10 75.2
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