Competition between corrosion attack and twinning on the corrosion fatigue crack initiation of magnesium alloys

Significance 

Magnesium alloys have attracted significant attention in various applications. Among the available magnesium alloys, I-phase strengthened Mg-Zn-Y-Zr alloys can have better mechanical properties at both ambient and elevated temperatures. When exposed to real service environments, they will inevitably subject to the interaction between cyclic loading and corrosion attack, which may lead to the occurrence of corrosion fatigue. Therefore, understanding of the corrosion fatigue behavior and underneath failure mechanisms is highly desirable for ensuring the service reliability and durability of Mg-Zn-Y-Zr alloys.

Although solid solution treatment method has been employed to eliminate the effects of inhomogeneous microstructure caused by thermal-mechanical processing, its effect on the corrosion fatigue behavior of magnesium alloys remains a research area. This requires the deep understanding about the difference in the corrosion fatigue behavior between samples with and without twinning activation.

Collaborative research by Dr. B. J. Wang (Shenyang Ligong University), together with Professor Daokui Xu, Professor En-hou Han and Dr. Shidong Wang (Chinese Academy of Sciences), Professor L.Y. Sheng (Peking University), and Professor Rongchang Zeng (Shandong University of Science and Technology) investigated the corrosion fatigue behaviors of an as-forged Mg-6.7%Zn-1.3%Y-0.6%Zr alloy before and after solid solution treatment in 3.5 wt% NaCl solution. The main objective was to determine the mechanisms that dominate their corrosion fatigue crack initiations. Also, they investigated the effects of solid solution treatment on the corrosion fatigue strength of the alloy. Furthermore, the results obtained were used for evaluating the possible transition of fatigue crack initiation mechanisms. The research work is published in International Journal of Fatigue.

Based on the S-N curves, the authors observed that the fatigue strength of the as-forged samples corresponding to 5×106 cycles was 30MPa, whilst that of the fatigue strength of solution-treated samples was 50MPa. This was attributed to the initiation of the fatigue cracks at the localized corrosion sites on sample surfaces. However, for all the solution treatment samples, the crack initiation was dependent on the localized corrosion and twin boundaries cracking when they were cyclically loaded both at low and high-stress amplitudes. The localized regions, therefore, provided a favorable path for the ingress of hydrogen into the magnesium matrix.

It was also necessary to establish the relationship between the crack initiation mechanism and the hydrogen embrittlement. Compared with the fatigue samples tested in air, the fatigue lifetime tested in NaCl was significantly shorter at the same stress amplitude, which is probably due to the occurrence of the localized corrosion at the twin boundaries considering the difference between the twinned and untwined regions. It was worth to be noted that the alleviation of the corrosion attack can suppress the influence of the hydrogen embrittlement on the corrosion fatigue resistance of magnesium alloys.

In summary, the research team successfully disclosed the effect of solution treatment on the corrosion fatigue behavior of as-forged Mg-Zn-Y-Zr alloy. It was noted that the corrosion fatigue performance of magnesium alloys is dependent on both the corrosion attack and twinning activation. Therefore, through suppressing the activation of twins and weakening the corrosion attacks, it is an effective way for improving the corrosion fatigue performance of magnesium alloys.

Reference

Wang, B., Xu, D., Wang, S., Sheng, L., Zeng, R., & Han, E. (2019). Influence of solution treatment on the corrosion fatigue behavior of an as-forged Mg-Zn-Y-Zr alloy. International Journal of Fatigue, 120, 46-55.

Go To International Journal of Fatigue

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