13th International Conference on Fracture June 16–21, 2013, Beijing, China Effect of yttrium content on the ultra-high cycle fatigue behavior of Mg-Zn-Y-Zr alloys D.K. Xu1*, E.H. Han1 1 State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China * Corresponding author: dkxu@imr.ac.cn Abstract In the super-long life regime, the fatigue behavior of as-extruded Mg-6wt%Zn-xY-0.8wt%Zr Mg alloys with Y content of 0, 1, 2 and 3 wt% have been investigated, respectively. The result indicates that for all measured S-N curves, a plateau exists in the regime of 5×106-108 cyc, and then the fatigue strength gradually decreases between 108 and 109 cyc. Therefore, only fatigue strength corresponding to 109 cyc can be determined. Compared with other alloys, the alloy with Y content of 2 wt% has the highest fatigue strength and its value is 105 MPa. SEM observations to fracture surfaces reveal that for all alloys, the fatigue crack mostly initiates at the surface or subsurface of samples failed within 106-109 cyc. Further observation indicates that the crack initiation is related with activated slip bands instead of phase particles and activated twins. Based on the measured results and Murakami equation, it demonstrates that the fatigue strength of alloys is more dependent on the hardness values. Keywords Mg alloy, super-long fatigue, slip band, fatigue properties 1. Introduction Magnesium alloys are currently used in cars for low stress applications such as covers and less frequently for the mechanically loaded structural components such as wheels, transmission housings and pedals [1]. Generally, car wheels need to be stressed at different amplitudes for several 108 cycles in service. For casting alloys, defects such as casting porosity and cavities are usually present and the fatigue properties are affected significantly by their shape and dimension [1-2]. Several material defects such as casting porosity, oxidation films and intermetallic inclusions, can act as crack initiation sites and reduce material’s fatigue strength in the super-long fatigue life regime [1-2]. Therefore, the fatigue strength of as-cast Mg alloys corresponding to 109 cycles is generally about 40-50 MPa [1]. In contrast, wrought alloys are basically defect-free and have superior mechanical properties, thus the evaluation of their fatigue properties is of great interest for understanding the intrinsic fatigue mechanism of Mg alloys [3]. In early reported work, researchers mainly focused on the fatigue behavior of wrought Mg alloys with the fatigue lifetime less than 107 cyc [2, 4, 5]. However, as for the ultra-high cycle (107-109 cyc) fatigue behavior of wrought Mg alloys, only a few research papers can be referred [6-8]. Additionally, when compared with other system Mg alloys, Mg-Zn-Y-Zr alloys are much stronger [9-10], and their tensile strength can reach up to 380 MPa. Previous work demonstrated that Y content can remarkably influence the microstructure and tensile properties of Mg-Zn-Y-Zr system alloys [9-12]. Therefore, it can be predicted that the change of Y content should have some effect on the fatigue behaviour of Mg-Zn-Y-Zr system alloys. However, so far, no related literature can be referred. The aim of this work is to disclose the crack initiation mechanism and to establish the relationship between Y content and the fatigue strength in the gigacycle regime by investigating the fatigue behavior of Mg-5.65%Zn-xY-0.8%Zr alloys with Y content of 0, 1, 2 and 3 wt%, respectively. 2. Experimental procedure The materials used in this study were the as-extruded Mg-Zn-Y-Zr alloys with different Y contents, which were prepared by special technology in magnesium alloy research department of -1-
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