Microfracture Test of Mg 12 ZnY Intermetallic Compound in Mg-Zn-Y Alloys
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Microfracture Test of Mg12ZnY Intermetallic Compound in Mg-Zn-Y Alloys Hajime Yoshimura, Shun Matsuyama, Mitsuhiro Matsuda, Masaaki Otsu, Kazuki Takashima, Yoshihito Kawamura Department of Materials Science and Engineering, Kumamoto University, 2-39-1, Kurokami, Kumamoto, Japan ABSTRACT A Mg-Zn-Y alloy including a Mg12ZnY intermetallic compound exhibits excellent mechanical properties as compared to conventional magnesium alloys. The superior mechanical properties of this alloy seem to originate from the Mg12ZnY intermetallic compound; however, the mechanical properties of Mg12ZnY itself have not yet been fully investigated owing to the small size of this compound. In this study, a microfracture test was performed to investigate the fracture properties of the Mg12ZnY intermetallic compound. The material used in this test was a Mg88Zn5Y7 alloy. Micro-sized cantilever specimens composed of Mg12ZnY, with dimensions of 10 × 20 × 50 μm3, were prepared selectively isolated from the Mg88Zn5Y7 alloy using focused ion beam (FIB) machining. Notches with a width of 0.5 μm and a depth of 5 μm were also introduced into the micro-sized specimens. Microfracture tests were performed using a mechanical testing machine for microscale materials. The fracture toughness values (KQ) of Mg12ZnY were 1.2−3.0 MPam1/2. TEM observations indicated that the KQ values were dependent on the crack orientation in Mg12ZnY, with the higher KQ values correlating with cracks propagating parallel to the c-axis of Mg12ZnY. This suggests that the fracture toughness of Mg-Zn-Y alloys can be improved by controlling the orientation of the Mg12ZnY compound. INTRODUCTION From an environmental point of view, magnesium alloys such as AZ31 and ZK60 are attractive light-weight structural materials owing to their low density, high specific strength, good damping properties, and the fact that these materials can be easily recycled. Consequently, magnesium alloys are incorporated into a variety of applications, including automotive parts and aerospace and electronic applications [1, 2]. However, their low strength and poor ductility hinder their widespread application. Therefore, great effort has been expended in improving the strength and ductility of magnesium alloys. Recently, Kawamura et al. reported that Mg-Zn-Y alloys with a long-period stacking ordered (LPSO) phase exhibit higher strength, better ductility, and superior heat resistance as compared to conventional Mg alloys [3]. These excellent properties originate not only from D-Mg matrix grain refinement but also from a novel precipitate with a LPSO phase [3]. The LPSO phase consists of a periodical stacking of close-packed planes, which correspond to (0001) in hcp. In a related study, Abe et al. reported a peculiar chemical order of Y and Zn atoms in Mg-based LPSO planes, that is, the periodical enrichment of Y and Zn atoms at particular close-packed planes [4]. Padezhnova et al. investigated the Mg-Zn-Y ternary phase diagram around the Mg corner and found a stable LPSO
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phase with a composition of Mg
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