Microscale Fracture Testing of Mg-Zn-Y
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1225-HH03-04
Microscale Fracture Testing of Mg-Zn-Y Shun Matsuyama1, Tetsuya Sakamoto, Masaaki Otsu1, Kazuki Takashima1 and Yoshito Kawamura1 1 Department of Materials Science and Engineering, Kumamoto University, 2-39-1, Kurokami, Kumamoto, Japan ABSTRACT A microfracture testing technique was applied for investigating the fracture properties of Mg-Zn-Y alloys with a long-period stacking ordered (LPSO) phase. Three types of Mg-Zn-Y alloys with difference volume fraction of the LPSO phase―Mg99.2Zn0.2Y0.6, Mg97Zn1Y2, and Mg88Zn5Y7―were used. Microsized cantilever beam specimens with dimensions ≈ 10×20×50 µm3 were prepared from Mg-Zn-Y alloys by focused ion beam (FIB) machining. Notches with widths of 0.5 µm and depths of 3.5–5 µm were also introduced into the specimens by FIB machining. Fracture tests were successfully conducted using a mechanical testing machine for microsized specimens at room temperature. The fracture toughness values (KIC) could not be obtained as the specimen size was too small to satisfy the plane strain condition. Hence, provisional KQ values were considered. The KQ values of the Mg97Zn1Y2 alloy were 0.8–1.2 MPam1/2, and those of the Mg88Zn5Y7 alloy were 1.2–3.0 MPam1/2. The volume fraction of the LPSO phase in the Mg88Zn5Y7 alloy was higher than that in the Mg97Zn1Y2 alloy, and this indicates that the LPSO phase increases the fracture toughness of Mg-Zn-Y alloy. As the fracture in the Mg99.2Zn0.2Y0.6 alloy specimen occurred in a ductile plastic deformation, it was impossible to evaluate KQ values of this specimen. INTRODUCTION Magnesium (Mg) alloys are considered environmentally friendly materials owing to their low density and recyclability. Several conventional Mg alloys such as those in the AZ and ZK series have been increasingly used in automotive parts. However, the mechanical properties of these conventional Mg alloys, such as strength and elongation, are inferior to those of Al alloys. These mechanical properties of Mg alloys must be improved in order to extend their application range. It was recently reported that Mg-Zn-Y alloys with a long-period stacking ordered (LPSO) phase exhibit high strength and superior heat resistance property as compared to those of conventional Mg alloys [1]. These alloys have high potential for use in motor vehicle components and in the casing of electronic appliances, because of their superior mechanical properties [2, 3]. These alloys consist of an α-Mg phase and an LPSO phase [4]. The superior mechanical properties of the Mg-Zn-Y alloys seem to originate due to the presence of both the fine-grained α-Mg phase and the lamellar phase, which consist of 2H-Mg and 18R LPSO phases, respectively [5]. It is therefore important to investigate the fracture toughness of each phase and interphase in these two-phase alloys. However, since the size of the constituent phase is in the order of micrometers, the present macroscale fracture tests cannot be used for the measurement of the fracture toughness of each phase and interphase. With this background, we have develo
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