Fatigue properties of Mg-Zn-Y alloys with long period orderd structure

Recently, Mg-Zn-Y alloys with high strength and good ductility, which have a long period order (LPO) structure, have been developed. Therefore, it is important to understand fundamental fatigue properties in such materials. In this study, the fatigue fracture behavior of Mg-96-Zn-2-Y-2 (two phase) alloy was investigated using a plain fatigue bending testing machine, which was originally developed for thin sheet specimen. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance at a frequency of 250-700 Hz occurs in the specimen due to forced vibration at the fixed end. A S-N curve for Mg-96-Zn-2-Y-2 alloy was obtained and the fatigue limit was estimated as about 180 MPa. The value corresponds to about 45% of 0.2% proof strength of the alloy. Two types of fatigue surface were observed in the alloy. One is striation like pattern and the other is relatively flat surface. For comparison of fatigue behavior, Mg-88-Zn-4-Y-7 (LPO single phase) and Mg-99.2-Zn-0.2-Y-0.6 (alpha-Mg single phase) alloys were also investigated. Fatigue limit of each alloys were 220 MPa and 140 MPa, respectively. Striation like pattern was also observed in fatigue surface of both alloys. These results show that a fatigue crack in Mg-Zn-Y alloys propagates with activation of basal slip at the crack tip.