Microstructure, generation of intermetallic compounds and mechanical strengthening mechanism of as-cast Mg-4Y-xZn alloys

The performance of as-cast Mg-4Y-xZn (x = 1, 2, 3, and 4 wt%) alloys with adding different content of Zn is thoroughly investigated. Results show that the alloy system mainly contains Mg24Y5 , Mg12YZn (X), and Mg3Y2Zn3 (W) phases. During the solidification process, with a Zn content of more than 1 wt%, the Mg24Y5 content gradually decreases and ultimately disappears, and the W-phase increases. The increase of Zn content account for the decrease of lattice parameters and volume of the alpha-Mg matrix. The yield strength and ultimate tensile strength of the Mg-4Y-xZn alloy increase with the Zn content varies from 1 to 4 wt%; the favorable values are 122 and 232 MPa. Moreover, the elongation first increases and then decreases; the optimal value is 11.9% as the Zn content reaches 3 wt%. For the alloy system, X-phase is the major strengthening phase owing to its 18R long-period stacking ordered structure; moreover, it has a crystallographic relationship of (0001)(Mg)//(001)(18R) and [(1) over bar2 (1) over bar0] mg //[010](18R) with the Mg matrix. Indeed, the tiny W-phase can cooperate with the X-phase in order to enhance the performance of the material in question by inhibiting X-phase deformation during the tensile process. Nevertheless, if W-phase is the only second phase in the alloy, the ductility of the alloy is reduced.