Effect of Zn on the microstructure and mechanical properties of as-cast Mg-7Gd-3Y-1Nd-0.5Zr alloy

The microstructure and mechanical properties of as-cast Mg-7Gd-3Y-1Nd-xZn-0.5Zr (x=0, 0.5, 1 and 2 wt%) alloys have been investigated by optical microscopy (OM), scanning electron microscopy equipped with energy dispersive spectrum, transmission electron microscopy (TEM), X-ray diffraction and tensile tests at room temperature (RT). Experimental results reveal that the microstructure of the alloy without Zn contains α-Mg and Mg5RE phase, the microstructure of the alloy with 0.5% Zn consists of α-Mg, (Mg, Zn)3RE phase, Mg5(RE, Zn) phase and stacking fault. The addition of 1% and 2% Zn results in the disappearance of the Mg5(RE, Zn) phase, but the stacking fault can be seen more clearly. Moreover, a new block-like long period stacking ordered (LPSO) phase is observed in grain boundaries with increasing Zn content up to 2%. TEM analyses indicate that the Mg5RE, (Mg, Zn)3RE and Mg5(RE, Zn) phases have a face-centered cubic (f.c.c.) structure with lattice constants of 2.22 nm, 0.73 nm and 2.23 nm, respectively. The new block-like LPSO phase belongs to 10H-type. The tensile tests at RT exhibit that the alloy containing 1% Zn shows the optimal mechanical properties and the ultimate tensile strength (UTS), yield strength (YS) and elongation are 187 MPa, 145 MPa and 3.1%, respectively. As indicated by fracture analyses, the fracture modes of the alloys with 0% and 0.5% Zn are typically intercrystalline fracture, whereas both intercrystalline and transcrystalline fractures are observed in the alloys with 1% and 2% Zn.