Effect of Zn Content on the Microstructure and Mechanical Properties of the Extruded Mg-6Gd-4Y (wt%) Alloy

Microstructures and mechanical properties of the Mg-6Gd-4Y (wt%) alloys with and without 1wt% Zn additions were investigated. The results show that the as-cast microstructure of the Mg-6Gd-4Y alloy consist of α-Mg matrix and Mg₂₄(GdY)₅ secondary phase. However, the as-cast microstructure of the Zn-containing Mg-6Gd-4Y-1Zn alloy consist of α-Mg matrix, Mg₂₄(GdY)₅ secondary phase and Mg₁₂Y₁Zn₁ phase which has a 18R long period staking ordered (18R-LPSO) structure. After extrusion a 14H-LPSO phase is found in the Zn-containing alloy which is distributed between the 18R-LPSO strips in the as extruded microstructure. The formation mechanism of the 14H-LPSO phase is precipitation, and the reaction can be expressed as ‘α-Mg′ → α-Mg+14H’. Zn content has no obvious effect on the precipitation of β series. Aged (T6 and T5 treatment) conducted on both the Mg-6Gd-4Y alloy and the Mg-6Gd-4Y-1Zn alloy causes the formation of β′ precipitates. The T6-aged Mg-6Gd-4Y-1Zn alloy exhibits high tensile strength combined with good ductility, and the values of the yield strength (YS), ultimate tensile strength (UTS) and elongation are 309 MPa, 438 MPa and 6.8%, respectively. It is due to the coexistence of 18R-LPSO phase and finely dispersed distribution of the 14H-LPSO phase and the β′ precipitates in the microstructure.