Experimental study on the microstructure and mechanical properties of the Mg-Y-Zn alloys with LPSO phases tailored by CALPHAD method
Yongxin Ruan, Shibing Xia, Changrong Li, Cuiping Guo, Zhenmin Du
Abstract
Long-Period Stacking Ordered (LPSO) phases in the Mg-Y-Zn alloys can significantly improve comprehensive mechanical properties of the Mg-based alloys. The nominal compositions of the Mg 95 Y 3 Zn 2 and Mg 86 Y 8 Zn 6 (at%) alloys with different contents of LPSO phases were tailored by CALPHAD method, and the tested compositions of these two alloys were Mg 95.2 Y 2.6 Zn 2.2 and Mg 86.2 Y 7.7 Zn 6.1 , respectively. The microstructure evolutions of the studied alloys in the states of as-cast and isothermal treatments at different times were observed, and the mechanical properties of the as-extruded alloys after homogenization (short term, 12 h) and the isothermal treatment (long term, 240 h) at 500 ℃ were measured. After the isothermal treatment at 500 °C for 240 h, the phase constituent of the Mg 95.2 Y 2.6 Zn 2.2 alloy changed from the as-cast three phases (18 R + α-Mg + W-Mg 3 Y 2 Zn 3 ) into the equilibrium three phases (18 R + α-Mg + 14 H), while that of the Mg 86.2 Y 7.7 Zn 6.1 alloy completely changed from the as-cast four phases (18 R + α-Mg + W-Mg 3 Y 2 Zn 3 + 14 H) into the equilibrium two LPSO phases (18 R + 14 H), where the 14 H-type LPSO phase is formed by the four-phase invariant reaction of “18 R + α-Mg + W-Mg 3 Y 2 Zn 3 → 14 H” at 546 ℃ and the three-phase univariant reaction of “18 R + α-Mg → 14 H” below 546 ℃. The results of mechanical properties show that the yield strength of the Mg 86.2 Y 7.7 Zn 6.1 alloy composed entirely of LPSO phases is better than that of the Mg 95.2 Y 2.6 Zn 2.2 alloy composed of both LPSO phases and some α-Mg matrix, while the plasticity of the Mg 86.2 Y 7.7 Zn 6.1 alloy is not as good as that of the Mg 95.2 Y 2.6 Zn 2.2 alloy. Therefore, the amount of LPSO phases can be increased for pursuing high strength of Mg alloys , but a certain amount of α-Mg phase needs to be kept for retaining good plasticity. For this purpose, the alloy composition with a specific amount of the LPSO phases and α-Mg matrix in the Mg-Y-Zn system can be tailored by CALPHAD method.