Experimental study on the microstructure and mechanical properties of the Mg-Y-Zn alloys with LPSO phases tailored by CALPHAD method

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 Mg95Y3Zn2 and Mg86Y8Zn6 (at%) alloys with different contents of LPSO phases were tailored by CALPHAD method, and the tested compositions of these two alloys were Mg95.2Y2.6Zn2.2 and Mg86.2Y7.7Zn6.1, 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 & DEG;C were measured. After the isothermal treatment at 500 degrees C for 240 h, the phase constituent of the Mg95.2Y2.6Zn2.2 alloy changed from the as-cast three phases (18 R + & alpha;-Mg + W-Mg3Y2Zn3) into the equilibrium three phases (18 R + & alpha;-Mg + 14 H), while that of the Mg86.2Y7.7Zn6.1 alloy completely changed from the as-cast four phases (18 R + & alpha;-Mg + W-Mg3Y2Zn3 + 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 + & alpha;-Mg + WMg3Y2Zn3 -* 14 H" at 546 & DEG;C and the three-phase univariant reaction of "18 R + & alpha;-Mg -* 14 H" below 546 & DEG;C. The results of mechanical properties show that the yield strength of the Mg86.2Y7.7Zn6.1 alloy composed entirely of LPSO phases is better than that of the Mg95.2Y2.6Zn2.2 alloy composed of both LPSO phases and some & alpha;-Mg matrix, while the plasticity of the Mg86.2Y7.7Zn6.1 alloy is not as good as that of the Mg95.2Y2.6Zn2.2 alloy. Therefore, the amount of LPSO phases can be increased for pursuing high strength of Mg alloys, but a certain amount of & alpha;-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 & alpha;-Mg matrix in the Mg-Y-Zn system can be tailored by CALPHAD method.