Microstructures and mechanical properties of homogenization and isothermal aging Mg–Gd–Er–Zn–Zr alloy

The effects of homogenization and isothermal aging treatment on the mechanical properties of Mg–12Gd–2Er–1Zn–0.6Zr (wt%) alloy were investigated. The precipitated long-period stacking order (LPSO) structure and the aging precipitation sequence of the conditioned alloys were observed and analyzed, respectively. The results indicate that the 14H-LPSO structure occurs after the homogenization treatment and the β′ phase forms after the isothermal aging process. These two independent processes could be controlled by the precipitation temperature range. The significant increase in the elongation of the as-cast alloy after homogenization treatment is attributed to the disappearance of the coarse primary Mg₅(Gd, Er, Zn) phase and the presence of the 14H-LPSO structure. The precipitation sequence of the investigated alloy is α-Mg(SSS)/β′′(D0₁₉)/β′(cbco)/β. Furthermore, the yield tensile strength (YTS) and ultimate tensile strength (UTS) values of the isothermal aging alloy have a great improvement, which could be attributed to the high density of the precipitated β′ phase.

With the aging time prolonging, the β′ phase is precipitated in the alloy. Both the 14H-LPSO structure and the β′ phase coexist in the alloy after aging for 84 h at 498 K. The layer structures of 14H-LPSO are obviously observed along [11\(\overline{2}\)0]_Mg direction, which is shown in the high-resolution transmission electron microscopy (HRTEM) image. The 14H-type structure displays a stacking order with a period of c = 3.67 nm, which does not change during the isothermal aging process.