## 1 Introduction  
The ZM6 alloy (Mg-Nd-Zn-Zr) is a typical magnesium alloy that is strengthened by solid solution and aging processes and that has high strength, a small micropporosity tendency, excellent mechanical properties, and good casting properties. Thus, it is commonly used in the aerospace and satellite communication fields due to its obvious age hardening effect [1; 2; 3; 4]. Rare earth elements (RE) have a large solid solubility limit in magnesium, and that solid solubility decreases sharply when the temperature decreases, resulting in a large amount of supersaturation being obtained. For magnesium alloys without Al, such as ML10, ZE41, WE43, ZK60, AM-SC1, MRI202S, and ZM6, the grain refinement element is adopted [2; 3; 4]. The common heat treatment process is a solution treatment with the addition of artificial aging. The Mg-RE-Zr series alloy can be used at temperatures above 200 \({}^{\circ}\)C. As the diffusion rate of RE is slow, a dispersion strengthening phase containing RE can be precipitated in the aging process, which also provides the alloy with high thermal stability and excellent creep resistance [5; 6].  
The ZM6 alloy is a kind of Mg-Nd-Zn-Zr alloy, and most of the intermetallic compounds that are formed in the RE-Mg binary system are magnesium-rich phases. For light are earth metals, the REMg\({}_{2}\) compound is the Laves phase that belongs to the MgCu\({}_{2}\) cubic system. Meanwhile, for heavy rare earths, the Laves phase belongs to the MgZn\({}_{2}\) hexagonal system [7; 8]. The stability of these binary phases increases from light rare earth metals to heavy rare earth metals. The Mg-Gd-Nd-Zr and Mg-Gd-Y-Zr alloys have been studied, and the nucleation of the aging phase has been found to be promoted by introducing a large number of dislocations [9]. The precipitation sequence in Mg-Gd-RE is as following: SSSS (super-saturated solid solution)\(\rightarrow\)a\"(Mg\({}_{3}\)Gd, hcp, D0\({}_{19}\))\(\rightarrow\)a\"(Mg\({}_{3}\)Gd, fcc)\(\rightarrow\)a(Mg\({}_{3}\)Gd, fcc) [3]. When the Mg-Gd-Y-Zr alloy is at its peak-aged condition, aging at 200 \({}^{\circ}\)C for 10-16 h and fine a\" particles with a D0\({}_{19}\) structure (\(a\) = 0.64 nm, \(c\) = 0.52 nm) are the dominant strengthening phase. Moreover, a\({}^{\prime}\) precipitates with an fcc structure (\(a\) = 0.742 nm) are the dominant phase in the alloy following over-aging at 250 \({}^{\circ}\)C for 10 h [10]. [PERSON] and [PERSON] et al. [11; 12] studied the transformation mechanism of the precipitated phases in Mg-Zn and Mg-Al-Zn alloys during graded aging. It was found that the distribution and morphology of the precipitated phases changed significantly after graded aging compared to after single aging, resulting in the mechanical properties being significantly improved. However, as the aging behavior of the ZM6 (Mg\({}_{2.6}\)Nd\({}_{0.4}\)Zn\({}_{0.4}\)Zr) alloy is more complex, there are few reports about the multi-stage aging treatment of magnesium alloys, and their multi-stage aging behavior has not been revealed yet. Therefore, the present study observes the precipitation behavior during the two-stage aging treatment process and the effect of the aging temperature and aging time on the mechanical properties (yield strength, tensile strength and elongation) of both as-cast and as-compressed ZM6 alloys by adjusting and controlling the pre-aging stage during the two-stage aging process. In this paper, the phase composition, microstructure evolution, and age-strengthening mechanism of ZM6 alloys were investigated by means of XRD, DTA, and TEM. The morphology, size and distribution of the precipitation phases were controlled by the TSAT process, and the corresponding mechanical properties were tested through a comparison of those properties under various treatments in order to obtain excellent properties and to provide optimized process parameters for performance control in practical applications.