**3. Results and Discussion**

#### *3.1. Microstructure*

Figure 1 shows the XRD patterns of as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys. Among these alloys, the LAZ532-0.2Zr alloy is a typical α-Mg single-phase alloy with an hcp structure, while the LAZ1132-0.2Zr alloy is a typical β-Li single-phase alloy with a

bcc structure. When the Li contents were 7 wt.%, 8 wt.%, and 9 wt.%, the alloys were α-Mg+β-Li dual-phase alloys with an hcp+bcc mixed structure. Obviously, as the Li content increased from 5 wt.% to 11 wt.%, the matrix phase of the alloy changed from α-Mg singlephase to α-Mg+β-Li dual-phase and then to β-Li single-phase, and the crystal structure transformed from hcp structure to hcp+bcc structure and then to bcc structure, which are results consistent with the phase diagram of the Mg-Li binary alloy [12]. Except for the matrix phase, all alloys contained the AlLi phase (fcc structure), and the corresponding phase peaks become more and more obvious with increasing Li content. No phases containing Zn and Zr elements were detected in the alloys, which indicates that Zn and Zr exist in the alloy matrix in the form of solid solutions and do not form intermetallic compounds.

**Figure 1.** XRD patterns of as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys: (**a**) *x* = 5; (**b**) *x* = 7; (**c**) *x* = 8; (**d**) *x* = 9; (**e**) *x* = 11.

Figure 2 displays the SEM micrographs of as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys. The as-homogenized LAZ532-0.2Zr alloy was composed of a light gray α-Mg matrix phase, with a kind of white fibrous phase in the α-Mg matrix. According to the XRD results (Figure 1) and works in the literature [13,14], this fibrous phase can be identified as the AlLi phase, and it exists in the α-Mg matrix in the form of eutectic. As-homogenized LAZ732- 0.2Zr, LAZ832-0.2Zr, and LAZ932-0.2Zr alloys consisted of a light gray α-Mg matrix and a dark gray β-Li matrix. With the increase in Li content, the α-Mg phase changes from a smooth block to long strip, and its content decreases gradually. It can be seen from the local enlarged view in the lower left corner of Figure 2c that a white fibrous phase similar to that in Figure 2a is distributed in the α-Mg matrix and a white granular phase is distributed in the β-Li matrix. According to the XRD results (Figure 1) and the literature [15,16], both the fibrous phase in the α-Mg matrix and the granular phase in the β-Li matrix were the AlLi phase. The as-homogenized LAZ1132-0.2Zr alloy consisted entirely of a dark gray β-Li matrix phase, and a large number of AlLi particles were uniformly distributed in the β-Li matrix.

**Figure 2.** SEM micrographs of as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys: (**a**) *x* = 5; (**b**) *x* = 7; (**c**) *x* = 8; (**d**) *x* = 9; (**e**) *x* = 11.

Figure 3 depicts the element distribution of as-homogenized LAZ532-0.2Zr, LAZ832- 0.2Zr, and LAZ1132-0.2Zr alloys. Clearly, both the fibrous AlLi phase and the granular AlLi phase were rich in Al and Zn, and poor in Mg. It is worth noting that Li is too light to detect. The enrichment of Zn in the AlLi phase was mainly due to the high solid solubility of Zn in Al (83.1 wt.%) [17]. A part of the added Al element was combined with Li to form the AlLi phase, and the remainder was dissolved in the alloy matrix. Because the solid solubility of Al in Mg (12.7 wt.%) is much higher than that of Al in Li (extremely limited), the dissolved Al element was mainly distributed in the α-Mg matrix. A part of the added Zn element was enriched in the AlLi phase, and the remainder was dissolved in the alloy matrix. Because the solid solubility of Zn in Mg (6.2 wt.%) is lower than that of Zn in Li (12.5 wt.%), the solid solution concentration of Zn in the β-Li matrix was higher than that in the α-Mg matrix, but the difference was not great.

Figure 4 displays the TEM micrographs of the AlLi phase in the as-homogenized LAZ832-0.2Zr alloy. The AlLi phase distributed in the α-Mg matrix and β-Li matrix shows quite different morphologies. Fibrous AlLi phases with a width in the range of 0.02–0.11 μm are found in the α-Mg matrix, and round-like AlLi particles with a diameter in the range of 0.42–0.92 μm are observed in the β-Li matrix, which is consistent with the microstructure shown in Figures 2 and 3.

**Figure 3.** EPMA area analysis of (**a**) LAZ532-0.2Zr, (**b**) LAZ832-0.2Zr, and (**c**) LAZ1132-0.2Zr alloys.

**Figure 4.** TEM micrographs of (**a**) fibrous AlLi phase and (**b**) round-like AlLi phase in as-homogenized LAZ832-0.2Zr alloy.

### *3.2. Mechanical Properties*

Figure 5 shows the stress–strain curves and hardness of as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys, and the densities and mechanical properties of the alloys are listed in Table 1. The ultimate tensile strengths (UTS) of as-homogenized LAZ532-0.2Zr, LAZ732- 0.2Zr, LAZ832-0.2Zr, LAZ932-0.2Zr, and LAZ1132-0.2Zr alloys were consecutively 197.2, 185.4, 181.4, 168.7, and 143.7 MPa; their elongations (δ) were consecutively 10.1%, 11.6%, 21.5%, 24.7%, and 27.3%; and their hardness was consecutively 68.6, 64.1, 61.3, 58.9, and 56.2 HV. In addition, the specific strength decreased from 124.8 MPa/(g/cm3) to 100.5 MPa/(g/cm3) as the Li content increased from 5 wt.% to 11 wt.%, exhibiting excellent specific strength. Clearly, as the Li content increases, the strength and hardness of the alloy decreases while the elongation increases, which is consistent with the variation law reported in previous research [18,19]. This result is mainly attributed to the transformation of the crystal structure of the Mg-Li alloy from the hcp structure to the bcc structure. It has been reported that the hardness of the α-Mg matrix is higher than that of the β-Li matrix [20]. With the increase in Li content, the content of the soft β-Li matrix in the alloy increased gradually, and thus the hardness of the alloy decreased gradually. In the Mg-Li alloy, the hard α-Mg matrix with hcp structure showed higher strength and worse ductility, while the soft β-Li matrix with bcc structure showed lower strength and excellent ductility. For the dual-phase Mg-Li alloy, plastic deformation occurred preferentially in the soft β-Li matrix during the deformation process, and then the hard α-Mg matrix began to undergo plastic deformation when the stress transmitted from the β-Li matrix to the α-Mg matrix was greater than the elastic limit of the α-Mg matrix. This is because the β-Li matrix with bcc structure is softer and has more independent slip systems than the α-Mg matrix. Table 2 lists the independent slip systems and the critical resolved shear stress (CRSS) in Mg and Li at room temperature [21–24]. The common slip systems in Mg are → *a*  basal slip, → *a*  prismatic slip, → *a*  pyramidal slip, and → *c* + <sup>→</sup> *a*  pyramidal slip. The slip direction

of these → *<sup>a</sup>*  dislocation slips is <sup>11</sup><sup>−</sup> 20 direction perpendicular to the c axis, which cannot

coordinate the strain in the c axis, while → *c* + <sup>→</sup> *<sup>a</sup>*  dislocation slip along <sup>11</sup><sup>−</sup> 23 direction can coordinate the strain in the c axis. At room temperature, the CRSS required for the basal slip initiation of Mg is about 0.45–0.81 MPa, and the CRSS required for prismatic slip and pyramidal slip is about 100 times that required for basal slip. Therefore, the basal slip is the easiest to initiate during the deformation process of Mg at room temperature, but the prismatic slip and pyramidal slip are difficult to initiate. However, the basal slip of Mg can only provide two independent slip systems, which fails to meet the requirements of five independent slip systems in Von Mises criterion, and thus Mg has poor plasticity and is difficult to deform. Due to the limited slip systems of Mg, twinning often occurs to coordinate the c-axis strain during deformation at room temperature. Compared with

compression twins ({10<sup>−</sup> <sup>11</sup>} and {10<sup>−</sup> <sup>12</sup>}), tension twin ({10<sup>−</sup> 12}) is the most easily initiated twinning mode in Mg at room temperature because of its lower CRSS (2.0–2.8 MPa) and smaller shear displacement [25]. For Li with a bcc structure, the primary slip system is the {110}111 with the secondary slip systems of {112}111 and {123}111. It has been reported that the {110}111 slip system of Li has a lower CRSS (0.54–0.57 MPa), and the corresponding number of the independent slip system is 12, so Li exhibits good ductility during the deformation process [23]. Moreover, it has been considered that the excellent plasticity of the β-Li matrix is attributed to the nearly equal CRSS values of the primary slip system ({110}111) and secondary slip systems ({112}111 and {123}111) [24]. Therefore, as the Li content increases, the content of the β-Li matrix with bcc structure increases gradually, and accordingly the tensile strength decreases while the elongation increases. In addition, as the Li content increases, the AlLi softening phase increases gradually, as shown in Figure 1, and the contents of Al and Zn dissolved in the alloy matrix decrease, which is one of the reasons for the gradual decrease in the alloy strength, although the effect is not great.

**Figure 5.** (**a**) Stress–strain curves and (**b**) hardness of as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys.

Figure 6 depicts the fracture morphologies of as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys. The fracture surface of the as-homogenized LAZ532-0.2Zr alloy was full of cleavage planes and cleavage steps, showing typical cleavage fracture characteristics. Cleavage fracture is caused by the stacking of dislocation at the grain boundary or twin boundary, which usually occurs along the cleavage plane, and the main cleavage planes of the α-Mg

matrix with hcp structure were {0001} and {1 − 100} with low index. In addition to cleavage planes and cleavage steps, many dimples could be observed on the fracture surfaces of as-homogenized dual-phase alloys (LAZ732-0.2Zr, LAZ832-0.2Zr, and LAZ932-0.2Zr), and the cleavage steps decreased while the dimples increased with the increase in Li content. Because a dimple is the basic feature of the microvoid coalescence fracture, the fracture modes of as-homogenized dual-phase alloys are mixture fractures of cleavage fracture and microvoid coalescence fracture. In the early stage of microvoid coalescence fracture, the microvoids are formed by the fracture of the second phase (AlLi), the separation of the second phase from the matrix, and the separation of the phase interface, owing to the elastic and plastic difference between the second phase and the matrix. The fracture surface of the as-homogenized LAZ1132-0.2Zr alloy was full of dimples with almost no cleavage steps, so its fracture mode was a microvoid coalescence fracture. The fracture mode of the as-homogenized alloys changed from cleavage fracture to microvoid coalescence fracture with increasing Li content, indicating that the elongation of the alloy gradually became better, which is consistent with the variation trend of elongation in Figure 5a.


**Table 1.** Density and mechanical properties of as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys.

**Table 2.** Independent slip systems and the CRSS in Mg and Li at room temperature [21–24].


**Figure 6.** Fracture morphologies of as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys: (**a**) *x* = 5; (**b**) *x* = 7; (**c**) *x* = 8; (**d**) *x* = 9; (**e**) *x* = 11.

Figure 7 depicts the side surfaces of the fracture for as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys. In as-homogenized LAZ532-0.2Zr and LAZ732-0.2Zr alloys, the microvoids mainly came from the rupture of AlLi particles with larger sizes (marked by a red oval), and large microcracks could even be observed in the α-Mg matrix (marked by a yellow

oval), while no microvoids were found around the fibrous AlLi phase and tiny AlLi particles. In the as-homogenized LAZ832-0.2Zr alloy, the microvoids mainly came from the separation of AlLi particles from the matrix, and in addition the separation of the matrix phase interface (marked by a blue oval) due to the AlLi particles was extremely tiny. In as-homogenized LAZ932-0.2Zr and LAZ1132-0.2Zr alloys, microvoids caused by the rupture of AlLi particles and the separation of AlLi particles from the matrix were observed. Since the fracture mechanism of the α-Mg matrix is mainly cleavage fracture, large and straight microcracks could be observed in the α-Mg matrix. These microcracks grew through the grains and continued to expand, and finally showed the cleavage steps. The fracture mechanism of the β-Li matrix was a microvoid coalescence fracture with the basic feature of dimples, and the microvoids were formed by the rupture of the AlLi phase and the separation of the phase interface. According to statistics, AlLi particles with a diameter greater than 1.55 μm break easily during the tensile test, the smaller particles are prone to separate from the matrix, and extremely tiny particles cannot produce microvoids easily. The formation of microvoids at tiny particles is more difficult than at large particles, because the tensile stress required for the formation of microvoids is inversely proportional to the square root of particle size. Therefore, except for the fact that the increase of the β-Li matrix greatly improves the alloy elongation, tiny AlLi particles inhibit the formation of microvoids and are also conducive to the improvement of elongation.

**Figure 7.** Side surfaces of the fracture for as-homogenized Mg-*x*Li-3Al-2Zn-0.2Zr alloys: (**a**) *x* = 5; (**b**) *x* = 7; (**c**) *x* = 8; (**d**) *x* = 9; (**e**) *x* = 11.
