*3.2. Tensile Mechanical Properties of the ECAPed and ECAP-Rolled Alloy*

The tensile engineering strain-stress curves of the alloy after different-passes ECAP process and post rolling were obtained and presented in Figure 5, and the related mechanical parameters summarized in Table 2. Due to the amount of soft β-Li phase of the duplex structure, the cast alloy has limited strength and excellent ductility, presenting the extremely low yield strength (YTS, about 52 MPa) and high elongation to failure (*E*f, about 33%). One characteristic noteworthy is that, the cast alloy presents sufficient tension work-hardening ability, presenting the continuously strengthened tensile bearing capacity. Benefited from this advantage, the ultimate strength (UTS, about 102 MPa) of the cast alloy is nearly double of the YTS and also presents the excellent performance in the uniform elongation (*E*u, about 15%).

ECAP process improved the strength but decreased the ductility of the Mg-9Li alloy, and the mechanical performance changed with different ECAP passes. After ECAP for 4 passes, the YTS and UTS of the E4 alloy were improved to 88 MPa and 106 MPa, and the *E*<sup>u</sup> and *E*<sup>f</sup> decreased to 5% and 25% respectively. With increased ECAP passes, the 8-passes ECAPed alloy (E8) was further greatly strengthened with better ductility compared to the E4 alloy, of which the YTS and UTS are about 110 MPa and 133 MPa, and the *E*<sup>u</sup> and *E*<sup>f</sup> are about 7% and 24%. However, remarkable softening of the alloy occurred when the ECAP process was further increased to 16 passes. Compared to the E8 alloy, the E16 alloy presented lower YTS (about 100 MPa) and UTS (about 116 MPa). However, the ductility of E16 alloy was improved, of which the *E*<sup>f</sup> was about 31%, nearly the same as that of the cast alloy. However, it should be emphasized that the strength of the E16 alloy is still significantly higher than that of cast alloy, especially the YTS. Different to the sufficient work-hardening ability of the cast alloy during the tensile process, the ECAPed alloys presented a reduced difference between values of YTS and UTS, indicating their less tension work-hardening ability.

**Figure 5.** Tensile strain-stress curves of Mg-9Li alloys: (**a**) Cast and ECAPed alloys; (**b**) cast rolled and ECAP-rolled alloys.

Figure 5b presents the engineering strain-stress curves of the rolled alloys. Different to the tensile curves of the cast and ECAPed alloys, the tensile curves of the rolled sample showed the obvious dense serrated fluctuations after yielding which can be adjudged to be the C-type Portevin-Le Chatelier (PLC) effect [9]. The appearance of this phenomenon may be caused by the instability of the Mg atoms in the β-Li matrix after rolling. When plastic deformation occurs under the action of tensile stress, dislocation passes through these unstable solid-solution Mg atoms, which promotes the dissolution of these Mg atoms from the β-Li matrix. At the same time, the dislocation will be pinned at the grain boundary or the dislocation accumulation during the movement, so the interaction between the movable dislocation and the unstable dissolved Mg atom will cause the fluctuation of the tensile curve [9,35]. Significant strengthening and dramatic reduction in ductility occurred to the cast alloy after rolling process, of which the YTS and UTS increased to about 152 and 158 MPa, and the *E*<sup>u</sup> and *E*<sup>f</sup> decreased to about 3% and 16% respectively. Judging from the typical change in strength and ductility of the cast-rolled alloy, one can point out that strain-induced dislocation hardening is the major factor dominating the above phenomenon. During the rolling process, a large number of dislocations are induced within the grains. With further deformation, the edge dislocations and screw dislocations move along (0002) planes. Until the grain boundary is reached, a large number of dislocations in the grain boundary tangles further from the dislocation cell walls, further impeding the slip of dislocations.


**Table 2.** Tensile test properties of as-cast, ECAP and ECAP-rolled Mg-9Li alloy.

Post rolling also improved the strength and decreased the ductility of the ECAPed alloys. However, all the ECAP-rolled alloys had better ductility compared to the cast-rolled alloy. Among them, the E8R alloy had the best strength with satisfactory ductility, of which the YTS, UTS, and *E*<sup>f</sup> values reached 166 MPa, 174 MPa and 22% respectively. Compared to the cast alloy, the E8R alloy dramatically enhanced the YTS and UTS, which increased by 219% and 70% respectively. Compared to the cast-rolled alloy, the YTS and UTS increased by 10% and 9%. The most important thing is that, the E8R still kept satisfactory ductility after rolling while the cast-rolled alloy suffered dramatic reduction in ductility. Different to the E8 alloy, the rolling process endowed a quite limited strengthening to the E4 and E16 alloys, of which the YTS and UTS were all less than that of the CR alloy. This phenomenon may have a close relationship to the incompletely refined microstructure of the E4 alloy and the softening process of the E16 alloy.

Figure 6 presents the SEM fracture morphology of Mg-9Li alloys. Generally, as seen in Figure 6a, the cast alloy has a typical tearing fracture, indicating the excellent ductility. The fracture of the as-cast alloy shows major deep dimples in the β-Li phase induced by further growth of the voids due to the accumulation of dislocations and the few quasi-cleavage fracture along the α-Mg phase. In the E8 alloy (Figure 6b), the fracture surface is relatively flat with fewer marks of severe tearing compared to the cast alloy, indicating less plasticity. In addition, the fracture is majorly occupied by a mass of small shallow dimples while a few cleavage steps can be also observed. The great decrease in dimple size should be closely related to the grain refinement. Post rolling created more obvious flat fracture to both the cast and ECAPed alloy, indicating remarkable reduced ductility. As seen in Figure 6c, the CR alloy has an equal amount of dimples, mixed with large cleavage steps. Meanwhile, its dimples are much shallower than that of the cast alloy. As seen in Figure 6d, the E8R alloy also presents increased cleavages compared to the E8 alloy. However, the cleavage of this alloy was obviously less than that of the CR alloy, indicating the satisfactory and better ductility. From the above analysis, it can be concluded that the SEM fracture morphologies properly reflect the alternating ductility of the alloys after ECAP and post rolling.

**Figure 6.** SEM fracture morphology of Mg-9Li alloys: (**a**) Cast alloy; (**b**) E8 alloy; (**c**) CR alloy; and (**d**) E8R alloy.
