*4.1. Grain-Boundary Strengthening and Dislocation Strengthening*

Generally, thermoplastic processing greatly influences the mechanical properties of a metal from three aspects [36–38]. Firstly, the process eliminates the casting defects of the raw materials. Secondly, the coarse grains will be greatly refined due to the strain-induced dislocation evolution, resulting in the significant grain refinement. Thirdly, the coupling effect of heat and strain-induced dislocation entanglement will stimulate the dynamic recrystallization of the deformed matrix. For the Mg-9Li alloy, due to the bcc structure and its softer essence of β-Li phase, it will be firstly deformed with lots of

dislocations accumulated in the α/β phase interface, which inhibits the recovery of the β-Li phase but stores up enormous distortion energy [39]. With increase in ECAP passes and induced strain, a large number of dislocations will concentrate inside the deformed β-Li phase grains [26,40]. The increase in dislocation density leads to the sub-grain boundary becoming the nucleation point of recrystallization, eventually growing up to from new grains [18,41]. Therefore, dynamic recrystallization takes place preferentially in β-Li phase, which may lead to a limited strengthening and the potential softening of the alloy [42].

As revealed in the tensile curves, the E4 alloy has an improved YTS compared to the cast alloy with limited UTS and decreased ductility. The dynamic-recrystallization of β-Li phase and the limited grain refinement of the α-Mg phase is the major contributing factor to this phenomenon as well as the extremely limited further strengthening to the E4 alloy after further rolling. As revealed in the tensile curves, both E8 and E8R alloys presented the best strength, due to the combination of grain-boundary strengthening and dislocation strengthening from both α-Mg and β-Li phases. Generally believed, the finer the grain size during thermal plastic deformation, the more easily dynamic recrystallization occurs [40,43]. Due to the re-heating of the E16 alloy after 8-passes followed by an additional 8 passes, a high possibility of intensive dynamic recrystallization occurring to both α-Mg phase and β-Li phase due to the lattice distortion energy obtained from the further ECAP-induced plastic deformation results in the typical softening phenomenon of the E16 alloy.

TEM images of the E8 and E8R alloys are presented in Figure 7. Figure 7a presents the typical grain morphologies of deformed α-Mg phase. The selected electron diffraction pattern (SAED) in the bottom right corner of Figure 7a was observed in the ([4153]) zone axis. As marked, the crystal planes of (1110), (1102), and (1013) clearly shows the typical lattice feature of the Mg phase. After continuous ECAP process for 8 passes, obvious grain refinement of approximately 2 μm has been achieved in the deformed α-Mg phase. As shown in Figure 7b, the deformed Mg grains has been further refined to about 800 nm to 1.5 μm with improved grain boundary strengthening, satisfying the Hall-petch equation, stated as σ<sup>y</sup> = σ<sup>0</sup> + *kd*−1/<sup>2</sup> [44]. Figure 7c presents the typical intragranular dislocations of the E8R alloy. Based on the TEM microstructure analysis, it can be deduced that the strength enhancement of E8 and E8R alloys was dominated by the combined effect of sufficient grain-boundary strengthening and dislocation strengthening with additional induced straining at room temperature and the obtained finer grains. A number of researches have shown that the intensity and plasticity of the α-Mg phase in the hcp structure can be improved synchronously after severe grain refinement via severe plastic deformation (SPD) process [45]. Also, according to the reported results of the SPDed UFG Mg alloys [46,47], the great refinement of the Mg grains will benefit both high strength and high ductility of the α-Mg phase of the alloy. However, the large plastic deformation also leads to the grain refinement, as well as the strain-induced work hardening of the β-Li phase, leading to the observed decrease in ductility of the E8 alloy.

**Figure 7.** TEM microstructure of the Mg-9Li alloys: (**a**) 8-passes ECAPed (E8) alloy grain morphology; (**b**) E8R alloy grain morphologies; (**c**) intragranular dislocations of the E8R alloy.
