In this work, through performing microstructural characterization, tensile testing and failure analysis, the influence of electrochemical hydrogen charging on the microstructure and mechanical behavior of an as-cast Mg-8wt.%Li alloy was investigated. It revealed that after being hydrogen charged at 50 mA/cm² for respectively 3 h, 6 h and 18 h in 0.1 M NaCl solution, obvious HID occurred and the damage degree was gradually increased with the hydrogen charging time. For the sample being hydrogen charged for 3 h, micro pores with the diameter ranging from 10 ~ 30 µm were formed and preferentially present in α-Mg phase. Moreover, micro cracks with the length ranging from 10 ~ 50 µm mainly initiated in α-Mg phase, at α-Mg/β-Li interfaces and the peripheries of pores. With the increase of hydrogen charging time, the numbers of pores and cracks were obviously increased. Tensile results revealed that the hydrogen charging can simultaneously decrease the tensile strength and ductility of the alloy. Compared with the uncharged sample, the tensile yield strength, ultimate tensile strength and the elongation ratio to failure were respectively reduced by 5.7%, 7.3%, 31.7% for the 3h-charged sample and 24.6%, 24.8%, 67.0% for the 18h-charged sample. Failure analysis indicated that hydrogen charging can induce the brittle cracking of the alloy and the size of brittle cracking region being composed of quasi-cleavage facets and interfacial cracks on the fracture surfaces was increased with the hydrogen charging time.

Through exploring the stress corrosion cracking (SCC) behaviors of the as-cast Mg-8%Li and Mg-8%Li-6%Zn-1.2%Y alloys in a 0.1 M NaCl solution, it revealed that the SCC susceptibility index (I_SCC) of the Mg-8%Li alloy was 47%, whilst the I_SCC of the Mg-8%Li-6%Zn-1.2%Y alloy was 68%. Surface, cross-sectional and fractography observations indicated that for the Mg-8%Li alloy, the α-Mg/β-Li interfaces acted as the preferential crack initiation sites and propagation paths during the SCC process. With regard to the Mg-8%Li-6%Zn-1.2%Y alloy, the crack initiation sites included the I-phase and the interfaces of I-phase/β-Li and α-Mg/β-Li, and the preferential propagation paths were the I-phase/β-Li and α-Mg/β-Li interfaces. Moreover, the SCC of the two alloys was concerned with hydrogen embrittlement (HE) mechanism.

In this work, the tensile behaviors of Mg-8%Li and Mg-8%Li-6%Zn-1.2%Y alloys at ambient temperature were investigated and compared. It revealed that the plastic instability of Mg-8%Li alloy was quite remarkable and the variation range of serrated flowing stress can reach 5 MPa. For the Mg-8%Li-6%Zn-1.2%Y alloy, the formation of I-phase (Mg₃Zn₆Y) can simultaneously enhance the tensile strength and eliminate the plastic instability phenomenon, whilst its ductility was degraded. The in-situ tensile tests revealed that for the Mg-8%Li alloy, the severity and number of slip traces present in both α-Mg and β-Li matrix phases increased remarkably with the applied tensile strain. However, slip traces were quite fine in β-Li matrix phase and could be clearly observed when the applied tensile strain exceeded 18%. Due to the incompatibility of plastic deformation occurred in two matrix phases, the induced strain concentration at α-Mg/β-Li interfaces caused their subsequent cracking. For the Mg-8%-6%Zn-1.2%Y alloy, the I-phase distributed at α-Mg/β-Li interfaces suppressed the plastic deformation of α-Mg matrix phase and the tensile strain was dominated by the β-Li matrix phase, resulting in the disappearance of plastic instability. Moreover, the plastic strain would preferentially concentrate at I-phase/β-Li interfaces and subsequently induced the cracking of I-phase.