Inhibiting effect of I-phase formation on the plastic instability of the duplex structured Mg-8Li-6Zn-1.2Y (in wt.%) alloy

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.