Hot Deformation Behaviors Of An As-cast Mg-5Li-3Zn-Y Alloy

The hot deformation behavior of an as-cast Mg-5.21%Li-3.44%Zn-0.32%Y-0.01%Zralloy has been investigated using hot compression experiments. The rule of flow stress wasalso analyzed. The flow stress constitutive equation and the processing map for the alloy wereestablished respectively. The character of structure evolution of the alloy was also studied.Accordingly, the optimum parameters for hot working of the alloy were obtained, which wereverified by hot extruding test.The as-cast alloy has a mixture of α-Mg phase, β-Li phase, Mg0.97Zn0.03phase andW-Mg₃Zn₃Y₂phase which is RE-containing intermetallics. Obvious strain-hardeningphenomenon has been observed during tensile tests at room temperature. An excellentcombination of strength and ductility has been achieved with σ_0.2＝92.3MPa, σ_b＝165.7MPa,and δ=14.5%.The flow stress-strain curves of the alloy at all compression conditions exhibitstrain-hardening phenomenon, which becomes more obvious with decreasing deformationtemperature. The character of dynamic recrystallization takes a dominate role with lowerdeformation strain, whereas the character of strain-hardening takes a dominate role withhigher deformation strain, showing increasing flow stress-strain curves. A hyperbolic sinefunction can well describe the kinetics of the hot deformation behavior. The alloy exhibitshigh average hot deformation activation energy （Q≈136.76kJ/mol） and high average stressexponent （n≈5.28） due to the addition of RE elements. When the deformation temperature islower than250℃, second phase particle strengthening and self-diffusion of Mg mainlycontribute to the hot deformation mechanism. When the deformation temperature is higherthan250℃, the main deforming mechanism is dislocation climbing and self-diffusion of Mg.The processing map reveals two peak efficiency regions. Under these two regions, thesoften mechanism mainly contributed by DRX takes a dominate role, which is beneficial toductility shaping of the alloy. So the optimum parameters of strain rate for hot working of thealloy are T=300℃, ε=0.001s^-10.01s^-1and T=350⁴00℃、 ε=0.001s^-10.1s^-1.The main precipitations of the compressed alloy are MgZn₂phase and W-Mg₃Zn₃Y₂phase. While the volume fraction of β phase is relatively low and dynamic recrystallization ofβ phase has taken place immediately under all of the deformation conditions. The finedistributed MgZn₂phase depresses the dynamic recrystallization of α phase when the deformation temperature is lower than300℃. When the deformation temperature reacheshigher than300℃, MgZn₂phase gradually solves into the matrix. The disrupted W phaseprovides beneficial nucleation site for the DRX of α phase. The grain size after DRX ismainly determined by deformation parameters.Hot extrusion has been conducted on the experimental as cast alloy according to the theabove optimum parameters for hot working. The results show that strain-hardeningphenomenon can also be observed on the extruded alloy during tensile tests. Significantenhancement on mechanical properties and excellent combination of ductility and strengthhas also been observed on the extruded alloy, with σ_0.2=132MPa, σ_b=215MPa, and δ=20.9%.