Hot Deformation Behaviors Of An Extruded Mg-Li-Zn-RE Alloy

The hot deformation behavior of an extruded Mg-Li-Zn-RE alloy was studied by hot compression experiment. The rule of flow stress was also analyzed. The flow stress constitutive equation for the alloy was respectively established with power function, exponential function and hyperbolic sine function, and the processing map was plotted. The character of structure evolution of the alloy was investigated.The experimental alloy was composed ofα-Mg phase,β-Li phase and RE-containing intermetallics, which were located along the deformation direction like streamline after hot compression deformation. Full dynamic recrystallization (DRX) had occurred inβphase while recrystallization had not developed inαphase. RE-containing intermetallics formed in theα/βphase interface andβphase. The strength of the alloy was high at room temperature(σ0.2=256MPa,σb= 260MPa) and the elongation 8 was equal to 14%.The flow stress-strain curves of the alloy showed the character of dynamic recrystallization. It is not proper to fit the flow stress equation using a power function and an exponential function, however, a hyperbolic sine function can well describe the kinetics of the hot deformation behavior. The stress exponent n of the alloy was increased with the increasing of deformation temperature. The main deforming mechanism is dislocation climbing. The average hot deformation activation energy of the alloy was high (about 148kJ/mol). The mean value of Q increased with the rise of temperature and strain rate. Nevertheless, the value of Q is equal while the strain rate is 10s-1 and 0.01s-1.It showed that hot processing performance was good under all the experiment conditions from the processing map, which revealed three peak efficiency regions. The phenomenon of cracking, cavity and band was not observed in flow instable region. The optimum parameters of strain rate for hot working of the extruded Mg-Li-Zn-RE alloy are 250℃/10s-1 and 250℃/0.001s-1DRX took place quickly in all deformation process forβphase, while a phase dynamic recrystallization developed incompletely with increasing strain rate and decreasing temperature, with the recrystallization grains forming at the RE-containing intermetallics,α/βphase interface and the grain boundary. The average grain size of a andβphase increased with the strain rate decreasing and temperature rising. However, the average grain size of a phase became reduced with temperature rising in the temperature range of 400-450℃. RE-containing intermetallics and a phase accelerated the DRX process of P phase, and meanwhile DRX of a phase process was retarded.