| ME20M alloy(Mg-1.9Mn-0.3Ce alloy)is a widely used rare-earth-containing magnesium alloy,which has good corrosion resistance,strong weld-ability and excel-lent mechanical properties.As a structural material,ME20M magnesium alloy has been employed in the aerospace,automotive,and ship industries.ME20M alloy may service under complex conditions in engineering applications,for example,high-rate loadings at elevated and low temperatures.In addition,strength differential effect exists in the tensile and compressive deformations for magnesium alloy due to its crystal structure and processing method.Therefore,it is necessary to understand the influence of tem-perature and strain rate on such strength differential effect.In the present paper,exper-imental investigation,mechanism analysis and modelling on the tensile and compres-sive plastic deformation responses of ME20M alloy sheet were performed over a wide strain-rate range of 0.001-2500 s-1 and temperatures of 213 to 488 K.Uniaxial quasi-static and dynamic(0.001-1300 s-1)tensile tests for ME20M alloy sheet were performed along rolling direction(RD)and transverse direction(TD)at room temperature.The stress-strain responses show that the tensile behavior along RD is similar to that along TD,which indicates its in-plane anisotropy is weak.Uniaxial tensile tests for ME20M alloy sheet along RD within the strain rate and temperature ranges of 0.001-1300 s-1 and 213-488 K were carried out,and results indicate that the tensile behavior exhibits positive strain-rate dependence and negative temperature dependence.Both initial tensile yield stress and strain-hardening rate decrease with in-creasing temperature and decreasing strain rate.Microscopic observations show that basal and prismatic slips are the main deformation modes in the tensile plastic defor-mation,and a few {1012} tension twins are found in tension samples.The types of deformation mechanisms are independent of strain rate and temperature.It is found that a mixture of ductile fracture and brittle fracture is the principal fracture mode when the test temperature is equal to or lower than room temperature.While increasing the test temperature to 423 K or higher,the ME20M tensile samples are broken in a manner of ductile fracture.A combined Zerilli-Armstong constitutive model was proposed to de-scribe tensile behavior of ME20M alloy with hexagonal close-packed structure over wide temperature and strain-rate ranges.Uniaxial compressive experiments at quasi-static and dynamic strain rates(0.001-2500 s-1)for ME20M alloy sheet along RD,TD and normal direction(ND)were per-formed at room temperature.The measured stress-strain results show that the compres-sive stress-strain responses along RD and TD present "S" shapes with weak in-plane anisotropy.The compressive stress-strain responses along ND are strain-rate dependent,and there exists obvious out-of-plane anisotropy.Uniaxial compressive experiments for ME20M alloy sheet along RD at strain rates of 0.001-2500 s-1 and temperatures of 213-488 K were carried out.The acquired results show that initial compressive yield stress is approximately strain-rate independent,while its temperature dependence exists.The compressive strain-hardening rate exhibits positive strain-rate dependence and negative temperature dependence.Microscopic observations for compressive deformed samples show that basal slip and {1012} tension twins are the main mechanisms in the com-pressive plastic deformation,and effects of strain rate and temperature on the compres-sive plastic deformation mechanisms are weak.A modified Johnson-Cook constitutive model was employed to characterize the macroscopic compressive stress-strain re-sponses in RD over wide temperature and strain-rate ranges.Comparing the tensile and compressive responses of ME20M alloy along RD and TD at room temperature,it was observed that the strength differential effects for initial yield stress and strain-hardening rate increase with increasing strain rate,and the strength differential effect along RD is slightly greater than that along TD.Comparing tensile and compressive behaviors along RD at different temperatures and strain rates,it was found that the strength differential effect of initial yield stress increases with the decrease of temperature and the increase of strain rate.Strain-rate sensitivity of strength differential effect increases with increasing temperature during the plastic deformation stage.The asymmetry of the strain-hardening rate in tension and compression is due to the different contributions of twinning in tensile and compressive plastic deformation.The large proliferation of {1012} tension twinning is beneficial to the increase of strain-hardening rate.The contribution of {1012} tension twinning to the tensile plas-tic deformation is less,while the growth of {1012} tension twinning can be observed in compressive plastic deformation.The viscoplastic self-consistent(VPSC)crystal plasticity model was used to sim-ulate the tensile and compressive stress-strain responses and texture evolution of ME20M alloy.The simulation results indicate that VPSC model is capable of describing the tensile and compressive macroscopic mechanical responses in different directions over wide ranges of strain-rate and temperature,and the texture distribution from the pole figures simulated by VPSC model is similar to that obtained by experiments.The pole figures and the relative activity of deformation modes at different plastic strains were acquired by means of VPSC model,and the evolution of texture and deformation modes under different loading conditions was obtained.Pyramidal<c+a>slip occurs in tensile plastic deformation at low and room temperatures,and pyramidal<c+a>slip contributes more to plastic deformation with increasing strain rate.For compressive plastic deformation,{1012} tension twinning is depleted when the plastic strain is greater than 0.15,and pyramidal<c+a>slip takes place after the compressive plastic strain of 0.1,and the relative activity of pyramidal<c+a>slip increases with increasing strain rate.The relative activity of pyramidal<c+a>slip at low and high temperatures is lower than that at room temperature. |
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