Effect Of Grain Size On Adiabatic Shear Susceptibility Of Three Magnesium Alloys

For magnesium alloy components,the source of damage should not occur during actual use or manufacturing.The adiabatic shear band（ASB）,as a special indicator of material failure,has significant research significance.Grain size has an important impact on the mechanical properties of materials.This study combines the two aspects to systematically explore the effect of grain size on the adiabatic shear susceptibility of AZ31,ZK60,and ZW41 magnesium alloys.This study prepared AZ31（3μm,35μm,50μm）,ZK60（3μm,15μm,25μm）,and ZW41（1.5μm and 5μm）magnesium alloy with different grain sizes through equal channel angular pressing（ECAP）shearing and heat treatment.High strain rate dynamic compression tests of AZ31,ZK60,and ZW41 magnesium alloy cylindrical and hat-shaped specimens with different grain sizes were conducted using split Hopkinson pressure bar（SHPB）.The strain rate range was 1000s^-1to 1700s^-1,and the test temperatures were room temperature and 200℃.The microstructure of magnesium alloy specimens with different grain sizes before and after deformation was characterized and analyzed using optical microscopy（OM）,electron backscatter diffraction（EBSD）,and transmission electron microscopy（TEM）.A systematic study was conducted on the adiabatic shear behavior,microstructure evolution process,and typical microstructure of magnesium alloys with different grain sizes AZ31,ZK60,and ZW41.The adiabatic shear susceptibility was discussed,and the formation mechanism of dynamic recrystallization in the ASB of three magnesium alloys with different grain sizes was explored,and LS-DYNA is used to simulate the stress,strain,and temperature changes of the ASB of magnesium alloys with different grain sizes under high-speed impact by establishing Johnson-Cook constitutive equation.Dynamic compression was investigated by cylindrical specimens of AZ31,ZK60,and ZW41 magnesium alloy with different grain sizes.The results showed that the yield strength and peak stress of the magnesium alloy decreased with the increase in grain size.Based on the Schmid factor calculation,it can be seen that grain size has a significant impact on the deformation mechanism of magnesium alloy.As the grain size increases,the deformation mechanism of AZ31 magnesium alloy changes from pyramidal<c+a>slip to pyramidal<c+a>slip and{10 1 2}tensile twinning.The deformation mechanism of ZK60 magnesium alloy changes from pyramidal<c+a>slip to pyramidal<c+a>slip and{101 2}tensile twinning.In fine-grain ZW41 magnesium alloy,the main deformation mechanisms of the 1.5μm and 5μm specimens are pyramidal<a>slip and pyramidal<c+a>slip,respectively.Dynamic compression was performed on the hat-shaped specimens of AZ31,ZK60,and ZW41 magnesium alloys.It was observed that the width of the ASB was narrower in fine-grain magnesium alloys,and the ASB was diffused as the grain size increased.Based on the true stress-strain curves of magnesium alloys with different grain sizes under adiabatic shear states,it can be seen that as the grain size increases,the competition between thermal softening and strain hardening becomes more intense,and the stress-strain curve exhibits a multi-peak characteristic.According to the calculation results of stress collapse time and adiabatic shear toughness,it can be seen that as the grain size increases,the adiabatic shear susceptibility of magnesium alloys increases.The lower adiabatic shear susceptibility of fine-grain magnesium alloys is attributed to their higher strain hardening capacity.Under the adiabatic shear state,the three magnesium alloys are divided into matrix region,transition region,and ASB region based on their different degrees of deformation.In fine-grain specimens,there is no clear boundary between the matrix region and the transition region,and the grain orientation from the non-ASB region（transition region and matrix region）to the ASB region presents a concentration dispersion process.The grain orientation in the deformation region of large grain-size specimens exhibits random,textured,and random,respectively.Under the adiabatic shear state,the high-density twinning phenomenon appears in large-sized magnesium alloy specimens,while this phenomenon does not occur in fine-grained magnesium alloys.Under high-speed deformation process,twins with different twin variants divide grains in vertical,parallel,and intersecting ways to refine the grains and promote the formation of fine grains in the ASB.The deformation mode of grains in the non-ASB region of the 3μm AZ31 specimen is prismatic slip.The deformation mechanism of grains in the non-ASB region of the 3μm ZK60 specimen,15μm ZK60 specimen and 5μm ZW41 specimen are pyramidal<a>slip.Research has found that the ASB is mainly composed of dynamically recrystallized grains,and the dynamic recrystallization mechanism of magnesium alloys with different grain sizes under adiabatic shear state is different.Among them,the dynamic recrystallization within the ASBs of 3μm AZ31 and 3μm ZK60 specimens is dominated by twin-induced dynamic recrystallization and continuous dynamic recrystallization mechanisms.The ASBs in 15μm ZK60,35μm and 50μm AZ31 specimens are formed under the combined action of twin-induced dynamic recrystallization,continuous dynamic recrystallization,and discontinuous dynamic recrystallization mechanisms.The dynamic recrystallization within the ASB of the 3μm ZW41specimen is formed under the combined action of twin-induced dynamic recrystallization and discontinuous dynamic recrystallization mechanisms.The twin-induced dynamic recrystallization mechanism was included in the specimens of three different grain sizes of magnesium alloys,indicating that twinning is the main reason for the formation of ASBs in the three magnesium alloys under high-speed deformation conditions.Based on the modified Johnson-Cook constitutive equation,the LS-DYNA module is used to simulate the adiabatic shear of AZ31,ZK60,and ZW41 magnesium alloys with different grain sizes.The temperature,stress,and strain changes of three magnesium alloys with different grain sizes during adiabatic shear deformation can be dynamically understood and compared.The numerical simulation results are consistent with the experimental results,which proves the accuracy of the numerical simulation results.