Dynamic Mechanical Behavior And Deformation Mechanism Of Magnesium Alloys At High Strain Rates

Dynamic compressive and tensile tests are conducted respectively on AZ31B and AM30 extrusions along extruding and transverse directions,and on GW123K extruded rods along extruding direction,using a Hopkinson pressure bar and a reflecting Hopkinson tension bar apparatus.Dynamic tensile tests are conducted on AZ31B sheets along rolling,transverse and 45°directions,and on AM50 and AM60 high pressure vacuum die cast sheets.Applied stain rates are in the range of 500s-1～4500s-1.Optical microscope and SEM are used to analyze the microstructure and fracture surface of tested samples,and to observe the adiabatic sheering band of AZ31B and GW123K magnesium alloys.Dynamic mechanical behavior of the several magnesium alloys are analyzed and compared.Micro-deformation mechanisms,strain rate sensitivity and fracture mechanism under differently applied stress conditions are discussed.The dynamic compressive results of GW123K,AZ31B and AM30 along extruding direction are simulated using modified Johnson-Cook model.The results show that, the dynamic mechanical behavior of AZ31B and AM30 extrusions is relevant to the conditions of applied force. When compressed stress is vertical to c-axes(compresson along extruding direction),extension twining is firstly activated,and yield stress is low,and yield stress is insensitive to strain rates.When strain is about 8%,the crystals easy to twining are consumpted.Non-basal-plane slips are activated.The effect of deformation hardening and strain rate hardening becomes significant.The strain-stress curves rise.The second derivative of the strain-stress curves changes a sign when strains are at 8%.When tensile stress is vertical to c-axes(tension along extruding direction),extention twining is not activated.The yield stress is higher.non-basal-plane slips are the main mechanism of plastic deformation.When stress axes are parallel to c-axes(compression or tension along transverse direction),extension twining is not activated(compression along extruding direction),or activated but it cannot be coordinative (tension along extruding direction),which produce little contribution to deformation.The deformation is dependent to non-basal-plane slips.Contraction twining also takes part in deformation,but it is not the main deformation mechanism of magnesium alloys.Basal plane{0002} texture is formed during the extruding process of AZ31B and AM30 extrusions.The mechanical properties are much different between extruding direction and transverse direction because of the formation of the texture,and anisotropy is significant. Asymmetry of tension to compression appears along extruding direction.The ratio of compressive yield stress to tensive yield stress along extruding direction of AZ31B is 0.31, and the ratio of AM30 is 0.38.But the asymmetry between tension and compression along transverse direction is almost not existed.Because the c-axes of {0002} texture of AZ31B sheet formed during rolling process is vertical to the sheet plane,the stress-strain response of AZ31B sheet at rolling,transverse and 45°directions is almost the same.The yield stress of high pressure vaccum die cast AM50 sheet is lower than that of high pressure vaccum die cast AM60,but fracture strength and fracture strain are higher than that of AM60.The increase of Al element can improve dynamic yield stress of the alloys,but makes the plasticity of the alloy lower.The strain rate sensitivities of manesium alloys are relevent to deformation mechanism and chemical composition of the alloy. When extention twining is the main deformation mechanism,the strain rate sensitivities of the alloy are very low.When non-basal-plane slips are the main deformation mechanism,the strain rate sensitivities of the alloy become higher. The strain rate sensitivity order of several magnesium alloys from high to low is:AM60>AM50>AM30