Dynamic Mechanical Behavior And Microstructure Evolution Of Pre-twinning Magnesium Alloy Under High-speed Impact Loading

The magnesium alloys are widely used in weapons,aerospace,transportation and other fields because of their low density,high specific strength and high specific stiffness.In applications,magnesium alloy component are unavoidably subjected to high-speed impact loading under medium-high temperature.However,the low yield strength of magnesium alloy limits its application.It is reported that pre-twinning can improve the strength of magnesium alloy.Therefore,it is great significance to study the deformation mechanism of pre-twinning magnesium alloys under medium-high temperature impact loading.In this paper,AZ31 magnesium alloy with different twin densities was introduced by pre-deformation.Mechanical behaviors and microstructure evolutions of AZ31 magnesium alloy with different twin densities under medium-high temperature and high-speed impact loading were compared.Deformation mechanism of pre-twinning magnesium alloy under medium-high temperature impact loading was revealed.A mechanical constitutive model accurately predicting the medium-high temperature flow behavior was established.The main conclusions of the paper are as follows:(1)Pre-twinning can significantly improve the yield strength of AZ31 magnesium alloy during impact loading.The twins,divide the initial grains into small area,could serve as barriers to dislocation motion,These can effectively raise the yield strength of magnesium alloys.(2)At lower impact temperatures,the dominant microstructure for AZ31 magnesium alloys is crossing twins.As impact temperature increasing,deformation shear bands and twins are the main microstructure.At higher impact temperature,complete dynamic recrystallization occurs.The deformation critical temperature of dynamic recrystallization would be reduced because of twin density and strain rate increase.(3)The mechanical response is related to the competition among the shear band strengthen,the twin strengthen and the fine grain strengthen and depends on the dominant grain structure.At lower temperatures,the twin strengthen and the shear band strengthen determine the mechanical response.At medium temperature,the mechanical response depends on the twin strengthen and the fine grain strengthen.While at high temperature,the fine grain strengthen is the dominant strengthen mechanism and compensates for soften caused by the reduction of twin and shear bands and the increment of temperature.(4)The high-speed deformation of medium-high temperature is achieved by pre-twin inducing extension/contraction twin and base/non-base slip,the onset of contraction/double twins and base/non-base slip in grains.The medium temperature and high-speed plastic deformation is realized by twins,dislocation slip and deformation shear band at different deformation stage.The high temperature and high-speed plastic deformation is realized by twins,dislocation slip,transition shear band and recrystallization at different deformation stage.(5)The twinning strengthening is introduced to modify the Johnson-Cook constitutive model.The modified Johnson-Cook constitutive model can accurately predict the effect of twinning on the flow behavior.The correlation coefficient(R)is 0.9744 and the average relative error(AARE)is 4.51%,indicating that the modified Johnson-Cook constitutive modelcan well forecast the high-temperature flow behavior of the pre-deformed AZ31 magnesium alloy.