Multi-Scale Study On Plastic Deformation Mechanism Of Extruded Magnesium Alloy Under Combined Stress

Magnesium alloys have attracted more and more attention in the fields of automobile,medical and health care,3C products and so on,due to excellent properties,such as low density,good bio-compatibility,high specific strength and stiffness.However,magnesium alloys have HCP crystal structure,with strong anisotropy,poor plastic deformation ability at room temperature and unstable properties at high temperature,which greatly limit the practical application.Thus,it is of strategic significance to study the plastic deformation mechanism and microstructure evolution of magnesium alloys.At present,the research on the plastic deformation mechanism of magnesium alloys is mostly focused on the uniaxial stress state,which is quite different from the multiaxial or combined stress state in the actual forming process.Therefore,in order to clarify the deformation mechanism of magnesium alloys in the practical forming process,the plastic deformation mechanism of extruded magnesium alloy and magnesium single crystal under combined stresses are studied by experiment and simulation.The main research contents and results are as follows:(1)The plastic deformation mechanism of extruded magnesium alloy under shear-tension and shear-compression combined stresses were studied experimentally.A new type of shear-tension specimens(STSs)and shear-compression specimens(SCSs)corresponding to different shear stress ratios are designed.The mechanical curves under quasi-static loading at room temperature were measured,and the evolution of metallographic structure and texture were observed.The effects of different shear stress ratios on mechanical properties and microstructure evolution were analyzed.The effect of shear stress ratio on failure mechanism was also analyzed by fracture scanning.It is found that the deformation of extruded magnesium alloy under combined shear-tension stresses is mainly dominated by basal and prismatic slips and supplemented by twin deformation.On the contrary,the deformation under combined shear-tension stresses is dominated by basal slip and twins and supplemented by prismatic slip.(2)The plastic deformation process of magnesium single crystal under uniaxialstress,shear-tension stress and shear-compression stress was studied based on molecular dynamics.The results show that the ability of plastic deformation decreases with the increase in shear stress ratio under combined stresses.Under shear-tension combined stresses,the plastic ability of Mg single crystal is better when it is subjected to tensile stress along ]0110[ direction and shear stress along ]0121[ direction due to the twinning activated.But under shear-compression combined stresses,the plasticity of Mg single crystal is poor when it subjected to shear stress along ]0121[ direction and compressive stress along ]0110[ direction,which is more conducive to the growth of the shear band and the formation of cracks at the edge of the model.(3)Based on the idea of multi-scale research,the micro-and nano-scale correlations of plastic deformation mechanism of extruded magnesium alloys and magnesium single crystals under combined stresses were analyzed qualitatively.