Microstructure Evolution And Mechanical Properties Of Magnesium Alloy Prepared By Dynamic Extrusion

Magnesium alloy is the lightest structural engineering material and has broad application prospects in aviation,aerospace,ship transportation and other fields.Most magnesium alloys have very low strength at room temperature,and their densely packed hexagonal crystal structure limits the number of independent slip systems of plastic deformation,which results in poor plasticity at room temperature.Up to now,although the research on strengthening and toughening methods for magnesium alloys is quite hot,there are still many problems to be solved.In this study,AZ31 magnesium alloy was taken as the research object,and the magnesium alloy grains were refined by equal channel angular extrusion.At the same time,two dynamic extrusion techniques were proposed in magnesium alloy samples with different grain sizes to introduce high density deformation twins,aiming at preparing a kind of AZ31 magnesium alloy with strong and tough combination,and studying the strengthening and toughening mechanism behind it.This paper provides a new method and theory for strengthening and toughening magnesium alloys.Firstly,the finite element technique was used to study the plastic deformation during dynamic extrusion.Then,the microstructures of samples in different states were analyzed by optical metallography microscope,scanning electron microscope and transmission electron microscope.The effects of grain size,texture and deformation twins on the mechanical properties of magnesium alloy were systematically studied.The main conclusions of this paper are as follows:(1)Firstly,high density {10-12} deformation twins were introduced into the fine-grained AZ31 magnesium alloy by dynamic extrusion technology,and the high density dislocation structure was reduced by annealing at medium temperature.Finally,the pre-twin fine-grained AZ31 magnesium alloy showed good strength and ductility.The analysis of dynamic extrusion and annealing samples shows that the higher Schmidt factor is helpful to improve the yield strength of magnesium alloy along the extrusion direction.(2)The high density twin boundary can be used as a barrier for dislocation movement during the process of tensile plastic deformation,which significantly improves the strength of AZ31 magnesium alloy.At the same time,twin boundaries provide more dislocation nucleation locations and slip systems by changing local crystal orientation,which helps to improve plasticity.(3)Secondly,high density {10-12} deformation twins were introduced into magnesium alloy samples with different grain sizes by the combination of equal channel angular extrusion technology and dynamic compression.The pre-twin magnesium alloys also showed good strength and ductility.After the grains were refined by equal channel Angle extrusion,the magnesium alloy samples with bimodal microstructure without deformation twins showed the best tensile ductility,and the tensile ductility and uniform elongation of the magnesium alloy samples were reduced after the grain size was further refined.(4)Introducing high-density deformation twins in fine-grained magnesium alloys can effectively improve the plasticity of magnesium alloys,because twins can provide more nucleation points for dislocation and inhibit the proliferation of dislocation motion during plastic deformation.The strain hardening ability of magnesium alloy is mainly determined by the limited dislocation proliferation and interaction in the slip sliders of the substrate.The introduction of high density deformation twins can provide a new nucleation point for dislocation and effectively improve the strain hardening ability of magnesium alloy.