Study On Twinning Behavior In Mg-Al-Ca-Mn Magnesium Alloys

Due to the poor room formability,strong tension-compression yield asymmetry and mechanical anisotropy,the application of magnesium and its alloy was limited to some extent.Many kinds of methods were applied to improve the properties of magnesium alloy,in order to meet different engineering requirements.And the improvement of yield strength,anisotropy and the tension-compression asymmetry of magnesium alloy through deformation twin introduced by pre-strain,has gradually become one of research highlights.Besides,the plastic forming of magnesium alloy generally processed under high temperature.Thus,the research on plastic forming of magnesium alloy under high temperature has important application value and theoretical significance.In this paper,the extruded Mg-Al-Ca-Mn magnesium alloy was selected as the subject of this study.Different pre-compressions were conducted.The effects of pre-compression on the re-compression behaviour of extruded Mg-Al-Ca-Mn magnesium alloy at room temperature were investigated,by means of optical microscopy(OM),strain hardening rate.The effect of twinning on dynamic recrystallization during high temperature plastic deformation was studied by means of different compression directions.At the same time,the O lattice model is used to further analyze the basic characteristics of different types of twin.The main conclusions can be summarized as follows:(1)Pre-compression along ED was conducted on extruded Mg-Al-Ca-Mn magnesium alloy.The {10-12} extension twinning introduces a crystallographic lattice re-orientation of 86° in twinned regions,resulting in the formation the twin texture with c-axis aligned parallel to the compressive load.After the ND(normal)pre-compression,a small amount of {10-11} twin is produced and contains some {10-12} twins,but the twinning density is lower than the ED direction.For the pre-compressed samples,when re-compression was conducted along ND,the phenomenon of de-twin will occur.While the pre-compression of the specimen along the ND direction for re-compression along the TD direction results in {10-12} twins again.(2)The introduction of twinning by pre-compression has an important effect on the mechanical properties of subsequent compression.The yield strength and the ultimate compressive strength are improved,and the anisotropy is weakened.The strain hardening behavior of the alloy is closely related to the initial texture and {10-12} twin.Due to the pre-compression of the introduction of new texture components,making the secondary compressive stress-strain curve and the work hardening capacity and pre-deformation there are significant differences.(3)Mg-Al-Ca-Mn alloy hot-compressed at high temperature,the flow stress is affected by the deformation temperature.When the strain rate is constant,the flow stress decreases with increasing temperature.The strain hardening rate shows that the main deformation is dominated by dislocation slip at higher temperature.With the increase of the strain,the heat shrinkage at the same temperature tends to be a stable value.The main reason is that the softening effect of the work hardening and the dynamic recrystallization reaches a equilibrium state.(4)Compressed at 200 ? to 300 ? in the ED direction produces a strip-like stretched twin.With the increase of the hot compression temperature,the number of twins decreases gradually.When the temperature increases to 350 ? and 400 ?,the twin crystal structure is replaced by the newly generated dynamic recrystallization structure.ED direction compression Since twinning provides more dynamic recrystallization nucleation energy,dynamic recrystallization is more pronounced than ND compression,and grain refinement.(5)Mg-Al-Ca-Mn alloy secondary compressed at room temperature,the peak compressive strain hardening rate of is obviously lower than that of pre-compression,which is closely related to the effect of preset twins.On the one hand,Preset twins can refine the grain,improve the strength;On the other hand,it also can adjust the slip system to promote the sliding process.Pre-compression annealing eliminates dislocations and causes the twin boundaries to migrate,exhibiting softening of the annealing process.Pre-compression annealing,eliminating dislocations,so that the twin boundaries of the migration,the performance of the annealing process for the softening effect.(6)O-lattice model simulates {10-12} tensile twinning,{10-11} compressive twinning and the second twin interfacial relations of 37.5 ° and 30.1 ° {10-11}-{10-12},calculated by geometrical relations The obtained shear is very close to the actual value and provides an effective and reliable method for studying the twin deformation mechanism.