Researches On The Microstructural Evolution Of Twin Dynamic Recrystallization In Magnesium Alloy

As a light weight and high specific strength material,magnesium alloys are regard as the most promising structural materials for applications in aerospace,automation and computer industries.However,owing to the lower symmetry crystal structure,magnesium alloys have a limited number of slip systems,which results in poor ductility at room temperature and widely limits their industrial application.Twin dynamic recrystallization widely exists at low and medium temperatures,it could refine the grain and improve magesium deformability effcetively.Therefore,it is significant to do researches on twin dynamics recrystallization.Among all the twin systems,{10(?)2} tension is commonly observed at the early deformation stage and has great influence on subsequent deformation.In this paper,{10(?)2} tension twin dynamics recrystallization microstructural evolution is investigated under different conditions by molecular dynamics method.The works are as follows:1.The coherent {10(?)2} twin boundary evolution behaviors were studied by molecular dynamics simulation under different shear directions and strain rates.The results showed that the twin was thickened under[(?)011]shear direction and shrunken with shearing in the opposite direction.It was found that the critical resolved shear stress of {10(?)2} twin boundary migration increased with the increase of the strain rate.In addition,the twin boundary migration distance was not related to the strain rates and shear directions,it was only affected by shear strain.By analyzing the atoms' displacement,it was concluded that the {10(?)2} twin migration was achieved by both the shear and the atomic shuffling.Every atom would be affected by the shear,and different shear directions would cause opposite move directions,which led to twinning or detwinning.The atom shuffling was only used for adjusting the glide twin boundary and mirror-symmetric twin boundary structure evolution.2.The incoherent twin boundary evolution was investigated under shear deformation.The results showed that incoherent twin boundary would transform into coherent twin boundary with twin boundary migrating.However,the coherent twin would recover to incoherent twin boundary in the unloading process.Due to the existence of defects,the coherent twin boundary migration would be hindered,which resulted a serrated shape.It was concluded that the defect was the main reason for formation of incoherent twin boundary.The transformation of coherent and incoherent twin boundary indicated that incoherent twin boundary migration was achieved by both shear and atom shuffling.Stacking faults were also observed in incoherent twin boundary migration process.They were related to the odd twin steps,and 1/6[(?)010]M and 1/6[(?)010]T were the main reason for their formation.Stacking faults formed in this way usually run through the matrix or twin,and only when I-TD met II-TD,could them stop inside the grain.3.Magnesium polycrystal containing {10(?)2} twin was compressed under different directions.The results showed that the dynamics recrystallization phenomena only occurred when loading normal to the twin boundary.It was found that twin dynamics recrystallization microstructure evolution process could be divided in two steps:1)basal partial dislocations nucleated near twin boundary,leading to large areas of stacking faults;2)non-basal slips nucleated in the stacking faults region continuously,forming the new crystal.When loading parallel to twin boundary,twin boundary migration dominated the deformation,which released the strain energy and inhibited the nucleation of dynamics recrystallization.4.Magnesium bicrystal of {10(?)2} twin was studied under different compression directions.The results showed that dynamics recrystallization occurred when compressed parallel and normal to twin boundary.The structure evolution process was same as that in polycrystal,it was also divided into two steps:1)basal partial dislocation nucleated and moved,forming large area of stacking faults and rotating the basal plane;2)partial dislocation nucleated on new basal plane,making the structure recover to HCP structure.By adding simulations,it was found that the confine of the first slip was the essential condition for DRX nucleation,the stronger the confine was,the earlyer the DRX nucleated.When compressed in 45 degrees direction to twin boundary,the twin boundary migration dominated the deformation process.The movement of twin boundary was achieved by motion of twin dislocations and tranformation of twin boundary and BP planes.