Atomistic Simulation Of Twin-Twin,Twin-Grain Boundary,Twin-Dislocation Interactions In Magnesium

Although magnesium alloys have excellent properties,the poor formability at room temperature has limited their application.The twin-twin,twin-grain boundary(GB)and twin-dislocation interactions are frequently observed in deformed magnesium alloys,which influence the nucleation and propagation of twins significantly,and play an important role in their plastic deformation.In this thesis,the interactions of twins with GBs,dislocations and twins were investigated using molecular dynamics simulations.Furthermore,the related mechanisms were studied by analyses of stress distribution,disregistry and atomic shuffles.As the shear strain increases,the transverse propagation of the single {1012} twin is faster than its thickening,while the growth modes of two adjacent twins vary considerably with their relative positions,and even the asymmetric growth mode appears.Moreover,the stress is low around the twinning planes and prismatic/basal(PB)interfaces.The interactions of the stress field proximity to the interfaces greatly influence the twinning growth modes and the asymmetric stress distribution leads to the asymmetric growth of twins.With the temperature and strain rate increasing,the stress distribution and twin growth become more symmetric.To provide an insight into twin transmission event,the interactions between a twin and GBs with different misorientation angles were studied.It is shown that twin transmission tends to occur at low angle GBs,while twin propagation is always hindered by high angle GBs.Moreover,this work contributes to understanding the essential mechanism of twin transmission.Not only the favorable orientations(e.g.,at low angle GBs),but also stress concentrations near GBs are important to the occurrence of twin transmission.A stress concentration around the GB is observed when the original twin impinges on the boundary,leading to twin nucleation in the neighboring grain accomplished via a shuffle mechanism.The transmitted twin variant should be the one which can accommodate the strain from the original twin efficiently to relax the stress at the GB.Taking the three-dimensional nature of twins into consideration,the interactions of dislocations with both twin boundaries(TBs)and lateral boundaries of twins were investigated.The results show that when a basal dislocation enters the(1011)TB,a stacking fault forms on the(1011)plane of the twin.The faulted region is erased and a two-layer twinning dislocation(TD)is generated after the second basal dislocation moves toward the boundary,due to dislocation slip and atomic shuffle.Accompanying the glide of the TD,the TB migrates,leading to the twin growth.In addition,when a basal dislocation interacts with the lateral side of twin,de-twinning occurs attributed to a shuffle mechanism.The investigations about twin-defect interactions are beneficial to understand the nucleation and propagation of twins.It is demonstrated that the stress distribution plays a vital role in the twin nucleation and growth.The stress concentration near the GB induces twin nucleation in the neighboring grain through a shuffle mechanism.With regard to twin growth,the front of a growing twin is made of twinning planes and PB interfaces.Besides,the stress value in front of the twin influences the speed of growth and the interactions of the stress field determine the twinning growth modes.In addition,twinning or de-twinning occurs due to the dislocation-twin interactions.