Study On The Nucleation And Growth Of {10?2} Twin In Close-packed Hexagonal Metals By Molecular Dynamics Simulations

The{10?2}twin is important in coordinating the strain in the c-axis direction of the hexagonal metal.However,the current mechanism for the nucleation and growth of{10?2}twins remains controversial.In this paper,through molecular dynamics simulations,the mechanism of{10?2}twin nucleation and growth is studied.First,by constructing nanostructured magnesium,it was found under uniaxial tensile test strain{10?2}twins tend to be heterogeneously nucleated at the grain boundaries or at the crack tips.In the nucleation process,the BCC mediation phase was found.We proposes a simple geometric model of the{10?2}twin nucleation based on the BCC mediation phase.In this model,the main mechanism is to form the BCC mediation phase through the slip of the two intrinsic basal planes,and then the newly formed basal plane is slipped,thus obtaining the geometrical framework of the crystal rotation from the matrix lattice to the{10?2}twin lattice.Through the uniaxial tensile test of the prismatic/basal boundary,it is found that there are two disconnection dipoles on each intrinsic prismatic plane.Only one of these two disconnection dipoles is the nucleation site of the{10?2}twin.When the dislocation dipole is activated,the twin crystal grows along the[101?0]direction.During the slip of the step,the cylinder of the substrate is converted to the base of the twin.Based on the structural analysis of the dislocation dipole,it is concluded that the atomic layer slip in the twin nucleation process is not limited to the simple one-way movement of the single prismatic plane.The actual atomic movement strategy is the interaction of the two prismatic plane.The slip and the interaction of the basal plane are simultaneously performed.Under this atomic displacement strategy,it can be well explained that the prismatic/basal boundary,the basal partial dislocation,etc.,which is often found in experiments and is often accompanied by{10?2}twin nucleation.In addition,in the polycrystalline tensile simulation,it is also found that the grain boundary migration of magnesium in the gradient nanostructure is an important mechanism for coordinated plastic deformation.It has been reported that{10?2}prismatic/basal boundaries exist widely on the win boundaries,and grain boundary migration occurs at prismatic/basal boundaries.During this grain boundary migration,there is nucleation of{10?2}twin boundaries.Since the{10?2}twin and rismatic/basal boundaries are essentially<112?0>tilt boundaries,we construct a series of<112?0>asymmetric tilt boundaries to study the grain boundary migration and the microscopic mechanical conditions of the{10?2}twin nucleation and growth in hexagonal close-packed metals.In order to study the influence of grain boundary misorientation and loading direction,we propose a mechanical model of uniaxial stress-driven grain boundary migration,in which the grain boundary orientation varies from 0~o to 90~o.It is also considered to apply two types of uniaxial loads which direction is parallel or perpendicular to the grain boundary.Model predictions were compared to molecular dynamics.The results show that under two loading conditions,the uniaxial stress-driven grain boundary migration(with{10?2}twin boundary nucleation)can only occur within a certain range of the tilting boundary misorientation.The critical strain that initiates grain boundary migration changes in a non-monotonic manner with the difference grain boundary misorientation.The proposed model provides new insights into the grain boundary migration in hexagonal close-packed metals.Through molecular dynamics simulation and continuum mechanics models,it has also been found that when the local gradient of the elastic energy difference across the prismatic/basal boundary or the mean value of the grain boundary along the prismatic/basal boundary increases sharply,the twin will nucleate.The critical strain of the nucleation of the twins on the prismatic/basal boundary can be obtained by indicater based on local elastic energy.These results show that under the uniaxial load,the{10?2}twin in the close-packed hexagonal metal nucleate on the prismatic/basal boundary.Both the elastic energy difference gradient and the spatial average of the elastic energy difference can be used as a physical indicator.According to the simulation results and the mechanical model,the position and critical strain of the{10?2}twin nucleation on the prismatic/basal boundary are obtained.