Molecular Dynamics Study On The Plastic Deformation Mechanism In Hexagonal Closed-packed Metals

With the development of manufacture and nanotechnology,the nano-mechanical devices have been widely used.It is found that the mechanical behaviors of metallic materials at nanoscale are very different from those at macroscale.The mechanical behavior is usually dominated by the microstructure and its deformation mechanism.Therefore,it is important to understand the plastic deformation mechanism from micro-field.At present,there has many difficulties and limitation for both traditional theory and mechanical experiments in explaining the mechanical properties of nano-materials.The molecular dynamics?MD?simulation,as an important method,can contribute to clarify the plastic deformation mechanism by providing the explicit detail of atoms movement.In this work,MD simulations are conducted to study the microstructure evolution process of the hexagonal close-packed(_hcp)metals.We focus on the growth mechanisms of different defects in plastic deformation stage.The main researches and results are listed as follow:?1?The pyramidal<c+a>dislocation in _hcp Mg is studied via the calculation of generalized-stacking-fault energy?GSFE?and MD simulation.According to the y surface of {1011} plane and its adjacent planes {3032} and {3034},the possible slip paths of {1011}<1123>dislocation are obtained.Combined with the atomic simulation in Mg under c-axis compression,the dynamic dissociation of pyramidal dislocation is observed.The y surface of {1122} plane and MD simulation results also show the slip path of {1122}<1123>dislocation.In addition to the direct slip path of pyramidal<c+a>dislocation,the other mode is fulfilled by two partial dislocations.By contrast,the maximum energy of partial dislocation slip is lower than the unstable stacking energy of full dislocation slip.Thus,the existence of partial dislocation is relatively reasonable.?2?The initial plastic deformation mechanisms of _hcp Mg nanopillars under compression and tension with different orientations are studied by MD simulations.The initial plastic deformation mechanisms include prismatic<a>slip,basal<a>slip,pyramidal<c+a>slip and crystal reorientations.The different crystal reorientations,e.g.basal/prismatic?BP?transformation," {1011}-{1012} "-like double twin and shear band,are particularly studied.Firstly,under c-axis tension,the BP transformation plays an important role in plastic deformation.The nucleation mechanism is described as the atomic shuffle.The BP interface migration is relied on the interfacial disconnections movement via shuffling.Moreover,the disconnections relatively tend to migrate towards the[1210]direction rather than the[1010]/[0001]direction since the accumulation of mismatches along the[1010]/[0001]direction can impede the disconnections movement.Furthermore,the boundary between the matrix and reoriented crystal is consisted of BP interface and {1012} twin boundary?TB?or series of BP/PB interfaces.The transformation between each other can be fulfilled by the glide and accumulation of interfacial disconnections.Secondly,along and near a-axis tension,the double twin-like structures are obtained.Two forms,which are respectively dominated by {1012} TB and BP interface,are expressed on the boundary between the two reoriented crystals.At the same time,a boundary between the reoriented crystal and matrix shows a rugged appearance in addition to the smooth{1011} TB.The nucleation of this grain boundary relies on multiple dislocations,and its migration is dominated by local atomic rearrangement.The orientation relationship between the various boundaries and the loading direction ultimately dominates the competition of different reotiented crystals.Besides,the shear band is another mode of crystal reorientation under special orientated loading.Its boundary migration depends on the movement of interfacial dislocations.?3?The molecular dynamics simulation is used to study the plastic deformation process of _hcp Ti nanopillars with circle,octagon and square cross sections.The main researches are focused on the _hcp-_fcc phase transition and the effect factors on its formation.The results show that the phase transition is nucleated by the accumulation of multiple non-adjacent partial dislocations.{1010}_hcp||{l110}_fcc?and<0001>_hcpl||<001>_fcc are used for describing the corresponding interface and direction.The surface and orientation effects are two important factors for the phase transition.The researches reveal that {1122} free surface and?[1010]-axis orientation loading are necessary conditions for the _hcp-_fcc phase transition.The surface energy difference between coarse {1122} plane and close-packed plane{111} may promote the transformation process.Meanwhile,the phase transition can be activated by only small prismatic shear stress.The present researches reveal the mechanisms of pyramidal dislocation dissociation,crystal reorientation,boundary glide and phase transition in _hcp Mg and Ti.They can provide a theoretical guidance on the material design for improving the strength and plasticity,and also have a great significance in the development and utilization of materials.