Molecular Dynamics Study On Mechanical Behaviors Of Grain Boundaries And Nanotwins

Nanocrystalline (NC) metals and their alloys have been wildly used in the applied industries for their superb mechanical behaviors. The enhancement of the strength and ductility are mainly due to the refinement of the grains size to a nanometer level, usually under 100nm. Plane defects such as grain boundaries (GBs), coherent twin boundaries (CTBs) and incoherent twin boundaries (ITBs), play a crucial role on the plastic deformation of the polycrystalline metal. They serve as defects nucleation sources which can accommodate the plastic deformation very well. Dislocation activities may also be hindered by the GBs, CTBs or ITBs which improves the dislocation storages abilities. As a result the ductility would be highly enhanced by the rich volume of dislocation. Moreover, the plane defects significantly exert effect on the intergranular and the transgranular crack propagations. Due to the dislocation emission from the plane defects or crack, crack tips may be badly blunted, and subsequently crack propagations are impeded. This feature are benefit to the materials fracture toughness. Thus it is very important to investigate the materials which contain plane defects, and reveal their deformation mechanisms. Molecular dynamics (MD) method are competent in presenting microstructure evolutions details in atomistic scale by simulating models. Thus in this work, molecular dynamics simulations are employed to study the mechanical behaviors and plastic deformation mechanisms of bicrystal and nanotwinned materials. The main results are listed as follow:1. α-Fe bicrystal models which contains the{112}(110) grain boundary are simulated under tensile loadings. Plastic deformation mechanisms and intergranular crack propagations are investigated. It is found that phase transition induced by the partial dislocation slipping along the grain boundary interface and subgrains generations are the main plastic deformation mechanisms. Phase transition can be captured mainly in the early stage of the plastic deformation, and with the strain increasing, subgrains gradually dominate the deformation. Temperature and strain-rates exert influences on the mechanical behaviors of the bicrystal models. Relative high temperature and high strain-rates are benefit to the subgrains generations which enhances the materials ductility. Intergranular crack propagation are affected by the crack position and temperatures due to the different plastic deformation mechanisms around the crack tip. It is found that subgrains generations ahead of the crack can prevent cleavage fracture especially under the high temperature.2. {1012}<1011) nanotwinned magnesium with different orientations and twin boundaries spacing are studied. Intergranular crack propagation are also concerned considering crack position and twin boundaries spacing effects. When the tensile loadings is perpendicular to the twin boundaries interfaces, basal dislocation emission, stacking faults and fcc phase transition are the main plastic deformation mechanisms. With the loadings orientations changing, twinning dislocation inducing twin boundaries migration and basal dislocation motion compete to dominate the deformation process. Different crack positions lead to various crack propagation pattern due to the dislocation nucleation abilities. Moreover a ductile to brittle transition is captured by changing the twin boundaries spacing. Essentially, it is the dislocation densities that influences the materials fracture toughness.3.{1011}{1012) nanotwinned magnesium with pre-existing line defects and different twin boundary spacing are studied to investigate the mechanical behaviors and plastic deformation mechanisms. Because of the stability of the{1011}(1012) twin boundary, it can hardly serve as defects nucleation sources. Thus the model without pre-existing defects yields under a high strain and possesses very low average flow stress. The introduction of the line defects can remarkably enhances the ductility of the nanotwinned materials under both compressive and tensile loadings. Basal and pyramidal slip are the main plastic deformation mechanisms. However due to the different volume of the slip, the tension-compression asymmetry are capture during the simulation processes.4. Because of the different twin boundaries in{1012}{1011} and{1011}{1012} nanotwinned magnesium, materials have distinct mechanical behaviors and plastic deformation mechanisms. This can be attributed to the different abilities of generating dislocations along twin boundaries interfaces and twin boundaries impeding crack propagation.