Investigation Of The Plastic Deformation Mechanism Of Nanotwinned And Nanocrystalline Mg By Molecular Dynamics Simulations

Metal materials are indispensable structural and functional materials in the national economy,aerospace,transportation,marine equipment and national defense.The design and preparation of metals with high strength and ductility have always been the forefront of materials science.As the lightest metal structure material,magnesium has a bright application prospect.However,due to the few equivalent slip systems that can be activated at the same time,the ductility of magnesium is poor.The nano-structuring of magnesium and magnesium alloys can greatly improve their mechanical properties,thus it has received more and more attentions.In general,the mechanical properties of materials are closely related to their microstructure and deformation mechanism.Because molecular dynamics method can accurately capture the evolution process of material internal microstructure,it has become an effective research method to reveal the relationship between microstructure and macro mechanical behavior of materials in recent years.Therefore,in this paper,the microstructure and plastic deformation mechanism of nanostructures in hcp magnesium are studied by molecular dynamics simulations.The effects of temperature,grain size and strain rate on the plastic deformation mechanism are also studied.It is expected to provide theoretical guidance for improving the strength and toughness of Mg and Mg alloys by designing of nanostructures.The main research work and progress are as follows:1.The atomic scale model of {10(?)1}-{10(?)1} double contraction nanotwinned magnesium is established,and its interface structures are characterized.The plastic deformation mechanism of double contraction nanotwinned magnesium is further studied.The results show that there are three typical interface structures in double contraction nanotwinned magnesium,including the {10(?)1} coherent twin boundary(CTB),symmetric tilt grain boundaries(STGBS)and asymmetric tilt grain boundaries(ASTGBS).The basal slips and pyramidal slips dominate the plastic deformation mechanism of double contraction nanotwinned magnesium under different loading conditions.Under mechanical loadings,the corresponding grain boundary defects along STGBs and ASTGBs act as sources for nucleating and emitting basal and pyramidal dislocations,enhancing the plasticity.Meanwhile,the densely distributed CTBs in DCTWs act as strong barriers for dislocation motion,strengthening the material.2.The deformation mechanism and size effect of [1(?)10] textured nanocrystalline magnesium under uniaxial tensile loadings are studied.First,the structure of grain boundary in the nanocrystalline magnesium is studied after a heating-cooling relaxation.It is found that when the misorientation angle of initial grain boundary is close to the misorientation angle of low-energy interfaces such as twin boundaries,BP interface and {20(?)1} STGB,the grain boundary will change into a structure dominated by low-energy interfaces after the heating-cooling relaxation.When the interface of the initial grain boundary is located at or deviates from the corresponding{30(?)2} or {50(?)2} symmetrical tilt grain boundary slightly,the grain boundary will evolve into a longer {30(?)2} or {50(?)2} symmetrical tilt grain boundary after the heating-cooling relaxation.When the misorientation angle of initial grain boundary is less than 10° or close to 180°,the grain boundary will change into the interface structure consisting of alternate arranged dislocations after the heating-cooling relaxation.For the grain boundary which does not meet the above conditions,the original interface structure is still maintained.Under uniaxial tensile loadings,the plastic deformation mechanisms of [1(?)10] textured nanocrystalline magnesium are mainly basal slips and pyramidal slips,and the basal slips and pyramidal slips prefer to nucleate and emit at the inclined {10(?)1},{10(?)3} and 180° grain boundaries.The nucleation site is mainly at the junction of twin boundary and interface defect or the interface dislocation on the inclined 180° grain boundary.3.The effects of temperature and strain rate on the plastic deformation of the [1(?)10]textured nanocrystalline magnesium are studied.The simulation results show that the plastic deformation under uniaxial tensile loadings is dominated by basal slips and pyramidal slips at low temperature(5 K).The increase of temperature can promote the migration of {10(?)2} twin boundaries,BP interfaces and {10(?)1} twin boundaries on grain boundaries.The further increase of temperature(300 K)can promote the generation of new {10(?)2} twin or {10(?)1} twin on the grain boundaries.The strain rate variation in a certain range studied in this work has no effect on the young's modulus of the model,and the deformation mechanism of nanocrystalline magnesium under uniaxial tensile loading of different strain rates is still dominated by the basal slips and pyramidal slips.4.The models of {10(?)2} twin boundary interact with basal and pyramidal stacking faults are established,and the effects of basal and pyramidal stacking faults on{10(?)2} twin boundary migration are studied.The results show that the {10(?)2} twin boundary can cross the basal stacking fault and continue to migrate,thus the basal stacking fault has almost no blocking effect on the migration of the {10(?)2} twin boundary.When the {10(?)2} twin boundary meets with the pyramidal stacking fault,the {10(?)2} twin boundary is blocked by the pyramidal stacking fault,and a ladder like interface structure is formed near the pyramidal stacking fault.The pyramidal stacking fault has a stronger blocking effect on the migration of the {10(?)2} twin boundary.