| As a new type of light metal structural material,magnesium alloy has excellent performance and high specific strength,which has great development potential and research value in aviation,automobile and other manufacturing industries.On the other hand,magnesium alloys have defects such as low strength and poor plasticity at room temperature,which limit their wide application.In the deformation process of magnesium alloy subjected to external loads,its unique close-packed hexagonal structure causes not only crystal slip,but also twins,twin boundary,holes and cracks,which also have important effects on the plastic deformation.However,traditional mechanical experiments can only characterize and study its mechanical properties and texture changes from a macroscopic perspective,and it is difficult to explore the deformation of its internal structure from a microscopic level.Therefore,it is very necessary to use computer simulation to simulate magnesium and its magnesium alloy from a microscopic level.In this paper,the interaction of grain distribution,grain boundaries,pores and cracks in monocrystalline and polycrystalline magnesium has been studied from the microscopic point of view by molecular dynamics method.The plastic deformation mechanism of monocrystalline magnesium and polycrystalline magnesium has been investigated by analyzing the stress-strain,atomic displacement and dislocation propagation.The research content and main conclusions of this paper are as follows:1.The Numerical simulation of tensile loading along the C-axis was carried out on a single crystal magnesium model with preset cracks and holes,and compared with the metallographic structure changes of the actual magnesium alloy tensile test,the influence of hole defects at different positions in magnesium crystals on crack propagation was explored.The results show that under the same characteristic size,the yield strength of the single crystal magnesium model is affected by the voids at different location.When the voids are located in the crack [10(?)0]direction,the yield strength of the model is the lowest.The void can alleviate the stress concentration of the crack in the void direction,and affect the twins growth rate of the crack.The twin growth rate near the void of the crack is relative slow,and the twin growth rate is the slowest when the void is located at [10(?)0] direction.The voids has little influence on the propagation direction of the crack.Generally,the crack extends in the tensile direction after combining with the void,and the overall appearance is a more symmetrical cone expansion.But,when the void is located in the crack [0001]direction,there is no obvious connection between the cracks and the voids.2.By applying tensile load to the magnesium twin-crystal model with hole defects,the plastic deformation of the twin-crystal magnesium at different interface rotation angles is studied.After analyzing the stress-strain data and microstructure of the microscopic magnesium twin-crystal model in the plastic deformation process,it is found that:(1)In uniaxial tensile deformation,dislocation nucleation is more likely to occur at the interface defects of grain boundaries due to the large atomic distribution gap at the grain boundaries.Base plane and conical slip are the main deformation modes at the grain boundaries.With the increase of grain rotation Angle,the growth direction of dislocation shifts along with the direction of grain base plane.(2)Under tensile load,the existence of holes in grain boundaries does not affect the plastic deformation mechanism,but the size change of holes is small.Compared with other locations of grain boundaries,induced dislocation nucleation is more likely to occur in holes.(3)Asymmetrical grain boundaries formed by mutually perpendicular grain distributions During the stretching process,the stress distribution in the system preferentially concentrates on the grains parallel to the(0001)base plane and the stretching direction,and a large number of holes are emitted and holes are formed in the grains,and the further combination of holes and vacancies forms cracks distributed along the(10(?)2)plane and((?)012)plane.In the plastic deformation of the grain whose base plane is perpendicular to the tensile direction,the stress concentration is relieved by the ellipsoidal dislocation growth in the1/3 < 1(?)20 >and < 1(?)00 >direction,and the grain boundary is gradually formed during the tensile process.3.The deformation process of polycrystalline magnesium models with 64 grains,128 grains and 256 grains under uniaxial tensile load was simulated,and compared with the tensile tests with 1%,5% and 10% compression.Through the analysis of the stress-strain data and microstructure of magnesium polycrystalline model and magnesium alloy in the process of microplastic deformation,it is found that under the action of external tensile load,the more grains in the same volume of magnesium crystals,the smaller the grains,the more different orientation between grains.Grain refinement is beneficial to grain rotation and grain boundary movement,and the activation stress of twinning is lower,and it is easier to coordinate crystal deformation.With the increase of the number of grains and the decrease of grain size,the proportion of atoms in the grain boundary will also increase.When the plastic deformation of magnesium crystal occurs under the action of tensile load,dislocation emission and slip at the grain boundary can provide a large degree of deformation for magnesium,and increase the toughness and plastic deformation ability of the material.During the stretching process,the grain arrangement direction changes little,there are different degrees of deviation along the stretching direction,and dislocation nucleation is more likely to occur at the grain boundary,but there is no transgranular expansion. |
