Molecular Dynamics Simulations Of Mechanical Properties And Deformation Mechanisms Of Nano-polycrystalline Al

Compared with macro bulk materials,nano-polycrystalline metal materials have more excellent mechanical properties,which has attracted extensive attention of scholars around the world and carried out a lot of related research.In this paper,the effects of different average grain size,twin content,temperature and alloy element Ti on the mechanical properties and deformation mechanism of nano-polycrystalline Al under tensile loading were studied by molecular dynamics simulation.The main conclusions are as follows:（1）For different average grain sizes,the simulation shows that the relationship between yield stress and grain size can be described by the classical Hall-Petch relationship until the grain size decreases to 5.91 nm.However,when the grain size is less than 5.91 nm,the relationship between yield stress and grain size becomes an“inverse”Hall-Petch relationship.This inverse relationship is mainly related to the dislocation density after the relaxation of the model,and the variation trend of yield stress is the same as that of dislocation density.In addition,the stacking fault tetrahedral structure generated within the grain increases with the decrease of grain size.Following with plastic deformation,the stacking fault tetrahedral structure will evolve into a more complex stacking fault structure.As the movement of dislocations,dislocation entanglement will occur within the grains.（2）Twins were added to the nanocrystalline Al with an average grain size of 5.67nm.The calculation results show that at 300 K and large twin boundary spacing,the main deformation mechanism is that the Shockley partial dislocation nucleates at the grain boundary,and the Burgers vector of this partial dislocation is b=₆112.With the decrease of twin boundary spacing,twin boundary also becomes the location of Shockley dislocation nucleation,resulting in the decrease of yield stress.The movement and disappearance of twin boundaries is mainly due to the nucleation of a large number of dislocations at the twin boundaries,and with the plastic deformation,new deformed nanotwins will be produced.When the temperature changes to 600 K,the atomic motion in the grain increases with the increase of temperature,and the binding force between atoms decreases,resulting in the decrease of yield stress and Young’s modulus.The movement of grain boundaries and twin boundaries are the main deformation mechanism of nano-polycrystalline Al at high temperature.Compared with 300 K at room temperature,the dislocation slips and the new deformation twins decrease with increasing temperature at 600 K.At the same time,high temperature will increase the content of disordered atoms in nano-polycrystalline Al.（3）The nano-polycrystalline Al structure with an average grain size of 5.67 nm was selected,in which the alloy element Ti atom was randomly added.The calculation results show that the tensile strength of nano-polycrystalline Al is increased by adding alloy element Ti,and the dislocation type in nano-polycrystalline Al is mainly Shockley dislocation.With the tensile loading,the stacking fault structure is formed by dividing dislocations in nano-polycrystalline Al.In addition to stacking faults,the deformation mechanisms in nano-polycrystalline Al include dislocation entanglement,stacking fault tetrahedron and Lomer-Cottrell lock.However,compared with pure nano-polycrystalline Al structure,these structures are relatively less produced.Therefore,the alloy element concentration is an important factor affecting the tensile strength and plastic failure mechanism of nano-polycrystalline Al.