Investigation Of Deformation Behaviors Of Metallic Nanoparticles And Nanowires Based On Molecular Dynamics

In this thesis,deformation behaviors of nanomaterials,including Cu nanoparticles, eore/shell-type Cu/Ag nanoparticles,defect-free nanowires,twinned nanowires and Al-Cu alloy nanowires were studied by molecular dynamics simulation method.Then,some mechanical properties and deformation mechanisms of lower-dimension nanomaterials were obtained.The compressive deformation of Cu nanoparticles with diameter of 4 nm,6 nm and 8 nm was examined by using molecular dynamics simulation.The compressive stress increased in a zigzag fashion with strain,accompanying with the temperature zigzag rise and the potential energy zigzag rise.The analysis of atomic structure demonstrated that the compressive stress rise and drop corresponded to that the defective atoms aggregated to form clusters and that the clusters decomposed into single atoms,respectively.The essence of compressive deformation of nanoparticles is a repeated process of storing and releasing energy,as is similar to the stick-slip motion in nanoscale friction process.The tmiaxial compressive deformation of core/shell-type Cu/Ag nanoparticles was simulated by molecular dynamics,revealing the role of nano-phase boundaries in the mechanical deformation.The simulations show that single type of Shockley partial dislocations glide across {111} slip planes and then resulted in elongated Cu cores.Twinnings, which originated from the movement of a pair of anti-parallel dislocations,were observed in the compressed core/shell-type Cu/Ag nanoparticles.The ultra-high atomic level stress in the phase boundary can ensure the movement of a pair of anti-parallel dislocations.The uniaxial tensile loading of defect-free Cu and Ni nanowies was carried out by utilizing molecular dynamics simulation.The results of structure relaxation showed that the surface stress can drive crystallographic lattice reorientation,in which the axial crystal orientation of nanowires transformed from <100> to <110> with the help of Shockley partial dislocations of Burgers vector <112>/6.Especially,we observed the rotation movement of slip plane in Ni nanowires.The axial tensile loading can drive reverse process of above reorientation.Under tensile loading and unloading,the Cu nanowires exhibit recoverable strains up to 50%.The uniaxial tensile deformation of twinned Cu nanowires was examined to reveal the strengthening mechanism of twinning boundary by utilizing molecular dynamics simulation. The initial configurations of nanowires,including the side length of cross section varied from 3 to 6 nm and twin thickness varied from 1.25 to 10 nm,were created basing on new twinning mechanism with zero macroscopic strain.The results showed that twinned nanowires were strengthened by twinning boundaries and the highest increment reached 60%.The slop of stress-strain curves and atomic structural evolution indicated that the strengthening process consists of two competitive processes,including extending elastic deformation and twinning boundaries blocking the movement of dislocations.Therefore,a theory model,that the ratio of twinning boundary area to surface area in twinned nanowires decided the energetic nucleation of dislocation,was built.When the ratio is more than 0.3,the process of extending elastic deformation can be observed in our simulation.A molecular dynamics simulation was utilized to study Al-Cu alloy nanowires for obtaining mechanical properties and deformation mechanisms by changing the ratio of Al atom to Cu atom.The simulation results showed that surface stress can force AlCu nanowires into high-elastic states and induce slip and clusters in AlCu₃ nanowires.Two distinct types of deformation mechanisms were observed.One is that the AlCu nanowires underwent the changes of elastic deformation,amorphous transition,necking,and breaking.The other is because the(0(?)1) twinning and crystal region coexist,the calculated tensile curves of AlCu₃ nanowires show many yield points,and unloading can lead to elastic recovery of 4%～9%.We increased the percentage of Al atom to build Al₃Cu and Al₁₅CU nanowires,which showed a good plastic but lower strength than other nanowires.