| The dynamic deformation behavior of metal materials under shock loading is very complex,resulting in insufficient understanding of dynamic plastic deformation.To better understand the dynamic plastic deformation behavior of materials,a more comprehensive understanding of the dynamic response of materials is required.It is generally identified that the plastic deformation of materials under high strain rate conditions such as shock loading is dominated by gliding dislocations.As a consequence,understanding the motion properties of dislocations is necessary.Niobium has high melting point,corrosion resistance,excellent ductility and formability,can improve the strength and toughness of steel alloys,and is applied for aerospace,defense and so on.Lead is chosen as a candidate material for strategic weapons due to its low melting point,high density and phase transition on high pressure.Due to the limitation of loading and diagnostic technology,it is difficult to adequately explain and analyze the instantaneous microstructure and changes in physical property of materials under dynamic high pressure.Molecular dynamics(MD)simulation can provide the analysis of microstructural evolution at the atomic scale,combined with the characteristic time scale of the picosecond scale,making it an important means to conduct the physics research under dynamic highpressure.The accuracy of results for molecular dynamics simulation heavily depends on the interatomic potential to describe the interactions between atoms.However,the existing potentials of niobium lack an accurate description of the impact properties,such as the existence of an artificial structural phase transformation from BCC to HCP or the deviation of the Hugoniot relation curves from the experimental results;For existing potentials of lead,there are deficiencies in describing the high-pressure plasticity and phase transformation behavior at finite temperatures,making it difficult to conduct relevant simulation researches.For this purpose,the improved interatomic potentials of niobium and lead are constructed and systematically tested.The testing results show these potentials can well reproduce the basic physical properties of niobium and lead by systematically testing the new potentials,such as lattice constant,binding energy,elastic constant,vacancy formation energy and melting point,etc.The physical properties related to high-pressure properties are also reasonably predicted by these potentials,such as stacking fault energy,equation of state under high pressure,etc.Furthermore,after applying the potentials to dynamic compression,we find that the new potential of niobium can correctly describe the shock Hugoniot relation curve and predict stability of the structure under higher pressure.The new potential of lead can reasonably describe the thermodynamic properties of lead and the pressure of phase transition from FCC to HCP under high pressure.Therefore,the potential of niobium and lead constructed in this dissertation are suitable to study the plastic deformation mechanism under shock loading and the plasticity and phase transition at finite temperature,respectively.The motion properties of dislocations in niobium and lead under the applied shear stress are studied by the constructed potentials.The results show that at low applied shear stress,dislocation velocity is proportional to applied shear stress and inversely proportional to temperature,consistent with the phonon damping model.At higher shear stress,different types of dislocations exhibit different motion properties.There is a subsonic plateau at which velocity saturates in edge dislocations.In contrast,for screw dislocations there is a critical velocity that connects a temperature-dependent linear regime and a temperature-independent nonlinear regime.In addition,the transonic motions of the 1/2<111>{112} edge dislocations of niobium and the edge dislocations of lead were observed in the simulation.After calculating the dislocation velocities using the different potentials of lead,it is found that different potentials predict different dislocation mobility and this difference is proved to be related to the stacking fault energy described by the potentials.The crystallographic orientation dependence of shock-induced plasticity of single crystal niobium is also studied.The results indicate the plastic deformation mechanism is dominated by twinning for the shock compression along the [001] and [110] crystallographic directions,plastic deformation is dominated by dislocation multiplication and slip for the shock compression along the [111] crystallographic direction.The twinning mechanisms for the shock along the [001] and [110] directions are analyzed by the lattice model combined with the transition state theory,indicating that uniaxial compression along the specific directions plays the key role for the transformation path of deformation twins.The dynamic responses of lead under shock compression also exhibit strong anisotropy.The results show the single crystal lead exhibits anisotropy behavior in the propagation of shock wave,plastic deformation and phase transformation.The phase transitions are observed in the simulation results for shock compression along the [001] and [110] directions,only plasticity is observed for the shock compression along the [111] direction.With the increase of shock strength,the coupling between plasticity and phase transition is observed.Plasticity(slip,twinning)precedes the phase transition and provides the nucleation sites for the transformation.Two different phase transition mechanisms are observed for the dynamic phase transition process of lead under shock compression.For the shock compression along the [001]crystallographic direction,FCC lead forms a BCC-like lattice under uniaxial compression,followed by dislocation slip to form stacking faults and deformation twinning,and HCP phase nucleates based on these defects;for the shock compression along the [110] crystallographic direction,virtual melting occurs first,followed by recrystallization and phase transformation. |
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