Molecular Dynamics Simulations Of Melting And Shock Of Defective Copper

Molecular Dynamics(MD) is a brand new method to explore real world. Ma-croscopic properties of materials can be studied and understood at the atomic and molecular scale. Recently along the development of molecular dynamics potential functions and advance of computation speed, MD becomes more and more mature. MD simulations have been applied in many earth science fields, e.g. geophysics. MD simulations are very helpful to study and understand the state and properties of materials in the Earth interior, especially to study the structures, compositions, anisotropies and equations of state(EOS) of the materials in the lower mantle and outer core, even in the inner core. Melting is one of the most important processes for materials in the Earth interior. To understand this process, we conduct MD simulations to study the melting behaviors.Thermodynamic behaviors of nano-scale materials are explained in this the-sis.Numerical method of simulating thermodynamics behaviors of materials by MD is elucidated in detail. Melting behaviors of single crystal copper with defective structures, melting behaviors of copper under hydrostatic and shock wave loading to high pressures and spallation of copper under shock wave loading are simula-ted with MD.The mechanical behaviors of copper during spallation are analyzed. Moreover, elastic and plastic behaviors of copper nanoparticles impact on a rigid wall are also simulated.The methodology of simulating the thermodynamic behaviors of solid ma-terials by molecular dynamics method is elucidated indetail.Some key techniques ,such as the control of temperature and stress, potential functions are emphasized.Defective structures are omnipresent in real solids, especially in the interior of the Earth. MD simulations are performed to investigate melting behaviors of cop-per with defective structures. This thesis focuses on stacking faults(SF) and grain boundary(GB).Both intrinsic and extrinsic SF induce negligible reduction in the temperature at melting. The existence of SF makes it is impossile to observe homo-geneous and heterogeneous nucleations simultaneously, but only slightly increases the nucleation rate and probability of nucleation at heterogeneous nucleations sites. Two representative types of symmetric (110) tilt GBs are explored, one isΣ=11/(113)/50.48°with low GB energy and the other isΣ= 27/(552)/148.41°with high GB energy. The results show within the GB region, continuous local melting precedes discontinuous bulk melting, while continuous solid state disor- dering may precede local melting. ForΣ= 11/(113)/50.48°with low GB energy, premelting of the GB region is negligible and local melting occurs near the ther-modynamic melting temperature. The GB region as a whole is superheated by about 13%before its bulk melting. The the case ofΣ= 27/(552)/148.41°with high GB energy, considerable premelting is observed for local melting, while the bulk melting occurs with negligible superheating.Melting behaviors of copper under hydrostatic and shock wave loading to high pressures are studied. In the case of hydrostaic melting of copper, equilibrium mel-ting curve are obtained by both two-phase method and superheating-supercooling hysteresis method. They show very similar results. The amount of superheating or supercooling is independent of pressure and keep a constant nearly. For shock-induced melting of copper, we investigate melting under shock wave loading along three main crystallographic directions:(100). (110) and (111). Melting behaviors under shock wave loading along (100) and (110), (111) are very different. Fr the former, solid undergoes about 20%superheating before it melts with a pronoun-ced temperature drop. For the latter, melting is quasi-continuous and premelting about 7%is observed.MD simulations are carried out to study spallation in solid and liquid copper incuded by planar shock loading at exteme strain rates. It is a reasonable firs-order approximation deducing spall strengthσsp and strain ratesεfrom free surface velocity history. The anisotroy inσsp is pronounced for weak shocks and decreases for stronger shocks. Voids are nucleated at a defective site in a solid. For weak solid shocks, spallation occurs without tensile melting, for stronger shocks or if the temperature right before spallation Tsp is sufficiently high, spallation may be accompanied or preceded by partial melting. Tsp appears to have a dominant effect on spallation for the narrow range ofεstudied here.σsp decreases with increasing Tsp for both solid and liquid copper andσsp-Tsp follows an inverse power law for liquids.Elastic and plastic behaviors of copper nanoparticles impacting on a rigid wall are studied. The copper nanoparticles can be highly elastic during impact. Thermal fluctuations induce large fluctuations in the velocity restitution coefficient for small particles, and thus cause superelastic. Copper nanoparticles are still highly elastic for larger particles. The reduced restitution coefficient is due to plasticity during impact, which gives rise to irreversible heating.