Molecular Dynamics Simulations On The Mechanisms And Dynamic Properties Of Melting In Shocked Copper

Large-scale molecular dynamics simulations are performed to explore the solid-liquid phase transitions in single crystal, bi-crystal and nanocrystalline Cu under shock loading conditions. The microjet production and jet breakup behaviors from single crystal Cu with a triangular grooved surface defect under shock loading are also investigated. Specifically,(1) We have built large-scale parallel molecular dynamics program (MOASP) for short-range interaction potentials based on the JASMIN infrastructure. Different kinds of potential models, ensemble and boundary control methods and mirco-structure analysis tools are integrated in the program. MO ASP has been successfully used to study the dynamic responses of metals such as shock plasticity, shock-induced phase transition, dynamic damage and the like.(2) Solid-liquid phase transitions in single crystal Cu during shock loading are studied using large-scale molecular dynamics simulations. The critical condition, dynamic properties and orientation-dependence are discussed in detail. We find that although the equilibrium states far behind shock front converge to the same Hugoniot, the pathways from meta-stable states right behind the shock front to the final equilibrium states and the resulting microstructures are strongly orientation-dependent.(3) The effects of nanovoid on the shock melting behavior of single crystal Cu are investigated. The long-time evolutions of the shocked region are captured by employing an absorbing boundary method. The simulation results show that prior to homogeneous melting of bulk solid, heterogeneous local melting in the void collapsed region dominates the melting process due to the effect of hot spot as well as the low energy barrier upon melting of the highly disordered, collapsed void region. As the hot spot cools, the liquid nucleus either re-crystallizes or retains liquid state and grows with time depending on the shock intensities.(4) Given the ubiquity and importance of grain boundaries (GB) in real materials, we investigate the shock melting of Cu bi-crystals with different types of twist GBs. It is found that the local melting properties of the shocked GB region are closely related to the GB energy. In the case of high energy GB, prior to the homogeneous melting within the constituent grains, continuous partial melting with considerable premelting of the shocked GB region precedes bulk melting with negligible GB superheatin, while solid state disordering may precede the partial melting. In contrast, for the low energy GBs, the melting process is very similar to that of a shocked perfect crystal.(5) We perform large-scale molecular dynamics simulations to study shock-induced melting transition of idealized hexagonal columnar nano-crystalline Cu model. Once again, it is found that although the Hugoniots of the columnar nano-crystalline Cu are similar for different loading directions, the exact melting dynamics and local melting features are highly anisotropic with respect to the shock directions. We observe rich phenomena with regards to wave propagation, and local melting and re-crystallization phenomena, including premelting and superheating, supercooling and recrystallization, as well as shock-induced grain boundary partial annihilation and the like.(6) In addition to the melting dynamics of perfect and defective Cu under shock compression, the solid-liquid phase transitions in single crystal Cu during release are also studied. Due to different microstructures in the far-behind-shock region for various loading directions, the release melting dynamics upon release are also highly anisotropic. The critical conditions and statistical properties of melting upon different release directions are obtained. The relationship between release melting and the release wave profiles as well as release path is also discussed.(7) We have performed large-scale molecular dynamics simulations to explore the microjet behavior including jet production and jet breakup from a grooved Cu surface under shock loading. It is found that the mircojet mass as well as the head speed of the microjet increases linearly with the post-shock particle velocity. The ejecta distributions after jet breakup are obtained with the help of the cluster analysis method. It is shown that the size distribution of ejecta exhibits a scaling power law which can be described well by the percolation theory independent of the simulated shock strengths and groove sizes.