Microscopic Mechanisms Of Deformation And Phase Transition Under High-strain-rate Loading

Shock physics is the subject studying physical and chemical response of condensed matter under high-strain-rate loadings. It aims at predicting, estimating and analyzing the dynamic behavior of material and its structure. The research of shock physics includes theoretical study, simulation and experiments. In the present dissertation, we study the deformation, damage and phase transition through theoretical derivations, molecular dynamics simulations, and backed by experiments.The main contents are:(1) A quantitative model is built to describe the stress required for homogeneous partial dislocation nucleation under different loading conditions in Lennard-Jones single crystals. In this model, hydrostatic pressure and all of the deviatoric non-Schmid stresses are considered to have influence on critical resolved shear stress.Molecular dynamics simulations along 25 different loading orientations are employed to examine our model. The simulation results show that our model predicts the exact slip systems and the critical resolved shear stress within reasonable errors. Replacing Schmid factor with newly defined parameter SF* results in good agreement with our simulations.(2) We investigate shock response of single crystal and nanocrystalline pentaerythritol tetranitrate. In single crystals, shock-induced plasticity is consistent with RSS calculations and the steric hindrance model of PETN. In polycrystals,hotspot formation is directly related to GB friction and GB-initiated crystal plasticity,and the exact deformation is dictated by grain orientations and resolved shear stresses.GB friction alone can induce hotspots, but the hotspot temperature can be enhanced if it is coupled with GB-initiated crystal plasticity, and the slip of GB atoms has components out of the GB plane.(3) We investigate spallation in liquid copper at high strain rates induced by planar shock loading with classical molecular dynamics simulations. Spallation simulations are performed at different loading conditions. Loading may have pronounced effects on spall strength. The acoustic method for deducing strain rate and spall strength from free surface velocity is discussed and compared to direct simulations. The effects of temperature rise induced by shock wave, tension attenuation, sound speed, and density on the accuracy of the acoustic method are examined?Modifications to the choice of sound speed and density are proposed to improve the accuracy of the acoustic method.(4) We perform molecular dynamics simulations to investigate homogeneous nucleation and growth of nanovoids in liquid Cu. We characterize in detail the atomistic cavitation processes by following the temporal evolution of nanovoids,analyze the nucleation behavior with the survival probability (SP) methods and mean first-passage time (MFPT), and discuss the results against calculations of classical nucleation theory (CNT). Independent CNT predictions of critical size and nucleation rate are in agreement with the simulation results. Different from previous works, we adopt Tolman's equation to describe the evolution of surface tension with cavity size rather than assume a constant one.(5) We demonstrate that homogeneous crystal nucleation in liquid Cu can be realized under effective supercooling, via quasi-isentropic compression or ramp wave loading with a particle velocity achieved within a ramp time. The simulations yield the ramp time- particle velocity- supercooling relations for homogeneous crystallization. A ramp wave loading with ramp time ?100 ps is essentially isentropic for liquid Cu. Homogeneous nucleation can also be achieved with shock loading in initially supercooled liquids.(6) Molecular dynamics simulations of cavitation are carried out in Lennard-Jones and Cu liquids. These studies confirm that temporal and spatial scales are equivalent in the tensile strengths both of the liquids. Predictions based on smallest-scale MD simulations of Cu for larger scales are consistent with independent simulations, and comparable to experiments on liquid metals. We analyze these results in terms of classical nucleation theory and show that the equivalence arises from the role of both size and strain rate in the nucleation of a daughter phase. Such equivalence is expected to hold for a wide range of materials and processes and to be useful as a predictive bridging tool in multiscale studies.(7) We demonstrate the double-elastic wave in cubic crystals through linear elastic theory, molecular dynamics simulations and gas-gun experiments. This kind of wave can only propagate in certain low-symmetric orientations. Along high-symmetric orientations such as [100] and [110], elastic wave is in one-wave form. Double-elastic wave induces grain rotation, which is tied to initial plastic deformation.