An atomic study on the shock-induced plasticity and phase transition for iron-based single crystals

Understanding the dynamic lattice response of solids under the extreme conditions of pressure, temperature and strain rate is a scientific quest that spans nearly a century. Critical to developing this understanding is the ability to probe and model the spatial and temporal evolution of the material microstructure and properties at the scale of the relevant physical phenomena—nanometers to micrometers and picoseconds to nanoseconds. While experimental investigations over this range of spatial and temporal scales were unimaginable just a decade ago, new technologies and facilities currently under development and on the horizon have brought these goals within reach for the first time. The equivalent advancements in simulation capabilities now mean that we can conduct simulations and experiments at overlapping temporal and spatial scales. In this article, we describe some of our studies which exploit existing and new generation ultrabright, ultrafast x-ray sources and large scale molecular dynamics simulations to investigate the real-time physical phenomena that control the dynamic response of shocked materials.

Shocked materials at the intersection of experiment and simulation

A model of a wide-range semi-empirical equation of state for metals is presented. The specific heat and Gruneisen coefficients of ions and electrons are functions of temperature and density. At low temperatures, the heat capacity varies according to Debye theory. The removal of the degeneration of the electron gas with increasing temperature is taken into account. The effect of ionization on the thermodynamic functions is effectively taken into account. The equation of state allows the calculation of states in a two-phase liquid-vapor region. This model was used to develop the equations of state for Ta, W, Al, and Be. For its range of applicability, the equation of state contains a relatively small number of free parameters, most of which have a physical meaning. Comparison of calculations of various isolines using equations of state with experimental data and calculations based on other models show that the equations of state for Ta, W, Al, and Be, describe most experimental data for these substances. At ultrahigh pressures and temperatures, calculations using the equations of state are in good agreement with calculations using the Thomas-Fermi model with corrections.

Equation of state model for metals with ionization effectively taken into account. Equation of state of tantalum, tungsten, aluminum, and beryllium

The present work is devoted to classical molecular dynamics investigation into microscopic mechanisms of the bcc-hcp transition in iron. The interatomic potential of EAM type used in the calculations was tested for the capability to reproduce ab initio data on energy evolution along the bcc-hcp transformation path (Burgers deformation + shuﬀe) and then used in the large-scale MD simulations. The large-scale simulations included constant volume deformation along the Burgers path to study the origin and nature of the plasticity, hydrostatic volume compression of defect free samples above the bcc to hcp transition threshold to observe the formation of new phase embryos, and the volume compression of samples containing screw dislocations to study the effect of the dislocations on the probability of the new phase critical embryo formation. The volume compression demonstrated high level of metastability. The transition starts at pressure much higher than the equilibrium one. Dislocations strongly affect the probability of the critical embryo formation and significantly reduce the onset pressure of transition. The dislocations affect also the resulting structure of the samples upon the transition. The formation of layered structure is typical for the samples containing the dislocations. The results of the simulations were compared with the in-situ experimental data on the mechanism of the bcc-hcp transition in iron.

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MD modeling of screw dislocation influence upon initiation and mechanism of BCC-HCP polymorphous transition in iron

Multimillion atom non‐equilibrium molecular dynamics simulations for shock compressed iron are analyzed using Fourier methods to determine the long scale ordering of the crystal. By analyzing the location of the maxima in k‐space we can determine the crystal structure and compression due to the shock. This report presents results from a 19.6GPa simulated shock in single crystal iron and compare them to recent experimental results of shock compressed iron where the crystal structure was determined using in‐situ wide angle x‐ray diffraction.

Shock Induced α‐ε Phase Change in Iron: Analysis of MD Simulations and Experiment

Multimillion atom non-equilibrium molecular dynamics simulations for shock compressed iron are analyzed using Fourier methods to determine the long scale ordering of the crystal. By analyzing the location of the maxima in k-space we can determine the crystal structure and compression due to the shock. This report presents results from a 19.6 GPa simulated shock in single crystal iron and compare them to recent experimental results of shock compressed iron where the crystal structure was determined using in-situ wide angle x-ray diffraction.

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Shock Induced (Alpha)-(Epsilon) Phase Change in Iron: Analysis of MD Simulations and Experiment

Laser-induced Ramp Compression of Tantalum and Iron to Over 300 GPa: EOS and X-ray Diffraction

In‐situ x‐ray diffraction was used to study the response of single crystal iron under shock conditions. Measurements of the response of [001] iron showed a uniaxial compression of the initially bcc lattice along the shock direction by up to 6% at 13 GPa. Above this pressure, the lattice responded with a further collapse of the lattice by 15–18% and a transformation to the hcp structure. The in‐situ measurements are discussed and results summarized.

Direct Observation of the α‐ε Transition in Shocked Single Crystal Iron

In-situ x-ray diffraction was used to study the response of single crystal iron under shock conditions. Measurements of the response of [001] iron showed a uniaxial compression of the initially bcc lattice along the shock direction by up to 6% at 13 GPa. Above this pressure, the lattice responded with a further collapse of the lattice by 15-18% and a transformation to a hcp structure. The in-situ measurements are discussed and results summarized.

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J H Eggert

Papers

Shock Induced α‐ε Phase Change in Iron: Analysis of MD Simulations and Experiment

Shock Induced (Alpha)-(Epsilon) Phase Change in Iron: Analysis of MD Simulations and Experiment

Laser-induced Ramp Compression of Tantalum and Iron to Over 300 GPa: EOS and X-ray Diffraction

Direct Observation of the α‐ε Transition in Shocked Single Crystal Iron

Direct observation of the alpha-epsilon transition in shocked single crystal iron