Session W3: PS-2: Phase Transitions

Combined Radiometry and Velocimetry for Phase Change Detection in Shock Loaded Tin

Reliable detection of phase transitions in multiphase materials studied in dynamic experiments has been a continuing challenge in shock physics. We report results from simultaneous radiometry and velocimetry measurements of dynamically loaded tin under a LiF window. When tin melts on release at the tin/LiF interface, a radiometric signature is observed at multiple wavelengths along with the velocimetry signature. The temperature of the tin/LiF interface is also obtained. Calculations of melt on release using a multiphase EOS for tin provide additional insight into the dynamics of the material when compared to the observables.   

A Gallium Multiphase Equation of State

A new SESAME multiphase gallium equation of state (EOS) has been developed. The equation of state includes two of the solid phases (Ga I, Ga III) and a fluid phase. The EOS includes consistent latent heat between the phases. We compare the results to the liquid Hugoniot data. We will also explore refreezing via isentropic release and compression.   

Empirical Multiphase EoS Modelling Issues

With a change in pressure and temperature various materials undergo solid-solid phase transitions where the crystal structure changes. Melting of materials is also another phase transition. Across a phase boundary discontinuities in energy, density, and properties of the material are seen. Even small changes can have a significant effect such as wave splitting under shock loading. The changes will affect the equation of state and strength of a material and thus should be included in an accurate model. The kinetics of the phase transition may also need to be accounted for. A thermodynamically consistent multiphase EoS model is needed. In developing an empirical model a lack of data will likely be an issue, especially for higher pressure phases. When implementing into a hydrocode robustness and time efficiency of the method used needs to be considered. These issues are discussed here, with tin used as an example material.   

Impact Response of Pure Iron Between 150 and 1273 K

Yield and spall strengths of pure polycrystalline iron (99.995{\%} Fe) were studied in a series of VISAR-monitored planar impact experiments with initial sample temperature ranged from 150 to 1273 K. It was found that decaying with temperature yield strength of $\alpha $-iron has two local maxima; the first one in the vicinity of the iron Curie point, 1030 K, and the second one just below the temperature of the $\alpha -\gamma $ transition in iron, 1180 K. It was also found that both the pressure and the volume effect of the shock-induced $\alpha -\varepsilon $ transformation (revealed by the two-wave structure of the recorded waveforms) decrease with the test temperature achieving their minima in the vicinity of the $\alpha -\gamma $ transformation. Heating of the iron above the temperature of the $\alpha -\gamma $ transformation is accompanied by the increase of both the $\alpha -\varepsilon $ transformation pressure and its volume effect.   

Modelling of Multiple Phase Transitions Under Shock in Ice

For several decades, experiments have shown the behavior under shock of materials featuring phase transitions. They exhibit shock splitting and rarefaction shocks. If the main phenomena are now understood, most studies are limited to the qualitative behavior, and a quantitative analysis of experimental results remains to be done in most cases involving complex materials, waves or geometries. In former studies, we have proposed a thermodynamic description of multiphase material in equilibrium (up to ten phases in bismuth), shown its ability to be used in hydrocodes, and demonstrated the good qualitative results obtained on tin at different impact velocities. We have also raised the numerical problems involved by phase transitions in hydrocodes. In the present study, we have undertaken calculations on ice to reproduce quantitatively an impact experiment on ice with double phase transition. We have built a six phases equation of state for ice which reproduces experimental equilibrium phase diagram and Hugoniot curves. Our first hydro calculations provide the pressure levels, but exhibit an elastic precursor and kinetics of phase transition effects. We explain how we take these phenomena into account to obtain a good agreement between numerical and experimental results.   

Large-Scale Molecular Dynamics Simulations of the fcc-fcc Volume Collapse Transition in Shocked Cesium

We have utilized large-scale classical molecular dynamics simulations to study the isomorphic fcc-fcc transformation in shocked cesium perfect crystals. Ackland and Reed [1] developed an interatomic potential to describe the similar volume collapse transition in Cs, adding an internal variable for the relative occupation of two ($s$ and $d)$ electronic bands on each atom to an embedded atom method (EAM)-like model. Using an orientation imaging map algorithm, we find a significant dependence upon initial crystallographic orientation: shock compression in the [001] direction leads to a product with a predominantly [011]-like texture, while [111] loading accomplishes the volume collapse transition without any crystallographic rotation. A three-wave (elastic-plastic-product phase) structure is also observed for shock pressures around 5 GPa in the [111] case, while the [001] plastic wave is overdriven prior to the onset of transformation. \\[4pt] [1] G. J. Ackland and S. K. Reed, \textit{Two-band second moment model and an interatomic potential for caesium}, Phys. Rev. B \textbf{67}, 174108 (2003).