Ramon Ravelo

Virtual melting as a new mechanism of stress relaxation under high strain rate loading

Generation and motion of dislocations and twinning are the main mechanisms of plastic deformation. A new mechanism of plastic deformation and stress relaxation at high strain rates (109–1012 s-1) is proposed, under which virtual melting occurs at temperatures much below the melting temperature. Virtual melting is predicted using a developed, advanced thermodynamic approach and confirmed by large-scale molecular dynamics simulations of shockwave propagation and quasi-isentropic compression in both single and defective crystals. The work and energy of nonhydrostatic stresses at the shock front drastically increase the driving force for melting from the uniaxially compressed solid state, reducing the melting temperature by 80% or 4,000 K. After melting, the relaxation of nonhydrostatic stresses leads to an undercooled and unstable liquid, which recrystallizes in picosecond time scales to a hydrostatically loaded crystal. Characteristic parameters for virtual melting are determined from molecular dynamics simulations of Cu shocked/compressed along the and directions and Al shocked/compressed along the direction.

Large-scale molecular dynamics simulations of shock induced plasticity in tantalum single crystals

We report on large-scale non-equilibrium molecular dynamics (NEMD) simulations of shock wave compression in Ta single crystals. The atomic interactions are modeled via a recently developed and optimized embedded-atom method (EAM) potential for Ta, which reproduces the equation of state up to 200 GPa. We examined the elastic-plastic transition and shock wave structure for wave propagation along the low index directions: (100), (110) and (111). Shock waves along (100) and (111) exhibit an elastic precursor followed by a plastic wave for particle velocities below 1.1 km/s for (100) and 1.4 km/s for (111). The nature of the plastic deformation along (110) is dominated by twinning for pressures above 41 GPa.

Shock compression and spallation of single crystal tantalum

We present molecular dynamics simulations of shock-induced plasticity and spall damage in single crystal Ta described by a recently developed embedded-atom-method (EAM) potential and a volumedependent qEAM potential. We use impact or Hugoniotstat simulations to investigate the Hugoniots, deformation and spallation. Both EAM and qEAM are accurate in predicting, e.g., the Hugoniots and γ - surfaces. Deformation and spall damage are anisotropic for Ta single crystals. Our preliminary results show that twinning is dominant for [100] and [110] shock loading, and dislocation, for [111]. Spallation initiates with void nucleation at defective sites from remnant compressional deformation or tensile plasticity. Spall strength decreases with increasing shock strength, while its rate dependence remains to be explored.

Non-equilibrium molecular dynamics simulations of spall in single crystal tantalum

Ductile tensile failure of tantalum is examined through large scale non-equilibrium molecular dynamics simulations. Several loading schemes including flyer plate impact, decaying shock loading via a frozen piston, and quasi-isentropic (constant strain-rate) expansion are employed to span tensile strain-rates of 108 to 1014 per second. Single crystals of 〈001〉 orientation are specifically evaluated to eliminate grain boundary effects. Heterogeneous void nucleation occurs principally at the intersection of deformation twins in single crystals. At high strain rates, multiple spall events occur throughout the material and voids continue to nucleate until relaxation waves arrive from adjacent events. At ultra-high strain rates, those approaching or exceeding the atomic vibrational frequency, spall strength saturates near the maximum theoretical spall strength.

Large-Scale Molecular Dynamics Simulations of Shock-Induced Plasticity in Tantalum Single Crystals

We report on large-scale non-equilibrium molecular dynamics (NEMD) simulations of shock wave compression in Ta single crystals. The atomic interactions are modeled via a recently developed and optimized embedded-atom method (EAM) potential for Ta, which reproduces the equation of state up to 200 GPa. We examined the elastic-plastic transition and shock wave structure for wave propagation along the low index directions: (100), (110) and (111). Shock waves along (100) and (111) exhibit an elastic precursor followed by a plastic wave for particle velocities below 1.1 km/s for (100) and 1.4 km/s for (111). The nature of the plastic deformation along (110) is dominated by twinning for pressures above 41 GPa.

High strain rates effects in quasi-isentropic compression of solids

We have performed large‐scale molecular‐dynamics (MD) simulations of shock loading and quasi‐isentropic compression in defective copper crystals, modeling the interatomic interactions with an embedded‐atom method potential. For samples with a relatively low density of pre‐existing defects, the strain rate dependence of the flow stress follows a power law in the 109–1012 s−1 regime with an exponent of 0.40. For initially damaged, isotropic crystals the flow stress exhibits a narrow linear region in strain rate, which then bends over at high strain rates in a manner reminiscent of shear thinning in fluids. The MD results can be described by a modification of Eyring’s theory of Couette shear flow in fluids.

Molecular dynamics studies of thin-films of Sn on Cu

Using Molecular Dynamics, the evolution dynamics of Sn on the (111) and (100) surfaces of Cu have been investigated as a function of coverage and temperature. The interaction potentials are described by modified embedded atom method (MEAM) potentials. The calculated diffusion activation energies of Cu in Sn and Sn in Cu agree reasonably well with experimental values. The authors find that the structure of the overlayer depends on the morphology of the substrate and remains stable up to temperatures of the order of 75% of the melting temperature of the substrate at which diffusion of Sn into the substrate and Cu atoms onto the overlayer is observed.

Free energy calculations of Cu-Sn interfaces

Excess free energies of solid Cu-Solid Sn and Solid Cu-liquid Sn have been calculated employing an adiabatic switching formalism in a Molecular Dynamics framework. The atomic interactions are described by modified embedded atom method potentials which includes the angular dependence of the electron density to describe bond bending forces necessary to model covalent materials.

Isospin-asymmetric nuclear matter

This study uses classical molecular dynamics to simulate infinite nuclear matter and study the effect of isospin asymmetry on bulk properties such as energy per nucleon, pressure, saturation density, compressibility and symmetry energy. The simulations are performed on systems embedded in periodic boundary conditions with densities and temperatures in the ranges

Test of a new heat-flow equation for dense-fluid shock waves

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