Phase transition behind a shock front in polycrystalline copper

Molecular dynamics simulation of shock-wave loading of copper and titanium

“…As we had showed earlier [5,6] the average grain size in nanocrystalline copper exerts a minor effect on the dependences of the shock wave velocity on the piston velocity and of the pressure on the density behind the front. For the material with the grain size of about 2 nm, the temperature behind the front is 10% higher than for the materials with greater grain sizes.…”

“…In this case we observe the effect which was called virtual melting in [8]. The mechanism of this crystal structure damage is related to the fact that, due to the nonhydrostatic character of loading in bulk behind the shock front, high tangential stresses arise, which are related to the deviator part of the stress tensor [5,6,8]. During compression, the tangential stresses achieve the maximum about 0.2 GPa for a calculation presented on Fig.…”

“…But this crystallization process goes in much longer time scale and needs employment of special MD algorithms. One of them is hugoniostat [14] which performs well for a sharp shock wave which is observed in copper with pressure higher then 100 GPa [5,6]. But for relatively weak shock waves ( 40 GPa P ) in titanium for which we have showen the splitting of a shock front in several parts with different order and time scale hugoniostat does not perform properly.…”

“…2, and, then, its sharp decrease takes place accompanied with a significant decrease in the number of atoms having the crystal-like coordination. According to our previous study of shock waves in copper [5,6] the virtual melting is followed by crystallization of material in a new phase if this state is more preferable from thermodynamic point of view. But this crystallization process goes in much longer time scale and needs employment of special MD algorithms.…”

“…The interatomic interactions were described by MEAM potential of Hennig, which is able to describe all three solid phases of titanium [12]. The details of MD simulations can be found in [5,6].…”