Simulation of the shock-wave effect on the amorphous Fe$_{80}$P$_{20}$ alloy

N. M. Sozonova, A. Yu. Drozdov, V. Ya. Bayankin

Physical-Technical Institute, Ural Branch of the Russian Academy of Sciences, Izhevsk, Russia

Abstract: Shock-waves are one of the means of amorphous glasses modification. So far, the effect of surface segregation and its interaction with the structure changes in amorphous materials has been insufficiently studied. Under laser effect on the solid there appear shock-waves due to sharp expansion of the strongly heated area during a short time heating of the surface layer [1, 2].
In this work we have carried out simulation of the shock-wave propagation in the amorphous Fe$_{80}$P$_{20}$ alloy. Using the LAMMPS software [7], an amorphous system consisting of 32000 atoms was created. The embedded atom method potential [8, 9] was chosen to simulate the Fe-P system. The potential of the embedded atom describes the properties and structure of the alloy better than the pair potential. The time step was 10$^{-16}$ s.
To produce the amorphous structure, a model crystallite in the shape of a rectangular parallelepiped with the linear dimensions of $29 \times 29 \times 460$ angstrom (in the directions of X, Y, Z) was heated up to the temperature higher than that of melting (2500 K). Then the alloy obtained was cooled down to the temperature 10 K at the cooling rate of 1011 K/s. The given cooling rate can be achieved under the irradiation with the laser of picosecond duration [10]. The model sample obtained was brought to the equilibrium state.
After obtaining the amorphous alloy the propagation of the shock-wave was simulated. The initial state of the model describes the moment of contact of the projectile moving at the speed V$_{imp}$ along the OZ axis with the motionless target. As a result, it was found out that in the amorphous Fe$_{80}$P$_{20}$ alloy under the shock-wave effect with the initial velocity of 5000 m/s the function of the radial distribution retains the form characteristic for the amorphous state.
The wave front passes through the whole sample for 7 ps, the linear dimensions of the modeled sample increasing along the direction of the wave movement by 6,5 %. According to the data obtained the velocity of the shock-wave propagation in the material was calculated. The calculations showed that it was equal to 6500 m/s. This value was 40 % higher that the velocity of sound in the amorphous Fe$_{80}$P$_{20}$ alloy that is equal to 4620 m/s at room temperature [11].
Judging by the form of the radial distribution function after the shock-wave propagation, one can make a conclusion that under the given effect there is no crystallization of the substance. After the action of the projectile and the shock-wave passage through the model crystallite, the sample is heated from the temperature of 300 up to 1030 K. The investigation of the atoms redistribution in the amorphous Fe$_{80}$P$_{20}$ alloy after the shock-wave propagation in 7 ps after the shock suggests that the phosphorous atoms begin to form clusters evenly distributed throughout the whole volume of the sample.
In this work the changes of the amorphous Fe$_{80}$P$_{20}$ alloy structure under the shock-wave effect were studied. Despite the fact that the system studied heats up to the temperature higher than that of crystallization, the latter does not occur. Most likely it is so because of the absence of the crystalline phase nuclei, which is accounted for by the high cooling rate while obtaining the amorphous state of the Fe$_{80}$P$_{20}$ alloy. The number of nucleation centers and the rate of crystals growth depend on the degree of overcooling. With high degrees of overcooling the number of nucleation centers and the growth rate are equal to 0 since at low temperatures the diffusional mobility of atoms is low, which reduces the system transformation ability.

Keywords: shock effect, amorphous alloy, computer simulation, shock-wave.

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