PSI - Issue 65

I.V. Kosarev et al. / Procedia Structural Integrity 65 (2024) 127–132 I.V. Kosarev, E.A. Korznikova, S.V. Dmitriev / Structural Integrity Procedia 00 (2024) 000–000

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Fig. 2. Frequency responses calculated with the four interatomic potentials considered. The amplitude A is given relative to the equilibrium lattice parameter ρ0. The groups of DNVMs are indicated at small amplitudes for each potential.

As can be seen from the results obtained for all four potentials, significant discrepancies are observed for both small and large amplitudes. The differences in the results in the region of small amplitudes tell us about significant discrepancies in the description of the phonon dispersion curves. For example, for the two EAM potentials the lowest frequency has the group G1, and for L2001_MEAM and B2020_GAP potentials the group G2. For A2003_EAM and O2009_EAM potentials the largest small-amplitude frequency show DNVMs of group G4, while for the rest two potentials the DNVMs of group G3. The change in the order of the groups with respect to the value of the small amplitude phonon frequencies shows not only quantitative but also qualitative discrepancies. At small amplitudes in the region where the force is linearly dependent on the displacement, the frequency remains unchanged; the presence of such a plateau in the region of small amplitudes is observed for all considered potentials, except for A2003_EAM. Such a change of the frequency in the region of small amplitudes, as for A2003_EAM, is unphysical and shows the inaccuracy of the potential in the region of small amplitudes, where the harmonic approximation should work and a weak dependence of the frequency on the amplitude should be observed, He et al. (2023), Guvenc et al. (2023). A distinction is made between the hard type of nonlinearity, when the frequency increases with amplitude growth, and the soft type of nonlinearity, when the frequency decreases with amplitude growth. Thus, group G1 for the EAM potentials has a hard type of nonlinearity, and the L2001_MEAM and B2020_GAP potentials predict practically unchanged frequency, except for DNVM 1 of the B2020_GAP potential, which has a soft type of nonlinearity. All DNVMs of group G2 for all potentials have a soft type of nonlinearity, a similar situation was observed for tungsten, Kosarev et al. (2024), which is characteristic of group G2. DNVMs of group G3 have a hard type of nonlinearity for the EAM potentials, the L2001_MEAM and B2020_GAP potentials show a different type of nonlinearity. Group G4 DNVMs have a hard type of nonlinearity for all considered potentials.

4. Conclusion

For all four considered interatomic potentials, neither qualitative nor quantitative agreement is observed in the calculated frequency responses of DNVM, see Fig. 2. There is a difference in the frequencies of both low-amplitude phonons and in the nonlinear part of the oscillations at large amplitudes. In addition, the A2003_EAM potential exhibits unphysical behavior in the small amplitude region, which is expressed as a linear variation of frequencies as