Resumen de Collective dynamics of bulk metallic glasses studied by molecular dynamics simulations
Araceli Vallés Sales
The development of new materials impacts on all branches of engineering and, in particular, in aerospace engineering. Metallic glasses (MG) are relatively newcomers to the world of materials science and have excellent mechanical properties; its study is mandatory to allow its technological implantation. The macroscopic mechanical properties of a material are linked to its atomic structure. In particular, the fracture behaviour of brittle materials is initiated by the generation of vibrational modes. In metallic glasses, with amorphous structure, the vibrational spectrum has specific features. In this work, the vibrational properties of metallic glasses are examined by Molecular Dynamics simulations. The study was focused in binary systems, which were simulated using different interatomic potentials: Lennard-Jones (LJ), Morse and the semiempirical Embedded atom (EAM). As in Pd-based metallic glasses, the ratio of masses of both species was high, namely 2 in Lennard-Jones potentials and 1.67 in Morse and EAM potentials. The large scale simulations allowed us to simulate systems with nm-scale heterogeneities frozen-in during the quenching process. Different relaxation states were obtained by changing the quenching rates of the simulated MGs. The collective vibrational atomic dynamics of metallic glasses is a longstanding subject of debate. The origin of the excess of vibrational modes known as Boson Peak (BP) is not clear. In the systems analysed the dependence of the BP position and intensity on the system size is found to be weak. On the contrary, the BP intensity increases with the quenching rate, while its position shifts slightly to smaller frequencies. The results obtained by using realistic, semiempirical EAM potentials compare well with the experimental data available in glasses of similar compositions. The dynamic structure factor, S(q, ¿) is also computed in large systems to get information on the behaviour of acoustic excitations at low wavenumbers. The dominant frequencies O L,T (q) are determined for each considered wavevector, in order to compute the relation of dispersion of longitudinal and transverse phonons. In all studied cases the width of the peak, GL,T(q), increases as the frequency increases. A linear region at low wavenumbers and a bending when approaching the limit of the first pseudo-Brillouin zone are found. This behaviour is the same than that observed experimentally by Inelastic X-Ray scattering. The macroscopic sound speed is obtained for wavenumbers tending to zero. The values obtained with EAM and Morse potentials are in qualitative agreement with those obtained experimentally in systems of similar composition. The Ioffer-Regel limit (IR), where the coherence length of the phonon is similar to the phonon wavelenght, was computed. It is found that the Ioffe-Regel frequency decreases slightly when applying faster quenching rates. The longitudinal Ioffe-Regel limit was found at frequencies higher than the Boson peak frequency for all the cases, although the diference in EAM systems is much reduced. Contrary to the typical results obtained in the LJ systems, using EAM potentials in both Cu20Pd80 and Cu50Pd50 the longitudinal IR limit is very close to the position of the BP while the transversal IR limit is found well below. This behaviour is coherent with that found by IXS measurements. It is inferred that the EAM potential increases the interaction between the longitudinal modes and the BP excess states. Finally the fragility of the studied systems was obtained by calculating the viscosity at different temperatures. Lennard-Jones systems showed a much larger fragility than EAM and Morse systems. However, even systems simulated with more realistic potentials showed fragility values much higher than those obtained experimentally. This is attributed to the extremely high quenching rates used in simulations, as it is known experimentally that fragility increases with the quenching rate.
