Ab initio Calculation of Configurational and Vibrational Entropies of Amorphous Alloys
R. Mahjoub, K.E. Laws and M. Ferry
School of Materials Science and Engineering, University of New Saouth Wales, Sydney, NSW 2052, Australia
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Configurational entropy is known to be related to the kinetic properties of metallic glasses [1] and connected to the amorphous system potential energy surface [2]. A local minimum in the potential energy landscape represents the configurational potential energy and entropy scales with the population of available minima at a certain energy and temperature level [3]. The calculation of the configurational entropy of amorphous system can lead us to predicting important physical and mechanical properties of the glassy materials.
The structural configuration of Mg-based metallic glasses has been simulated using ab initio molecular dynamics. The resulting configuration has been Voronoi tessellated to approximate the free volume distribution and hence the configurational entropy of the amorphous systems.
Furthermore, we have used the ab inito simulated configuration to find the dynamical matrix and hence the vibrational density of states (VDOS) which can be used to calculate other phonon properties such as vibrational entropy, free energy and specific heat Cv. In the low frequency VDOS of amorphous phase, the excess relative to the Debye contribution (Boson Peak) is regarded as a characteristics of the glassy state. It is believed that the vibrations that destabilize the crystalline phase can be related to Boson Peaks [4]. The differences in the vibrational entropy of materials may be related to the chemical and structural origins. The number of such assessments is growing due to the increasing reliability of the thermodynamic effects of vibrational entropy for different materials [5].
We acknowledge the financial support of the Australian Research Council (ARC) for funding of this work via the ARC Centre of Excellence for Design in Light Metals (CE0561574) and ARC Discovery grants scheme (DP120102863). Computational resources were provided by Intersect Australia Ltd and the computation was performed on the NCI National Facility in Canberra, Australia.
References
[1] G. Adam and J. Gibbs, J. Chem. Phys. 43 139 (1965).
[2] P. Debenedetti and F. Stillinger, Nature 410 259 (2001)
[3] Y. Cheng and E. Ma, Prog. Mater. Sci. 56 379 (2011).
[4] G. Greaves et al. Nature Mater. 10 823 (2011).
[5] B. Fultz Prog. Mater. Sci. 55 247 (2010).