Thermal processing and enthalpy storage of a binary amorphous solid: A molecular dynamics study
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Robert Maaß Department of Materials Science and Engineering, University of Illinois at Urbana Champaign, Urbana, Illinois 61801, USA (Received 11 January 2017; accepted 6 June 2017)
Using very long molecular dynamics simulations of duration up to a microsecond of physical time, temperature protocols spanning up to five orders of magnitude in time are performed to investigate thermally activated structural relaxation in a model binary amorphous solid. The simulations demonstrate significant local structural excitations (LSE) as a function of increasing temperature and show that enthalpy rather than internal potential energy is primarily responsible for relaxation. At low temperatures these LSE involve atoms whose displacements are smaller than a typical bond length, whereas at higher temperatures approaching that of the glass transition regime, bond-length displacements occur in the form of string-like motion where one atom replaces the position of another. Such thermally activated excitations are observed to mainly involve the smaller atom type. The observed enthalpy changes can be correlated with the level of internal hydrostatic stress homogenization and icosahedral content within the glassy solid.
I. INTRODUCTION
Amorphous solids, such as bulk metallic and network glasses, constitute a class of materials which attract both scientific and engineering interest.1,2 Despite their out-ofequilibrium nature, these materials have impressive elastic (in terms of extension) and plastic (in terms of strength) properties. However, their ductility upon yield is limited because of the emergence of local plastic instabilities, leading to immediate failure under tension and the formation of a few dominant shear bands under compression.3,4 Two widely used material preparation protocols used to influence the mechanical properties of the amorphous solid are that of aging and rejuvenation.5 Aging involves careful thermal annealing procedures, which induce structural relaxation and a reduction in energy, resulting in improved elastic properties and a higher yield, but at the expense of ductility. Rejuvenation generally involves structural excitation and an increase in energy, tending to delocalize plasticity and increase ductility, but at the expense of a well defined yield point and thermal stability. Recently, a number of quite different rejuvenation protocols have been investigated, in particular temperature quenching/ramping between ambient temperatures Contributing Editor: Franz Faupel a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.251
(well below the glass transition temperature) and liquid Nitrogen temperatures,6 and stress relaxation/creep at ambient temperatures under strains and stresses which are almost two orders of magnitude below the yield point.7,8 The implication of these works is that the underlying structural excitations, which collectively lead to plasticity, must be thermally driven or driven by thermal-stresses which develop during cooling. The amorphous atomic str
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