Stretched and compressed exponentials in the relaxation dynamics of a metallic glass-forming melt

Abstract

The dynamics of glass-forming systems shows a multitude of features that are absent in normal liquids, such as non-exponential relaxation and a strong temperature-dependence of the relaxation time. Connecting these dynamic properties to the microscopic structure of the system is challenging because of the presence of the structural disorder. Here we use computer simulations of a metallic glass-former to establish such a connection. By probing the temperature and wave-vector dependence of the intermediate scattering function we find that the relaxation dynamics of the glassy melt is directly related to the local arrangement of icosahedral structures: Isolated icosahedra give rise to a liquid-like stretched exponential relaxation whereas clusters of icosahedra lead to a compressed exponential relaxation that is reminiscent to the one found in a solid. Our results show that in metallic glass-formers these two types of relaxation processes can coexist and give rise to a dynamics that is surprisingly complex.

Stress breaks universal aging behavior in a metallic glass

“…Indeed, ref. 29 demonstrates that compressed decorrelations are linked to the activity of clusters of icosahedra, whereas a stretched decorrelation is due to liquid-like isolated icosahedra. Thus, the applied stress may here disrupt parts of the icosahedral network, and the initial ( t w < 5 × 10 3 s) time-dependent increase in β can be interpreted as a result of structural reconfigurations during the restoration of the icosahedral network.…”

“…The shape parameter β , also called the stretching exponent, quantifies a deviation from pure exponential decay ( β = 1), with β < 1 being characteristic of mass transport in liquids. The case of β > 1 is currently debated in the literature and may be related to the release of internal stresses 17 or the dynamics of specific structural units 29 . In the following, we will focus on the application of Eq.…”

“…Thus, compressive loading leads to a change from a compressed decorrelation to a stretched decay, followed by simple exponential dynamics. While it is impossible to experimentally reveal the underlying physics for this change of β , recent computer simulations indicate that the value of β is linked to the predominant activity of differently sized structural units in the MG 29 . Indeed, ref.…”

Fast contribution to the activation energy of a glass-forming liquid

“…For a temperature down jump, the system is self-retarded because the structure approaches one of slower structural relaxation. For a temperature up jump, the system is self-accelerated because the structure converges to one of faster relaxation (19,(25)(26)(27). Consequently, temperature down jumps result in relaxation functions that approach equilibrium faster and are more "stretched" than those of temperature up jumps to the same temperature, compare the red and blue curves in Fig.…”

“…Although aging and degradation of materials are often due to chemical reactions, physical aging is important for amorphous solids like polymers and oxide glasses during production as well as in subsequent use. The experimental study of physical aging requires measurements of high accuracy and considerable patience; simulations of aging are likewise demanding in terms of computational power requirements (17)(18)(19). Aging has been studied by monitoring density (1), enthalpy (2,3), dc and ac electrical response (20)(21)(22), etc.…”