Structural origin of dynamical relaxation in undercooled Cu50Zr50 metallic liquid

Abstract

Molecular dynamics (MD) simulation has been performed to explore the correlation between dynamic characteristics and structural heterogeneity in undercooled Cu50Zr50 melt based on the embedded atom method. It is found that atoms in the undercooled liquid can be classified into three groups in terms of the probability distribution of atoms' displacement, named ‘fast atoms’ (FAs), ‘intermediate atoms’ (IAs), ‘slow atoms’ (SAs). Meanwhile, the undercooled liquid can be divided into three temperature regime: as the temperature is higher than 1120 K, the distribution of FAs become network connectivity, in which the SAs region are surrounded; as the temperature is in the range from 1120 K to 900 K, the network of FAs region decompose into several isolated region gradually; after that the single relaxation separates into α- and β-relaxation at temperature lower than 900 K, the FAs region become isolated islands, while the SAs have connected into networks where the FAs regions are surrounded, which cause the significant decreasing of atomic mobility and sharp increasing of the relaxation time. The FAs, located in loose packaging and isolated region, which corresponds to β-relaxation reflected by the time-scale of intermediate plateau in mean-squared displacement (MSD) and intermediate scattering function (ISF). The SAs which develop into the backbone of the system and surround clusters formed by FAs, are responsible for the glass formation and have a connection with α-relaxation. The compressive deformation in the Cu50Zr50 metallic glass shows that the ‘fast atoms’, anticipating in β-relaxation of undercooled liquid, mostly are located in large shear strain zones and the structural heterogeneity of undercooled liquid inherits into the glass. Since metallic glass (MG) can be regarded as frozen undercooled liquid, our study is helpful for understanding the structural origins of dynamical relaxation in MGs.