Molecular Dynamics Study of Model Systems Showing Dynamic Heterogeneity
Creators: Wang, Xinyi
Publisher: name: University of Alberta Library
Year: 2022

<p>Glass-formation is a ubiquitous phenomenon that is often observed in a broad class of materials ranging from biological matter to commonly encountered synthetic polymer, as well as metallic and inorganic glass-forming (GF) materials. Despite the many regularities in the dynamical properties of GF materials, the structural origin of the universal dynamical properties of these materials has not yet been identified. In the current thesis, we employ the methodologies for characterization of dynamics in numerous GF liquids to study the ‘dynamic heterogeneity&#39; (DH) in model metallic glass systems under three scenarios. In particular, we investigate the dynamics with in the mobile interfacial layer of secondary prismatic plane of hexagonal ice, metallic GF liquids, and deformed metallic glasses by molecular dynamics simulation and test the metrology drawn from the field of glass-forming liquids. Relaxation mode is examined above and below glass-transition temperatures. Firstly, the width ξ of the mobile interfacial layer varies from a monolayer to a few nm as the temperature is increased towards the melting temperature Tm in the study of interfacial dynamics in hexagonal ice. The dynamics within this mobile interfacial layer prove to be “dynamically heterogeneous” in a fashion that has many features in common with glass-forming liquids over a reduced temperature range, 2/3 &lt; T / Tm &lt; 1. We also find that the common non-Gaussian diffusive transport, decoupling between mass diffusion and structural relaxation, and stretched exponential relaxation. String-like cooperative motion is identified within the mobile layer, confirming that the mobile layer evidently shares many common features with GF liquids. Then, we extend the study on dynamic heterogeneity arising in the coarse-grained polymeric GF liquids to metallic GF liquids with vastly different molecular structure and chemistry (i.e., Cu-Zr, Ni-Nb and Pd-Si). By identifying the lifetimes of mobile and immobile clusters, we confirm the ‘universality’ of DH phenomenon. While the mobile clusters arise from molecular diffusion, the immobile clusters are dominated by alpha relaxation. Finally, we examine the relaxation mode in the low-temperature regime. Model metallic glasses Cu-Zr systems with different thicknesses under different temperatures are investigated. We observe that the applied stress acts as excitation instead of diffusion to particles and makes them ‘mobile’, and these ‘mobile’ particles form the soft spots initiating the formation of shear band (SB) region. The ‘Debye-Waller factor’ provides a useful method for estimating the shear modulus of the entire material and, by extension, the material stiffness at an atomic value. This metrology further proves that that SB formation indeed occurs through the strain-induced formation of localized soft regions that percolate within the SBs in our deformed metallic glass with free surfaces. The current thesis work clearly provides us a better understanding of the nature of universal relaxation from a dynamic heterogeneity perspective. </p>