We test the localization model (LM) prediction of a parameter-free relationship between the α-structural relaxation time τ α and the Debye–Waller factor 〈u 2 〉 for a series of simulated glass-forming Cu–Zr metallic liquids having a range of alloy compositions. After validating this relationship between the picosecond (‘fast’) and long-time relaxation dynamics over the full range of temperatures and alloy compositions investigated in our simulations, we show that it is also possible to estimate the self-diffusion coefficients of the individual atomic species (D Cu, D Zr) and the average diffusion coefficient D using the LM, in conjunction with the empirical fractional Stokes–Einstein (FSE) relation linking these diffusion coefficients to τ α . We further observe that the fragility and extent of decoupling between D and τ α strongly correlate with 〈u 2 〉 at the onset temperature of glass-formation T A where particle caging and the breakdown of Arrhenius relaxation first emerge.
Claims are often found in the literature that metallic materials can be nanocrystallized by severe plastic deformation (SPD). However, SPD does not generate a well-defined nanocrystalline (NC) material, which can be achieved by subsequent annealing/recovery treatment. In this study, molecular dynamics (MD) simulation is employed to study the effect of annealing on structure and mechanical properties of cyclic deformed NC α-iron, which simulates SPD-processed α-iron. It is demonstrated that grain boundaries in the deformed NC α-iron evolve to a more equilibrium state during annealing, eliminating or minimizing the residual stress. The annealing treatment increases the system's strength by reducing dislocation emission sources, and improves material ductility through strengthening grain boundaries' resistance to intergranular cracks. The results indicate that the annealing treatment is an essential process for obtaining a well-defined NC structure with superior mechanical properties.
We investigate collective molecular motion and the self-diffusion coefficient D of water molecules in the mobile interfacial layer of the secondary prismatic plane (112¯0) of hexagonal ice by molecular dynamics simulation based on the TIP4P/2005 water potential and a metrology of collective motion drawn from the field of glass-forming liquids. 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 T, in accordance with recent simulations and many experimental studies, although different experimental methods have differed in their precise estimates of the thickness of this layer. We also find that the dynamics within this mobile interfacial ice layer is "dynamically heterogeneous" in a fashion that has many features in common with glass-forming liquids and the interfacial dynamics of crystalline Ni over the same reduced temperature range, 2/3 < T/T < 1. In addition to exhibiting non-Gaussian diffusive transport, decoupling between mass diffusion and the structural relaxation time, and stretched exponential relaxation, we find string-like collective molecular exchange motion in the interfacial zone within the ice interfacial layer and colored noise fluctuations in the mean square molecular atomic displacement 〈u〉 after a "caging time" of 1 ps, i.e., the Debye-Waller factor. However, while the heterogeneous dynamics of ice is clearly similar in many ways to molecular and colloidal glass-forming materials, we find distinct trends between the diffusion coefficient activation energy E for diffusion D and the interfacial width ξ from the scale of collective string-like motion L than those found in glass-forming liquids.
Due to the industrial importance of α-iron-based polycrystalline materials, their grain boundary (GB) structures and properties need to be well characterized and understood in order to optimize the materials through effective GB engineering. In this study, a molecular dynamics (MD) simulation study was performed to investigate a series of 〈1 1 0〉 symmetric tilt grain boundaries (STGBs) and asymmetric tilt grain boundaries (ATGBs) in α-iron. It is shown that the GB energy is proportional to the GB volumetric expansion. During uniaxial deformation, 〈1 1 1〉{1 1 2} twinning appears to be more competitive or easier than 〈1 1 1〉{1 1 2} dislocation emission from the GB at yielding. For bicrystal systems containing STGBs the yield strength obeys the Schmid law, while for ATGB bicrystal systems the yield strengths are mainly determined by the local stress rather than overall stress and average GB energy. The higher degree of atomic disordering in the ATGB regions generates larger local stress fluctuation and thus facilitates local defect emission when subjected to external stresses.
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