Computational Investigation Of The Dynamic Crossover Mechanism In Metallic Glass-forming Liquids

The formation of metallic glasses undergoes a process from metallic glass-forming liquids to glassy solids.The dynamic change during the glass transition is significant,but is not accompanied by structural evolution that can clearly be observed in experiments.Due to the nature of liquid-solid inheritance,the structure and properties of metallic glass-forming liquids have a crucial influence on the structure and properties of glass solids.Therefore,understanding the relationship between structure and dynamics of metallic glass-forming liquids is of great importance for an in-depth understanding of metallic glass nature and the regulation of glass solid properties.However,the dynamical behavior of metallic glass-forming liquids is very complex,and there are many phenomena like liquid-liquid phase transition,fragile-to-strong transition,and dynamic slowdown,etc.The existence of these phenomena poses new challenges for an in-depth understanding of the structural evolution of metallic glass-forming liquids during solidification.In this paper,we investigate the dynamic transition of metallic glass-forming liquids in different temperature intervals and reveal the key structural features behind them by combining molecular dynamics simulation methods and experimental techniques with a variety of metallic glass-forming liquids as the object of study.The main points and conclusions of the full disertation are summarized as follows:(1)The abnormal viscosity drops of CuZr binary alloy,CuZrAl,and CuZrTiNi melts was investigated by gradually changing the composition of the alloy system.It was found that the abnormal viscosity drop disappears when the fraction of doping elements(Al,Ti,Ni)becomes large enough.For the CuZr binary alloy,the range of compositions in which the viscosity anomaly exists is almost identical to the range of compositions in which the CuZr system forms bulk metallic glasses.Using molecular dynamics simulations,the evolution pattern of fragile icosahedral-like clusters with temperature is found to be responsible for viscosity drop.Contrary to the trend that the degree of connectivity of the clusters keeps increasing during cooling,the icosahedral-like clusters show anomalous behavior at temperatures where the viscosity decreases,i.e.,a decrease in the degree of connectivity of these clusters occurs.An analysis based on the inheritance characteristics of the different states of the melt further validates the prediction of the crystallization behavior based on the evolution of the fragile icosahedral-like behavior.The research demonstrates that both bulk metallic glass formation and dynamical anomalies in melts originate from these same structural patterns,indicating the significant role of fragile icosahedral-like clusters in liquid dynamics.(2)A quantitative prediction of the characteristics of the fragile-to-strong transition in metallic glass-forming liquids was achieved by focusing on the activation behavior of the atoms during the cooling process,instead of the more general relaxation behavior.The results show that this dynamic transition arises from the transition from free to confined diffusion behavior of the atoms and is a self-regulating behavior of the atomic positions involved in the activation event,which arises due to the appearance of local cooperative motions of the nearest neighboring atoms.Entropy plays a dominant role in this transition,and the configurational entropy turn marks the emergence of the fragile-to-strong transition phenomenon.When the fragile-to-strong transition occurs,the enthalpy of the different liquids undergoes a fixed change and also reaches a steady state at the structural level.This result is verified again by calculating the change of the atomic participation fraction Pi in quasi-localized low-frequency soft modes.This work demonstrates a close connection between the characteristics of atomic energy fluctuations reflected by atomic activation events and the complex dynamical behavior of disordered systems,and establishes a general rule for the thermodynamic and structural features behind fragile-to-strong transition.(3)The structural origin of the dynamic transition at the model coupling temperature Tc is investigated through molecular dynamics simulation on CuZr liquids.The results show that the evolution of the topological connectivity behavior of the icosahedral clusters is responsible for this dynamic transition.Tc is the temperature where the connectivity between the icosahedral clusters reaches its maximum and the dynamical heterogeneity of the system starts to remain stable.Below Tc,a fractal topology appears on the medium range order,and an icosahedral cluster with a fixed connectivity pattern can be considered as a fractal structural unit.The fractal dimension D of the icosahedral network is calculated by using the existing fractal analysis method,and the fractal dimension increases monotonically with increasing pressure,demonstrating that the fractal behavior of the icosahedral network is an inherent feature of metallic glasses.This fractal behavior is also present in clusters with high local fivefold symmetry.This work elucidates the structural origin of dynamic transition in deep supercooled liquids of metallic glasses and also demonstrates the important role of icosahedral clusters in revealing the fractal behavior of metallic glasses.