On The Fragile-To-Strong Transition Of Metallic Liquids

Since the first report of the fragile-to-strong transition (F-S transition) phenomenon in water was reported in 1999, it has aroused great interest of condensed matter scientists. The investigation of glass-forming liquid dynamics is important for revealing the nature of the glass transition and liquid freezing characters and also for understanding the mutual competition between the glass transition and crystallization. The outcome of such investigations will be crucial for improving the glass forming ability (GFA) of metallic glass-forming liquids. As is known, metallic glasses are often obtained by fast quenching from the high-temperature melt. Clarifying the dynamic behavior of metallic liquids in the entire temperature region from a temperature above the liquidus temperature Tliq to the glass transition temperature Tg is critical to exploring the structural and thermodynamic evolvement.In this work, we discovered the occurrence of the F-S transition in both bulk and marginal metallic glass-forming liquids and found a method for describing the F-S transition behavior. The difference between bulk and marginal metallic glass-forming liquids in the F-S transition was investigated for the relation between the F-S transition and the GFA. Based on Mauro-Yue-Ellison-Gupta-Allan (MYEGA) model, a model was established to capture the liquid dynamics with/without the F-S transition. The relation between the F-S transition and the fragility of superheated melt for metallic liquids has been discussed. The influence of experimental approach on the F-S transition has also been studied.The following work has been carried out:(1) The general and special features of the F-S transition for bulk metallic glass-forming liquids (BMGFLs) have been studied and described in terms of a physical model.By thermodynamic and dynamic approaches, we studied the liquid fragility of several Gd-, Pr-, Sm-, La- and Ce- BMGFLs in both the supercooled liquid region near Tg and high-T region above Tliq. The composition dependence of the F-S transition of BMFLs has been discussed. It is found that the F-S transition is not limited to some known liquids like water, SiO2 and BeF2. But it is probably a general behavior of BMGFLs.According to the liquid fragility difference between the high-T liquid and the supercooled liquid, a quantitative characteristic method, i.e.,f=m'/m for the fragile-to-strong transition of BMGFLs is established. Here, the parameter m quantifies the liquid fragility in the supercooled region near Tg while m'quantifies that above Tliq.(2) The general and special features of the F-S transition for marginal metallic glass-forming liquids (MMGFLs) have been studied and described in terms of a physical model.In thermodynamic and dynamic approaches, the research on the liquid fragility has been carried out in the temperature region above the liquidus temperature Tlip and supercooled region near the glass transition temperature Tg. The F-S transition and its composition dependence for Al-Ni-La, Al-Ni-Pr and Al-Ni-Nd MMGFLs have been investigated. The F-S transition is possibly a general behavior of MMGFLs.The quantification for the F-S transition of MMGFLs was carried out to obtain the F-S transition parameter f.The relations among the F-S transition, supercooled liquid fragility and GFA were discussed for both MMGFLs and BMGFLs. It is concluded that, the F-S transition parameter f is inversely proportional to the supercooled liquid fragility parameter m. For BMGFLs in our work, the larger the F-S transition parameter, the greater the GFA is. However, for MMGFLs, the smaller the F-S transition parameter, the greater the GFA is.(3) With two relaxation regimes, i.e., fragile and strong regimes, a new viscosity model has been proposed for capture of liquids with the F-S transition.On the basis of MYEGA model, we proposed a generalized viscosity model: where,η∞is the viscosity at infinitive temperature, T is the temperature, W1 and W2 are weighting factors for C1 and C2 physically and C1 and C2 correspond to two constraint onsets for different structural mechanisms.This model can not only capture the dynamic viscosity character of liquids with the F-S transition involving the fragile and strong regimes but also that without such transition.According to the proposed viscosity model, we calculated the F-S transition temperature for metallic glasses. From the calculation results, the F-S transition occurs at the temperature Tg/T= 0.61-0.81 in Angell plot, i.e., T= (1.23-1.64) Tg.(4) Discussion on relation between the F-S transition and the fragility of superheated melt and influence of various liquid fragility determination approaches on the F-S transitionThe relation between the F-S transition and the fragility of superheated melt was discussed for Sm-, La-, Gd-, Cu-and Al-based metallic liquids. No strict linear relation between the F-S transition and the fragility of superheated melt was found.Three-point beam bending method and thermal scanning method were adopted for the glass transition temperature of Sm-, La-, Pr-, Gd- and Cu- bulk metallic glasses. The glass transition temperatures obtained from two methods are not always consistent. The dynamic viscosity of different bulk metallic glasses at Tg,10 varies but averages to 1012.3 pa s which shows the intrinsic resistances against atomic movement and time scales for structural rearrangement at the glass transition temperature are not constant for BMGs.The liquid fragility parameters from dynamic and thermodynamic methods were compared. There is not too large influence of various testing approaches on the liquid fragility and correspondingly the F-S transition.