Fragile-to-strong transition in metallic glass-forming liquids

It has been observed that many glass-forming liquids are transformed from fragile to strong liquids in a supercooled region upon cooling. This is the so-called fragile-to-strong (F-S) transition. Since its discovery in water, the F-S transition, as a frontier problem, as well as a hot issue, in condensed matter physics and material science, has aroused the considerable interest of researchers. It has been generally accepted that the F-S transition might be a universal dynamic behavior of metallic glass-forming liquid (MGFL). Studying the F-S transition is important not only for better understanding the nature of glass transition, uncovering the microstructural inheritance during the liquid-solid transformation, clarifying the structural competition during crystallization, improving the stability of MGs, but also for promoting the standardization during the production and treatment technology of MGs. In this paper, the general and special features of the F-S transition for bulk and marginal MGFLs are studied and described in terms of a physical model. A characteristic parameter f is introduced to quantify the F-S transition. With two relaxation regimes, on the basis of Mauro-Yuanzheng-Ellison-Gupta-Allan model, we propose a generalized viscosity model for capturing the liquids with the F-S transition. Using this model, we calculate the F-S transition temperature for metallic glass. From the calculation results, the F-S transition might occur around (1.36 +/- 0.03) T-g. By using the hyperquenching annealing-calorimetric approach, we find that the anomalous crystallization behavior occurs in both LaAlNi and CuZrAl glass ribbons. This phenomenon implies the existence of a thermodynamic F-S transition, which could be used as an alternative method of detecting the F-S transition in MGFLs. To date, the origin of the F-S transition is far from understanding. We find that the F-S transition in CuZr(Al) GFLs is attributed to the competition among the MRO clusters composed of different locally ordering configurations. By comparing the parameter f with the parameter r that characterizes the competition between the alpha and the slow beta relaxations in 19 MGFLs, we find that the slow beta relaxation plays a dominant role in the F-S transition and the extent of the F-S transition is mainly determined by the degree of the comparability in structure units between the beta and the slow beta relaxations. The existence of the liquid-liquid phase transition might also be the root of the F-S transition. The tendency of investigation of the F-S transition is also evaluated.