Study On The Dynamic Behaviors Of Metallic Glass-Forming Liquids

Metallic glass is a new kind of material developed in recent decades,which has attracted much attention,owing to its excellent over-all properties and obvious glass transitions.Although metallic glasses have been used in many fields,such as information and electronic devices,precision instruments,aerospace devices and so on,it is still lack of research on the evolution of the structure and the dynamic mechanism during the solidification process.Nevertheless,research on dynamic behaviors and structure evolution mechanism of metallic glass-forming liquids is of great significance for understanding and revealing the microcosmic essence of liquid-solid heredity,thereby improving the glass-forming ability and the standardization during the practical production of metallic glasses.Based on the above,we choose the study of metallic glass-forming liquids as a starting point,complemented by thermodynamic tests and microstructure characterization method,and focus on investigating the dynamic behaviors of metallic glass-forming liquids.An abnormal dynamic phenomenon was observed,which is contrary to the traditional theory by Adam-Gibbs that the dynamic properties and structural evolution with temperature of metallic glass-forming liquids should be monotomic.We explored the temperature dependence of the liquid structure changes during the abnormal dynamics,revealing the evolution of the micro-clusters in metallic glass-forming liquids.The main research contents and innovation results are as follows:(1)By investigating the dynamic behaviors of Fe-based metallic melts well above their respective liquidus temperatures,it has been found that there is a dynamic transition,where the slope of the viscosity-temperature relation of the Fe-based metallic liquid increases upon cooling,and a viscosity hysteresis between the heating and cooling processes.Combined with the concept of fluid cluster and supercooled liquid fragility,we proposed a parameter F,which is the characterization of the degree of the dynamic transition for melts,and established a relationship between the parameter F and the glass forming ability.By taking into account the thermodynamic effect accompanied with the dynamic transition in Fe-based melts,a possible scenario has been proposed from the aspect of the underlying structural evolution during cooling above the liquidus temperature.This work is of great significance to investigate the properties of Fe-based melts and improve the standardization of Fe-based metallic glass.(2)By using a differential scanning calorimeter(DSC),we investigated the dynamic behaviors in Cu46Zr42Al7Y5 metallic glass-forming liquids reflected by the sub-Tg relaxation and crystallization of hyperquenched Cu46Zr42Al7Y5 glass ribbons.Three exothermic responses to dynamic heating were observed upon heat flow curves,among which the first and the third one are the signatures of the normal sub-Tg relaxation and the crystallization process,respectively.The second exothermic peak,which is supposed to be the endothermic glass transition peak,is the performance of the abnormal dynamic behavior in supercooled liquids.Combined with the changing tendency and the activation energy of the second exothermic peak and crystallization peak,the origin of the second exothermic peak is attributed to the partial overlap between the exothermic peak to the formation of ordered clusters and the endothermic peak to the glass transition.The evolution of local ordered structures and the competition between two different local favored structures in supercooled liquids are analyzed.It is considered that the evolution of cluster ordering might be caused by the large difference between the Zr-Y mixing enthalpy and the Al-Y(or Cu-Y)mixing enthalpy.Our work gives a new way to distinguish between phase separation and clusters motion scenarios.