Amorphous alloys are metallic liquids "cloaked in solids." Formed in the rapid cooling process of the metallic liquid, the amorphous alloy inherits the structure of the metallic liquid through glass transformation, and can also exhibit some physical properties in the liquid state in the solid state.
Understanding the structure and physical properties of high-temperature metallic liquids is essential to understanding the curing process of liquids and understanding the glass transition of amorphous alloys. The presence of liquid-liquid phase transitions in alloys as a frontier issue in the field of amorphous physics and materials has received widespread attention from researchers for more than a decade and has sparked a series of discussions. For example, why do long-range disordered liquids with atomic structure undergo phase transitions? Can new liquids produce new glass? Among all the questions that arise, "Is there a high-temperature liquid-liquid phase transition above the liquid phase line temperature?" "There has been a long debate, and there is an urgent need to consolidate credible experimental evidence to provide answers.
Recently, Academician Wang Weihua of the EX4 Group of the Key Laboratory of Extreme Conditions Physics of the Institute of Physics of the Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics and Cheng Qi, a doctoral student from the White Ocean Researcher Team, under the joint guidance of their doctoral supervisor Sun Yonghao, and Professor Jiri Orava of Jan Evangelista Purkyne University in the Czech Republic, utilized the latest ultrafast difference thermal analysis technology (Mettler Toledo FDSC 2+). ) demonstrates the existence of a new kinetically driven metal-liquid phase transition above the liquid phase line temperature.
By observing in situ the rapid cooling process of four ytterbium-zinc binary alloy (Yb-Zn) liquids, Cheng Qi found an exothermic peak with a starting temperature higher than the liquid-phase line equilibrium temperature (Figure 1). This exothermic peak is unique in that :(1) the reversible specific heat is unchanged when cooling (Figure 1) ;(2) No endothermic peak corresponds to it during reheating (Figure 2) ;(3) occurs again when it is cooled again. The above results show that the exothermic peak cannot be crystallization, precipitation and phase separation, but can only be a special liquid-liquid structure phase transition - it has no thermodynamic characteristics of the primary or secondary phase transition, only the kinetic behavior of the phase transition.
Fig. 1: During the rapid cooling of the YbZn metal liquid, an exothermic peak No. 1 appears above the equilibrium liquid phase line temperature (793 K), and the reversible specific heat (red rectangular symbol) corresponding to the exothermic peak remains unchanged.
Figure 2: YbZn's "heat-quenching" experiment confirmed that the exothermic peak No. 1 is a signal of the liquid-liquid phase transition.
The phase transition temperature of this kinetically driven liquid-liquid phase transition is 6-8% higher than the temperature of the equilibrium liquid phase line, which is similar to the temperature at which the viscosity of the metallic liquid undergoes a hyper-Arrhenius transition in the literature. Based on this phase change temperature, Cheng Qi et al. constructed for the first time a non-equilibrium phase diagram of a ytterbium-zinc alloy liquid (Figure 3).
Fig. 3: (a) Phase diagram of the liquid-liquid phase transition of the Yb-Zn binary alloy comprising the liquid-liquid phase transition; (b) the enthalpy of the enthalpy of the liquid-liquid phase transition of the four Yb-Zn metals with the cooling rate.
The ultrafast differential thermal analyzer used in this work is the latest product produced by METTLER TOLEDO in Switzerland, which has a heating rate of 60,000 degrees per second and a cooling rate of 40,000 degrees per second, capable of reaching an experimental temperature of 1000 degrees, suitable for characterizing the high temperature behavior of metal liquids and rapid cooling in situ to prepare amorphous alloys.
After measurement, the enthalpy value of the new liquid-liquid phase change in this work at a high cooling rate accounts for 10-12% of the total molten enthalpy; it is calculated that if measured at the 20 degrees per second cooling rate of conventional calorimeter, the enthalpy value of the liquid-liquid phase change accounts for only a few thousandths of the total molten enthalpy - can not be experimentally observed. Therefore, this new liquid-liquid phase transition can only be observed by high cooling rates. Ultrafast cooling techniques are key to discovering this kinetically driven liquid-liquid phase transition.
The significance of this work is to prove that there is a kinetic-driven liquid-liquid phase transition in metal liquids above the liquid phase line temperature, providing new ideas for people to design solid alloys by regulating liquids. The results were published in the Journal of Metallurgy I under the title "Kinetically facilitated liquid-liquid transition in a metallic liquid", the first author is Doctoral Student Cheng Qi, and the corresponding author is Sun Yonghao Distinguished Researcher and Professor Jiri Orava.
The article is linked to: https://doi.org/10.1016/j.actamat.2022.117834
The work was supported by the National Natural Science Foundation of China (51971239), the Pilot Program of the Chinese Academy of Sciences (XDB30000000), the Key R&D Program of the Ministry of Science and Technology (2018YFA0703603), the Natural Science Foundation of Guangdong Province (2019B030302010) and the Beijing Municipal Science and Technology Commission (Z191100007219006).
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