Thermal Analysis On The Ordering Processes Of Metallic Liquids

Metallic glass is a type of metallic material also termed as‘frozen liquid’.This is because metallic glass is formed by rapidly quenching the metallic liquid to be free of crystalline nuclei.Metallic glass inherits the structure from its forming liquid through vitrification and hence exhibits some properties of the liquid.Similar to the metallic liquid,metallic glass’s atomic structure is long-range disordered and short-range ordered.Such a short-range order includes both structural order,represented by the atomic packing,and chemical order,represented by the elemental distribution.Realizing the structural order and chemical order of the high-temperature metallic liquid is important for understanding those contained in the metallic glass.These structural and chemical order plays a significant role in controlling the solidification process and understanding the glass transition of metallic glasses.The investigation of structural order and chemical order in metallic liquid has always been a frontier problem in the field of liquid’s physics and materials science,arousing an extensive discussion in recent decades.For example,“Are there any high-temperature liquid-liquid phase transitions really existing above the liquidus temperature?”,“How can we distinguish liquid-liquid phase transition from liquid-liquid phase separation?”,“How can we avoid the precipitation of high-temperature metallic liquid?”These questions have long been argued by researchers,and answers are urgently needed by providing more solid experimental evidence.However,it is difficult to characterize and investigate metallic liquids by conventional means due to the liquid’s characteristics of high temperature,fast transition and disordered long-range atomic structure.Therefore,this thesis used a fast differential scanning calorimetry（FDSC）to study the metallic liquid.The FDSC instrument with a high-temperature sensor enables measurements up to 1273 K at cooling rates as high as40000 K s^-1 and heating rates as high as 60000 K s^-1,which is suitable to study the thermal behavior of high-temperature metallic liquid and form the metallic glass in-situ through rapid quenching.By altering the quenching temperature,the structural order and chemical order of the metallic liquid can be in-situ manipulated.This thesis reports that the chemical order affects the glass-forming ability and the crystallization behaviors,and the structural order enables a kinetically facilitated liquid-liquid phase transition.The main results of the thesis are summarized as follows.Altering the quenching temperature of an off-eutectic metallic glass of Yb-Mg-Zn-Cu changed its glass-forming ability.When cooling was started,a lower quenching-temperature suppressed precipitation and eased the glass form,whereas a higher quenching-temperature promoted the precipitation of a Zn-rich solid solution and deteriorated the glass formation.A new up-quenching strategy was introduced that made some off-eutectic compositions glassy.This work sheds light on the origin of metallic glass:a solution instead of a liquid.Altering the quenching temperature of an Au-based metallic glass proved that the“invisible”quenched-in nuclei,as reported in literature,was a high-temperature exothermic peak on the cooling curve of fast calorimetry when quenched from a high quenching-temperature.The quenched-in nuclei originated from a liquid-liquid phase separation,took place at a temperature that was 137 K higher than the melting temperature,and were removable when heated up to higher temperatures.On the other hand,the quenched-in nuclei would not affect the crystal-nucleation on cooling if the quenching temperature and cooling rate were properly adjusted to a suitable range due to the lack of interaction between nuclei and liquid.This thesis pointed out that the preparation of chemically-homogeneous glass required not only high cooling rate but also high quenching-temperature;and that the self-doped glass needed to be further categorized as doping-unaffected and doping-affected self-doped glasses.This work emphasized the importance of high-temperature liquid-liquid phase separation,found out the experimental conditions to eliminate or restrict the liquid-liquid phase separation and clarified the distinction and connection between the quenched-in nuclei and crystal formation.Altering the quenching temperature of four Yb-Zn binary alloys revealed an exothermic peak that had an initiation temperature 6-8%higher than the equilibrium liquidus temperature.The reversible specific-heat remained unchanged during this exothermic event;the exothermic peak was repeatable on cooling curves but its theoretically reversed endothermic peak was not detected on heating curves;a critical quenching temperature existed,and only when the metallic liquid was heated above this temperature could this exothermic event be triggered.This work proved that the exothermic peak occurring above the liquidus temperature was a kinetically facilitated liquid-liquid phase transition.Altering the heating rate of the Cu-Zr-Al metallic glass enabled the quantitative characterization of glass-crystal transition in the metallic liquid.By combining the conventional DSC,fast DSC,and resistive heating apparatus,this work achieved a range of heating rate spanning over 6 orders of magnitude from 10^-2 K s^-1 to 10⁴ K s^-1,within which the crystallization mechanism of Cu-Zr-Al metallic glass was revealed,involved both structural ordering and chemical ordering,and the continuous-heating-transformation（CHT）and time-temperature-transformation（TTT）diagrams were constructed.The new diagrams not only highlighted the crystallization mechanism of metallic glass but also guided the preparation of a glass-crystal composites material with enhanced ductility related to the B2-Cu Zr metastable crystalline-phase formation.The experimental results reported in this thesis could deepen our understanding on the key issues in the field of metallic liquid,including high-temperature liquid-liquid phase transition,liquid-liquid phase separation and precipitation in metallic liquid,guide the preparation of metallic glasses and metallic glass-crystal composites,and pave a way for acquiring the metallic material with specific properties by a new strategy:adjusting the metallic liquid.