Thermophysical Properties,Dynamics And Structure Of Liquid-liquid Phase Transition In Pd-based Glass Forming Liquids

Due to its structural characteristics of"long-range disorder and short-range order",metallic glasses（MGs）have special properties in mechanics,chemistry and physics.However,the current application range of MGs is very limited.One of the reasons is that the basic research on MGs is still insufficient,especially the backward level of structural characterization hinders the development of MGs.Recently,a large number of studies have shown that there is a phase transition between two different liquid phases in amorphous alloy melts,that is liquid-liquid phase transitions（LLPT）.The study of LLPT is of great scientific significance for understanding the nature of melt structural transformation and expanding the application of amorphous liquids.Focusing on the Pd-Ni-Cu-P MGs system,this thesis studies the atomic structural evolution of LLPT during cooling by means of flash differential scanning calorimetry（FDSC）analysis combined with ab initio molecular dynamics（AIMD）and Reverse Monte Carlo（RMC）simulations and clarifies the evolution law of the local atomic structure of the melt with different dynamic conditions and compositions.The key findings of the study are as follows:（1）Evidence of the abnormal endothermic LLPT was first found in Pd₄₃Ni₂₀Cu₂₇P₁₀alloy liquid.The FDSC captured a tiny anomalous endothermic peak around 893K during cooling.Show by the AIMD simulations,the change of the local environment of P atom at short-range to medium-range scale during the LLPT is the atomic mechanism of the structural transformation.Cu atoms and surrounding coordination P atoms form a new octahedral skeleton structure during the LLPT,which hinders the formation of specific icosahedral clusters.The occurrence of LLPT has a significant impact on both thermodynamic and dynamic behavior.（2）The influence of P content on the cooling process of Pd-Ni-Cu-P MGs liquid was systematically studied.As the P content in the system increases from 10%to 20%,the LLPT exists,but the temperature of the LLPT increases.The increase of P content leads to a decrease in the distance of the nearest neighbor atom in the alloy liquid,the hindrance effect of the LLPT on the icosahedral clusters becomes stronger,and the icosahedral clusters in the alloy liquid after the LLPT decrease.At the same time,the influence of LLPT on dynamic gradually becomes stronger.（3）The structure of the three MGs were characterized by X-ray diffraction,and the effect of P content on the atomic structure of Pd-Ni-Cu-P MGs after solidification was systematically studied by combined AIMD and RMC simulations.The study found that as P content increases,it is not easy to bond between P and P,while P atoms tend to form bond pairs with Pd and Ni atomsThis thesis helps to deeply understand the structural nature of LLPT in MGs.