Temperature And Pressure Induced Atomic Structural Evolution And Phase Transition In Disordered Metals

A combined experimental and theoretical study of the structural signature of Au55Cu25Si20 metallic glass(MG),its behaviors under high-pressure and high-temperature,structural evolution of metallic liquids(Ca,Al)and their corresponding Ca72.7Al27.3 alloy under high temperature and pressure,are presented.A brief introduction,experimental methods and behavior of the studied materials under extreme conditions of temperature and pressure are documented.The structural signature of Au55Cu2sSi20 MG has been elucidated that local atomic packing in this glass is mainly composed of 9,10 and 11-coordinated polyhedra rather than icosahedral-like clusters,in which the small sized clusters prefer avoiding each other while large ones prefer connecting themselves.This anomalous atomic packing structure limits its glass forming ability(GFA)and results in the anomalous shoulder of the first peak in structural factor.With the replacement of 5 at.%Cu atoms by Ag atoms,the spatial connectivity and the atomic packing density in Au55Cu20AgsSi20MG are largely increased,enhancing the GFA,as observed in experiments.The pressure-induced structural change in an Au55Cu25Si20 MG is investigated up to 36.8 GPa by employing in-situ high pressure X-ray diffraction together with theoretical ab initio molecular dynamics simulations.Here we report pressure-induced amorphous-amorphous phase transition in Au55Cu25Si20 MG around 13.1 to 15.8 GPa during compression,which is reversible upon decompression.The theoretical investigations confirm the experimental observation,revealing structural differences in both low and high pressure Au55Cu25Si20 MG,i.e.,different pressure dependences of peak position,volume,energy,density,total and partial coordination number,Bader charge and volume.The charge transfer between elements and local atomic arrangement could be the origin for the pressure-induced amorphous-amorphous phase transition in Au55Cu25Si20 MG,which is very appealing as a source of information from physics point of view.A liquid-liquid transition(LLT)has rarely been reported in metallic liquids.In the Au55Cu25Si20 alloy,however,we discovered a temperature-induced liquid-liquid transition in a temperature range of about 700-900 K by detecting "reversible ?-anomalies" of thermal expansion coefficient between two liquid states deduced from in-situ high temperature synchrotron x-ray diffraction.Both experimental XRD,heat flow,and specific heat capacity measurements and AIMD calculation results with different temperature dependent first peak position of Au-,Cu-,Si-centered partial pair correlation function,coordination number of the Au-,Cu-,Si-centered first shell,Bader volume and charge of Au,Cu,and Si atoms,and total energy and volume of the system below and above the transition provide the evidence for the existence of LLT in the liquid Au55Cu25Si20 alloy,This is the first time to report the existence of LLT in Au55Cu25Si20 metallic liquids,triggering more challenging studies in disordered systems.Behavior of the liquid Ca under various pressures reveal different crossovers and the comparisons of the pressure-dependent bond angle distribution and bond orientation order data of liquid Ca with solid crystalline phases imply that pressure-dependent atomic structural evolution in liquid calcium is,to some extent,similar to that of solid crystalline calcium.The relationship between the atomic structure and dynamic of liquid aluminum(Al)has been studied as a function of pressure.It's found the fortuity of crystalline-like phase first evolves into the metastable body centered cubic(bcc)phase,followed by face centered cubic(fcc/hcp),different from the crystallization mechanism proposed by Desgranges et al.for the supercooled liquid Al in which the metastable phase is avoided.Moreover,the crystallization in liquid A1 is distinctly correlated with bond orientational order(Q6)fluctuations,instead with density fluctuations.Interestingly,density calculation as the function of temperature with two fast and slow cooling rates,differs the idea proposed by Desgranges et al.for polymorph selection.Structure,dynamics and the mechanism of pressure induced structural evolution in Ca72.7Al27.3 glass forming liquid alloy is elucidated.Our results clearly reveals that,molten Ca72.7Al27.3 alloy undergoes structural and electronic liquid-liquid transition under pressure,analogous to AAT observed in the solid Ca72.7Al27.3 MG.These finding suggest that A1 aggregates already exist in the liquid Ca72.7Al27.3 glass forming liquid instead of common belief that they form during quenching.