Study On The Coupling Relationship Between Thermophysical,Structural And Dynamical Properties Of Al And Al-Fe Melts

Thermophysical,structural and dynamical properties of liquid metals play an important role in nucleation,crystallization growth and glass transition.Even though there are many studies on those properties,the relationship between them is still unclear.Consequently,they are the research focus of material science and condensed matter physics.In this work,using molecular dynamics simulation,we studied the thermophysical,structural and dynamical properties of a monoatomic melt(using liquid Al as the model)and a binary melt(using Al-Fe liquid alloys as the model)to explore the possibly underlying coupling relationship.One fundamental and characteristic thermophysical property of liquid metals is its undercooling ability.By experimental measurement and theoretical analysis,we found that the undercooling of liquid metals is closely associated with experimental conditions.Generally,the impurity and oxidation in liquid metals can vigorously decrease the measured undercooling.So,the intrinsic undercooling,i.e.,the difference between the homogeneous nucleation temperature and the liquidus line T_L,is always higher than the available experimental undercooling.The experimental result of aerodynamic levitation indicated that there still exists the possibilities to improve the measured undercooling.Combined the classical nucleation theory with the solid/liquid interface energy theory,we pointed out that the intrinsic undercooling has a upper limit which is 0.67T_L,we also derived an expression connecting the intrinsic undercooling with the molar fusion entropy.The expression implies that the intrinsic undercooling is actually determined by other thermophysical properties.The solidification of liquid metals is strongly influenced by cooling rate.A faster cooling rate leads to a lower solidification point and a larger undercooling.As cooling rate increases,fcc and hcp order structures decrease and disordered structure increases.If cooling rate exceeds the critical glass transition velocity,glass transition takes place at T_g and the final state becomes amorphous structure.By analyzing the liquid structure under different cooling rates,we identified that a slower cooling rate favors a more pronounced solid-like cluster in liquid.This finding is consistent with the prediction of classical nucleation theory,which can provide new insights into the mechanism underlying the effect of cooling rate on solidification behavior.With the decrease of temperature,there is a continuous and persistent change in liquid structure,thermophysical and dynamical properties.For liquid structure,the fraction of atomic clusters obviously increases and the free volume decreases.For thermophysical properties,enthalpy(H)and potential energy(P_e)are gradually reduced while density(?)has a steady rise.For dynamical properties,the diffusion coefficient(D)decays,whereas the viscosity(?)grows,following an approximate Arrhenius relationship.However,after a carefully examination we found that one Arrhenius fitting is not enough to characterize the evolution of D and?.A kink is found at T_x~1.2T_L.At T_x,we also found the breakdown of SER and the abnormal enhancement of dynamical heterogeneity.A fine structural analysis demonstrated that icosahedral cluster and local five-fold symmetry has a sudden rise at T_x,indicating a close correlation between liquid structure change and dynamics behavior.This work argues a couple relationship between the thermophysical,structural and dynamical properties of liquid metals.